SESS 2_16022011_TPR3.pub Il Quaternario Italian Journal of Quaternary Sciences 24, (Abstract AIQUA, Roma 02/2011), Congresso AIQUA Il Quaternario Italiano: conoscenze e prospettive Roma 24 e 25 febbraio 2011 DETECTING AND MODELING ACTIVE STRUCTURES IN NORTHERN CALABRIA: NEW INSIGHTS FROM DRAINAGE NETWORK MORPHOMETRIC ANALYSIS. Enrico Santoro1, Luigi Ferranti2, Carmelo Monaco1, Frank J. Pazzaglia3 & Pierfrancesco Burrato4 1Dipartimento di Scienze Geologiche, Università di Catania, Catania 2Dipartimento di Scienze della Terra, Università “Federico II”, Napoli 3Department of Earth and Environmental Science, Lehigh University, Bethlehem (PA) 4Sezione di Sismologia e Tettonofisica, I.N.G.V., Roma Corresponding author: E. Santoro ABSTRACT: Santoro E. et al., Detecting and modeling active structures in northern Calabria: new insights from drainage network morphometric analysis.(IT ISSN 0394-3356, 2011). A detailed morphometric analysis of the drainage network on the eastern side of the Pollino mountain range (North- eastern Calabria) was performed in order to gain new insights on the active tectonic frame, the landscape evolution state and the response time of the hydrographic network to the tectonic deformations. The uniform coastal lithology and climate allow us to isolate the tectonic signal from the fluvial morphometric analysis. Previous work, based on detailed mapping and lateral correlation of ten marine terrace orders (Middle Pleistocene – Holocene) between the Pollino range and the southern border of the San Nicola plain (Basilicata), and on analysis of structural, geodetic, geophysical and seismic data, allowed to hypothesize the active uplift of two fault-propagation folds, along the Pollino and the Valsinni ridges, linked to the high-angle transpressive shear zones that affected the region since the Early Pleistocene. The quantitative analysis of the longitudinal profiles was performed in order to test and improve the above mentioned deformation model. Linear regressions of the logarithms of channel gradient and drainage area data was used to find convexities (knickpoints) and to determine the concavity index (θ) and the steepness index (ks). Based on the θ and ks values the river paleo-longitudinal profiles were reconstructed in order to correlate each knickpoint with a marine terrace and the following glacial eustatic regression. The river vertical incision rates and the along coast variation of the knickpoints elevation linked to the same glacial eustatic regression are in perfect agreement with the deformation model. The rivers that flow through the uplifting Pollino and Valsinni anticlines are characterized by an increase of the: 1. longitudinal gradient; 2. vertical incision rate; 3. knickpoints elevation. They react to the higher uplift rates increasing their steepness in order to gain sufficient energy to incise the river-bed and to reestablish the previous equilibrium state. The hydrographic network response time (~105 year; that is the time for the knickpoint migration through the entire basin) shows that the tectonic morphometric anomalies are linked to active landscape deformation processes. Finally, the drainage network morphometric analysis has confirmed the deformation model derived from the paleo-shoreline profiles and from structural-geophysical data, suggesting that the analyzed sector of the Southern Apennine is still affected by a transpressional tectonic regime. RIASSUNTO: Santoro E. et al., Individuazione e modellazione di strutture attive in Calabria settentrionale: nuove evidenze dall’analisi morfometrica del reticolo idrografico. (IT ISSN 0394-3356, 2011). Un’analisi morfometrica di dettaglio delle principali fiumare che drenano il versante orientale del Pollino (Calabria Nord- orientale) ha consentito di acquisire nuove conoscenze riguardo il quadro tettonico attivo, lo stato evolutivo del paesaggio e i tempi di risposta del sistema idrografico alle deformazioni imposte dalla tettonica. La possibilità di determinare indipendentemente i fattori di controllo di natura climatica, litologica e tettonica rende l’area di studio un ottimo banco di lavoro per l’analisi dei processi tettonici attivi attraverso l’applicazione delle tecniche morfometriche. In lavori precedenti, sulla base del rilevamento di dieci ordini di terrazzi marini (Pleistocene Medio – Olocene), tra il versante orientale del Pollino e il limite meridionale della Piana di San Nicola (Basilicata) e di dati strutturali, geofisici, geodetici e sismici era stato ipotizzato che l’area è interessata dal sollevamento di due anticlinali di crescita, in corrispondenza delle dorsali del Pollino e di Valsinni, relazionate a zone di taglio transpressive, ad alto angolo e a direzione NO-SE, attive a partire dal Pleistocene Inferiore. Con l’obiettivo di verificare e implementare il suddetto modello di deformazione è stata realizzata un’analisi quantitativa dei profili longitudinali delle fiumare che ha consentito di individuare numerosi knickpoint. La ricostruzione dei paleo- profili longitudinali delle valli, attraverso l’utilizzo dei parametri ks (indice della pendenza) e θ (concavità), ricavati dai grafici logaritmici area del drenaggio – gradiente, ha permesso di datare le convessità di origine eustatica (regressioni glaciali) attraverso la correlazione con i terrazzi marini. I risultati ottenuti (tassi di incisione verticale delle fiumare, variazione lungo costa dell’altezza delle convessità correlate ad uno stesso ciclo glaciale e dei parametri ks e θ) sono in perfetto accordo con il sollevamento delle suddette anticlinali. In corrispondenza di queste ultime i corsi d’acqua reagiscono ai maggiori tassi di sollevamento attraverso un aumento dei gradienti finalizzato all’acquisizione dell’energia necessaria ad erodere e attraversare la soglia tettonica in sollevamento. Il risultato è un aumento dei tassi di incisione fluviale fino ad un massimo di 0,7 mm/a. I tempi di risposta del sistema idrografico (~105 anni; tempo necessario ad una convessità per migrare attraverso l’intero bacino idrografico) indicano che le anomalie morfometriche relazionabili alla tettonica sono necessariamente legate a processi di deformazione del paesaggio tuttora attivi. In definitiva, l’analisi morfometrica conferma il modello di deformazione precedentemente proposto sulla base dei profili delle antiche linee di riva e dei dati strutturali e geofisici, evidenziando che il settore di catena investigato è tuttora interessato da tettonica transpressiva. Key words: morphometric analysis, knickpoints, active transpression, North-eastern Calabria. Parole chiave: analisi morfometrica, convessità, transpressione attiva, Calabria Nord-orientale. 132 - 134 133 The longitudinal profile of bedrock channels may yield valuable information about the landscape vertical deformation processes, the rock mass erosional strength, the eustatic cycles and the climatic factors controlling the landforms. A number of studies have been successful in the evaluation of recent deformation features using quantitative analysis of longitudinal profiles (e.g., Merritts and Vincent, 1989); the realization of this goal, however, remain limited to the areas were local calibration of model parameters is possible (KIRBY & WHIPPLE, 2001). In this study, inspired by previous results obtained through the mapping of the marine terraces and the reconstruction of the relative paleo-shorelines, we have performed a detailed morphometric analysis of the drainage network on the eastern side of the Pollino mountain range (North-eastern Calabria; Fig. 1). The uniform coastal lithology and climate allow us to isolate the tectonic signal from the fluvial morphometric analysis. This region is located on the northeastern tip of the Calabrian arc, which lies above the westerly- plunging Ionian slab, and a combination of lithospheric and crustal processes concurred to rapid Late Quaternary uplift. FERRANTI et al. (2009) and SANTORO et al. (2009) mapped ten terrace orders up to 663 m a.s.l., correlated between the coastal slopes of Pollino range across the Sibari Plain, facing the Ionian Sea side of North-eastern Calabria and eastern Basilicata. Based on the terrace chronology, uplift along the Pollino coastal range, in the last ~600 ka, occurred at an average rate of 1 mm/a, but was characterized by the alternation of more rapid (~2.2 mm/a) and slower (~0.7 mm/a) periods of displacement. Besides, spatial variability in uplift rates is recorded by the deformation profile of terraces parallel to the coast, which document small-wavelength (~20 km) and amplitude (up to ~100 m) local undulations superposed to the regional uplift pattern, which is the dominant tectonic signal (SANTORO et al., 2009). Based on the structural, geodetic, geophysical and seismic data the paleo-shorelines undulations were interpreted as fault-propagation folds linked to the transpressive shear zones, NO- SE directed, that affected the region since the Early Pleistocene. This hypothesis is confirmed by the results of the numeric modelling of the shear zones cutting the coastal area where the major deformations were identified. The good match between the modelled vertical co-seismic deformation and the paleo-shorelines deformation profiles testifies that the transpressional regime was active during the Middle-Upper Pleistocene. Through an iterative comparison between the paleo-shoreline profiles and the vertical co-seismic deformation it was possible to define the geometric and kinematic fault parameters that best explain the marine terraces deformation. The final deformation model predicts the active uplift of two anticlines along the south-eastern flank of the Pollino range and the Valsinni Ridge (hereafter named Pollino and Valsinni anticlines, respectively; Fig. 1). The morphometric analysis was performed to test and improve the above mentioned deformation model. Linear regressions of the logarithms of channel gradient and drainage area data was used to find convexities (knickpoints) and to determine, the concavity index (θ) and the steepness index (ks). The method is based on the shear-stress incision model, predicated on the hypothesis that the bedrock-channel-erosion rate (E), in volume per unit channel area, is a power-law function of basal shear stress (τb): E = kbτb a, (1) where kb is a dimensional coefficient dependent on dominant erosion process, rock resistance and sediment load and a is a positive, process- dependent constant. Combining the shear-stress incision model with a statement of conservation of mass, assuming that uplift and erosion are uniform in a drainage basin and solving for the river longitudinal slope is possible to write: S = ksA-θ, (2) The coefficient ks (steepness index) and the exponent θ (concavity index) are, respectively, the gradient-axis-intercept and the slope of the linear regressions through the logarithms of channel gradient and drainage area data. We limited our analysis to the bedrock channel region; the results obtained from area controlled by hillslope/colluvial or alluvial processes are, indeed, not reliable. The rivers longitudinal profiles, extracted from DEM 10x10 m, were corrected averaging the elevation through a mobile 200 m window, in order to reduce the gradient data dispersion derived by DEM errors. Following SNYDER et al. (2000) and WHIPPLE (2004) we used a fixed reference concavity (θ = 0.45) in order to calculate a normalized steepness index (ksn); this permitted the comparison between channels characterized by different drainage areas (e.g., KIRBY et al., 2003). The ksn values increase regularly toward the Pollino and Valsinni anticlines; in other words, the rivers flowing through the areas that are affected by the higher uplift rates tend to establish a steeper longitudinal profile in order to have the energy sufficient to erode the uplifting tectonic barrier. Several knickpoints (steeper channel segments characterized by convex-upward shapes and θ<0) were detected along the longitudinal profiles of the trunk streams. Being the analyzed rivers adjusted to the sea level (drainage network base level), it is reasonable to hypothesize that some of the knickpoints are related to the glacial-interglacial Quaternary cycles. To test this idea, for each Detecting and modeling active structures ... 134 Ms. received: Testo ricevuto il knickpoint the reconstruction of the interglacial profile was performed through the ks and the θ values calculated for the respective river. An eustatic origin was considered valid if: 1. the paleo- longitudinal profile crosses a marine terrace; 2. at least three knickpoints are related to the same marine terrace (same eustatic regression); 3. the knickpoints correlated to the same eustatic cycle are characterized by migration rates and positions that are in direct relationship with the landward drainage area. For some of the knickpoints the eustatic origin is confirmed by the presence of fluvial terraces aligned with the relative paleo- longitudinal profiles; the terraces are, reasonably, the consequence of the erosive wave triggered by the eustatic regression and progressively transmitted to the entire drainage basin through the knickpoint landward migration. Generally, the elevation of a knickpoint genetically related to an eustatic sea level regression is influenced by the drainage area (migration rate) and the trunk stream longitudinal gradient. As a consequence, holding constant the drainage area the knickpoint elevation can be influenced by gradient changes determined by differential uplift rates; the higher the uplift rate the higher the gradient and the knickpoint elevation. In the study area this theoretical prediction is perfectly confirmed: all the knickpoints related to the same marine terrace are always characterized by an increase of the elevations toward the Pollino and Valsinni anticlines (Fig. 1) where the higher uplift rates determine a rise of the river gradients. The maximum difference in elevation between the paleo-longitudinal profiles and the present river- bed was evaluated in order to determine the mean fluvial incision rates. As forecasted by the deformation model, the rivers flowing through the Pollino and Valsinni anticlines are characterized by the higher incision rates (Fig. 1). These streams, in order to adjust their equilibrium profile disturbed by the tectonic deformation, increase their gradient and vertically incise through their valley. Finally, based on the migration rates, it was possible to determine the drainage network response time considered to be the time necessary for a convexity to migrate through the entire drainage basin. All the analyzed rivers show a low response time (~105 yr) that highlights the youthfulness of the tectonic deformation; indeed, all the tectonic signals older than 105 yr can be considered completely erased by the fluvial erosive processes. The longitudinal profile analysis have permitted to confirm the results independently reached with the marine terraces, the structural and geophysical analysis and the fault modelling, evidencing that compressional regime is still active in this sector of the Southern Apennines. REFERENCES FERRANTI L., SANTORO E., MAZZELLA M.E., MONACO C. & MORELLI D. (2009) - Active transpression in the northern Calabria Apennines, southern Italy - Tectonophysics, 476 (1-2), 226-251. KIRBY E., WHIPPLE K.X., TANG W. & ZHILIANG C. (2003) – Distribution of active rock uplift along the eastern margin of the Tibetan Plateau: Inferences from bedrock channel longitudinal profiles – J. Geophys. Res., 108 (B4), 2217, doi: 10.1029/2001JB000861. KIRBY E. & WHIPPLE K. (2001) – Quantifying differential rock-uplift rates via stream profile analysis – Geology, 29 (5), 415-418. MERRITTS D. & VINCENT K.R. (1989) – Geomorphic response of coastal streams to low, intermediate and high rates of uplift, Mendocino triple junction region, northern California – Geol. Soc. Am. Bull., 101, 1373- 1388. SANTORO E., MAZZELLA M.E., FERRANTI L., RANDISI A., NAPOLITANO E., RITTNER S. & RADTKE U. (2009) - Raised coastal terraces along the Ionian Sea coast of northern Calabria, Italy, suggest space and time variability of tectonic uplift rates - Quaternary International, 206, 78-101. SNYDER N.P., WHIPPLE K.X., TUCKER G.E. & MERRITTS D.J. (2000) – Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in Mendocino triple junction region, northern California – Geol. Soc. Am. Bull., 112, 1250-1263. WHIPPLE K.X. (2004) – Bedrock rivers and the geomorphology of active orogens – Ann. Rev. Earth Planet Sci., 32, 151-185. Fig. 1 – Knickpoints elevation and incision rates compared with the deformation model. Altezze dei knickpoints e tassi di incisione fluviali comparati con il modello di deformazione. E. Santoro et al. January 13, 2011 13 gennaio 2011