3-D geological model of the Central High Atlas fold-and-thrust belt

Author(s):  
Pablo Santolaria ◽  
Antonio M. Casas ◽  
Pablo Calvín ◽  
Tania Mochales ◽  
Hmidou El-Ouardi ◽  
...  

<p>The Atlas system, an ENE-WSW intracontinental chain in the NW of Africa, grew because of the inversion of Mesozoic extensional basins during the Cenozoic convergence between the African and European plates. The Central High Atlas (CHA) is located in the mid-western sector of the chain and is characterized by (i) the presence of an Upper Triassic décollement, (ii) thick Lower-Middle Jurassic sedimentary sequences, and (iii) the occurrence of diapirs and igneous bodies which are especially common in the central part of the chain. The northern and southern borders are characterized by fold-and-thrust-systems involving the Paleozoic basement and the Mesozoic cover and showing significant displacements, especially towards the South.</p><p>Framed on a multidisciplinary structural project aiming to reconstruct the 4-D structure of the CHA, the purpose of this work is to gather a vast constraining dataset into a present-day, regional, 3-D geological model of the CHA fold-and-thrust belt. This 3-D reconstruction gives special weight to along- and across-strike variations of the geometry of the basement and cover structures and the distribution of salt and igneous bodies. The 3-D model is founded by 23 serial cross-sections, constrained by surface geology and more than 1900 structural data and complemented by geophysical modelling. The model considers regional structures having enough lateral continuity and so we ruled out minor, local features. Stratigraphically, we considered 5 horizons: (1) the top of the Triassic located below the detachment level, and partially equivalent to the top of the basement, (2) the base of the Jurassic succession (i.e. the top of the detachment level), (3) the Lower-Middle Jurassic limit and, towards the southern and northern fronts and foreland basins, (4) the bases of the Cretaceous and (5) the Cenozoic succession.</p><p>The reconstruction of the 3-D model entailed a strong feedback between the model and the cross-sections. The incipient 3-D model helped to refine the lateral consistency between cross-sections regarding branch and tip lines, cut-offs, fault angles, etc., and so to improve and further constrain them.</p><p>Thick to thin skinned deformation dominates the eastern, central and northwestern areas of the chain while thick skinned deformation occurs in the westernmost transects. The chain is defined as an asymmetric, doubly verging fold-and-thrust belt. A north-dipping, basement regional fault represents the main rooting structure of the CHA. Its geometry varies from and almost horizontal (West), to a 10°-12° (Centre) and 15° (East) thrust ramp surface. As this fault intersects the cover, it splits into a regional thrust front with several thrust branches. To the north, antithetic basement faults change to a thrust relay system as they intersect the Mesozoic sequence. Along the core of the chain, the structural style is characterized by open salt bodies, welded diapirs and steep thrusts having relatively limited lateral continuation. The Toundoute Unit, located in the central-western sector represents a basement-and-cover thrust stack where the basement is exhumed and crops out.</p><p>This 3-D structural model provides the bases for further 4-D reconstruction of the CHA and, at the same time, served as a constraining approach for cross-section construction.</p>

2020 ◽  
Author(s):  
Pablo Calvín ◽  
Juan J. Villalaín ◽  
Antonio M. Casas-Sainz ◽  
Teresa Román-Berdiel ◽  
Pablo Santolaria ◽  
...  

<p>The Jurassic carbonates of the Central High Atlas (CHA) are affected by a widespread and homogeneous chemical remagnetization. This is an interfolding remagnetization (dated in ca. 100 Ma by comparison with the GAPWP of the African plate) that separates two deformational events; the first one is related to the basinal period in the Atlas (Triassic and Jurassic times) that is responsible of the thick Jurassic sequence that crops out in the CHA. The second one is associated with the tectonic inversion during the Cenozoic caused by the African and European plates convergence, which resulted in the uplift of the cordillera.</p><p>Using the Small Circle tools, it is possible (i) to obtain the remagnetization direction and then (ii) use it as reference to obtain the paleodips of each site (i.e. the paleodips at the remagnetization time). This methodology applied to interfolding remagnetizations allows restoring the present-day geometry to the one at ca. 100 Ma and therefore separating the structure associated to the extensional and compressional periods.</p><p>This work is framed on an ambitious research project (CGL2016-77560-C2-P, Spanish Ministry of Science and Innovation) in which 700 AMS/paleomagnetic sites and additional 1000 bedding data are integrated to unravel the Mesozoic and subsequent Cenozoic evolution of the CHA. Based on the aforementioned datasets, field work, geophysical (gravimetric and magnetic surveys) constraints and the construction and restoration (at the remagnetization time) of cross-sections, the ultimate goal of the project is to reconstructed and restored the 3-D geometry of the CHA fold-and-thrust belt.  </p><p>Here in particular, we present the paleomagnetic data supporting the calculated paleodips. We also analyze how the deformation associated with each of the two deformational events is distributed along the study area. The comparison of dip-domains maps showing the present day attitude and also the pre-inversional one allows analyzing how extensional deformation is more or less associated with particular structures and to understand the importance of the different processes that acted during this period (i.e. deformation associated with extensional faults, salt walls, igneous intrusions, etc.).</p>


2020 ◽  
Author(s):  
Stéphane Bodin ◽  
Jan Danisch ◽  
Malte Mau ◽  
Francois-Nicolas Krencker ◽  
Alexis Nutz ◽  
...  

<p><span>Mesozoic sea-level fluctuations have been a matter of debate for several decades, especially the extend and origin of sea-level cycles that have a periodicity of about 1 Myr or less. The debate lies in the main driving mechanism for sequence development (global sea-level or sediment flux variations) as well as the reason behind water exchanges between the continents and the oceans (glacio- or aquifer-eustatism). In this study, we focus on the carbonate-dominated sedimentary record of the Bajocian (Middle Jurassic) in the Central High Atlas Basin of Morocco. Several aspects make this basin an appropriate location for discussing Middle Jurassic sea-level changes. Firstly, the outstanding exposures of the High Atlas Mountains, with continuous exposures for 10s of kilometres, allow to describe and track sedimentary packages and their bounding surfaces from proximal to distal settings. Moreover, a combination of ammonite and brachiopod biostratigraphy with carbon-isotopes chemostratigraphy allows to temporarily constrain their development, which permits to correlate and compare the Central High Atlas sedimentary record to other basins. Finally, due to high-subsidence rates, thick Bajocian sedimentary sequences have accumulated, minimizing condensation and hiatus that might prevail in other basins due to a lack of accommodation space creation. Two Bajocian long-term transgressive-regressive (T-R) packages are observed throughout the basin. They are modulated by several medium-term T-R packages, that have each an approximate duration of 1 Myr. These sequences can also be correlated on a basinwide scale. Combined with sedimentological and facies analyses, architectural evidence along proximal-to-distal transect illustrates that several of the medium-term sequences are characterized by the presence of a falling stage and lowstand systems tracts, demonstrating that medium-term T-R stacking patterns are not solely linked to fluctuation in sediment supply, but also to episodes of relative sea-level fall of at least 30m of amplitude. This is confirmed by backstripping analysis performed in a composite section from the center of the Basin. Comparison with Bajocian deposits from France and Scotland, where good biostratigraphic dating is also available, shows that similar contemporaneous sea-level fall can be observed, highlighting their potential global character. The two long-term Bajocian sequences are more difficult to correlate on a global scale, suggesting that they are rather primarily linked to fluctuation in regional sediment supply or dynamic topography processes. The exact cause of the Bajocian medium-term sea-level falls is currently unknown, but it is here interesting to note that a relatively cool globate climate has been postulated for the Middle Jurassic, suggesting that glacio-eustasy was their most likely driver.</span></p>


PalZ ◽  
2017 ◽  
Vol 92 (2) ◽  
pp. 219-240 ◽  
Author(s):  
Ján Schlögl ◽  
Tomáš Kočí ◽  
Manfred Jäger ◽  
Tomasz Segit ◽  
Jan Sklenář ◽  
...  

PalZ ◽  
1989 ◽  
Vol 63 (1-2) ◽  
pp. 5-14
Author(s):  
Rainald Brede ◽  
Manfred Hauptmann ◽  
Hans-Georg Herbig

2021 ◽  
Author(s):  
Hamza Skikra ◽  
Khalid Amrouch ◽  
Youssef Ahechach ◽  
Muhammad Ouabid ◽  
Abderrahmane Soulaimani ◽  
...  

<p>The Moroccan High Atlas mountain range is an aborted Mesozoic rift basin that was moderately shortened during the Late Cretaceous‒Cenozoic inversion. The range is currently featured in its central part by the presence of conspicuous S-shaped open gentle synclines where Middle Jurassic strata crop out, with sub-horizontal bottom, separated by 15-to-80-km narrow faulted anticline ridges with two distinct directions: ENE and NE. The tight anticline ridges are cored by Triassic continental red-beds intruded by the CAMP basalts and subsequently by Upper Jurrasic‒Lower Cretaceous alkaline magmatism. Regional cleavage with very low-grade anchi- to epi-zonal metamorphism are depicted along several structures of the High Atlas, particularly the NE-trending anticlines. The sedimentary layers thickness, on the other hand, gets thinner towards the faulted anticlines with the development of intraformational truncations. The structural history of the High Atlas syncline-topped anticlinal ridges remains a controversial matter. Any attempt to reconstruct the evolutionary process of such folded structures must take into consideration the following circumstances:</p><ul><li>After a Triassic rifting episode followed by the establishment of Liassic carbonate platform, the High Atlas basin underwent a wide spread exhumation event at the time interval between the Middle Jurassic and Lower Cretaceous leading to the deposition of continental detrital series and sedimentary hiatus;</li> <li>The upward motion was accompanied with the emplacement of alkaline magmas in the Central High Atlas;</li> <li>A complex halokinetic history characterizes the Central High Atlas salt province during both pre-orogenic and orogenic stages;</li> <li>During the Late Cretaceous‒Cenozoic, the High Atlas experienced a moderate crustal shortening which was focused essentially within the range’s borders;</li> </ul><p>In order to bring new insights to the structural history of the High Atlas folded structures, a structural investigation was carried out in Tirrhist and Anemzi ridges. In each station, fractures measurements were taken, and oriented samples were collected for micro-structural analysis. First paleo-stress inversion in some stations reveals the presence of pre-folding bedding-parallel maximal horizontal stress oriented NE to NNE. For a deep analysis of pre syn and post-folding stresses history, we use a calcite stress inversion technique, namely Etchecopar’s method, to unravel the paleo-stresses orientations and to quantify the differential stresses during the different episodes of deformation. The present work is a preliminary attempt to quantify tectonic stresses in the hinterland of an arguably weakly deformed orogenic belt.</p>


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