


































Energy and Earth Science 
Vol. 6, No. 2, 2023 

www.scholink.org/ojs/index.php/ees 

ISSN 2578-1359 (Print)   ISSN 2578-1367 (Online) 

 

1 

 

Original Paper 

Detection of Ring Structures and Their Surrounding Tectonic 

Pattern in South-Algeria, North-Mali and North- Niger based on 

Satellite Data 

Barbara Theilen-Willige
1
 

1
 Technische Universität Berlin (TUB), Faculty IV (retired), Berlin, Germany 

 

Received: March 21, 2023      Accepted: March 30, 2023      Online Published: April 25, 2023 

doi:10.22158/ees.v6n2p1                URL: http://dx.doi.org/10.22158/ees.v6n2p1 

 

Abstract 

This study is focused on the detection of circular features with different sizes, origins, and state of 

erosion as well as on their surrounding tectonic pattern based on different satellite images of Southern 

Algeria, Northern Mali and Northern Niger. Sentinel 2- and Landsat 8/9-images and Sentinel 1- and 

ALOS L-band Phased Array Synthetic Aperture Radar (PALSAR)-radar data help to identify larger 

ring structures and smaller circular features, most of them related to magmatic intrusions into the 

subsurface, but also to cosmic impacts, with varying ages and state of erosion. Some of them seem to 

be unknown so far as they are partly covered by aeolian sediments and become only visible on radar 

images. Digital Elevation Model (DEM) data and the DEM derived morphometric maps support these 

investigations in a GeoInformation System (GIS) embedded environment. Some of the ring structures 

are only visible on morphometric maps, traced by circular arrangements of slope gradients or 

concentric drainage patterns. The large, circular structures and the smaller circular features such as 

volcanic features (cinder cones, calderas, maars, impact craters) were digitized and merged in a GIS 

with available geologic information. 

Keywords 

ring structures, structural analysis, remote sensing, S-Algeria 

 

 

 

 

 



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1. Introduction 

Although a large amount of research has been carried out related to the geologic history and structural 

and tectonic inventory in Southern Algeria and adjacent areas (Liégeois et al., 2005; Azzouni-Sekkal et 

al., 2007; Fezaa et al., 2010; Yahiaoui et al., 2014; Bouzid et al., 2015) combined evaluations of 

different satellite data integrated into a GeoInformation System (GIS) are investigated, whether they 

can contribute to further additional geologic knowledge Especially the combination of Landsat 8 and 9 

thermal band-, and Sentinel 2 optical images, and Sentinel 1-, ALOS PALSAR- and SIR-C-radar 

images allows the detection of surface-near structures (Paillou et al., 2006).  

After evaluations of optical and radar satellite images and digital elevation data from Southern Algeria, 

Northern Mali and Northern Niger it became obvious, that there are some circular structures visible on 

the satellite images and on morphometric maps that were neither documented on available geologic 

maps, nor described in the geologic literature and, thus, seem to be unknown so far. The reasons for 

this vary according to the specific situation. Some structures are buried underneath younger sediments 

and extended aeolian covers and not visible in the field, but still on satellite radar data because of the 

concentric arrangement of the drainage pattern or ring-shaped tonal anomalies on the images. Therefore, 

this study aims to contribute to the systematic detection and inventory of ring structures and their 

adjacent tectonic pattern, as far as possible based on remote sensing and GeoInformation System (GIS) 

methods.  

The combination of different remote sensing data is used in the scope of this study to derive an 

overview of structural information and focus on an inventory of circular features of different sizes and 

origins and their surrounding tectonic pattern. Such an inventory is important for example for 

hydrogeologic investigations as the groundwater flow is influenced by those structures because of 

permeability changes and for the mining and energy industry. For example, larger ring structures have 

an impact on multi-aquifer groundwater flow in the different groundwater levels. The larger fault 

systems and geologic structures such as dikes affecting the groundwater flow should be mapped 

carefully using the potential of thermal and radar satellite data as far as possible for this purpose. The 

long and thick, low-permeability dikes often act as barriers for the topography-driven groundwater 

flow.  

As mineral occurrence and deposits of economic value are often related to ring structures evaluations 

of satellite data might be of use for further detailed exploration. 

Another reason for the detailed inventory of circular structures is the monitoring of potential 

geohazards. The position of larger ring structures related to plutons seem to have an influence on 

earthquake activity (Theilen-Willige, 2022). The different circular forms created by volcanic activity 

might be a risk for the land use and infrastructure, even in this arid environment with a low population 

density. Not only the smaller eruptions have to be considered, but also the numerous dikes along fault 

zones and zones of weakness along structures.  



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Another example for the importance of geohazard monitoring is the collapse of sinkholes in karst areas 

affecting the safety of infrastructure.  

Among the geohazards cosmic impacts of a larger meteorites or even asteroids have to be taken into 

account as well. The documented impact craters in Algeria, Mali and Niger are proof of this potential 

risk in the geologic history. The Earth Impact Database (EID) created by the Planetary and Space 

Science Centre (PASSC), Canada, comprises a list of confirmed impact structures from around the 

world on land surfaces. To date, there are 190 confirmed impact structures in the world-wide database. 

It can be assumed that with ongoing research further impact craters will be discovered, some of them 

may be in the scope of this study. The low settlement density and land use restricted to water 

availability in this arid environment allows the conservation of craters, although often covered by 

aeolian sediments. Special attention is focused on smaller, bowl-shaped craters that can be related to 

volcanic maars and calderas, karst depressions or impact craters.  

The next figure shows an overview of cosmic impact craters in the investigation area covering Algeria 

and N-Mali and N-Niger (Figure 1).  

 

 

Figure 1. Overview of Documented Impact Craters in Algeria. N-Mali and N-Niger Based on the 

Planetary and Space Science Centre (PASSC), Canada, Impact Crater Data Base 

 

 

 

 



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2. Geographic and Geologic Overview 

The investigation area includes the Hoggar mountains in S-Algeria with height levels up to 2900 m, the 

Air mountains in N-Niger reaching height levels up to 2000 m and the Adrar des Iforas in NE-Mali 

with height levels between 800-900 m (Figure 2). The climate is characterized by an arid environment 

with a hot desert climate.  

The Hoggar Massif in Algeria is associated with exposed Pan-African basement extending over an area 

greater than 500 000 km
2
 (English et al., 2016), belonging to the Tuareg Shield composed of terranes. 

Most of the Tuared Shield is situated in Algeria (Hoggar) and extends in the SW into the Adrar des 

Iforas in Mali mountains and in the SE into the Air mountains in Niger. The Hoggar mountains form 

the main part of the Tuareg shield, which principally comprises Archaean/Palaeoproterozoic and 

Neoproterozoic terranes, composed of Precambrian lithologies and unconformably overlain by 

subhorizontal Palaeozoic sediments. The Tuareg shield amalgamated during the late Neoproterozoic 

Pan-African orogeny as a result of the convergence of the West African craton (WAC) and the Saharan 

craton. The major structural domains of the Tuareg Shield are the “Polycyclic Central Hoggar” to the 

east and the “Western Hoggar”, or “Pharusian Belt”, to the west (Kourim et al., 2014). The terranes 

collided, welded and moved along subvertical shear zones to be finally squeezed between the West 

African craton to the west and the Saharan metacraton to the east (Azzouni-Sekkal et al., 2003).  

 

 

Figure 2. Height Level Map of the Investigation Area Based On GEBCO Elevation Data 

 



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Geochronological results demonstrate that the Eastern Hoggar had not stabilized by c. 730 Ma, but was 

subjected to a late Ediacaran tectono-magmatic episode at c. 575-555 Ma. This episode is younger and 

unlinked to the Pan-African orogeny that occurred further west in the Tuareg Shield (Fezaa et al., 2010). 

A first stage comprised the accretion of oceanic island arcs on these cratons and on microcratons during 

the period 900-680 Ma. Relics of these terranes, including ophiolites and eclogites, are preserved as 

thrust sheets on more rigid bodies (Liégeois et al., 2003, 2008). The second stage was the regional 

northerly tectonic escape of the Tuareg terranes due to oblique collision with the WAC. During that 

stage, the metacratonization of the Central Hoggar microcontinent occurred, the squeezing of this rigid 

body, which was torn into several moving blocks. During the main Pan-African phase (625-580 Ma) 

the Central Hoggar microcontinent was dissected by N-S-oriented mega-shear zones that induced 

several hundreds of kilometers of relative displacement and allowed the emplacement of high-K 

calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement 

of subcircular plutons with alkaline affinity. Linear lithospheric delamination beneath these mega-shear 

zones may occur under such circumstances, allowing a drastic increase in heat flow and melting of the 

crust. Post collisional and anorogenic high-level alkaline plutons are aligned on the same megashear 

zones, particularly along craton margins (Liégeois et al., 2003; Fezaa et al., 2010). 

Figure 3 provides a geologic overview of the main geologic units according to the data provided in the 

web portal OneGeology by the British Geological Survey British Geological Survey (BGS). 

 

 

Figure 3. Geologic Overview of the Tuareg Shield according to the 1:10M-scale Geological Map 

of Africa-BGS, 1:5,000,000 Scale, Downloaded from the OneGeology, 

http://portal.onegeology.org/OnegeologyGlobal/ 



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Recent volcanic activity from Upper Eocene to Quaternary in age (35 to nearly 0 Ma) is associated in 

the Hoggar area with a crustal swell of 1000 km in diameter, probably the product of a mantle plume. 

In response to stress resulting from the Africa-Europe collision, volcanism may be generated by 

adiabatic pressure release of an uprising asthenosphere. The reactivation of preexisting shear zones and 

fractures generated during the Pan-African (late Neoproterozoic) orogeny (inducing limited linear 

lithospheric delamination at the lithosphere-asthenosphere interface along these mega-shear) played an 

important role. There are several volcanic districts located in Hoggar, Massif, N-Mali and N-Niger 

(Figure 3, Liégeois et al., 2003, 2008; Yahiaoui et al., 2014; Bouzid et al., 2015). The first volcanic 

event produced Miocene composite flood lavas (plateau basalt) essentially made of alkali olivine 

basalts. Trachytic and phonolitic plugs are associated with this phase zones (Liégeois et al., 2003, 

2005).   

 

3. Data and Methods 

The interdisciplinary approach used in the scope of this research comprises evaluations of remote 

sensing data, geological, and topographic data, integrated into a GIS environment. Satellite imageries 

and Digital Elevation Model (DEM) data were used then for generating a GIS data base and combined 

with different geodata (infrastructure, land use) and available geologic maps. Infrastructural and land 

use shapefiles from Algeria were downloaded from the Geofabrik’s download server. 

Digital Elevation Model (DEM) data from the Shuttle Radar Topography Mission (SRTM), and 

ASTER DEM data (both with about 30 m spatial resolution) are covering the whole investigation area. 

The Advanced Land Observing Satellite-1 (ALOS), Phased Array type L-band Synthetic Aperture 

Radar (PALSAR) from the Japan Aerospace Exploration Agency (JAXA) provided data with 12.5 m 

spatial resolution, however, not a complete coverage. The satellite data were downloaded from open 

sources such as the USGS/Earth Explorer, the Sentinel Hub/ESA, and the Alaska Satellite Facility 

(ASF).   

3.1 Digital Image Processing of Different Optical and Radar Satellite Data  

Satellite data were processed and evaluated such as Sentinel 1-C-Band, Synthetic Aperture Radar (SAR) 

and ALOS PALSAR L-Band data and optical Sentinel 2 images, and Landsat optical data (Landsat TM 

and Landsat 8 and 9 of the Operational Land Imager-OLI) using digital image processing software as 

the Sentinel Application Platform (SNAP)/ESA and ENVI/L3Harris Geospatial Solutions as well as the 

geoinformation systems ArcGIS/ESRI and QGIS. Digital image processing of LANDSAT 5 Thematic 

Mapper and Landsat 8/9-The Operational Land Imager (OLI) data was carried out by merging different 

Red Green Blue (RGB) band combinations with the panchromatic Band 8 to pan-sharpen the images. 

Three color composites were generated for the purpose of lithological zones mapping. Especially, when 

combing the thermal bands of optical satellite data in a RGB image structural features are enhanced due 

to brightness differences. When analyzing satellite data traces of structures such as faults, folding and 



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domes are often clearly visible on satellite images.  

By using the band combination of 2, 7 and 10 of Landsat 8/9 even deeply eroded ring structures can 

become visible as demonstrated by the example below from N-Mali (Figure 4). SNAP from ESA 

provided the tools as well for the processing of radar data.   

Synthetic Aperture Radar (SAR) space-borne satellites operate by using microwave radiation with a 

frequency in a range of wavelengths, for example, the L-band (23 cm-1.30 GHz and 24 cm-1.25 GHz), 

C-band (6 cm-5.0 GHz), (Mansour et al., 2022). L-band radar coverage was acquired by the JERS-1 

satellite of the Japanese space agency (JAXA), C-Band coverage by the Sentinel 1 mission of ESA. 

SAR imagery can differentiate topographic variation with a high spatial resolution. Radar imagers send 

out pulses of radio waves and collect the return signals after the waves were reflected of surfaces. The 

smoothness, roughness, and density of a surface affect the return signal. Unlike optical instruments, 

which can only see the surface, radar can penetrate loose, dry materials such as sand sheets. 

ALOS/PALSAR L-Band data are characterized by the optimal conditions of penetration in desert areas 

depending on the grain size, moisture, roughness of the surrounding geology and radar backscattering 

mechanism (Mansour et al., 2022). In very dry soils, L-Band SAR (1.25 GHz) can intrude into the 

subsurface down to several meters. Dark image tones are associated in general with flat areas because 

the incident radar signals were largely reflected from their “radar-smooth” surfaces in a mirror-like 

fashion away from the satellite antenna.  

 

 

Figure 4. Deeply Eroded Ring Structure in N-Mali Visible on a Landsat 9-Scene (RGB, Bands 2,7 

and 10) 



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Paleodrainage systems become visible due to their radar-smooth reacting sediment fill. Thus, ring 

structures forming circular basins and depressions are often clearly detectable on radar images. The 

evaluation of radar data also helps in the building of more complete geological maps and in support of 

future water and mineral prospecting in arid regions (Paillou et al., 2006, 2017). Evaluations of C- and 

L-Band radar data prove to be of great value in the arid environment of the investigation area for the 

detection of geologic structures underneath aeolian covers. The visibility of ring structures on radar 

images depends in this arid environment mostly on the radar-illumination geometry and the surface 

properties. 

3.2 Evaluations of DEM Data 

Mosaics were created based on the various DEM data. In the scope of this study evaluations of 

morphometric maps derived from different DEM data play an important role because ring structures 

sometimes can be traced more clearly on slope gradient, height level or drop raster maps than on other 

satellite images. Optical, digital processed and enhanced satellite images are then correlated with the 

morphometric maps. Figure 5 shows an example of a combined analysis of different satellite data.  

 

 

Figure 5. Combined and Comparative Analysis of Different Satellite Data (Optical Data-Sentinel 

2, Radar Data-Sentinel 1, SRTM DEM Derived Height Level and Slope Map) for the Detection of 

Ring Structures and-S-striking Shear Zones 

 

 



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3.3 Structural Evaluation 

The shapefiles related to ring structures, lineaments, structural features, karst phenomena and volcanic 

features were digitized visually based on the different satellite images. Especially Sentinel 1 and ALOS 

PALSAR radar images reveal larger fault zones, for example by dislocations of lithologic units that can 

be digitized easily. In the scope of this study the following types of linear and curvilinear features were 

mapped: lineaments (as a neutral term for linear features without identifying their origin), probable 

fault zones, major fault zones and structural features. As traces of structural features are digitized the 

deformations due to stress such as synclines or anticlines, bedding structures, or traces of foliation in 

metamorphic rocks, often become visible as dense, arc-shaped, parallel lines.    

 

4. Inventory of Ring Structures and Their Surrounding Structural Pattern Based on Satellite 

Data  

In the following chapters an overview of different circular structures and their expressions on satellite 

images is presented. In most cases their origin can be derived, however, in some cases clarifying of 

their origin remains for future investigations. The multiple overprinting of the structures during their 

geologic history often has led to a complex lithologic and tectonic development and composition. The 

structural pattern was analyzed, whereby the numerous dikes and dike swarms in the investigation area 

are of great value as the dikes indicate zones of “weakness” allowing the intrusion of magma. The 

spatial arrangement of mostly basic and ultrabasic dikes, their orientation and their density provide 

hints about the dynamic stress pattern. When digitizing the circular, structural features, it can be 

subdivided between several types of ring structures: 

a) traces of larger ring structures by a circular outline of the drainage pattern and circular tonal 

anomalies on the satellite images such as shown in Figure 6,  

b) distinct expressed larger ring structures related to plutonic rocks forming often high-altitude domes 

and hills in the landscape with circular outline (Sieber & Theilen-Willige, 1984),  

c) large circular basins with surrounding concentric hill ranges. Some are forming bowl-shaped craters.  

The origin of this type of ring structure can be complex. One explanation might be volcanic activity 

leading to the development of a caldera. Another one might be the erosion of a pluton with different 

resistivity of the lithologic units to erosion. Along concentric faults of plutons magma often intruded 

later building ring dikes. 

d) Smaller volcanic features such as volcanic cones, bowl-shaped craters (often related to maars and 

some to cosmic impact craters) and undefined smaller depressions such as deflation hollows.  

The main types of ring structures according to their sizes, geomorphologic appearance and origin are 

summarized in Figure 6.  



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Figure 6. Main Types of Sizes and Origins of Ring Structures 

 

Figure 7 provides an overview of the mapped circular features. Of course, due to the large dune fields 

and aeolian covers this inventory based on satellite images cannot be complete. When combining the 

inventory of ring structures with lineament analysis and the known fault systems extracted from 

geologic maps, it becomes obvious that the larger ring structures are concentrated along the major shear 

zones (Figure 7). Post collisional and anorogenic high-level alkaline plutons are aligned on these 

mega-shear zones (Black et al., 1979; Liégeois et al., 2003, 2005, 2007). The NNW-SSE, NNE-SSW 

and N-S-oriented Pan-African mega-shear zones are cross cutting lithologic units over hundreds of 

kilometers, up to 1000 km from Middle Algeria to Northern Niger and Mali, visible even through the 

youngest sedimentary covers. These shear zones consist of parallel segments, partly moving relatively 

to each other. Sometimes “graben-like” structures can be observed on the satellite images and on height 

level maps. Recent tectonic uplift might be the reason (Azzouni-Sekkal et al., 2007). The large 

shear-zones were reactivated during later orogenic events and are still overworked by ongoing 

neotectonic movements, traced by abrupt displacements and magmatic intrusions, especially by parallel, 

linear dikes. The shear zones have been displaced and intruded by magmatic bodies forming dikes, 

maars and scoria cones, most of them of Pleistocene and Holocene ages (Liégeois et al., 2005; 

Yahiaoui et al., 2014). Dikes and single, smaller volcanic scoria cones occur concentrated along the 

shear zones, as well as along larger ring structures.  

 



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As these shear zones played an important role for the emplacement of magmatic bodies, a more 

detailed analysis of their structure was carried out using Landsat 8 and 9, Sentinel 2 and radar satellite 

data. Figure 8 demonstrates the appearance of a shear zone near the southern border of Algeria as 

visible on a Landsat 9 RGB scene and Figure 9 its structural evaluation.  

The varying geomorphologic properties of the shear zones depend on the regional geomechanical stress 

situation and lithologic composition. Dikes intruded later into shear zones are forming linear ridges 

over long distances. Reactivations of pre-existing fault structures under the current tectonic stress field 

have an impact on the development of dikes.  

The topographic cross section of the shear zone shown in Figure 9 and reveals the almost flat 

environment, intersected by ridges and hill chains whenever dikes intruded into these zones of 

weakness (Figure 10). However, most of them are buried underneath sedimentary covers. Whenever 

the loose sedimentary covers or other lithologic layers comprise more than several meters, they cannot 

be detected even on L-Band radar images. The fact that distinct expressed N-S-striking fault zones are 

clearly visible on the different satellite images, cross-cutting youngest sediments, as well as the 

intrusion of numerous dikes and scoria cones into the fault segments seems to support the assumption 

that the shear zones are still reactivated due to ongoing tectonic stress and movements. 

 

 

 

 

 

 

 

 

 

 

 



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a) 

 

b) 

Figure 7 a and b. Structural Evaluation Combined with Known Fault Zones Pan-African Mega 

Shear Zones, 1:10M-Scale Geological Map of Africa-SIGAfrique-Bedrock Age, French 

Geological Survey (BRGM), France), Downloaded from the OneGeology Portal  

 



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Figure 8. Landsat RGB, Bands 2,7 and 10 Scene of the Shear Zone and a Distinct Visible Crater 

on the Lower Left of the Scene. The WSW-ENE Oriented Stripes Are Caused by Wind Erosion 

 

The topographic W-E-profile of the crater and the shear zone is presented in Figure 10. The crater rises 

above the environment with more than 100 m. The height levels of the shear zone vary with the dike 

occurrence.  

Another part of the Pan-African shear zone in the central part of the Hoggar Massif between 4° and 5° 

longitude is revealed by ALOS PALSAR L-Band data (Figure 11 a-d). The parallel segments can be 

detected clearly. Dikes are visible as very light lines along the shear zone due to their strong radar 

backscatter.  

In the volcano-tectonic regions of S-Algeria, N-Mali and N-Niger, dike propagation from shallow 

magmatic chambers is controlled by the interaction with the local and regional stress fields, whereas 

their variations result from a combination of factors such as the geometry of pressurized magma 

chambers and the local geologic structure and the pre-existing fractures. When dike swarms are 

associated with flood basalts, their full extent and distribution are often difficult to assess because of a 

lack of exposure (Hou, 2011). Dike systems are typically hidden beneath volcanic piles or youngest 

sedimentary covers.  

 

 



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Figure 9. Structural Evaluation of the Scene in Figure 9 

 

 

Figure 10. W-E-topographic Cross Section of The Shear Zone and the Crater (Left Side) Based 

on SRTM DEM Data 



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Figure 11. a) and b). ALOS PALSAR L-Band-scene (Horizontal Polarization HH) of a Large, 

Pan-African Shear Zone in the Hoggar Massif (Red Frame-enlargement Shown in c and d) 

c) BingMap, and d) ALOS PALSAR-Scenes visualizing Dikes along the Shear Zone in the SW of 

the Hoggar Mountains 

 

An overview of dikes visible on the different satellite data is shown in the next figure (Figure 12 a and 

b). The density of dikes was calculated to visualize those areas with a higher dike concentration (Figure 

12 a). The relatively higher dike density might be explained by uprising magma, that (by updoming the 

strata above) intensified fracturing processes and the reactivation of existing fault zones. Dikes intruded 

into these zones of weakness. The highest density of dikes striking predominantly NNW-SSE was 

detected in the Adrar des Iforas area in N-Mali (Figure 12 b) within the Kidal-terrane, mainly 

comprising basement gneisses and calc-alkaline granitic plutons (Bosch et al., 2016). A higher density 

of dikes could be used as an indicator for a relatively stronger tectonic stress in the affected area, at 

least at the time period of their development.  

 

 

 

 



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Figure 12 a. Density Concentration of Dikes Digitized Based on the Different Satellite Data 

 

 

Figure 12 b. Highest Dike Density in the Adrar des Iforas Area in N-Mali and Main Dike 

Orientations 

 

 



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Larger circular structures appear on the different satellite images as demonstrated in the next figures. 

3.1 Large Ring Structures Related To Plutons Visible By Traces of a Circular Outline Because of the 

Drainage Pattern and Circular Tonal Anomalies on the Satellite Images 

Pluton emplacement mechanisms and surrounding conditions play an important role. The ring 

structures mostly related to alkaline magmatism (535-525 Ma) as described by Liégeois (2008) and the 

surrounding area are intruded later, mainly during the Cenozoic reactivation (35-0 Ma) by ring dikes, 

radial dikes and volcanic lava fields.  

The advantage of radar images becomes evident as the drainage network and structural features are 

clearly traced on the Sentinel 1- C-Band radar image, whenever sheets of aeolian sediments are 

covering rock units as shown in Figure 13 a and b. 

 

                                                                                                                                                                    

Figure 13 a and b. Comparison of a BingMap-scene with a Sentinel 1 Radar-scene Indicating a 

Buried Ring Structure in SW-Algeria 

 

The Advanced Land Observing Satellite (ALOS) L-band Synthetic Aperture Radar (PALSAR, 

Japanese Space Agency-JAXA) complex single look (SLC) images are characterized by the optimal 

conditions of penetration in these desert areas depending on the grain size, rare moisture, surface 

roughness, radar backscattering mechanism and radar illumination geometry (Sieber & Theilen-Willige 

1984; Theilen-Willige, 1986; Mansour et al., 2022). The ALOS/PALSAR L-band waves can propagate 



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up to several meters, and, thus, may detect faults and structural features buried underneath loose 

sedimentary covers. The next figure (Figure 14) demonstrates the better evaluation feasibilities of the 

L-Band radar data to detect a circular structure in comparison with optical satellite data due to the 

traces of a concentric drainage pattern. Thus, circular structures nearly invisible in the field and on 

optical satellite images because of sedimentary covers can be detected and mapped precisely on the 

radar image. The detection of the drainage system underneath younger sedimentary covers plays a role 

in case of rare flash floods in these arid areas because the surface water infiltrates through the loose 

sediment covers predominantly into the older drainage systems and, thus, contributes to groundwater 

storage. 

 

 

Figure 14 a and b. World Imagery-scene provided by ESRI (a) in Comparison with the ALOS 

PALSAR L-Band Single Horizontal Polarized (HH) Radar Scene (b) Indicating a Multi-ringed, 

Circular Structure Forming a Basin with a Diameter of about 8 km and a Concentric Drainage 

Pattern in SE-Algeria 

 

Another example is presented in Figure 15 a, b, c and d by comparing optical satellite data with the 

ALOS PALSAR L-Band (single horizontal polarized-HH), radar scene showing traces of a 

multi-ringed structure in South-Algeria with a diameter of about 12 km. Whereas the optical satellite 

data such as Landsat (Figure 15 c) reveal “stripes” caused by wind erosion in the main wind direction, 

the radar image allows a more detailed insight of the structural setting. The origin of this structure can 



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be explained by an uprising magmatic body updoming the strata above and leading to ring faults that 

are traced by the drainage pattern on the radar scene in dark concentric lines (Figure 15 b). However, 

this structure has a strong geomorphologic similarity with a complex cosmic impact crater as well by 

showing a central hill ring (central uplift?) surrounded by ring depressions and concentric hill ranges. 

There seems to be a ring graben outlining the structure. The ring structure was intersected later by dikes 

in the eastern part. This structure should be investigated in the field to get rock samples for mineralogic 

analysis to get more knowledge about its origin. 

 

 

Figure 15 a-d. Detection of a Circular, Complex Structure in S-Algeria Based on Different 

Satellite Data 

 

3.2 Larger Circular Basins and Depressions 

The thermally emitted radiation or brightness temperature observed over deserts in Algeria supports the 

detection of reflection anomalies using satellite-based data. When evaluating Landsat 8 and 9 RGB 

images involving the thermal bands large circular basins with surrounding concentric hill ranges, often 

consisting of ring dikes, become clearly visible (Figure 16). Large basins with more than 10 km in 

diameter are intersected by younger, parallel dike intrusions and dike swarms. 

3.3 Smaller Ring Structures in Volcanic Areas 

Different volcanic fields are situated in the Hoggar mountains in South Algeria in height levels up to 

3,000 meters with the lava domes and lava sheets, maars and scoria cones rising above the surrounding 

terrain. The topographic highlands are composed of Precambrian basement uplifted since the Pliocene 



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and of overlying lava plateaus, domes and spines, scoria cones, and valley-filling lava flows. The scoria 

cones have similar morphologies, characterized by the formation of an external erosion cliff, 10 to 30 

m high, carved in the ejecta. A pediment developed at the cliff base, about 100 m wide, surrounding the 

cones, often covered by aeolian sediments (Yahiaoui et al., 2014). Some are forming craters and maars. 

 

 

Figure 16. Landsat 8-RGB Scene of Ring-shaped Basins (Eroded Alkaline Domes) Surrounded 

by Ring Dikes in South Algeria 

 

The Atakor massif is located at the center of the Hoggar swell within the Tuareg shield. It is 

characterized by volcanic activity during three discrete Neogene igneous episodes, separated by fairly 

long periods of quiescence (Azzouni-Sekkal et al., 2003, 2007). The evaluation of optical and radar 

satellite data contributes to a more detailed knowledge of the deep-seated fault zones facilitating the 

uprise of magmatic intrusions. In the area of the Atakor and Tahalra volcanic field prominent W-E and 

N-S oriented lineaments are prevailing (Figure 17, Figure 18). The lavas lie on Precambrian granitic 

and gneissic rocks (Azzouni-Sekkal et al., 2007). Uprise of magma occurred predominantly where N-S 

striking, large shear zones are intersected by W-E oriented lineaments. Whereas the occurrence of 

volcanic activities in the Atakor and Tahalra fields seem to be influenced by larger fault zones, the next 

examples of the Egéré volcanic field, Idlès, Tamanrasset Province, show the influence of pre-existing 

ring structures such as domes with concentric faults on the emplacement of younger magmatic bodies 

as well (Figure 19 a and b). Developmental sequences of intrusions can be observed. The concentric 

arrangement of scoria cones follows both, the ring faults and larger intersecting fault zones. 

 

 



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Figure 17. Structural Analysis Based on Optical and Radar Satellite Images of The Atakor 

Volcanic Field (Landsat 9-RGB Image in the Background Indicating Lava Fields in Orange and 

Yellow Colors). The Line Direction (Rose) Diagram Is Calculated Based on Lineaments in the 

Red Indicated Area 

 

 

Figure 18. Structural Analysis of the Talhara Volcanic Field Based On Satellite Data 

 



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Figure 19 a and b. Sentinel 2-scene of the Western Part of the Egéré Volcanic Field (a) with a 

Concentric Arrangement of the Scoria Cones and Lava Fields and (b) Structural Evaluation of 

the Sentinel 2-scene  

 

3.4 Smaller Cosmic Impact Craters 

Most of the documented known impact craters in the investigation area are smaller, bowl-shaped, 

simple impact craters such as the Talemzane crater in N-Algeria (Figure 20). The Talemzane crater is 

embedded in in Senonian or Eocene limestones and features an up to 70 m high rim. Limestones are 

strongly fractured, upturned, and -in the upper rim-overturned. Large, ejected blocks of limestone are 

scattered around the outside of the crater. Breccia dikes are intersected by the crater wall, and detrital or 

reworked monomict breccia is found at the crater floor near the rim. Quartz is a rare constituent in the 

limestones, but Lambert et al. (1980) reported high pressure modifications like planar elements in some 

quartz grains. These authors estimated the age of the structure at <3 Ma because of the limited degree 

of erosion observed (Reimold & Köberl, 2014). 

 



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Figure 20. Sentinel 2-RGB-scene of the Talemzane Impact Crater 

 

The next figure (Figure 21 a) provides an overview of ring structures forming craters, most of them 

related to magmatic activity (maars, calderas). However, some of them might be related to cosmic 

impacts. Figure 21 b shows the diameter of crater-shaped ring structures, most of them with a diameter 

of about < 1 km, up to 1-2 km. When evaluating the different satellite data further craters were 

identified that are assumed to be created by a cosmic impact due to their similarities to the known 

impact craters and should be recommended for more detailed mineralogical and geophysical 

investigations to verify their origin. Examples of craters that might be formed by a cosmic impact and 

not documented so far are presented in the next figures (Figure 22 and Figure 23), a potential complex 

crater with a diameter of about 4 km in Figure 24.  

As mineral deposits are often linked to these types of ring structures (Reimold & Koeberl, 2005; 

Osinski et al., 2012), more detailed investigations would make sense. The evaluation of different 

satellite data helps to focus cost- and time-intensive field research. 



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Figure 21 a. Ring Structures with Crater Morphology (Red Points)  

 

 

Figure 21 b. Diameters of Crater-like Ring Structures 

 

5. Conclusions 

Evaluations of different satellite data, enhanced with digital image processing methods, allow a more 

detailed and systematic inventory of circular structures, their different types, sizes and their 

surrounding tectonic pattern, even the detection of so far unknown ring structures. When analyzing the 

different satellite data, it becomes obvious that majority of the larger ring structures can be identified 

and correlated with available geologic information, whereas the smaller circular features (with 

exception of those situated within volcanic areas) are sometimes difficult to determine regarding their 

origin. Nevertheless, the inventory of circular features based on satellite images will support further 

investigation in the field by providing a data base for focused field research. In the arid environment of 

the investigation area ALOS PALSAR L-Band data were especially useful for this purpose because of 

the penetration capability of long-wave radar signals into dry, loose sedimentary covers.  



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Figure 22 a and b. Landsat 9- and Sentinel 1-scene of a Potential Impact Crater in S-Algeria 

 

 

Figure 23. Sentinel 2-scene of a Potential Eroded Impact Crater Site in SE-Algeria, a Ring 

Structure with a Diameter of about 2 km, Embedded in Paleoproterozoic Calc-alkaline Plutonic 

and Anatectic Layers and Neoproterozoic to Ordovician Sedimentary Layers (according to 

CGMW-BRGM 1:10M Geological Map of Africa,-3rd Edition) 



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Figure 24. Satellite-scenes of a Potential Complex Impact Crater in S-Algeria with a Central 

Elevation (?) 

 

Acknowledgement 

The author is grateful for the input and support of the editor and team of Energy and Earth Science and 

the reviewers. 

 

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