Geologica Acta: an international earth science journal ISSN: 1695-6133 [email protected] Universitat de Barcelona España Guimerà, J.; Arboleya, M.L.; Teixell, A. Structural control on present-day topography of a basement massif: the Central and Eastern Anti-Atlas (Morocco) Geologica Acta: an international earth science journal, vol. 9, núm. 1, marzo, 2011, pp. 55-65 Universitat de Barcelona Barcelona, España Available in: http://www.redalyc.org/articulo.oa?id=50522124006 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Geologica Acta, Vol.9, Nº 1, March 2011, 55-65 DOI: 10.1344/105.000001643 Available online at www.geologica-acta.com Structural control on present-day topography of a basement massif: the Central and Eastern Anti-Atlas (Morocco) 1 2 2 J. GUIMERÀ * M.L. ARBOLEYA A. TEIXELL 1 Departament de Geodinàmica i Geofísica, Facultat de Geologia, Universitat de Barcelona (UB) Martí i Franqués s.n. 08028 Barcelona, Spain. E-mail: [email protected] 2 Departament de Geologia, Facultat de Ciències, Universitat Autònoma de Barcelona 08193 Bellaterra, Spain * Corresponding author ABSTRACT The Anti-Atlas basement massif extends South of the High Atlas, and, despite a very mild Cenozoic deformation, its altitude exceeds 1500m in large areas, reaching 3305m in Jbel Sirwa. Structural contours of the present elevation of a polygenic planation surface (the High Erosional surface) and of the base of Cretaceous and Neogene inliers have been performed to characterize the major tectonic structures. Gentle Cenozoic WSW-ENE- and N-S- trending folds, of 60 to100km wavelength, reactivate Variscan structures, being the major contributors to the local topography of the Anti-Atlas. Reactivated thrusts of decakilometric to kilometric-scale and E-W trend involving the Neogene rocks exhibit a steep attitude and a small displacement, but they also produce a marked topographic expression. The resulting Cenozoic horizontal shortening along N-S sections across the Anti-Atlas is about 1%. The position of the major anticlinal hinges determines the location of the fluvial divides of the Warzazat basin and the Anti-Atlas, and a structural depression on one of these hinges (Jbel Saghro anticline) allowed the formerly endorheic Warzazat basin to drain southwards. The first Cenozoic structures generating local topography are of pre-mid Miocene age (postdated by 6.7Ma volcanic rocks at the Jbel Saghro), whereas the youngest thrust movements postdate the Pliocene sedimentary and volcanic rocks (involving 2.1Ma volcanic rocks at Jbel Sirwa). In addition to these features, the mean elevation of the Anti-Atlas at the regional scale is also the result of a mantle thermal anomaly reported in previous works for the entire Atlas system. KEYWORDS Cenozoic contraction. Crustal folding. Uplift. Tectonic topography. Anti-Atlas. Morocco. INTRODUCTION 3305m in the Jbel Sirwa, 2701m in the Amalou-n-Mançour (Jbel Saghro), and 2526m in the Imgand (Central Anti- The Anti-Atlas of Morocco is a mountainous region Atlas). Being dominated by Proterozoic and Palaeozoic located south of the Warzazat and Sous basins, which are the rocks, the Anti-Atlas contains occurrences of marine southern foreland basins of the High Atlas (Fig. 1). The present Cretaceous sediments indicating that such elevations are topographic elevation of the Anti-Atlas is over 1500m in the signal of a Cenozoic uplift. However, much of the extensive areas, exceeding 2000m in several places (Fig. 2): internal structure of the Anti-Atlas was formed during the 55 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas 6º W W N MEDITERRANEAN SEA A O C E Rif N C I T Middle Atlas N A L T A High Atlas i-Atlas ATLAS Ant Errachidia Hama 8º W Tinejdad da HIGH Tinghir du Guir Erfoud sin ch arzazat Ba rake W Mar Warzazat S L A T A Zagora Taliwine Taznakht Agadir in I - T em Sous Bas N Thrust Fig. 4 A Kem-K South Atlas Thrust Fig. 8 Cenozoic Cretaceous Tata 0 50 100 km Triassic and Jurassic Fig. 2 Pre-Mesozoic rocks FIGURE 1 Geological map of the Anti-Atlas and the Central and Western High Atlas, simplified after Hollard (1985). Location of Figs. 2, 5 and 8 is shown. Fig 1 Panafrican and Variscan orogenies (late Proterozoic and late this study illustrates how a geomorphological analysis can Palaeozoic, respectively) while the Cenozoic deformation provide grounds to the tectonic interpretation in regions is considered to be very mild (Choubert & Marçais, 1952; where the sedimentary record is scarce. Hoepffner et al., 2005). The high topography of the High Atlas thrust belt is GEOLOGICAL SETTING the combination of crustal shortening and long-wavelength thinning of the lithosphere, affecting the mountain belt Geologically, the Anti-Atlas is composed of a and the peripheral plains on a much wider scale (Teixell Proterozoic Pan-African basement overlain by post-Pan- et al., 2003, 2005; Missenard et al., 2006). The differential African Uppermost Proterozoic and Palaeozoic rocks, topography between the High Atlas and the adjacent deformed during the Variscan orogeny (Choubert, 1952; elevated plains results from tectonic shortening, manifested Choubert & Marçais, 1952). The Variscan deformation by well developed imbricate thrust systems. This is not so caused in the inversion of late Proterozoic and Palaeozoic straightforward in the Anti-Atlas, where these systems are intracratonic basins, remelting in a thick-skinned fold lacking. Choubert (1952) described the overall structure of and-thrust-belt (Burkhard et al., 2006; Toto et al., 2008; the Anti-Atlas as a vast basement arch (pli de fond), which Guimerà & Arboleya, 2008). resulted from the remobilization of a previous fold of Variscan age, a view followed by Teixell et al. (2003), who Directly overlying Precambrian and Palaeozoic rocks, a interpreted it as a lithospheric-scale fold. A closer inspection Cretaceous succession starts with pre-Cenomanian fluvial evidences that topography is not homogeneous within the red beds, followed by Cenomanian-Turonian marine Anti-Atlas (Fig. 2) and, as pointed out by Choubert (1952), limestones and red-beds of Senonian age (Choubert, 1952). major faults do involve Mesozoic and Cenozoic rocks. The pre-Cenomanian red beds, markedly unconformable The aim of this paper is to analyze the tectonic origin of on the pre-Mesozoic basement, cover a peneplain (Robert- topography on the basis of 1) the attitude of several surface Charrue, 2006) and are preserved in isolated outcrops, markers that can be reconstructed from preserved erosional essentially on the northern and eastern side of the Anti- features and sedimentary inliers within the Anti-Atlas, and Atlas (Choubert, 1950; Choubert, 1955-56, Ferrandini et 2) the field analysis of the contractional structures that al., 1985) (Fig. 1). These pre-Cenomanian clastic sediments involve Neogene rocks and morphological surfaces. In any derive from the Sahara region, and were deposited by case, Post-paleozoic rocks are rare in the Anti-Atlas, and alluvial systems, whose regional slope was north-directed Geologica Acta, 9(1), 55-65 (2011) 56 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas (Guillocheau et al., 2007). The Cenozoic is largely confined dips steeply to the North and had a reverse slip during the to the Warzazat and Sous basins (Fig. 1), where it begins Neogene (Missenard, 2006; Sébrier et al., 2006; Guimerà with Palaeocene to Middle Eocene marine limestones, et al., 2006 and 2008) (Fig. 3 B; Fig. 5, section 2). followed by continental Eocene red beds (Choubert, 1950; Choubert, 1955-56; Gauthier, 1957). After a marked hiatus, the Aït Kandoula Formation (El Harfi et al., 2001) METHOD OF STRUCTURAL ANALYSIS is composed by alluvial and lacustrine terrigenous and carbonate sediments of Middle to Upper Miocene age In view of the scarcity of post-Palaeozoic deformation (Tesón, 2009). The Aït Kandoula Fm. constitutes the bulk markers, the method of analysis of the structures of the infill of the Warzazat basin (ca. 1000m), and crops responsible for the uplift of the Anti-Atlas consists of out in isolated exposures in the Sirwa massif, the western structural contour mapping of reference surfaces. More part of the Sous basin, and within the Anti-Atlas (Choubert, than 900 points of elevation measurements were acquired 1945; Gauthier, 1957; Görler et al., 1988), where it from SRTM90 DEM (about 90m of pixel size, available constitutes a valuable marker for Neogene deformation. It at http://srtm.csi.cgiar.org) and our own field-based has not been documented whether the Anti-Atlas was ever geological mapping on satellite orthoimages. After this, we completely covered by Mesozoic or Palaeogene sediments. have drawn georeferenced contour maps of the present-day Previous authors have assumed that most of it was exposed elevation of the sub-Cretaceous unconformity, the HES during these epochs (Choubert, 1952; Riser, 1988; Robert- and the base of the Neogene, using Surfer software with Charrue, 2006; Malusà et al., 2007). the Kriging gridding method. The results of contouring are presented in Figs. 4, 6 and 8. A characteristic feature of the Anti-Atlas is a widespread planation surface called the High Erosional Surface (HES) For the sub-Cretaceous unconformity and the base of or Pre-hamadian Surface by Choubert, (1952). This the Neogene, elevation measurements were acquired from author recognized this surface as polygenic in origin and geological maps. For the HES, elevation measurements attributed its development to an indeterminate period from were taken, after the recognition of this surface in the field the beginning of the Tertiary to the Aquitanian. Moreover, (Fig. 3 A, B, C and F), from 3D visualizations of satellite we have observed that North of the Sirwa region the orthoimages draped on the SRTM90 DEM.