untitled 148 Dynamic development of the thin-skinned thrust faulting The dynamic development of the thin-skinned thrust faulting in the Rubjerg Knude Glaciotectonic Complex is presented as a sequence of restoration stages. Thus, the progressive deformation of thin-skinned thrust faulting and related syntectonic depositional devel- opments are illustrated in sequentially restored cross- sections beginning with the proximal Moserende Sec- tion and concluding with the Ulstrup Section in the most distal part of the thrust-fault complex. The basis of each restoration sequence is the balanced profile (Plate 2A), and the end stage is identical with the thrust- fault cross-section (Plate 2B), including the interpre- tation of the unexposed ramps and flats in the subsur- face. The most proximal sections, the Mårup Kirke and the Ribjerg Sections were interpreted individually in the preceding chapters, and are not included here. In a summary scheme (see Fig. 123), it is concluded that the dynamic development was a process of con- tinuous progressive deformation. Thus, although the following description is concentrated on the individual sections, it should be kept in mind that there is over- lap between sections, and that the whole system was mobile. Thus a displacement of 5 m on one thrust might be followed by 10 m on a more proximal thrust and 7 m on a more distal thrust depending on the local conditions. This is the reason why a number of displacements appear to be out-of-sequence, but with- in limits that respect the lowest décollement level, and that displacements along the most distal, leading-edge thrusts were the last to be activated. It is therefore also evident that displacement along a leading-edge ramp may correspond to a translation along a flat in a proximal section. Moserende Section The thrust-fault development in the Moserende Sec- tion is regarded as normal progressive piggyback thrusting from the proximal towards the distal part. The MR12 thrust sheet was probably the first to be thrust onto the relatively thinner piggyback basin on the back of MR11 after a c. 10 m thickness of Rubjerg Knude Formation sediments had been deposited. This is included in the first stage of the sequential restora- tion (Fig. 112, stage 1). A total of eight stages have been dif ferentiated, of which stages 1–6 are illustrat- ed in Fig. 112. The stage preceding the deformation is shown in Plate 2A, and the final stage terminating the deformation is reconstructed in Plate 2B. Moserende stage 1. The initial thrusting started with 40 m displacement of MR12 over the back of what was to become MR11. This thrusting was rooted down to the 20 m intermediate décollement level. During accumulation of a 20 m thick succession of sediments in the piggyback basins above the MR13 and MR12 sheets, the thrusting progressed with ramping of MR11 over the Rubjerg Knude Formation on top of what was to become MR10. This thrusting involved ramp- ing and translation of the lower segments of MR13– MR11 from the 30 m flat level onto the 20 m flat level. The trailing-end segments of the Moserende Section were contemporaneously over-thrust by MK01, the frontal thrust of the Mårup Kirke Section, which is rooted in the 40 m décollement level. The accumulat- ed displacement of thrusting of MR13, MR12 and MR11 is estimated at about 150 m. Moserende stage 2. Thrusting of MR09 initiated this stage. The MR09 thrusting ramped up from the 40 m décollement level, and a single duplex formed during stacking of the lower MR09 thrust segment. The MR09 sheet was displaced c. 40 m over the MR08 piggyback basin. Contemporaneously, MR10 was thrusted over MR09 and the MR10 hanging-wall flat extended from the top flat level down to the 40 m décollement level. The MR13–MR11 thrust sheets were then passively translated on the trailing lower segment of MR10. Moserende stage 3. Initial imbrication of the MR08– MR05 thrust sheets resulted in an accumulated dis- placement of c. 200 m. The ramping was rooted in the 40 m décollement level along which the main transla- tion of the trailing-end thrust sheets of the Moser- ende Section took place. The thrusting involved a complex relationship between MR07 and MR06 that may be interpreted as a connecting splay duplex (Mi- tra & Sussman 1997). Above the L/R-unconformity, the deposits of the Rubjerg Knude Formation probably reached a thickness of 20 m. Moserende stage 4. The frontal part of the section was activated by c. 40 m translation of MR1 along the 30 m 149 décollement level over the lowermost trailing-end seg- ments in the Stortorn Section. MR02 and MR03 fol- lowed this translation, whereas MR04 ramped up one level from the 40 m décollement level to the 30 m flat level that resulted in the initial ramping of MR04 up over the Rubjerg Knude Formation on the back of MR03. The continued displacement consequently re- orientated thrust sheets MR05–MR07 into more steep- ly dipping orientations. The trailing-end thrust sheets from MR08 and northwards were translated passively during this displacement. Moserende stage 5. The frontal displacement of MR01 continued along the 20 m flat level over the trailing- end segment of the Stortorn Section. MR02 ramped up along the footwall ramp at the trailing end of MR01 during a fault-bend rotation, which also included the lower segment of MR01u. A vertical thrust separation of c. 10 m brought MR02 up along the northern termi- nation of the MR01 piggyback basin. During the pas- sage of two intermediate ramps, an irregular anticline formed on the back of MR02 that had significant im- plications for the synsedimentary structures formed in the MR02 piggyback basin (see description of the Moserende Section, above). During MR04 thrusting, MR03 was imbricated along the upper 10 m flat level and the MR03b and MR03c thrust segments started to break through the piggy- back basin. From the rear, MR04 was pushed by MR05 which had to pass up over the fault-bend-folded seg- ment MR04u. Together with MR06 and MR07, the MR05 thrust sheet moved up to the highest level indicated by the L/R-unconformity, situated c. 20 m above sea level on the back of MR06 and MR07, and their thrust faults were steepened into a nearly vertical position. Moserende stage 6. In the frontal part of the Moser- ende Section, MR01 picked up a lower segment and thrust up to the 20 m flat level, which consequently also elevated the piggyback basin up into its present high level. The trailing-end ramp of MR01 formed the footwall ramp for the MR02 thrusting, which resulted in a fault bend of MR02 as well as MR03. This was followed by the final displacement of 18 m along the leading MR03 thrust. Minor adjustments and re-orien- tation of MR04–MR07 followed the ramping of MR03, and the trailing-end thrust sheets MR08–MR13 were passively displaced by translation along the 40 m dé- collement level. Moserende stage 7. During this stage, a complex du- plex was formed by thrusting of the frontal lower seg- ments, which also including the trailing-end segments of the Stortorn Section. Moserende stage 8. The final displacement along the leading thrust-fault ramp in the Moserende Section progressed up along the Stortorn trailing-end foot- wall ramp. Moreover, the fault-bend folding due to thrusting in the Stortorn Section brought the thrust sheets into their present steeply dipping orientation. Moserende Section: summary data Balanced length (L0): 1120 m Cross-section length (L1): 650 m Shortening (ΔL): 470 m Compression: 40.2% Stortorn Section The most important development in the Stortorn Sec- tion was the change from the lowermost 40 m décol- lement level to the 30 m décollement level. The ramp, or progressive development of lower ramps, which marked the change, is here referred to as the Stortorn lower segment footwall ramp, and was located some- where near the thrust between ST04 and ST03. Thus, the ST03–ST01 thrust sheets had their lower décolle- ment level at 30 m, whereas the thrust faults related to ST04–ST10 were rooted in the 40 m décollement level. Formation of a duplex complex comprising the low- ermost thrust segments exposed the Stortorn Forma- tion, the oldest strata involved in the thrusting. In the frontal part of the section, a complex stacking of low- er segments, remaining in the subsurface from dis- placement in the Grønne Rende Section, resulted in duplex formation that elevated ST01–ST03 about 30 m above the reference level. Due to arguments present- ed later (see Grønne Rende Section) the duplex stack- ing had to have been contemporaneous with the short- ening of the Grønne Rende Section. In the Stortorn Section, seven stages have been differentiated of which stages 1–5 are illustrated in Fig. 113. Stortorn stage 1. This stage is a direct continuation of the displacement in Moserende stage 4. In the Stor- torn Section, deformation was initiated by imbrica- tion of ST07, ST09 and ST10 with an accumulated dis- placement of about 50 m. This resulted in a ramping 150 Skærumhede Group Active thrust fault Rubjerg Knude Formation Moserende 2 MR13 MR12 MR11 M MK1 Moserende 3 MK1 MR13 MR12 MR11 Moserende 4 MK1 MR Moserende 5 MK1 MR13 Moserende 6 MK1100 m N S Moserende 1 MR13 MR12 MR11 MR10 MK1 Fig. 112. Dynamic model of progressive defor- mation in the Moserende Section illustrated in six sequential restoration cross-sections. The six stages demonstrate steps in the development between the balanced cross-section and the structural cross-section (Plate 2); thus the starting and final positions are not shown. The red lines indicate the active displacement surfaces in each deformation stage. The basic décollement sur face is the 40 m flat level. From this, the flat levels rise by 10 m onto the reference level (L/R- unconformity) defined as the 0-level. Note (1) that the final two stages (7, 8) discussed in the text are not illustrated, and (2) that the thrust- sheet terminology in Figs 112–122 is simplified (i.e. MR 3 on Fig. 112 is equivalent to MR03 in the text). 151 Stortorn footwall ramp Stortorn footwall ramp Stortorn footwall ramp MR10 MR9 MR8 MR7 MR6 MR5 MR4 MR1 ST10 MR10 MR9 MR8 MR7 MR6 MR5 MR1 ST10 MR1 ST10 R13 MR12 MR11 MR10 MR9 MR8 MR7 MR6 MR5 MR4 MR3 MR2 Stortorn footwall ramps 3 MR12 MR11 MR10 MR9 MR8 MR7 MR6 MR5 MR4 MR4u MR3a MR3u MR3c MR3b MR2 MR1a MR1u MR1 ST10 ST10 Stortorn footwall ramps MR13 MR12 MR11 MR10 MR7 MR6 MR5 MR4 MR3c MR3b MR3a MR2 MR1 ST10 ST10u ST10u MR8 MR9 Stortorn footwall ramp MR9 MR8 ST10 MR1 152 100 m N S Skærumhede Group Active thrust fault Rubjerg Knude Formation MR1 ST10 ST9 ST8 ST7 ST6 Stortorn 2 ST5 MR1 MR1 ST10u ST10 ST9 ST8 ST7 ST6 ST5uST8uST8uST9u Stortorn 1 Stortorn footwall ramp ST10MR1 ST9 ST6 ST7 ST8 Stortorn 4 Stortorn footwall ramps Stortorn 5 Stortorn footwall ramps ST10 MR1 ST10 ST9 ST8 Stortorn 3 Stortorn footwall ramps Fig. 113. Dynamic model of progressive deformation in the Stortorn Section illustrated in five sequential restoration cross-sections. The cross-sections demonstrate steps in the development between the balanced cross- section and the structural cross-section (Plate 2); thus the starting and final positions are not shown. The red lines indicate the active displacement sur faces in each deformation stage. Note that the duplex segments GR u refer to elements that had to be deformed contem- poraneously with the shortening taking place in the Grønne Rende Section; duplex seg- ments ST s refer to horse/splint segments. The final two stages (6, 7) discussed in the text are not illustrated. 153 ST4 RF6 ST1s ST1ST2 ST3 ST4 ST4s ST4s ST4s ST4s ST2s ST1s Stortorn lower ramp Rubjerg Knude Fyr ramp frontal footwall ramp ST5 ST5 ST4 ST2ST3 Rubjerg Knude Fyr footwall ramp Rubjerg Knude Fyr footwall ramp Stortorn lower segment footwall ramps Rubjerg Knude Fyr footwall ramp Stortorn lower segment footwall ramps ST9 ST7 ST6 ST5 ST4 ST3 ST2 ST1 RF6GRu GRu GRu ST8 ST6 ST5 ST4 ST3 ST2 ST1 ST2s RF6 Stortorn lower footwall ramp RF6 ST1s ST1 ST1s RF6 154 up of ST10 from décollement level 40 m to flat level 30 m, along which the translation displacement took place. Most of the ST09 thrust sheet was also ramped up by the formation of a lower duplex structure. Both ST10 and ST09 were affected by fault-bend folding, which created a major distortion of the L/R-unconform- ity surface in the uppermost part of the thrust sheets. Stortorn stage 2. During an accumulated displacement of about 200 m related to the ST10 and ST09 thrusts, ramping progressed with development of the first imbrications of ST08 and ST06. Due to ramping from the lowest décollement level to the 30 m flat level in the trailing end of ST06, a fault bend affected the ST07– ST10 thrust sheets that were translated piggyback on the ST06 thrust sheet. This contributed to the steep- ening up of the ST07–ST10 thrust structures. In the frontal part of the section, the imbricate thrust- ing was initiated at ST01–ST03. Accumulation of sedi- ments referred to the Rubjerg Knude Formation reached a maximum thickness of about 20 m, notably in the synformal troughs of ST03 and ST09 that formed during the progress over the ramps below. Stortorn stage 3. At this stage, ST05 was thrust about 40 m up over the footwall ramp on the back of ST04. The ST05 thrust was rooted in the 30 m flat level, and during a passage of a lower ramp from flat level 30 m to 20 m, the initial fault-bend-fold resulted in undula- tion of the L/R-unconformity at the top of the ST05 thrust sheet. The ST06 thrust sheet progressed over the footwall flat of ST05, and both thrust faults were rooted down to the 30 m flat level along which the main translation of the sheets emplaced piggyback on ST06 took place. The ST08 thrust sheet was finally displaced along the upper flat at the top of the ST07 piggyback basin. Consequently, most of the 20 m thick succession in this piggyback basin was preserved and indicates the maximum level of sediment accumula- tion in the Rubjerg Knude Formation during stage 3. Thrusting of the ST08 sheet along the footwall ramp on the back of ST07 resulted in a further steepening of ST09 and ST10, while the frontal elevated parts of the ST08–ST09 thrust sheets became subject to erosion. The trailing-end lower segments of ST10–ST07 were over-thrust by the frontal parts of MR01 and MR02, corresponding to stage 7 in the Moserende Section. The accumulated displacement in Stortorn stage 3 was of the order of 320 m. Stortorn stage 4. During this stage, the ST04 thrust sheet was thrust 40 m over the piggyback basin of ST03, and ST05 was thrust about 70 m over the upper flat on top of the piggyback basin of ST04. During this relatively large displacement of ST05, two lower duplex segments were picked up from the lower 40 m décollement level. After ramping over the Stortorn lower ramp, the duplex segments participated in the thrusting up along the footwall ramp on the back of ST04. The lower trailing-end segments of the Stortorn Section were finally thrust up along the steep foot- wall ramp on the back of ST10 and subsequently the frontal parts of the Moserende Section were brought into their present upright orientation. Erosion and re- deposition affected the piggyback basins on ST05 and ST08, whereas thrusting over ST07 and ST04 sealed these piggyback basins. The accumulated displace- ment reached about 410 m. Stortorn stage 5. A substantial displacement, in the order of 80 m, took place along the leading thrust in the Stortorn Section at this relatively late stage of de- velopment of the structures at Stortorn. However, this is only a small amount of the accumulated displace- ment (c. 500 m) which is of the same order of magni- tude as that taken up by the duplex stacking of the lower trailing-end segments of the Rubjerg Knude Fyr and Grønne Rende Sections. The ramping and thrust- ing of ST01–ST04 over this duplex structure explains the high elevation of the L/R-unconformity and over- lying piggyback basins in the frontal part of the Stor- torn Section. The formation of the duplex stack com- prising the lower duplex segments annotated GRu in Fig. 113 would have taken place only after the imbri- cate thrusting in the Grønne Rende Section developed (see below). The combination of displacement at the leading edge in one section and stacking of lower duplex segments in another, indicates a continuous progressive thrust-fault evolution. During the propagation of ST05, the trailing end of ST04 was involved in a duplex formation that result- ed in fault-bend folding of the earlier formed ST05 lower duplex at the Stortorn lower ramp. The piggy- back basin on the back of the ST05 thrust sheet was deformed into a north-verging syncline due to steep- ening. A similar re-orientation is seen in the thrust- isolated piggyback basins in ST10 and ST09. A marked diapirism and remobilisation of mud in the ST01–ST03, ST05–ST07 and ST09 thrust sheets sug- gests that the diapirism was related to the intensity of ramping, especially when the ramping involved the 155 Fig. 114. Dynamic model of progressive deformation in the Rubjerg Knude Fyr Section illustrated in four sequential restoration cross-sections. The cross-sections demonstrate five stages in the development between the balanced cross-section and the structural cross-section (Plate 2). The red lines indicate the active displacement surfaces in each deformation stage. Note that the tip of the RF04 thrust sheet was displaced by normal faulting during syntectonic deposition in the RF03/RF04 piggyback basin. lower level segments and fault-bend folding of these segments. Stortorn stage 6. The final ramping of lower segments from décollement level 40 m to flat level 30 m at the base of ST05 terminated the translation along the low- ermost 40 m décollement level. For the sections fur- ther south, the lower décollement level was situated at the 30 m level. In the frontal part of the section, continued minor compression steepened the thrust structures, and the tips of ST02 and ST03 were eroded and deposited in the piggyback basin of ST01. Stortorn stage 7. The structural complex, including the 156 ST01 thrust sheet and the underlying duplex struc- ture, became fault-bend-folded during the thrust prop- agation related to the progressive deformation in the Rubjerg Knude Fyr Section. Stortorn Section: summary data Balanced length (L0): 1125 m Cross-section length (L1): 570 m Shortening (ΔL): 555 m Compression: 49.3% Rubjerg Knude Fyr Section The Rubjerg Knude Fyr Section roots into the 30 m décollement level. The most striking features devel- oped in the Rubjerg Knude Fyr Section are the large olistoliths in the piggyback basin that were derived from the collapse and gravity gliding of a projecting segment of ST04. Five stages in dynamic development have been distinguished, which are illustrated by four cross-sections in Fig. 114. Rubjerg Knude Fyr stage 1. During sedimentation of the first 10 m of sand of the Rubjerg Knude Forma- tion, the RF05, RF04 and RF03 thrust sheets were thrust up along their footwall ramps. RF04 was displaced 90 m along the upper flat level (10 m level) before the frontal part propagated up along the upper ramp. With a displacement of about 30 m, this brought the nose of the RF04 thrust sheet up into the open air, above the sedimentation level of the Rubjerg Knude Forma- tion. The displacement on the other two thrusts amount- ed to c. 20 m, implying an accumulated displacement of 70 m. Rubjerg Knude Fyr stage 2. The exposed nose of the RF04 thrust sheet slumped down along a normal fault into the piggyback basin of RF03. At the same time, the frontal nose of RF03 was eroded away and sedi- mentation of the Rubjerg Knude Formation onlapped and covered these features. At the leading edge of the section, thrusting was initiated that brought RF01 and RF02 up over what was to become the trailing- end segments of the Grønne Rende Section. Rubjerg Knude Fyr stage 3. The frontal imbrication of RF01 and RF02 progressed during sedimentation up to about 20 m above the main L/R-unconformity lev- el. The RF05–RF06 thrust sheet ramped up onto the intermediate flat above the trailing-end segment of RF04. The RF04 thrust sheet was displaced about 70 m up along the relatively steep footwall ramp at the trailing end of RF03. Due to the fault-bend folding of RF04, the RF05–RF06 hanging-wall ramp was rotated into a vertical position. Rubjerg Knude Fyr stage 4. When the second ‘drop’ of the frontal part of thrust sheet RF04 took place, a c. 45 m long slab of the relatively thin thrust-sheet nose slumped down along a normal fault with a vertical separation of more than 10 m. The ‘drops’ may be regarded as two break-back sequences of the RF04 thrust sheet (in the terminology used by Mitra & Suss- man 1997; see Figs 99, 100). Sediment accumulation continued in the piggyback basin to a thickness of more than 30 m, including the ‘dropped’ noses of RF04. The final accumulation in the piggyback basin took place while the displacement in the Rubjerg Knude Fyr Section was concluded more than 500 m laterally to the south. The translation progressed along the 20 m flat level on top of what was to become the lower trailing-end segments of the Grønne Rende Section. Rubjerg Knude Fyr stage 5. The continued displace- ment of RF04 resulted in structural propagation of this sheet above its own piggyback basin with the ‘dropped’ thrust noses. Stage 5 in the Rubjerg Knude Fyr Section is interpreted to have been contempora- neous with stage 7 in the Stortorn Section in which compression brought the thrust sheets into their final, steeply inclined position. Rubjerg Knude Fyr Section: summary data Balanced length (L 0 ): 525 m Cross-section length (L 1 ): 260 m Shortening (ΔL): 265 m Compression: 50.5% Grønne Rende Section The impressive imbricate fan composed of 12 upright thin thrust sheets is the essential element in the Grønne Rende Section. As a consequence of the displacement in the imbricate fan, 550 m of trailing-end lower seg- ments were left behind to be stacked in a duplex be- low the frontal part of the Stortorn Section (Fig. 113). 157 Four stages have been differentiated in the develop- ment of the Grønne Rende Section, the first three of which are illustrated in Fig. 115. Grønne Rende stage 1. The initial thrust-fault frame- work was a low-angle imbrication, about 20° on each upper hanging-wall ramp, which rooted down to the upper 10 m flat level. During thrusting, the upper thrust sheets were split up into three main segments with leading thrust faults below GR02, GR06 and GR11/ GR12 which ramped down to the main level of de- tachment in the 20 m flat level. The initial displace- ment of the imbricate fan is regarded to have been 20 m on each thrust. This implies that the accumulated dis- placement sums up to 240 m. GR01 was not affected by thrusting in the first stage, and 240 m of its lower trailing-end segment was consequently not displaced during this stage. The sediments of the Rubjerg Knude Formation at- tained a maximum thickness of 15 to 20 m during this stage, with decreased thicknesses on the back of the GR06–GR08 thrust sheets, which were elevated to the highest position. Grønne Rende stage 2. The imbricate thrusting pro- gressed with a displacement of 50 m on each thrust. This implies that the hanging-wall flats were fault-bend- folded while they passed the footwall ramps, result- ing in a dramatic steepening of the thrust sheets. Be- low GR10–GR12, the GR07u and GR08u lower seg- ments formed a duplex structure that resulted in ele- vation and complex ramp-propagation folding of the sheets above. The accumulated displacement implies an increase in length of the trailing-end segment of GR01 in the order of 500 m, allowing for some adjust- ments due to the irregular duplex deformation. Sedi- ment thicknesses in the piggyback basin in the front- al part of the section increased to 25–30 m. Grønne Rende stage 3. Finally, the leading-edge thrust was activated and GR01 was displaced 50 m up along its footwall ramp. The GR01 thrust roots in the lower 30 m décollement level, and the displacement of the hanging-wall flat up along the footwall ramp resulted in steepening of all the early-formed thrust elements (GR02–GR13). The displacements of the individual thrust sheets range between 60 and 70 m. The thrusting resulted in the final, almost vertical, orientation of the thrust sheets. In the rear part of the section, complex defor- mation of the duplex below GR10–GR13 was reflect- ed in unusual folding of the beds in the GR13 thrust sheet where folds with horizontal axial planes were formed due to gravity collapse of the piggyback ba- sins. Grønne Rende stage 4. This stage concluded the thrust- ing of the leading hanging-wall ramp-and-flat over the footwall ramp in the trailing end of the Stenstue Rende Section and the subsequent final rotation of the GR02– GR05 thrust sheets. In the trailing end of the section, the RF01 and RF02 sheets concluded the displacement by thrusting from the trailing-end segments of GR12 up over the footwall ramp onto the back of GR13. Moreover, GR13 was rotated into an upright position whereby the horizontal axial planes became vertically orientated (Plate 1). Grønne Rende Section: summary data Balanced length (L0): 1080 m Cross-section length (L1): 423 m Shortening (ΔL): 657 m Compression: 60.8% Comment. The lengths are measured from the foot- wall ramp between RF01 and GR13 to the footwall ramp between GR01 and SS06, near the thrust trunca- tion of the L/R-unconformity. Stenstue Rende Section Two markedly different structural complexes were formed during the development of the Stenstue Rende Section. They were mainly caused by the displace- ment of the same thrust sheet (SS01) when it was dis- placed 200 m over the upper flat on top of the piggy- back basin in the Sandrende Section. The frontal part of SS01 above the footwall flat of the SR04 thrust sheet is one of the complexes. The other structural com- plex is the chaotic breccia and gravity slumping in the northern part of the section that formed as the piggy- back thrust sheets were transported over a minor an- tiformal stack in the central lower part of the section. The progressive dynamic development in the Sten- stue Rende Section is described in terms of five stages, the first four of which are illustrated in Fig. 116. Stenstue Rende stage 1. Four minor imbrications with an accumulated displacement of 70 m initiated the 158 development in the Stenstue Rende Section. At the leading-edge thrust, a minor connection splay sepa- rated SS02 and SS03. Most of the thrusting was locat- ed at the upper 10 m flat level for a distance of about 180 m, in the northern part of which it was eventually rooted down to the lower décollement level. The dis- placement of the SS05 thrust sheet followed the same system, but with a smaller translation along the upper 10 m flat level. At the trailing end, SS06 was thrust up along a steep footwall ramp, and here the formation of duplex structures was probably initiated. The thick- ness of sediment (Rubjerg Knude Formation) that had accumulated by this stage amounted to 10 m. Stenstue Rende stage 2. The leading-edge thrusting shifted to the SS01 thrust sheet, which was displaced 20 m up along the footwall ramp (the trailing end of SR04 in the Sandrende Section). The SS01 thrust fault extended down via an intermediate ramp to the 20 m flat level, and about 200 m from the leading footwall ramp it stepped down the lower ramp to the 30 m décollement level. At the upper hinge of the lower ramp, SS01 was folded into a fault-bend anticline, a small detachment anticline. Along the foreland-dip- ping limb of the anticline in the SS01 thrust sheet, a normal fault was formed that displaced the tip of the SS02 thrust sheet. Furthermore, the SS03 thrust sheet Fig. 115. Dynamic model of progressive deformation in the Grønne Rende Section illustrated in three sequential restoration cross-sections; the final stage (4) described in the text is not illustrated. The red lines indicate the active displacement surfaces in each deformation stage. Note how the shortening due to the displacement along the 20 m flat level resulted in the substan- tial length of the ‘left over’ lower duplex segment between the 20 and 30 m flat level. 159 became steeply inclined, and the initial imbrication of the thin SS03 thrust sheet resulted in the separation of SS03 from SS04. In the trailing end of the Stenstue Rende Section, duplex stacking of the lower segments in SS06 result- ed in elevation of the L/R-unconformity more than 10 m above the mean level. The accumulated displace- ment ranged up to 160 m. Stenstue Rende stage 3. Thrusting of SS04 progressed on the upper flat over the piggyback basin of SS03 with a frontal displacement of 80 m. The hanging- wall flat of SS04 ramped up along the footwall ramp of SS03 and during this translation the nose of SS05 became fault-bend-folded into a syncline with a steeply dipping southern limb. The trailing end of the SS04 thrust sheet was translated along the 10 m flat level; it was pushed from the rear by the ramping of the trail- ing end of the SS05 thrust sheet whereby the SS06 thrust sheet also steepened up. Sediment thicknesses in the piggyback basins increased to c. 20 m, and the accumulated displacement ranged up to 240 m. Stenstue Rende stage 4. The dramatic major foreland thrusting of the SS01 thrust sheet, which included about 200 m displacement of the hanging-wall ramp 160 Fig. 116. Dynamic model of progressive deformation in the Stenstue Rende Section illustrated in four sequential restoration cross- sections; the final stage (5) described in the text is not illustrated. The cross-sections demonstrate the development stages between the initial and final positions displayed in the balanced and the structural cross-sections in Plate 2. The red lines indicate the active displacement sur faces in each deformation stage. 161 over the piggyback basin of the Sandrende Section, occurred contemporaneously with the formation of an antiformal stack above the trailing end of SS01. The creation of the antiformal stack had already been initia- ted by the earlier formation of the minor detachment anticline at the ramp splitting the lower segments of SS01 (the SS01u segments). A duplex duplication of the lower SS01u segments accentuated the anticline, and finally the SS03 thrust sheet riding piggyback on SS01 was folded into an anticline with a steep fore- land-dipping southern limb (Fig. 90). Along this limb, a normal fault developed that displaced the frontal part of the SS04 thrust sheet. A chaotic soft sedimen- tary fault breccia was formed during the stretching and fault separation of SS04 (Fig. 91). Due to an extra push from the rear, the SS05 thrust sheet was displaced a further 30 m to the south, which resulted in the formation of a huge southerly overturned slump fold above the normal fault zone (Fig. 89). The accumulated displacement totals about 470 m. The displacement of the SS01 hanging-wall flat up along the steeply dipping footwall ramp constrains the sequential thrusting of the Stenstue Rende rela- tive to the Sandrende thrusting. Thus stage 4 could not have begun before the maximum sedimentation in the piggyback basin was accomplished in the Sand- rende Section. The initial SS01 thrusting could be re- garded as a growth fault, whereby the syntectonic accumulation of sand added to the steepening of the footwall ramp. The present vertical to northerly over- turned orientation of the SS01 hanging-wall flat and ramp resulted from differential thrusting and fault-bend of the SS01u lower hanging-wall ramp. Note also the re-orientation of the normal fault at the tip of SS02, which due to the same deformation was bent into a horizontal position. Stenstue Rende stage 5. This stage corresponds to stage 6 in the Sandrende Section, wherein the SS01 thrust sheet riding piggyback on SR04 was displaced by normal faulting (Fig. 117, stage 6). Stenstue Rende Section: summary data Balanced length (L 0 ): 760 m Cross-section length (L 1 ): 285 m Shortening (ΔL): 485 m Compression: 62.5% Comment. The lengths are measured from the foot- wall ramp between GR01 and SS06 to the footwall ramp between SS01 and SR04. If the compression was calculated from the leading-edge thrust tip of SS01 to the trailing-end footwall ramp of SS06, L1 amounts to 455 m, ΔL = 305 m and the calculated compression would only be 40.1%. Sandrende Section The dynamic development of the Sandrende Section was formerly interpreted as a combination of diapir- ism and normal faulting caused by volume exchange during thrust propagation (Sadolin et al.1997). The model presented here aims at an explanation of the development purely based on a thin-skinned thrust- fault model including dif ferential ramping and duplex formation. Thus, the diapirism is interpreted to be an effect of ramping and fault-bend folding growth, sim- ilar to the model of Mitra & Sussman (1997), but also including mud-mobilisation and exaggeration of back- limb thrusting. The normal faulting occurring in the Sandrende Section is interpreted as the effect of dif- ferential ramping of a lower trailing-end segment that created foreland-dipping features above a hanging- wall ramp propagation along an intermediate foot- wall flat. Six stages of dynamic development have been dif ferentiated in the Sandrende Section (Fig. 117). Sandrende stage 1. After initial deposition of a 3–5 m thick succession of Rubjerg Knude Formation sedi- ments, the SR04 thrust sheet started thrusting about 50 m over the upper flat. The dip of the footwall ramp was relatively gentle, only c. 14°, and in the 15 m flat level the thrust fault may be traced along a minor flat segment on top of the lower trailing-end segment of SR03 (SR03u). From the minor intermediate flat, the thrust fault rooted down to the 30 m décollement lev- el along a 20° dipping footwall ramp of SR03u. Note that an upper and lower SR04 hanging-wall ramp was introduced subsequently. Sandrende stage 2. Translation of the lower SR04 hang- ing-wall ramp along the intermediate flat established the anticline in the central part of the SR04 thrust sheet. The SR03 thrust sheet started to propagate towards its foreland along the upper 10 m hanging-wall flat, and the frontal part of SR03 was displaced 50 m over the upper flat on top of the piggyback basin of SR02. The tip of the SR02 thrust sheet propagated up along a growth-fault ramp, which caused the steeply dipping 162 163 orientation of the northern boundary of the piggy- back basin at the top of the SR01 thrust sheet. The accu- mulated displacement ranged up to about 150 m, in- cluding the initial thrusting of SR01. Sandrende stage 3. During stage 3, the thickness of the sediments of the Rubjerg Knude Formation reached 20 m in the piggyback basins in the Sandrende Sec- tion. In the basin at the top of the SR04 thrust sheet, the thickness varied considerably. The reason for this variation is that the top of the anticline above the SR04 lower hanging-wall ramp was subjected to ero- sion while deposition continued in the frontal part, south of the anticline, as well as in the basin north of the anticline. On the foreland-dipping flank of the anticline, minor sets of normal growth faults governed sedimentation (Fig. 87). The tip of the SR04 thrust sheet suffered minor erosion before deposition re- sumed during thrust propagation. This is document- ed by the angular onlap relationships described by Sadolin et al. (1997). Sandrende stage 4. The thrusting of SR04 continued with 50 m further displacement. Below the trailing end of the SR04 thrust sheet, the SR03u lower seg- ment was picked up and displaced onto the footwall ramp of SR02. This minor duplex and ramp thrusting accentuated the SR04 hanging-wall anticline, and nor- mal faulting on the foreland-dipping limb progressed. Above the crest of the SR03u detachment anticline, a significant normal fault complex developed. Here in the SR04 thrust sheet, a dense network of conjugate normal faults (Fig. 85) resulted from lateral extension due to flexural slip bend over the upper hinge of the lower footwall ramp. Sandrende stage 5. The thrusting of SR01 propagated up along the lower and intermediate footwall ramp of the trailing segments of the Brede Rende Section. Dur- ing this ramping, the hanging-wall flat of SR02 pro- gressed up over the piggyback basin of SR01. The tips of the SR03 and SR04 thrust sheets thus experienced fault bending up along the footwall flat of SR02. The atypical northerly overturned tip at the top of the SR02 sheet probably formed due to accentuated reverse fault- ing along a former established back-thrust. In the trail- ing part of the section, a minor satellite splay thrust de- veloped, which broke through the SR04 thrust sheet from the hanging-wall flat to the footwall flat below SS01. Sandrende stage 6. The final development of the Sand- rende Section was dominated by complex duplex for- mation and fault-bend folding of the SR01 thrust sheet below the frontal part of SR02. During the thrust pro- pagation over the footwall ramp of the trailing-end segments of the Brede Rende Section, a fault-bend- folded syncline was formed in SR01, which resulted in normal fault displacement of the SR01 piggyback basin and the overlying frontal part of the SR02 thrust sheet. Similar normal faulting affected the SS01 thrust sheet, which had over-thrust the piggyback basin on the back of SR04. Due to the intense ramping and folding of SR01 and its underlying duplex (SR01u) into an antiformal stack, mud of the Lønstrup Klint Forma- tion was remobilised in SR01, which intruded through the hanging-wall flat of SR02 to form the diapir in the Sandrende Section. Sandrende Section: summary data Balanced length (L 0 ): 775 m Cross-section length (L 1 ): 440 m Shortening (ΔL): 335 m Compression: 43.2% Comment. The lengths are measured from the foot- wall ramp between SS01 and SR04 to the footwall ramp between SR01 and BR08, at the level where the ramps cut the L/R-unconformity. The volume lost in diapir- ism has not been considered, and a reduced amount of compression would result by measuring L 1 from the tip of the SR01 thrust sheet to the SR04 footwall ramp. Brede Rende Section The development of normal faults associated with foreland-dipping features of hanging-wall ramps trans- Facing page: Fig. 117. Dynamic model of progressive deformation in the Sandrende Section illustrated by five restoration cross- sections. Note that stages 2 and 3 include syntectonic sedimentation of the Rubjerg Knude Formation, mainly related to stage 2, and the thrust-fault configuration conclud- ing stage 3. The cross-sections demonstrate the development stages between the initial and final positions displayed in the balanced and the structural cross-sections in Plate 2. The red lines indicate the active displacement surfaces in each deformation stage. 164 lated along footwall flats has already been demon- strated in the previous sections. One of the best ex- amples of such a normal fault relationship occurs in the Brede Rende Section. An essential element for this development was the formation of a long thrust sheet, translated laterally more than 150 m along the upper flat. This is demonstrated by the seven stages of de- velopment recognised in the Brede Rende Section, as illustrated by the five cross-sections in Fig. 118. Brede Rende stage 1. The first stage differentiated here is the initial sedimentation of about 3–5 m of the Ru- bjerg Knude Formation. This corresponds well with the thickness of sediments deposited initially above the L/R-unconformity in the Sandrende Section; this unit is considered to represent pre-thrust sedimenta- tion, i.e. the sediment record prior to piggyback basin formation. Brede Rende stage 2. Accepting that the thinnest pre- served section of the Rubjerg Knude Formation indi- cates the timing of the earliest thrusting, then thrust- ing in the Brede Rende Section was initiated with the displacement of the BR03 thrust sheet. The frontal part of BR03 was displaced about 50 m over the upper flat corresponding to the relative foreland in front of the leading edge of thrusting. The BR03 thrust fault pro- bably rooted down to the 30 m décollement level. However, translation in the upper 10 m flat level can- not be excluded, and in this case the beds disturbed by hydrodynamic brecciation might be interpreted as thrust flats. In the trailing end of the section, the BR06 hanging-wall ramp was the next thrust to break through and initiate the translation along the upper flat. Brede Rende stage 3. The BR06 thrust sheet was fur- ther displaced c. 50 m over the upper flat. The trailing end of BR06 was separated by a splay thrust at the footwall ramp, along which the BR07 thrust sheet prop- agated contemporaneously with piggyback thrusting of BR08. This stage is equivalent to the frontal thrust- ing during stage 5 in the Sandrende Section. Brede Rende stage 4. Sediment accumulation in the piggyback basins increased up to about 15 m. The marked dif ference in thickness of deposits is clearly seen by comparing the BR05 thrust sheet with the BR06 thrust sheet. The roof of BR05 was obviously capped at an earlier stage than BR06 where sediments accumulated to more than twice the thickness of that in BR05. Brede Rende stage 5. With a displacement of about 60 m, the BR05 thrust sheet propagated up along the footwall ramp of BR04 and onto the upper flat on top of the BR04 thrust sheet. Translation of the BR06 thrust sheet progressed c. 60 m along the upper flat. The thrusting rooted down to the 20 m flat level on top of the trailing-end segment of the BR03 thrust sheet (BR03u). The accumulated displacement amounted to 150 m, including the ramping and translation of the BR07 thrust sheet along the same 20 m flat level. Brede Rende stage 6. After the thrusting of BR05 and BR06 ceased, the BR04 thrust sheet was translated c. 80 m. The BR04 thrust fault included three ramps: an upper gently dipping ramp from the upper flat to the 5–10 m flat level, an intermediate ramp-bend of the BR03 thrust sheet due to the presence of the formerly established BR02 footwall ramp, and a lower ramp from the 10 m to the 20 m flat level. The translation of the BR04 lower hanging-wall ramp along the footwall flat of BR03 created the foreland-dipping bend that, combined with the bend due to the BR03 ramping, formed a syncline in front of the BR04 ramp anticline. The normal fault created parallel to the foreland-dip- ping features displaced the tip of the BR06 thrust sheet. The vertical offset on the normal fault amounted to c. 20 m, which also included the displacement caused by the offset in front of the BR05 thrust tip. Brede Rende stage 7. Finally, the leading-edge thrust- ing of the section propagated over the trailing end of the Kramrende Section. Above the footwall ramp of BR01, a minor antiformal stack was formed and sub- sequently an irregular duplex formation affected the BR01 thrust sheet during the last stage of deformation in the Brede Rende Section. This phase developed into diapirism that intruded towards the thrust fault between BR01 and BR02. Facing page: Fig. 118. Dynamic model of progressive deformation in the Brede Rende Section illustrated in five sequential restoration cross-sections. The cross-sections demonstrate seven stages in the development between the balanced cross-section and the structural cross-section (Plate 2). The red lines indicate the active displacement surfaces in each deformation stage. Note that significant normal faulting occurred in the Brede Rende Section during stages 5 and 6 while the hanging-wall anticline in the middle part of the BR04 thrust sheet was formed. 165 166 Brede Rende Section: summary data Balanced length (L0): 815 m Cross-section length (L1): 440 m Shortening (ΔL): 375 m Compression: 46.0% Kramrende Section In the central part of the Kramrende Section, a major diapir developed during the progressive thrusting. The Kramrende diapir was the most distally located diapir in the thin-skinned thrust-fault system indicating that a certain amount of ramp propagation from a deeper décollement level (at least 30 m flat level) was need- ed for macroscopic-scale diapirism. South of the Kram- rende Section, the décollement level gradually changed to a shallower position and the intensity of ramping decreased. Seven stages of dynamic development have been differentiated in the Kramrende Section, as illus- trated in the five cross-sections in Fig. 119. Kramrende stage 1. The thrusting in the Kramrende Section was initiated with leading-edge propagation along the KR01 thrust fault, which constituted an up- per footwall ramp with a dip of 10°, a minor interme- diate flat at the 15 m flat level, and a c. 15° dipping lower ramp connecting the thrust fault to the 30 m décollement level. The displacement was in the order of 100 m along the upper flat, where almost no sedi- mentation of the Rubjerg Knude Formation took place. Kramrende stage 2. Subsequent to the early stage thrusting, the lowermost 10 m of the Rubjerg Knude Formation was deposited; the sediment thickness in the KR01 piggyback basin was probably a little less. Kramrende stage 3. The KR01 thrusting progressed about 60 m over the upper footwall flat of what was to become the MB04 thrust sheet, and the trailing end of the KR01 thrust sheet was elevated to the 15 m flat level by ramp propagation over the lower footwall ramp of MB04. A small duplex segment (KR01s) un- der the middle part of the KR01 thrust sheet was picked up in the thrusting and displaced to the upper foot- wall ramp hinge, where it formed a minor angular anticline. In the syncline between the anticline and the footwall ramp of KR01, the thickness of piggy- back basin sediment accumulation increased to about 15 m before the KR02 thrust sheet propagated c. 50 m up along the ramp, and the KR02 hanging-wall ramp partly capped the KR01 piggyback basin. The accu- mulated displacement ranged up to about 260 m. Kramrende stage 4. Thrusting of the KR03 thrust sheet was initiated up along the northerly dipping footwall flat of KR02. The KR03 thrust fault included an upper and a lower relatively steep (c. 23°) ramp. The top of the KR02 thrust sheet was probably exposed to ero- sion, and the Rubjerg Knude Formation is thus miss- ing in this part of the section. The piggyback sediment pile increased to a thickness of 20 m, as indicated by the sedimentary section preserved above the L/R-un- conformity at the top of the KR04 thrust sheet. From the trailing end of the KR01 thrust sheet, diapirism intruded through the footwall ramp and irregular mud diapirism developed in the KR02 thrust sheet. Kramrende stage 5. With a displacement of c. 30 m, KR03 thrusting propagated over the two ramps that resulted in the fault-bend folding of two anticlines separated by an intervening syncline. Kramrende stage 6. The KR04 thrust sheet was thrust over the fault-bend-folds formed in stage 5, simulta- neously with limited continuation of KR03 thrusting. Minor irregular duplex formation started to develop into mud mobilisation at the trailing end of the KR02 and KR04 thrust sheets. Kramrende stage 7. The final thrust propagation of the KR03 thrust sheet concluded with a displacement of 30 m up along the footwall flat of KR02. At the bend between the footwall flat and the footwall ramp of KR02, a remarkable set of reverse faults developed (Fig. 72). The KR04 thrust sheet, carried piggyback on KR03, was also displaced by the reverse faulting, a fact that testifies to the relative timing of KR04 piggy- back thrusting and KR03 ramp propagation. The re- verse faults are regarded as back-limb thrusts similar to the back-thrust features mentioned in stage 5 of the Sandrende Section. Minor back-limb reverse faults Facing page: Fig. 119. Dynamic model of progressive deformation in the Kramrende Section illustrated in five sequential restoration cross-sections. The cross-sections demonstrate seven develop- ment stages, of which stage 2 represents a purely deposition- al phase and stage 4 only includes minor displacement. The red lines indicate the active displacement surfaces in each deformation stage. 167 168 also developed at the crest of the fault-bend-folded KR01 thrust sheet. Polyphase diapirism evolved in the trailing end of the KR03 and KR04 thrust sheets, such that the primary thrust-fault framework was partially destroyed. Kramrende Section: summary data Balanced length (L0): c. 600 m Cross-section length (L1): c. 300 m Shortening (ΔL): c. 300 m Compression: c. 50% Comment. The lengths are measured from approxi- mate positions on the footwall ramps bounding the Kramrende Section and the data must therefore be regarded as tentative estimates. Martørv Bakker Section The development in the Martørv Bakker Section was dominated by the translation of a thrust sheet that was more than 600 m long and only 20–30 m thick. During nearly 400 m of displacement towards the fore- land, a lower segment transformed into a duplex that ramped at a relatively late stage and created a fault- Fig. 120. Dynamic model of progressive deformation in the Martørv Bakker Section illustrated in three sequential restoration cross-sections. The cross-sections demon- strate four stages in the development between the balanced cross-section and the structural cross-section (Plate 2). The red lines indicate the active displacement sur faces in each deformation stage. Note the significant depression formed in the hang- ing-wall block south of the Martørv Bakker normal fault. In this depression, diamictites interlayered with slump-slides were depo- sited. 169 bend-fold anticline and syncline pair. At the upper surface of the intervening limb between the fold pair, a foreland-dipping normal fault was formed, rather similar to the structural complex formed in the Brede Rende Section. Simultaneously with the sedimentation of a diamictite, three slump-slides filled the piggyback basin developed in a syncline created at the top of the hanging-wall block of the normal fault. The sequen- tial restoration stages are illustrated in three cross-sec- tions in Fig. 120. Martørv Bakker stage 1. Thrusting in the Martørv Bakker Section started with foreland thrusting of MB02, and translation of the trailing-end duplex that consti- tuted the KR01 thrust sheet emplaced piggyback on the MB04 thrust sheet, thrust up along the footwall ramp of MB03. The more than 600 m long MB02 thrust sheet was displaced c. 105 m over the footwall flat of MB01. The MB02 thrust fault included two ramps, an upper footwall ramp of MB01 and a lower ramp be- tween the 20 m flat level and the 30 m décollement level. The lower ramp was located below the central part of the MB02 thrust sheet, where it acted as the final step for the décollement level change to the 20 m footwall flat level. It is thought unlikely that signif- icant sedimentation occurred in the section during this stage. 170 Martørv Bakker stage 2. The MB01 thrust sheet was displaced about 100 m over the foreland of the Stens- næs Section along the leading-edge thrust. The MB01 thrust was rooted down to the 20 m flat level, and it can be traced further on to the 30 m décollement level by passing the central lower footwall ramp of MB02. Minor adjustments along the hanging-wall flat result- ed in formation of small duplexes along the thrust fault. In the trailing end of the section, the MB03 thrust sheet was thrust up over the footwall ramp of MB02, whereby an antiformal stack was formed due to the folding that also involved the MB04 and KR01 thrust sheets. At the base of the MB03 thrust sheet, the low- er segments formed an irregular duplex, which ac- centuated the antiformal stack. The accumulated dis- placement ranged up to 290 m. Martørv Bakker stage 3. The final thrusting of the Martørv Bakker Section was concluded by nearly 100 m displacement of the MB02 thrust sheet. The frontal hanging-wall ramp-and-flat was thrust over the piggy- back basin of the SN04 thrust sheet in the Stensnæs Section. During the thrusting, the lower segment MB02u was activated and formed a fault-bend-folded lower duplex. Above the hanging-wall ramp of the trailing-end segment (MB02u3), an anticline was formed at the surface of MB02 and a subsequent syn- cline above the hanging-wall/footwall flat became a piggyback basin. Martørv Bakker stage 4. The foreland-dipping limb of the fold pair at the top of the MB02 thrust sheet devel- oped into a southerly dipping normal fault. The c. 10 m deep piggyback basin was filled with diamictitic deposits and slump-sheets that glided down from the top of the antiformal stack. Deformation in the Mar- tørv Bakker Section concluded with the steepening up of the leading-edge thrust structures due to ramp bending in the Stensnæs Section. Martørv Bakker Section: summary data Balanced length (L 0 ): 1065 m Cross-section length (L 1 ): 675 m Shortening (ΔL): 390 m Compression: 36.6% Comment. The lengths are measured from the tip of the leading-edge hanging-wall ramp to the upper bend of the footwall ramp of the MB04 thrust sheet. Stensnæs Section In the Stensnæs Section, a number of conspicuous flexural slip folds occur which are interpreted to have resulted from the deformation that accompanied se- quential footwall ramp collapse and subsequent ramp displacement of minor duplexes. It is significant that they occur in relation to the final ramping from the lower 20 m flat level to the upper 10 m flat level. Eight stages have been differentiated in the development of the Stensnæs Section, of which stages 2, 4 and 6–8 are illustrated by the cross-sections in Fig. 121. Stensnæs stage 1. In contrast to the Martørv Bakker Section, an initial sediment thickness of 5 m of the Rubjerg Knude Formation is thought to have covered the Stensnæs Section. It should be noted, however, that typical Lønstrup Formation facies grade upwards into typical Rubjerg Knude Formation facies in this distal part of the Rubjerg Knude Glaciotectonic Com- plex; the L/R-unconformity is not clearly developed, and location of the formation boundary can be diffi- cult. The affinities of the sediment packet referred to above are thus debatable. Stensnæs stage 2. Thrusting in the Stensnæs Section was initiated with c. 100 m displacement of the SN02 thrust sheet over the upper flat. The thrust fault ramped down to the 10 m flat level, which separated the up- per and lower segments of the SN04 thrust sheet, si- multaneously with stacking the SN03 thrust sheet into a northerly dipping duplex complex along the foot- wall ramp of SN01. Stensnæs stage 3. Accumulation of the Rubjerg Knude Formation increased to a sediment thickness of 10 m. Sedimentation was restricted to the piggyback basin of the SN04 thrust sheet, as well as on the foreland south of the frontal tip of the SN02 thrust sheet. Stensnæs stage 4. The piggyback basin on the back of SN04 was sealed in by the overthrusting of the MB02 thrust sheet; this is equivalent to stage 3 in the Mar- tørv Bakker Section. Stensnæs stage 5. As a trailing-end structural complex to the Ulstrup Section, the thrust sheets of the Stens- næs Section were translated together with the UL02 thrust sheet over the Ulstrup footwall ramp onto the hanging-wall flat of the foreland. During ramping, the SN01 and SN03 thrust sheets were separated into small duplex segments. Flexural-slip folding and polyphase 171 UL1 intermediate footwall rampsUlstrup footwall ramp Stensnæs 2 Ulstrup footwall ramp MB1 SN4 SN4u SN4u SN1 UL2SN1 SN3 SN2 Stensnæs 4 Ulstrup footwall ramp MB2 MB1 SN4u SN4u SN4 SN3 SN2 SN1 SN1u UL2 ? Stensnæs 6 MB1 SN4u SN4 SN3 SN2 SN 4u SN1 SN1u UL2 UL1 MB2 ? ? Stensnæs 7 & 8 Stensnæs ramp MB2 MB4 SN4 SN3 SN2 SN1 SN1u UL2 UL1 Lønstrup Klint Formation Active thrust fault Rubjerg Knude Formation 100 m N S Fig. 121. Dynamic model of progressive deformation in the Stensnæs Section illustrated in four sequential restoration cross-sections. The cross-sections demonstrate five of the eight stages in the development described in the text between the balanced cross-section and the structural cross-section (Plate 2). The red lines indicate the active displacement surfaces in each deformation stage. hydrodynamic brecciation resulted from the ramping (Figs 53, 57, 58). The accumulated displacement ranged up to 35 m, whereas the length of the hanging-wall flat in the 10 m flat level amounted to 500 m. At the leading edge of thrusting, the UL02 thrust sheet initia- ted the thrusting up over a stepwise ramp. Stensnæs stage 6. During this stage, about 10 m of the Rubjerg Knude Formation was deposited in the pig- gyback basin at the top of the SN02 thrust sheet. The sedimentation level was probably up to 20 m above the L/R-unconformity, inferred from the elevated po- sition of the SN02 thrust sheet. However, this is un- certain and the sediments were either never deposit- ed or eroded away during later thrust elevation. In the northern part of the section, the SN04 thrust sheet propagated up along the footwall ramp of the earlier created SN02–SN03 duplex. This resulted in fault-bend folding of the SN04 thrust sheet and its piggyback basin as well as the overlying MB02 thrust sheet. This 172 stage correlates with stages 3–6 in the Martørv Bakker Section. Stensnæs stage 7. The SN01 thrust sheet was displaced about 50 m over its lower segment (SN01u), and together they were thrust onto the footwall ramp-and-flat of the UL02 thrust sheet. During the thrust-fault propagation of the UL02 thrust sheet over the footwall ramp of UL01, the SN01 and SN02 thrust sheets, piggyback translated on UL02, were bent into c. 30° dipping position. Fi- nally, the SN04 thrust sheet was displaced up along the footwall ramp of SN03 during dif ferential duplex formation along the SN04 hanging-wall ramp. Stensnæs stage 8. The frontal parts of the SN01, SN02 and SN03 thrust sheets, as well as the anticlinal crest of the UL02 thrust sheet (formed above the upper hinge of footwall ramp of UL01), were significantly eroded, and a local piggyback basin was formed above the transition between the Stensnæs and Ulstrup Sections. To the north of this piggyback basin, the elevated and exposed tips of the SN02–SN04 thrust sheets grav- ity-slumped out into the basin, where they were de- posited as olistoliths, 1–5 m in size. Stensnæs Section: summary data Balanced length (L0): 350 m Cross-section length (L1): 180 m Shortening (ΔL): 170 m Compression: 48.6% 173 Ulstrup Section Thin-skinned thrusting in the Ulstrup Section involved the remarkable translation of extensive, thin thrust sheets over the footwall flat of the foreland. Cohesion of the thrust sheet was probably increased by ground frost in the upper part of the thrust sheet, while the hanging-wall ramp-and-flat slid on a thin zone of mo- bilised mud. During translation, piggyback sedimen- tation varied considerably. Six stages have been dif- ferentiated in the development of the Ulstrup Section; stages 1–3 and 5 are illustrated by the cross-sections in Fig. 122 (see also Fig. 121). Ulstrup stage 1. Thrusting in the Ulstrup Section initia- ted with frontal ramping of the UL02 thrust sheet over a two-stepped footwall ramp of what was to become the UL01 thrust sheet. This ramping resulted in the for- mation of two, fault-propagating folded anticlines, which were separated by a shallow, broad syncline. The leading edge of the UL02 hanging-wall ramp was displaced about 25 m over the c. 5–10 m thick Rubjerg Knude Formation deposited in the foreland (at the top of UL01). The UL02 hanging-wall flat extended along the upper 10 m flat level for about 400 m, termi- nating to the north at the foreland footwall ramp root- ing down to the 20 m décollement level. Thrust pro- pagation up over this ramp formed a hanging-wall anticline at the trailing end of the UL02 thrust sheet. Between the anticline at the trailing end and the anti- cline at the upper footwall ramp of UL01, a piggyback basin formed in which glaciolacustrine sediments were deposited to form the small, ephemeral Ulstrup lake. Fig. 122. Dynamic model of progressive deformation in the Ulstrup Section illustrated in four sequential restoration cross- sections. The cross-sections demonstrate four of the eight stages in the development described in the text between the balanced cross-section and the structural cross-section (Plate 2). The red lines indicate the active displacement surfaces in each deformation stage. 174 Ulstrup stage 2. Thrusting along the UL02 hanging-wall ramp-and-flat progressed with an accumulated displace- ment of 230 m. The glaciolacustrine deposits of the ephe- meral Ulstrup lake participated in the ramp-propagat- ing-folding. During translation along the upper 10 m flat level, the trailing end of the UL02 thrust sheet was probably covered by sediments, which subsequently became eroded. This event in the Rubjerg Knude For- mation corresponded to stage 6 in the Stensnæs Section. Ulstrup stage 3. The long lateral thrusting of the UL02 thrust sheet along the upper flat resulted in 550 m of displacement, and at the lower trailing end, the hang- ing-wall ramp became detached to the upper footwall ramp of UL01. Above this ramp, conspicuous flexural- slip folds, similar to the folds developed in the Stens- næs Section, were formed in the UL02 thrust sheet. Ulstrup stage 4. Glaciofluvial sands were deposited upon an erosional surface capping the Ulstrup lake sediments (all Rubjerg Knude Formation). Depositional base level was probably equivalent to that experienced in stage 8 in the adjacent Stensnæs Section (see above). Ulstrup stage 5. The final foreland thrusting took place as the UL01 thrust sheet was thrust over the upper ramp of the foreland and propagated about 200 m to the south. When the anticline above the UL02 hang- ing-wall ramp approached the footwall ramp of the foreland, where a fault-bend formed continuously during the propagation of the UL01 thrust sheet, a narrow channel was formed in which coarse-grained glaciofluvial gravel was deposited (Figs 27, 122). The gravel also included redeposited frozen blocks of sand, testifying to the ground-frozen conditions of the en- vironment (Fig. 28). Ulstrup stage 6. At the leading edge of the UL01 thrust fault, the deformation concluded with 150 m of dis- placement over the upper footwall flat of the fore- land. During the translation of the UL01 thrust sheet over a minor depression in the foreland, a sandy mud volcano developed due to trapping of the high water pressure close to the leading-edge thrust. The sedi- ment extrusion resulted in chaotic disturbances in the central part of UL01. Ulstrup stage 7. The last stage of development in the Ulstrup Section involved sedimentation of the upper- most post-tectonic deposits of the Rubjerg Knude For- mation. The conglomerate (of stage 5) was covered by sand, and deposition in the foreland covered the leading-edge thrust at Tvonnet Rende (for location see Plate 1). Ulstrup Section: summary data Balanced length (L0): c. 1350 m Cross-section length (L1): c. 850 m Shortening (ΔL): c. 500 m Compression: c. 37% Summary of dynamic development The dynamic development of the complex is summa- rised in Figs 123 and 124. From the scheme in Fig. 123, it is clear that the Rubjerg Knude Glaciotectonic Complex developed in sequential progressive stages during syntectonic sedimentation of the Rubjerg Knu- de Formation. The thin-skinned thrust-fault complex developed mainly as piggyback thrusting with proximal thrust sheets being displaced contemporaneously with acti- vation of the distal thrust fault. During the advance of the thrust-fault complex, the position of the décolle- ment zone shifted progressively to deeper levels. The dynamic development can be summarised in eight steps that resulted in the formation of eight character- istic thrust-fault structure types (Fig. 124). Fig. 123. Summary scheme of the syntectonic sedimentary development in the Rubjerg Knude Glaciotectonic Complex. The deformation stages for each section, as described in the text, are indicated here by the symbol #. 175 1. Long lateral translation of a thin thrust sheet took place over the foreland. The ramp was rooted in the uppermost shallow décollement level 1 at a depth of c. 10 m from the top surface. 2. Ramps became rooted in décollement level 2, and the increase in ramp height, amounting to about 20 m, is regarded to be the cause of the duplex folding at ramp collapse. 3. The hanging-wall anticlines became dominant struc- tures with hinterland-dipping piggyback thrust sheets on the back limb. As the ramps extended down into décollement level 3, the height of the ramps increased and consequently the hanging- wall anticlines increased in size. 4. The antiformal stack developed, which included long-distance translated piggyback thrust sheets that were folded in a hanging-wall anticline. In relation to the antiformal stack, foreland-dipping thrust faults occur that were accompanied by nor- mal faults. 5. The prominent imbricate fan formed above décol- lement level 2. The initially gently to moderately dipping imbricated thrust sheets were re-orientat- ed into steeply dipping positions due to lateral translation of the imbricate fan along décollement level 3. 6. This step involved the subsequent deformation of the lower duplex segment not incorporated in the imbricate fan. This lower duplex segment was im- bricated and the sub-segments were displaced in- to a duplex stack during push from behind by a progressing hanging-wall ramp, rooting in décol- lement level 3. 7. This step involved differential duplex stacking and imbrication of thrust-fault sheets. The piggyback basins vary in elevation due to differences in du- plex stacking. Furthermore, the variation in duplex stacking reflects the shift from décollement level 3 to 4 (corresponding to a shift in the décollement surface from 30 to 40 m). 8. The fault-bend-folded duplex units were formed. The formation of these duplex units was only pos- sible because the four thrust-fault flat levels had developed, and thus the duplexes could be stacked and subsequently fault-bend-folded during maxi- mum compression and translation along décolle- ment level 4 (Fig. 124). The thickness of sediments that accumulated contem- poraneously in tectonically correlated piggyback ba- sins decreases from north to south. Thus the depo- centre was situated in front of the last activated thrust section, and the depocentre gradually shifted to a more and more distal position. Correlation of the syntec- tonic progressive development of the complex shows that sedimentation was contemporaneous with thrust- ing rather than there being an alternation between periods of active thrust faulting and periods of depo- sition. Furthermore, it indicates that the ice margin was not melting back during the formation of the com- plex but advanced in a continuous progressive gravi- ty-spreading process. 176 Ice Fault-bend-folded duplex Imbricated duplex stack Duplex stack Imbricate fan Antiformal stack Hanging-wall anticline Duplex folds at ramp collapse Foreland translated thrust sheet Distance T im e SN Fig. 124. Model of thrust-fault structure types formed during the progressive deformation of the thin-skinned glacio- tectonic thrust-fault complex. The model outlines a progressive development in eight steps resulting in the formation of eight characteristic thrust-fault structure types, the first to develop earliest and continuously in the distal part of the complex, and the last to be formed in the most proximal part of the complex (see text for details). 177 Lønstrup Klint Formation Rubjerg Knude Formation Stortorn Formation 178 Discussion The observations that form the basis for the descrip- tion of the structural geology, mechanical behaviour and dynamic development of the Rubjerg Knude Gla- ciotectonic Complex, raise important questions with respect to understanding the framework and nature of thin-skinned thrusting related to glacial deforma- tion; seven topics have been selected for further discus- sion below. The basis for understanding a structural complex is to describe the tectonic architecture and the range of structures it contains from microscopic to macroscopic scale. The discussion of thrust-fault archi- tecture leads to evaluation of the reliability of the balan- ced cross-section. Consideration of thrust brecciation and diapirism leads naturally to a focus on the thrust- fault dynamics, and the significance of the rate of defor- mation. The dynamics associated with the syntecton- ic deposits and the formation of piggyback basins merit discussion, as does the interpretation of a proglacial contra subglacial deformational setting. The final topic deals with the geological setting of the complex, inclu- ding the timing of the event that created it. Thrust-fault architecture A prerequisite for understanding the thrust-fault ar- chitecture is a familiarity with the terminology (see Appendix 2). The macroscopic structures encountered in thin-skinned orogenic belts are all recognisable in the glaciotectonic complex. Mesoscopic structures such as folds and faults are similarly recognisable. However, small-scale structures such as joints, cleavage and fabric are more difficult to recognise (except for hydrody- namic brecciation), and this may be one of the major dif ferences between soft sedimentary deformation and hard-rock deformation. It seems likely that joints and fractures in soft sedi- ments would be able to re-heal after deformation. Thus, a large number of minor reverse faults must have formed in thrust sheets during ramp propagation (Fig. 67), but appear to have disappeared again after sub- sequent thrust sheet propagation along the flat, as they have not been observed with the exception of the in situ positions related to ramp bend (Fig. 85). There is an approximation to a right-angle relationship be- tween the footwall ramp and the back-thrust faults, which indicates that an increase in the dip of the foot- wall ramp results in a decrease in the dip, in the op- posite direction, of the back thrust. Moreover, a steeper and higher footwall ramp also corresponds to an in- crease in displacement along the back-thrust fault. Thus one can regard the KR01 (Kramrende) back-thrust faults as structures related to initial faulting in the progres- sive deformation (Fig. 67), and the KR04 back-thrust splay faults as a structural element related to a devel- oped phase of progressive thrust-fault deformation (Fig. 72). Major back thrusting at the back of SR02 (Fig. 84) represents a mature phase in the progressive thrust faulting. The apparent lack of joints and frac- tures reflecting ramp propagation could probably be explained as having been absorbed in the hydrody- namic brecciation process. Among the structural elements analysed during the interpretation of the balanced cross-section, the du- plex structures create the most interesting problems. Firstly, the interpretation of the duplex imbricates in the Stensnæs Section provides an explanation for the complicated fold framework. Secondly, the interpre- tation of the duplex below the frontal part of the Stor- torn Section links the hidden duplex segments at the base of the Grønne Rende Section with the duplex stacking below ST01–ST03. Thirdly, the normal faults can be interpreted to have been related to the ramp- ing of lower duplex segments. If the normal faults are regarded as foreland-dipping faults or part of a fore- land-dipping duplex, the model for duplex formation suggested by Contreras & Sutter (1997) may be rele- vant for the understanding of the foreland-dipping faults. In their model for formation of foreland- or hinterland-dipping duplexes, they considered two fac- tors: u = distance of displacement along the upper flat, and s = length of duplex segment. In a regime where the ratio u/s is greater than one (u/s > 1), fore- land-dipping duplexes are formed; in a regime where u/s < ½, hinterland-dipping duplexes are formed. In regimes where ½ < u/s < 1 or u/s = 1, antiformal stacks or angular antiformal stacks, respectively, are formed. This corresponds well to the interpretation presented here of the Rubjerg Knude cross-section, where most thrust sheets are displaced by less than their length, and consequently the main orientation of thrust sheets is hinterland dipping. According to the model of Contreras & Sutter (1997), foreland-dip- 179 ping duplexes are formed when a duplex segment is displaced along an intermediate or upper flat for a distance equal to, or more than, its length. A conse- quence of this is that a roofing thrust sheet will be displaced in front of the foreland-dipping upper foot- wall flat, where normal faulting will take place. This is interpreted to be the case for the normal faults in the Martørv Bakker and Brede Rende Sections (see Fig. 66). The normal fault developed in the Stenstue Rende Section may also be regarded as an expression of the latter regime in the suggested model. A fore- land-dipping feature may well reflect the foreland-dip- ping limb of a hanging-wall anticline formed above a laterally displaced hanging-wall ramp. However, the normal fault-displaced thrust sheet must be thrust over the duplex segment before it was thrust faulted together with its roofing thrust sheet, and then as translation continued attached to the hanging-wall flat until the displacement was concluded by the normal faulting over the tip of the duplex segment. The consideration of duplex formation naturally leads to a focus on the changes in décollement levels. When a duplex segment is formed, there would nor- mally be an early décollement surface at a shallow level, succeeded by a shift to a deeper level connect- ed with a new footwall ramp. The upper décollement level (the 10 m level or corresponding gently dipping ramp to the leading edge) was probably the first to be formed in the proximal part of the complex and prob- ably also the last to form in the distal part (Fig. 124). From the cross-section, it is indicated that the 10 m décollement level extended for about 1 km, but end- ed up with a distance of only 600 m. The 20 m décol- lement level was the next to take over, and during the establishment of a related ramp, footwall ramp imbri- cation progressed, modifying the ramp transition con- necting the two décollement levels. The length of this décollement level might have been of the same scale, but only c. 400 m is preserved as a lower footwall flat. The 30 m flat level is the dominant décollement level extending from the middle of the Martørv Bakker Sec- tion to the middle part of the Stortorn Section, where finally the 40 m décollement level was developed. Note that the present-day position of the lowermost décollement surface is at about 45 m b.s.l. due to the regional, very gentle dip to the north of the L/R-un- conformity that serves as the reference level, corre- sponding to the 0 m flat level. From the cross-section, it can be seen that the dips of the ramps increase from gentle (5–15°) in the zone between the upper surface flat to the 10 m flat level, to 25° dips between the 10 and 20 m levels, and reaching up to 35° between the 20 and 30 m flat levels. The ramp dips with steeper angles in the cross-section, arise from over-steepen- ing or superimposed tilting during ramp propagation (Fig. 9). Thus, in the proximal part of the complex, dips between 35° and 45° are interpreted as the pri- mary dips of ramps rooting down to the deepest dé- collement level at 40 m, which is incorporated in the model for the fold-imbricate duplex units. The increas- ing dips of ramps are interpreted to have resulted from the increase in fracture angle as a function of increase in normal stress (change of levels) and increase in shear stress (increasing force required to move thrust sheets). This is implied from the shape of the Mohr- envelope in the Mohr diagram (Hobbs et al. 1976), and it is suggested to be a basic relationship for glacio- tectonic fracture and fault deformation (Pedersen 1996). Balanced cross-section In the balanced cross-section, the changes in dip ang- les are responsible for the insertion of a number of small triangular-shaped duplex segments, which are incorporated in the geometric construction and anno- tated as splints (horses) (Plate 2). It is not known how many splints exist in reality. A few have been recog- nised as structural identities (KR01 S in Kramrende), but it is likely that space deficits or excesses have been absorbed in mud-mobilisation or differential small-scale anastomosing fracturing. Dif ferential frac- turing and thrust-fault formation with a spacing of only 1 m has been documented in the Moserende Section (Fig. 27), indicating that the duplex segmentation does exist. Hence it is probable that a much more differen- tial translation took place than is indicated in the cross- sections of the dynamic model of thrust-fault propa- gation (Fig. 112). The reliability of the approximations in construc- tion inherent in a balanced cross-section is founded in the area balance. The main calculation of the bal- ance indicates that the shortening amounts to approx- imately 50%, with L 0 = 12 km and L 1 = 6 km. The area of the L 0 cross-section (L 0 multiplied by stratigraphic thickness) amounts to 340 000 m2, and the area of the L 1 cross-section (L 1 multiplied by measured thickness of the retrodeformed cross-section) is 382 500 m2. The dif ference amounts to 11%, which is interpreted as a consequence of the erosion of the thrust sheets in the proximal part of the complex (Grønne Rende Section – Ribjerg Section). The detailed calculations of areas 180 for the area balance are summarised in Table 1, and documented in Plates 2A and 2B. The amount of ero- sion indicated from the area balance differs markedly from the 80% erosion estimate by Gry (1941), and sup- ports the argument that Gry’s cylindrical thrust-fault model was incorrect. Considering the amount of erosion, the question arises: why is the preservation potential so great? Three factors are suggested here to answer this: (1) the steep- ly orientated thrust sheets were partly packed by the sand fill in the piggyback basins, (2) the thrust-fault deformation resulted in a strain hardening that con- solidated the complex, and (3) as the sole of the ap- proaching ice sheet advanced across the proximal part of the complex, the over-pressured pore water mi- grated from the hanging-wall ramps and flats of the thrust sheets to the hanging-wall flat of the ice sheet, facilitating the over-thrusting of the footwall block which subsequently comprised the thrust-fault complex. Thrust brecciation and diapirism In order for a thrust sheet to move, a fracture must be created that can develop into a plane of thrusting. The initial fracture is formed when the failure limit is reached in a system subjected to pressure (loading and lateral compression). A recurring question, and an apparent conflict in reasoning, is why fault planes develop, leaving the rest of the thrust sheet preserved? Since the sedimentary unit forming a thrust sheet is subjected to the same amount of confining pressure, it might be expected that a muddy mass of collapsed sedimentary units just as well could have been the result? It is well known that an increase in pore-water pres- sure results in failure and initialisation of fractures along surfaces of anisotropy, as described for orogenic sys- tems by Hubbert & Rubey (1959). However, in soft sedimentary deformation with lower confining pres- sure and smaller shear strength, as well as smaller coefficient of internal friction, the limits of fracture formation and complete collapse are much narrower. The structures developed in the Ulstrup Section re- flect these conditions. The anastomosing jointing and mud mobilisation at the tip of the UL01 thrust sheet reflect the stage of near collapse (Fig. 49). The thick zone of hydrodynamic brecciation along the hanging- wall flat reflects the same tendency towards collapse, and speculations about the influence of ground-fro- zen conditions on the preservation of the thin thrust sheets during translation over the foreland are rele- vant. Ground-frozen conditions are interpreted to have affected that part of the thrust sheets elevated above the ground surface; the freezing of the sediments in the thrust sheet may result in more brittle behaviour, whereby cracks formed (Figs 44, 46). However, due to the high pore pressure maintained along the hang- ing-wall flat, the cracks were filled with sand pumped into the cracks by the over-pressured pore water from the base of the thrust sheet. Hydrodynamic brecciation is evidently related to the hanging-wall ramp-and-flat. Brecciation was ini- tiated at an episedimentary stage with the formation of ball-and-pillow structures due to sediment load- ing. When the loading increased by over-thrusting, the ball-and-pillow formation progressed further and hydrodynamic brecciation was concentrated at the hanging-wall flat. Small-scale mud diapirism took place, with chaotic folding developing into polydiapirs (Figs 54, 55, 77, 78, 86, 88). Polydiapirism and mud-mobili- sation are considered to have developed simultane- ously and with an increasing degree of disordering and size of diapir in progressive stages of deforma- tion. Many of the mesoscopic diapir structures recog- nised in the Rubjerg Knude Glaciotectonic Complex can be compared with the multi-wavelength gravity structures described in the model analysis by Wein- berg & Schmeling (1992). The formation of large-scale diapirs is suggested to have been related to thrust- fault deformation of a deep-seated hanging-wall flat that propagated up to surface level along a set of rel- atively steep footwall ramps. During propagation up along a lower ramp to an intermediate flat, and ramp- ing from the 20 m intermediate flat level to the 10 m flat level, polysequential hanging-wall anticlines formed, and were subsequently destroyed by mud- mobilisation initiated from the deep-seated thrust zone of the hanging-wall flat. Some of the soft sedimentary xenoliths floating in the mud diapirs can be viewed as relicts of anticline crests (Fig. 74). Staircase-like ramp propagation is indicated for the Kramrende diapir and the Sandrende diapir, but is not so obvious in the case of the Brede Rende diapir. Intrusive remobilised mud is evidently related to the footwall ramp propa- gated hanging-wall flat of GR01, and the mud mobili- sation in the thrust sheets of the Stortorn and Moser- ende Sections are all easily identified with sequential ramping from the deepest décollement level. 181 Thrust-fault dynamics The difference in thrust-fault development that relates to the upper flat level (10 m) is very marked when the Ulstrup Section is compared to the Grønne Rende Section. Thus the foreland regime in the latest stage of deformation is characterised by thin, very long sheets subjected to horizontal translation over the foot- wall flat of the foreland. In contrast, the Grønne Rende Section probably formed an imbricate complex of smaller, moderately dipping thrust sheets when this section was adjacent to the foreland. There is no ob- vious reason for this difference, although minor dif- ferences in lithology and dif ferences in environmen- tal conditions (frozen or unfrozen ground) could be viewed as contributing factors. However, there is an invisible condition which must be considered, name- ly the velocity of deformation. At the initiation of any deformation, the velocity is zero; the velocity then increases until the displacement is brought to a halt at the edge of the foreland during decreasing velocity. Fast deformation results in more fractures than slow deformation. It is therefore suggested that the imbri- cate structures in the central part of the thrust-fault complex were initiated during the fastest advance to- wards the foreland and that the long-distance transla- tion of unbroken thrust sheets relates to decreasing velocity or slow advance. In a discussion of the velocity of thrust-fault propa- gation, the question of rates and timing is inevitable. The youngest dating of the Stortorn Formation is 30 000 years B.P., while the oldest dating of the Rubjerg Knude Formation is about 29 000 years B.P. and the oldest dating of the Ribjerg Formation is 26 000 years B.P. Thus, a time span of 3000 years is estimated for the calculated shortening of 6 km, which indicates an average velocity of 2 m per year. The peak velocity of the deformation must evidently have been more than 2 m per year, taking into ac- count the acceleration and deceleration. However, the velocity would also have been much higher if defor- mation had progressed in periodic steps rather than continuously. A step-like process would have involved periods of no movement alternating with higher ve- locity in the periods of advance. With respect to the Rubjerg Knude Glaciotectonic Complex, the summary of the dynamic development suggests that a continu- ous progressive deformation process characterised the formation of the complex (Fig. 123). Although the developments of the sections are described separate- ly above, the deformational overlap from one section to the next links the sections in a continuous dynamic development. Syntectonic deposition The concept of piggyback basins was originally relat- ed to large-scale regional orogenic settings (Ori & Friend 1984; Ricci Lucchi 1986). However, as applied here the term is used for the syntectonic deposits of the Rubjerg Knude Formation that were laid down in ba- sins structurally overlying moving thrust sheets. The initial depositional environment of the Rubjerg Knude Formation was a relatively flat lowland, dominated by shallow lakes in an outwash plain bounded by an ice margin to the north. The plain was probably gently dipping towards the north due to isostatic loading of the ice cap. Judging from the variation in thickness of the Rubjerg Knude Formation (30 m in the proximal part to only about 10 m in the distal), the dip of the plain was not more than 2°. As the thrust belt propagated southwards, the plain became separated into smaller, more or less isolated basins characterised by steep slopes and uneven re- lief. The most distinctive deposits in these basins are the sedimentary breccias and slumped thrust sheets derived from the tips of up-thrust thrust sheets. Three types of syntectonic slump/slide deposits can be differentiated. The first type involves deposition of coarse clasts up to metre size, which were rotated indicating transport as sedimentary clasts enveloped by sandy mud. This deposit type is regarded as being related to the distal part of the thrust-fault system, and is exemplified by the piggyback basin in the Stens- næs Section (Fig. 56). The second type is characte- rised by isoclinally folded slump sheets interlayered with matrix-supported coarse clastic diamictite. This indicates that the source was very close to the depo- centre, although the slump sheets were detached from their roots and were transported independently by gravity gliding into the basin. The piggyback basin in the Martørv Bakker Section represents this deposit type (Figs 23, 64). The third type comprises slump-folded sheet segments that can be traced directly, or correla- ted over short distances, back to the source of the thrust sheet; this type is regarded as being related to the proximal part of the system. The next step in the development would be that of thrust sheets displaced by normal faulting, but lacking depositional features such as sedimentary breccias. However, this type of dynamic development is strictly tectonic. The major 182 slump fold occurring in the Stenstue Rende Section (Fig. 89) may be regarded as a transition from a sedi- mentary to a tectonic regime. The deposition of recog- nisable thrust-sheet tips in the piggyback basins sup- ports the concept of a continuous thrust-fault process. Proglacial and subglacial deformation Glaciotectonic analyses distinguish between deforma- tion generated in proglacial and in subglacial regimes (Aber 1982; Croot 1988; Aber et al. 1989; Pedersen 1993, 1996, 2000). It has already been argued that the thin- skinned thrust-fault deformation of the Rubjerg Knude Glaciotectonic Complex is an example of proglacial deformation. The key evidence for this is the presence of intimately associated syntectonic piggyback basins. These basins must have been situated in front of the ice margin, with sedimentation taking place under open water, simultaneously with thrust-fault propagation. However, the subglacial deformation is represented locally by the 1 m thick glacitectonite occurring below the glaciotectonic unconformity that truncated the thrust-fault complex. This is found in the glaciolacu- strine beds at the top of the UL02 thrust sheet in the northern part of the Ulstrup Section. It can be argued that here the subglacial deformation penetrated down a depth of c. 5 m below the glaciotectonic unconform- ity. Mud diapirism and hydrodynamic brecciation oc- curred in this setting, probably caused by loading when the ice sheet overrode the sediments. The focus of subglacial deformation is at the Blå-unconformity (Fig. 32). In the northernmost 300 m of the cross-section, the effects of mud mobilisation increase to a point at which primary sedimentary as well as early structural features are completely destroyed. This phase of de- formation is interpreted to have taken place while the sole of the frontal part of the ice sheet was fixed to the trailing end of the thrust-fault complex. This also implies that the velocity of the thrust faulting was equal to the advance of the ice sheet. The advance of the ice-sheet load corresponds to the mechanics of grav- ity spreading (Pedersen 1987). The increasing propa- gating stress resulted in increasing mud mobilisation, and subsequently the overpressure was transmitted laterally by the mud fluid towards the foreland. The mechanism might well be compared to squeezing toothpaste out of its tube. At a certain stage, the fluid pressure was released, probably due to migration of all the hydrodynamic breccias, and the mobilised mud consolidated. Sub- sequent to consolidation, the frontal sole of the ice- sheet released contact with the Blå-unconformity and propagated over the thrust-fault complex formed in the foreland of the ice margin. During this process, subglacial shearing affected the top of the structure- less consolidated mud, and anastomosing as well as plane-parallel shear fractures were formed (Fig. 32). In soft sediment structural geology, the gravity- spreading model has been successfully applied to the geological setting of the mud lumps in the Mississippi Delta (Morgan et al. 1968; Pedersen 1987; Aber et al. 1989). It could therefore be suggested that a gravity- spreading model due to clastic progradation might be the deformation mechanism. However, there is no known delta setting at this time/place that could have provided the basis for this model, and furthermore, the sand units observed here only reach a third of the thickness of the 100 m delta-sand units in the Missis- sippi Delta setting. Finally, the presence of the glacio- tectonic unconformity and related glacitectonite is in- compatible with a sand sediment-spreading process. Although a delta setting has not been document- ed, it might be suggested that a slope similar to that of a megascopic delta foreset existed, and that the deformation was caused by major gravity gliding on this slope, or was simply due to uplift in the hinter- land. However, this is not considered likely. The iso- static rebound documented from the elevation of the Vendsyssel Formation reaches 60 m a.s.l. To this must be added the uplift due to the lowering of sea level; the area in the hinterland was thus an area of subsi- dence rather than uplift. Structurally, a gravity-gliding model would provide extensional normal fault sys- tems in the trailing end of the thrust-fault complex (Pedersen 1987). This is not compatible with the ob- served increase in compressional structures in the hin- terland, as documented in the cross-section and indi- cated by the balanced cross-section; a gravity-gliding model for the complex can therefore be rejected. Such larger glaciotectonic complexes are often so impressive that some geologists suggest that they were formed by orogenic activity (Lykke-Andersen 1992; K. Binzer, personal communication 1997). Disregar- ding the obvious glacial geological indications, there are two features that distinguish glaciotectonic com- plexes from basement-involved deformation: (1) the superficial detachment, and (2) the rate of translation. In the Rubjerg Knude Glaciotectonic Complex, there are no infracrustal rocks involved and the thrust sheets are not rooted down into a deep-seated hinterland source. The lowermost detachment level is 40 m be- 183 low the reference level, which is more or less coinci- dent with present sea level, and there are no indica- tions that the deformation extended below the 40 m level. The velocity of thrust-sheet motion in orogenic mountain ranges is of the order of 1 cm per year (Wilt- scko & Dorr 1983). In glaciotectonic systems, the ve- locity can be up to 100 times as fast, as documented by the velocity estimate of 2 m per year for the Ru- bjerg Knude Glaciotectonic Complex. Glacial geological conditions The Rubjerg Knude Glaciotectonic Complex is inter- preted to have formed due to the advance of the Nor- wegian Ice in the late Middle Weichselian. The Nor- wegian Ice melted back at the beginning of Late Weich- selian time and was succeeded by a renewed advance of the Scandinavian Ice Sheet from central Sweden. In that part of Denmark east and north of the Main Sta- tionary Line (Figs 1, 12), the direction of this advance was towards the south-west and the advance is thus referred to as the NE-Ice (Houmark-Nielsen 1987). The eastward advance of this ice towards Vendsyssel pro- bably formed the N–S-trending hilly landscape named Jyske Ås, the formation of which was contemporane- ous with deposition of the outwash plain represented by the Ribjerg Formation. When the NE-Ice advance reached the Rubjerg Knude Glaciotectonic Complex, it only resulted in minor superimposed deformation. The oblique orientation of the fold axis of the mega- slump in the Stenstue Rende Section might be due to such superimposed deformation, but in general very few glaciotectonic disturbances can be related to the NE-Ice advance. That overriding by the NE-Ice had so little ef fect may be attributed to strain hardening due to the preceding deformation, or the smoothing out of the landscape by the former glaciotectonic uncon- formity, which would facilitate the second overriding of the complex. Ground-frozen conditions could also have been a factor, since this would have prevented drainage from the ice sheet through the substratum, resulting in high pore-water pressures at the sole of the ice. The effect of this would have been to facili- tate easy and fast propagation over the complex, al- though it by then formed a hill in the landscape. The Norwegian Ice produced a hill-and-hole pair with Rubjerg Knude as the hill and the depression extending from Lønstrup northwards as the hole. Im- mediately after the melting back of the NE-Ice, the landscape was covered by the Vendsyssel Formation. A contour map of the base of the Vendsyssel Forma- tion (Fig. 125) thus provides a picture of the geomor- phology of the young glacial landscape unaffected by the succeeding 15 000 years of erosion. In Fig. 125, the hill-and-hole pair is readily identified and the gen- eral E–W morphological trends are well represented. To the east, this trend is truncated by a strong SE–NW hill-and-hole geomorphology, related to the NE-Ice. The trend of the thrust-fault belt of the Rubjerg Knu- de Glaciotectonic Complex can be followed from the coastline to the east for about 2.5–5 km. The eastern fringe of the complex has been eroded down to sea level, probably by the NE-Ice, and subsequently con- cealed by the Vendsyssel Formation. 184 185 Facing page: Fig. 125. Contour map of the pre-Vendsyssel Formation landscape; for location, see Fig. 13. Note the depression north of Lønstrup which represents the hole in the hill-and-hole pair morphology of a glaciotectonic complex; the correspond- ing hill is represented by the high at Rubjerg Knude. The map is based on data from the GEUS well database and from Plate 1. Conclusions Structural analysis of the Rubjerg Knude Glaciotec- tonic Complex, based on detailed photogrammetric measurements and field investigations, provides a geological cross-section through a low-friction thrust- fault system. Interpretation of the entire thrust-fault architecture included unexposed parts of the complex, and is based on the construction of a balanced cross- section. A model for the dynamic development dem- onstrates that deformation progressed continuously and involved formation of duplexes and mud diapirs. Although the thrust-fault structures were formed in a proglacial regime related to the advance of the Nor- wegian Ice (30 000 – 26 000 B.P.), the structures can be viewed as representing an almost complete model of thin-skinned thrust-fault systems. For descriptive purposes, the complex is subdivid- ed into 13 sections, which demonstrate the structural development from a proximal to a distal position in the thrust-fault system. Investigation of these sections provided the following main conclusions. 1. The structural elements in the Rubjerg Knude Gla- ciotectonic Complex comprise ramps and flats re- lated to hanging-wall and footwall positions, re- spectively. Hanging-wall anticlines and footwall synclines were formed due to thrust-fault propa- gation. Back-thrust faults were formed during up- per ramp-hinge propagation, and an irregular fold framework developed in relation to sequential duplex imbricate formation during footwall ramp collapse. Foreland-dipping normal faults were formed in relation to translation of duplex segments. 2. From the balanced cross-section, the shortening during thrust-fault deformation is calculated to have been c. 50%. About 11% of the initial stratigraphic unit subjected to thrust faulting is estimated to have been lost due to erosion. The décollement zone was at its deepest position (40 m) in the proximal sections, becoming shallower towards the foreland. Stacking of duplex segments is correlated with space problems created in the subsurface due to initial displacements at the upper levels. Stacking of du- plex segments correlates well with the elevation of the reference level in the system. 3. Hydrodynamic brecciation was dominantly relat- ed to the hanging-wall ramps and flats. Polydiapir- ism and mud mobilisation characterise the thrust zones. Mud mobilisation resulted in the formation of larger mud diapirs, and preferentially evolved during hanging-wall propagation from the décol- lement level up above sets of intermediate and upper footwall ramps. 4. Syntectonic deposition took place in piggyback basins overlying the thrust sheets. Thrust sheets exposed to erosion provided sediment to the ba- sins, and in some cases major lumps derived from the tips of thrust sheets slumped and slid as mega- blocks into the piggyback basins. 5. The thrust-fault deformation was caused by gravi- ty spreading at the front of an advancing ice sheet. Over-pressured mud formed an important part of the stress transfer. The average velocity of the thrust-fault displacement is estimated to have been 2 m per year. A 40 m thick succession of flat-lying sediments, extending for 12 km, was compressed into a thrust-sheet complex that was 6 km in length and up to 80 m thick. Acknowledgements The Geological Survey of Denmark and Greenland is thanked for supporting this project during the last 10 years. Initial investigations were carried out while the author held a senior stipend at the Geological Insti- tute, University of Copenhagen. The Carlsberg Foun- dation supported the project with a one year research grant, which is gratefully acknowledged; the Danish Research Agency is thanked for financial support for printing this bulletin. Keld Dueholm and the Institute of Survey and Photogrammetry are thanked for their co-operation and willingness to provide time and fa- 186 cilities at the photogrammetric instrument at the Tech- nical University of Denmark. Frants von Platen-Hallermund is thanked for assist- ance with the ARC-INFO transformation and ARC-VIEW editing, which provided the graphic display of Plates 1 and 2. Alice Rosenstand and Benny M. 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American Association of Petroleum Geologists Bulletin 67, 1304–1322. 190 Appendix 1 Thrust-fault terminology Thrust fault : A surface along which an overlying block is displaced relative to an underlying block. Relative to bedding, two different elements are distinguished in a thrust fault: the ramp and the flat. Ramp: A thrust-fault ramp cuts up-section in the di- rection of slip and dips towards the hinterland. The angle between bedding and the ramp is in general between 20° and 30° and will not exceed 45° due to general rules of initial fracturing. A ramp is linked to a lower flat at the lower ramp hinge and to an upper flat at the upper ramp hinge. A ramp may become fore- land-dipping in special cases, mainly related to trans- port along an upper flat. Flat: A thrust-fault flat is a bedding-parallel slip sur- face along which lateral displacement takes place. The lowermost thrust-fault zone in deformation complex- es is in general referred to as the décollement surface, décollement zone or décollement level. In the present description the ‘décollement level’ is the term used for the thrust fault between a thrust sheet and an un- displaced footwall block below a footwall flat. As flats develop at dif ferent levels, the flats above the décol- lement level are referred to as intermediate flats and the upper flat (identical with the roof thrust fault). The thrust-fault flats are referred to by their depth from the upper reference zero-level indicated from the balan- ced section. Thus the 20 m flat level is the horizontal thrust fault situated 20 m below the top reference level and 10 or 20 m above the décollement level. Thrust sheet: A thrust sheet is the block displaced over a thrust fault. In this study, the thrust sheets are annotated according to the section in which they oc- cur with two capital letters, referring to the named section, and a number referring to its position from leading edge to trailing end of the section. Thus, KR01 is the thrust sheet nearest to the foreland in the Kram- rende section. A thrust fault is referred to according to the thrust sheet it displaces. A thrust sheet is syn- onymous with the hanging-wall block. Hanging-wall block : The rock mass displaced over a thrust fault is a hanging-wall block. At the base, a hanging-wall flat and a hanging-wall ramp bound the hanging-wall block. At the roof, the hanging-wall block is capped by a top surface or a roof thrust fault. The roof thrust fault may constitute a footwall flat as well as a footwall ramp. Footwall block: The rock below a thrust fault is a footwall block. The footwall block is bounded by a footwall ramp, and the top of the footwall block con- stitutes a top surface and/or a footwall flat. Hanging-wall ramp: The segment of a ramp that bounds the hanging-wall block is a hanging-wall ramp. At the incipient displacement along a ramp, the hang- ing-wall ramp is thrust along a footwall ramp. When the hanging-wall ramp passes the upper ramp hinge, the hanging-wall ramp is thrust along a footwall flat. A hanging-wall anticline is always formed above a hanging-wall ramp. Hanging-wall flat: The bedding-parallel thrust-fault boundary below the hanging-wall block is a hanging- wall flat. When a hanging-wall flat is thrust up along a footwall ramp, the hanging-wall flat is re-orientated and becomes inclined towards the hinterland. When the hanging-wall flat is thrust along an upper footwall flat, the thrust fault again becomes bedding parallel. Footwall ramp: The inclined thrust-fault boundary of a footwall block is a footwall ramp. The footwall ramp is either the ramp boundary to the undisplaced foreland or it forms the trailing ramp boundary of a thrust sheet. In this study, the trailing footwall ramp is referred to using the annotation of the thrust sheet/ footwall block that underlies it. Thus the KR02 hang- ing-wall ramp is displaced up along the KR01 foot- wall ramp. Footwall flat: A footwall flat is always the top of a footwall block. A footwall flat is more or less horizon- tal unless it is re-orientated during the displacement of a thrust sheet up along a ramp. Hanging-wall anticline: When a hanging-wall block is thrust over an upper ramp hinge, the hanging-wall block is folded into an anticline with a foreland-dip- ping forelimb and a hinterland-dipping backlimb. This fold may also be termed a ramp anticline. During the progress of thrusting along the upper limb, the hang- ing-wall anticline develops into a flat-topped anticline. The flat-topped anticline may alternatively be regard- ed as a flat-lying thrust sheet with a foreland-dipping forelimb or a frontal thrust-sheet nose. However, it is important to note that above a hanging-wall ramp thrust along a footwall flat, a foreland-dipping sur- face is formed. 191 Footwall syncline: When a thrust fault propagates up along a ramp, an anticline–syncline pair is formed above, and in front of, the tip of the thrust fault, iden- tical to the formation of a fault-propagation fold. When the thrust fault finally breaks through the folded lay- ers, the fold pair is separated into a hanging-wall an- ticline and a footwall syncline. A footwall syncline therefore represents the gentle deformation below the footwall ramp; this deformation does not add signifi- cantly to the displacement along the thrust fault. The footwall syncline may also be regarded as a drag fold. The case where this is the only correct interpretation is along a growth fault. Here the sediments deposited syntectonically up against a hanging-wall ramp are successively bent into an overturned syncline. The footwall syncline is identical to a trailing syncline. Duplex: A duplex is one or more thrust-sheet seg- ments entirely bounded by thrust faults and thus over- lain by a thrust sheet. A thrust sheet bounded by thrust faults is called a horse, originally regarded as a minor rootless thrust-sheet segment. Some of the lower thrust- sheet segments described in this study are identical to horses, although the more neutral term ‘segment’ is adopted here. The formation of a duplex is related to the ‘footwall ramp collapse’ (Boyer & Elliott 1982), whereby progressive failure during thrust-fault prop- agation creates successively younger thrust faults be- low older ones. A parcel of thrust-sheet segments may be stacked to form a duplex complex. Imbricate fan : A branching thrust-fault complex in which the individual thrust faults reach the surfaces or top level is called an imbricate fan. An imbricate fan is termed a duplex if the upper boundary is a roof thrust. Antiformal stack: When a duplex is fault-bend-fold- ed over a footwall ramp, an antiformal structure sim- ilar to a hanging-wall anticline is formed. Due to the complex stratigraphic relationship within such a struc- ture, it is referred to as an antiformal stack. Piggyback thrusting: When an older thrust sheet rests on the back of a younger thrust sheet and is trans- ported due to the displacement along the thrust faults bounding the younger thrust sheets, it is called pig- gyback thrusting. Piggyback basin: Just as piggyback thrusting refers to transport of a thrust sheet, the term is also applied to a basin that accumulates sediments during transla- tion on the back of an active thrust sheet: the piggy- back basin (Ori & Friend 1984; Ricci Lucchi 1986). In this study, the term is mainly used in the description of an area of sedimentation between two thrust sheets. In general, the piggyback basin is deposited between a fault-bend thrust-sheet tip in the distal part of a thrust structure and bounded by a hanging-wall ramp at the proximal boundary of the basin. The term piggyback basin can only be applied to successions identified as having been deposited syntectonically. 192 Appendix 2 Specification of photogrammetric work The construction of the Rubjerg Knude cross-section (Plate 1) is based on a multi-model photogrammetric investigation of the cliff section, with the application of the method described by Dueholm (1992). A series of oblique photographs were taken from a Cessna fixed-wing aircraft in June 1993. The camera used for the photography was a Minolta XG2, which had been tested and calibrated for its optical specifications at the laboratory of photogrammetry at the Danish Tech- nical University. The films used were standard 24 × 36 mm colour diapositive. The photographs were taken with 66% overlap from a distance of 200–300 m with an inclination angle of c. 35°. From the series of pho- tographs, 70 samples were selected for setting up three sets of templates, which included 67 stereoscopic models. In the laboratory, the orientation of the stereo-models was carried out based on ground control points adapt- ed from two sets of vertical aerial photographs at a scale of 1:25 000, namely D9202 G 1365–66 and KMS 9203 A509–10 taken in May 1992. The strike of the section line is N15°E from Rubjerg Knude and south- wards. North of Rubjerg Knude, the strike is N24°E, which is nearly parallel to the direction of the coast- line along the beach. Fortunately, this is also a rea- sonable approximation of being perpendicular to the main concentration of structural strikes (bedding, thrust faults and fold axes; Fig. 10). A minor adjustment of the northern and southern section lines was subse- quently implemented to make the cross-section fit to the general plane of orthographic projection with a projection axis striking 107°. The stereoscopic instrument used for the investiga- tion was a Kern DSR 15 analytical plotter with a DEC VMS operating system and the special attached GEO- PROGRAM developed by Dueholm (1992). Five dif- ferent labels were used for the features outlined by the floating mark: line type 1 includes bedding traces, line type 2 includes the main unconformities, line type 3 was used for the contacts between geological units (members and formations), line type 4 outlines thrust faults, and finally line type 5 was used for topograph- ic features (dunes, scree cones, strandplain, rockfalls etc.). Digitalisation of the geological structures in the stereo-models was administrated in data files, each covering a plot-area. The plot-areas covered 500 m of the Rubjerg Knude cross-section, and 13 plot-areas were used for the analogue plotting of data digitised in the stereo-model. The digital data were stored for the later construction of the cross-section and the trans- formation for other programs applied for the manage- ment of the cross-section display. The orientation of models and setting up the sys- tem for the cross-section investigation took about one week, and the photo-geological compilation work was made over a period of three months in the autumn of 1993. The average progress was two models per day. The benefit of the multi-model analytical stereo-plot- ter is that features can be traced continuously from one model to the adjacent models. Thus one is not restricted to working model by model, but the compi- lation can be extended over several models using the same set of templates. By January 1994, the cross-sec- tion could be plotted out in a normal vertical projec- tion profile plan from the stored digital data with the application of the program facilities prepared by Due- holm (1992). The scale of the Rubjerg Knude cross- section in the draft versions is 1:500, and the accuracy of the plotted data is estimated to be better than 25 cm. In 1995–1996, the cross-section details observed in the photo-geological models were checked in the field, and in 1997 the templates were set up again for cor- rection, adjusting and compilation of details in the cross-section.