Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 DOI 10.1007/s00531-012-0773-4

ORIGINAL PAPER

Post-rift vertical movements and horizontal deformations in the eastern margin of the Central Atlantic: Middle to Early evolution of

Giovanni Bertotti • Mohamed Gouiza

Received: 25 June 2011 / Accepted: 31 March 2012 / Published online: 27 April 2012 Ó The Author(s) 2012. This article is published with open access at Springerlink.com

Abstract Wide regions of Morocco, from the Meseta to the angle to the present margin. As a working hypothesis, it is High Atlas, have experienced km-scale upward vertical suggested that regional shortening can explain the structural movements during to Early Cretaceous observations and the enigmatic vertical movements. times following the appearance of oceanic crust in the Central Atlantic. The area experiencing exhumation was Keywords Post-rift uplift Á Post-rift deformation Á flanked to the W by a domain of continuous subsidence, part Passive margin Á Morocco Á Essaouira basin of which is named the Essaouira-Agadir basin. Comparison with vertical movement curves predicted by lithospheric thinning models shows that only 50–60 % of the subsidence Introduction documented in the Essaouira basin can be explained by post- rift thermal relaxation and that \30–40 % of the observed The recent discovery of Late to Early Cretaceous exhumation can be explained by processes (in)directly km-scale vertical movements, subsidence and then exhu- related to the evolution of the Central Atlantic rifted margin. mation, affecting substantial parts of Morocco which had Syn-sedimentary structures in Middle Jurassic to Lower hitherto been considered as stable (Ghorbal et al. 2008; Cretaceous formations of the Eassouira-Agadir basin Saddiqi et al. 2009; Barbero et al. 2011; Ruiz et al. 2011) are common and range from m-scale folds and thrusts to represents a major development in Moroccan geology, km-scale sedimentary wedges. These structures systemati- opening new horizons for understanding the cally document coeval shortening generally oriented at high tectonic evolution of NW Africa. The exhumation com- ponent of these movements, in particular, is remarkable as it took place after the appearance of oceanic crust in the & G. Bertotti ( ) Central Atlantic, that is, during its passive margin devel- Department of Geoscience and Engineering, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands opment (e.g., Sahabi et al. 2004; Gouiza et al. 2010). e-mail: [email protected] Exhumation is clearly older than, and therefore, unrelated to Alpine contraction in the two orogens of NW Africa, G. Bertotti Á M. Gouiza namely the Rif and the Atlas. Upward movements are Department of Tectonics and Structural Geology, VU University Amsterdam, De Boelelaan 1085, unusual during the drifting stage, but our discoveries are 1081 HV Amsterdam, The Netherlands part of an increasing awareness that passive margins could e-mail: [email protected] be less passive than believed hitherto (e.g., Bache et al. 2011; Japsen et al. 2009). M. Gouiza Netherlands Research Centre for Integrated Solid Earth Science In the recent past, a number of studies by different (ISES), Amsterdam, The Netherlands groups (e.g., Ghorbal et al. 2008; Saddiqi et al. 2009; Ghorbal 2009; Barbero et al. 2011; Ruiz et al. 2011) have Present Address: concluded that rocks of the Meseta of N Morocco, the High M. Gouiza Department of Earth Sciences, Memorial University Atlas and parts of the Anti-Atlas, have experienced[1km Newfoundland, St. John’s, Canada of exhumation during Middle Jurassic to Early Cretaceous 123 2152 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 times. More low-temperature geochronology (low-T) work is Mountains in the N and the Atlas Mountains in the S and E, ongoing on samples from the Anti-Atlas and areas of southern both developed in association with the convergence between Morocco (Bertotti et al. 2010) and will undoubtedly result in Europe and Africa. The area between the two orogens is the new relevant information. The focus of the present study is on Moroccan Meseta. The Rif mountains, a mainly thin-skin- the tectonic regime driving the observed vertical movements. ned fold-and-thrust belt incorporating parts of the European To reach this goal, we compile data on vertical movements and African passive margins and of the intervening oceanic along a roughly W–E trending, *100-km-long section con- crust (e.g., Chalouan et al. 2001), is not of significant interest necting areas of strong Jurassic to Early Cretaceous subsi- to our study and will not be further considered. dence, the Essaouira-Agadir basin, with domains of strong Mesozoic sediments, recording phenomena taking place coeval exhumation, the Massif Ancien de Marrakech (MAM) during the time span of interest to this contribution, are (Fig. 1). We compare these estimates with values predicted by mainly found along the Central Atlantic and the High Atlas numerical models of rifted margin evolution and conclude that inverted rift systems, which developed in association with the a significant portion of such movements cannot be explained opening of the Central Atlantic and Tethyan Oceans. The by current models. To constrain the nature of the tectonics Mesozoic succession in the Meseta and other non-orogenic driving these enigmatic vertical movements, we identify and areas is much more discontinuous and typically characterized interpret syn-tectonic deformation structures in the Mesozoic by a major unconformity separating Upper Cretaceous mar- sediments of the Essaouira-Agadir basin cropping out to the W ine from metamorphosed Hercynian basement. of the area experiencing Jurassic to Early Cretaceous exhu- mation. Finally, we integrate horizontal deformations and The Meseta vertical movements to propose a tentative kinematic recon- struction for the Jurassic to Cretaceous evolution of the section The Meseta is a fairly flat-lying domain characterized by a from the Moroccan onshore in the Essaouira-Agadir basin Hercynian basement (e.g., Hoepffner et al. 2005; Michard region to the central part of the High Atlas. et al. 2008) overlain by scattered Triassic continental sediments and extensive Upper Cretaceous shallow marine carbonates. The upper boundary of the basement The geological framework at the contact with non-metamorphic sediments shows little palaeo-topography suggesting that the Hercynian orogen The main domains had been effectively peneplained prior to Mesozoic sedi- mentation (e.g., Michard 1976; Hoepffner et al. 2005). The present-day geology of northern Morocco (Fig. 1)is Triassic syn-rift structures, generally oriented N–S, host characterized by two orogenic belts of Tertiary age, the Rif limited thicknesses of mainly terrigenous sediments. The stratigraphic gap characterizing the central Meseta decreases toward the W, and in the offshore, the sedimentary succession is essentially continuous having experienced persisting subsidence throughout the syn- and post-rift stages of Central Atlantic opening (e.g., Hafid et al. 2008; Gouiza et al. 2010).

The Atlas system

The Atlas orogen begins in the W, offshore Essaouira- Agadir (e.g., Hafid et al. 2006) and continues onshore in ENE direction traversing Morocco, Algeria, and reaching eventually Tunisia (Frizon de Lamotte et al. 2000). The Atlas fold-and-thrust belt originates from the Eocene to inversion of a Triassic–Jurassic graben system developed during the opening of the Central Atlantic and, somewhat later, of the Tethyan ocean in the NE. Northeast of Marrakech, the Middle Atlas splits away from the High Atlas and joins the Rif in NE Morocco (Fig. 1). Shortening Fig. 1 Schematic geological map of Morocco (simplified from Hafid has been estimated at \25 % accommodated by predomi- et al. 2006). OB Ouarzazate basin; SB Souss basin; MAM massif ancien de marrakech. The inset shows the detailed trace of Fig. 3 and nantly thick-skinned thrusts (e.g., Teixell et al. 2003; the position of the investigated samples Arboleya et al. 2004). 123 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 2153

The High Atlas is traditionally subdivided into a western Tertiary contraction. Most normal faults were steep and a central part separated by a large area of crystalline accommodating a total extension of \10 % (e.g., Gouiza basement cropping out south of Marrakech known as the et al. 2010). The Central Atlantic and Atlas systems come MAM (Michard 1976) (itself part of the West Moroccan together in the area of the Essaouira-Agadir basin where Arc) (Fig. 1). The MAM is generally interpreted as a faults of the two trends are encountered and where strong morphologic high resulting from the Hercynian orogeny subsidence took place allowing for the deposition of a that separates the two segments of the High Atlas from the several kilometers thick Mesozoic sedimentary succession Triassic to Present (e.g., Michard 1976). This interpretation (e.g., Medina 1995; Hafid et al. 2008). was contested by German authors (e.g., Stets 1992) and has There is a general agreement that normal faulting ended been recently disproven by low-T geochronology (see before the late Liassic, at *196 Ma (e.g., Hafid et al. below). 2008). This is well documented in seismic lines in the Essaouira basin and in the Central Atlantic margin. The areas to the S of the High Atlas Depending on the interpretations, cessation of normal faulting coincides with or it is significantly older than the Geographically, the High Atlas passes toward the S to the first appearance of oceanic crust in the Central Atlantic Anti-Atlas with the partial interposition of two shallow (Sahabi et al. 2004; Davison 2005, respectively). In our foredeep basins, the Sous basin in the west and the study, we will follow the traditional approach assuming an Ouarzazate basin in the east (Fig. 1). The two basins are age of 175 Ma for break-up. separated by the Siroua plateau, which lies on the southern continuation of the MAM and forms a morphologic link between the High Atlas and the Anti-Atlas. Vertical movements and passive margin evolution The Anti-Atlas is a high mountain belt with summits reaching elevations of [3,300 m. The rough and high In this section, we compile data on Mesozoic vertical topography are at odds with the generally assumed very old movements along a *100-km-long transect along the age of its dominant deformation and its subsequent stable western part of the High Atlas from the Essaouira-Agadir character (e.g., Stets and Wurster 1982). Brittle deforma- basin in the W to the MAM in the East. We focus espe- tion is ubiquitous but commonly attributed to (late) cially on the areas experiencing upward movements as the Hercynian episodes (e.g., Burkhard et al. 2006; Cerrina- sedimentary record documenting subsidence have been Feroni et al. 2010). An Alpine age for at least part of the extensively described in the literature (e.g., Hafid et al. deformations has been proposed by Malusa` et al. (2007) 2008; Hafid et al. 2006;Zu¨hlke et al. 2004; Le Roy and and Guimera et al. (2011). Unfortunately, dating is difficult Pique´ 2001; Bouatmani et al. 2007; Ellouz et al. 2003). because of the scarcity of Mesozoic to Tertiary sediments. These movements (Fig. 2) are then compared with those predicted by rifting models. Kinematics of the NW Morocco rifted margin Observed movements The opening of the Central Atlantic and related basins was the most important process affecting Morocco during Tri- Upward movements in the MAM assic to Early Cretaceous times. Despite the substantial amount of studies dedicated to the topic, the ages of the A significant amount of low-T geochronological studies onset and end of rifting are debated (e.g., Sahabi et al. has been performed in the High Atlas during the last years 2004; Gouiza et al. 2010; Labails et al. 2010) and there (Missenard et al. 2008; Balestrieri et al. 2009; Ghorbal exist only very few estimates on the amount of extension 2009). Because of the lithological limitations imposed by experienced by the margin through time. the Apatite Fission Track and (U–Th)/He methods, prac- In accordance with most studies, we assume that rift- tically all samples measured by the different authors orig- related normal faulting began in times at inate from the MAM and, more specifically, from the *225 Ma (e.g., Laville et al. 2004) and created two main Paleozoic basement and its stratigraphic cover of Triassic extensional systems: a NNE–SSW trending one which led continental deposits. The most complete data set has been eventually to opening of the Central Atlantic and an ENE– presented in a PhD Thesis carried out at the VU University WSW striking system, the Atlas rift, which was abandoned Amsterdam to which we refer for specific information during the Early Jurassic. While normal faults in the (Ghorbal 2009). Samples have been taken from the north- Central Atlantic system are generally well imaged in ern foothills, the central domain around Jebel Toubkal and, seismic lines, this is less the case for extensional faults in further to the S, from the Siroua plateau (see inset in the Atlas because of their subsequent reactivation during Fig. 1). 123 2154 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165

Fig. 2 The sediments of the Essaouira-Agadir basin. a Schematic stratigraphic column (simplified from Hafid et al. 2008); b subsidence curves from 3 selected localities in the onshore portion of the Essaouira basin (from Bouatmani et al. 2007); position is indicated in Fig. 5

N Atlas foothills (NS0401) J. Toubkal (TO0405) S Atlas foothills (SI0401) 0 C 0 C 0 C 40 40 40 80 80 80 temperature 120 120 120 rift post-rift rift post-rift 200 100 0Ma 30 20 10 0Ma 200 100 0Ma 0Ma NW SE

40

80

120 4000

160

200 jurassic Cretaceous Tertiary 0m N sub-Atlas zone Toubkal Massif Siroua Plateau 20km

Fig. 3 Low-T geochronology data from a transect across the High He ages (squares). Upper panel time–Temperature curves for samples Atlas (see inset in Fig. 1 for location of transect). Lower panel from the N Foothills, the axial domain and the Siroua Plateau. topographic profile with apatite fission track (triangles) and (U–Th)/ Modified and simplified from Ghorbal (2009)

Apatite Fission Track and (U–Th)/He ages from all have been obtained. This implies that, following deposition, samples are Jurassic to Tertiary (Ghorbal 2009) (Fig. 3). these and the stratigraphically underlying Similarly to the Meseta, no Hercynian ages were found. basement have been buried to depths where temperatures Jurassic to Cretaceous fission track ages have been obtained were high enough to cause full annealing of the apatite from the northern and southern foothills of the High Atlas fission tracks and then moved upward experiencing cooling. (Fig. 3). Tertiary ages are only found in the axial part of the Samples with Tertiary ages were exhumed only during the mountain belt. The obtained values are systematically Atlas orogeny and bear no memory of their Mesozoic ver- younger than the deposition of the sediments and the tical movements (see Toubkal time–temperature path in the metamorphism affecting the rocks from which the apatites upper panel of Fig. 3).

123 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 2155

Time–temperature paths from representative samples margins evolution which predict gentle post-rift subsidence from the northern foothills and the Siroua plateau are shown driven by lithospheric cooling (McKenzie 1978). In more in the upper panel of Fig. 3. The geological conditions that recent years, dynamic models of post-rift margin develop- have been imposed on the modeling study are that the ment have highlighted the importance of processes such samples were close to the surface at the end of the Hercy- shoulder uplift, erosion-driven crustal flow (Burov and nian orogeny and immediately prior to the Late Cretaceous Cloetingh 1997) and mantle-seated processes of convection when they were covered by Upper Cretaceous marine car- and magmatism (e.g., Steckler 1985; Lucazeau et al. 2008). bonates. The t-T paths show that the samples started sub- These models typically predict localized uplift but provide siding in times probably in association with little information about the movement of physical particles continental rifting and reached a depth of [3 km where (geological bodies) and the associated exhumation and temperatures were high enough to cause annealing of the erosion, thereby making difficult a comparison with real- apatite fission track system. At 170–150 Ma, the same rocks world cases. In an extensive study on the evolution of the started moving upward and reached the surface prior to Central Atlantic margins of Morocco and Canada, Gouiza 100 Ma. For these samples, as well as all other samples et al. (2009) and Gouiza (2011) considered the effect of described by Ghorbal (2009), exhumation took place during secondary convection cells but were unable to predict an the drifting stage of Central Atlantic evolution. uplift/exhumation comparable to what observed in the transect of interest. Even in the most optimistic case, the Downward movements in the Essaouira-Agadir basin authors could attribute to secondary convection only an exhumation of few hundred meters that is an order of In a very schematic fashion, the Mesozoic sedimentary magnitude less than what observed. In addition, the wave- succession of the Essaouira-Agadir basin is characterized length of the predicted exhumation was significantly larger by an up to 8-km-thick sedimentary succession separated in than the observed one. We conclude that a significant por- a syn-rift and a post-rift part by an angular unconformity of tion ([60 %) of the observed upward vertical movements Toarcian age, interpreted as the break-up unconformity cannot be explained by rift- or post-rift-related processes. associated with the opening of the Central Atlantic (Med- ina 1995) (Fig. 2a). The post-rift Jurassic to Cretaceous The downward moving domain succession is up to 4 km thick and mainly composed of shallow water carbonates with episodes of continental To quantify and interpret the origin of the subsidence in sedimentation during the Middle Jurassic to Early Creta- Essaouira basin, we compare the backstripped subsidence ceous (e.g., Hafid et al. 2008 and references therein). curves shown in Fig. 2 with modeled curves predicted for Three of the numerous subsidence curves that have been different crustal thinning factors. The predicted curves proposed for different localities in the onshore part of the have been obtained using 2D numerical modeling of a Essaouira-Agadir basin are shown in Fig. 2b. They describe lithospheric extension (Kooi 1991; Kooi et al. 1992; vertical movements of Triassic rocks in the proximal, central Gouiza et al. 2010), which initiated in the Late Triassic and distal part of the Essaouira basin. These curves show a (*228 Ma) and ended in the Early Jurassic (*196 Ma) decrease in subsidence rates around ca. 200 Ma coinciding when normal faulting ceased in most rift basins. The with the local termination of normal faulting. Subsidence models assume uniform crustal and subcrustal thinning however continues and even accelerates after 150 Ma. during rifting and a flexural lithosphere characterized by (1) an effective elastic thickness defined by the 450 °C Comparison with model predictions isotherm, and (2) a 15-km-deep level of necking (i.e., similar to the lithospheric strength used by Gouiza et al. In previous sections, we have shown that the early passive (2010) in the Doukkala margin to the N). margin evolution of the Moroccan margin of the Central Inspection of Fig. 4 shows that initially, between 228 Atlantic was characterized by the coexistence of subsi- and 170–160 Ma, the subsidence in Essaouira basin is in dence in the W (Essaouira-Agadir basin) and exhumation agreement with the models as the observed curves follow in the E (MAM). The question we address here is whether relatively well the ones predicted for thinning factors the sign and magnitude of these vertical movements can be between 1.2 and 1.5. Although no quantitative analysis explained by passive margin development. exists, these values seem to require an extension signifi- cantly larger than what documented in the Essaouira basin. Upward moving domain In the Doukkala basin, to the North of the Essaouira basin, Gouiza et al. (2010) did perform such an analysis and The km-scale exhumation documented for the MAM is at concluded that the observed extension translated in thin- odds with classical, McKenzie-type models of rifted ning factors \1.2. 123 2156 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165

200 150 100Ma Essaouira PF30 DKS1 0m Fig. 11 N 1.1 PF18 TA 2000 1.2 Q 1.3 Imi-n-Tanoute DKS1 4000 1.5 PF18 InTw 1.7 Fig. 5 6000 syn-rift post-rift PF30 MAM

Fig. 4 Comparison between observed and predicted subsidence curves Central Atlantic in the Essaouira basin. Observed subsidence curves are from Fig. 2. Assumptions made for predicted subsidence curves are described in text. The vertical grey dashed lines marks the end of normal faulting; the thick Q Taroudant grey bar the appearance of oceanic crust in the Central Atlantic Souss valley 25km Agadir Q In accordance with classical models, subsidence rates are high during normal faulting and decrease after the cessation Tertiary Lower Liassic to Upper Jurassic of extension (dashed grey line in Fig. 4). No significant Upper Cretaceous Permian to Lower Liassic change is observed at 175 Ma when the first oceanic crust Lower Cretaceous metamorphic basement appeared in the Central Atlantic. At 160–150 Ma, however, the subsidence curves display substantial deviation from Fig. 5 Schematic geological map of the area between the MAM and predicted ones indicating renewed subsidence. During a the onshore part of the Essaouira basin (simplified from the sheet Marrakech of the 1:500,000 geological map of Morocco). TA Tidsi time span of 40–50 Myr, rocks moved downward by an , InTw Imi-n-Tanout wedge. The position of (synthetic) amount comparable to that experienced during rifting. The DKS1, PF18 and PF30 wells used for subsidence analysis (Fig. 4)is timing of renewed subsidence coincides with the onset of also shown. Names of wells are from Bouatmani et al. (2007) upward movements in the MAM and surrounding regions. The interpretation that the subsidence observed in the Ess- The wedge geometry aouira basin could not be entirely explained by thermal relaxation had already been made by Pique´ et al. (1998) but In the eastern part of the area (Figs. 5, 6), Hercynian associated with regional transtension. basement is unconformably overlain by Upper Permian to We conclude that realistic thinning models can only Lower Cretaceous sediments which typically show strikes explain less than 50 % of the observed post-rift subsidence. changing from N–S to E–W moving up-section thereby The remaining part (1–2 km) must be driven by different forming a clear fan opening from NE to SW. The Upper tectonic processes. This is a conservative estimate as the Permian to Lower Cretaceous succession is divided in two modeling curves have been obtained assuming a moderate parts by the Toarcian unconformity, a regional erosional strength of the underlying lithosphere. surface gently cutting down-section from SW to NE gen- erally interpreted as the breakup unconformity which Linking upward and downward moving domains: formed when oceanic crust appeared in the Central Atlantic the Imi-n-Tanout wedge (e.g., Medina 1995). Upper Permian to Liassic sediments are mainly The region to the SW of Imi-n-Tanout (Figs. 5, 6)isof with intercalations of volcanic rocks of the Central Atlantic particular interest because of the map-view wedge formed Magmatic Province (Verati et al. 2007) beautifully exposed by Triassic to Lower Cretaceous sediments. The wedge along the Argana valley (Fig. 5) (e.g., Medina 1995; links the MAM in the E with the Essaouira-Agadir basin in Baudon et al. 2009). Upper Permian to Liassic sediments the W thereby providing relevant information on the pinch-out between Upper Cretaceous and basement rocks kinematics of coeval upward and downward vertical to the W of Imi-n-Tanout (Figs. 5, 6) and thicken to the movements (e.g., Stets 1992). In contrast to other regions, SW reaching[6,000 m (Zu¨hlke et al. 2004). The thickness no evaporites are documented in the area and differential changes are interpreted as associated with (1) a roughly N– vertical movements must have a tectonic origin rather than S trending, E-dipping normal fault supposedly located to being driven by halokinesis. the W of the Argana valley (e.g., Fig. 2 of Medina (1995)) 123 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 2157 and (2) the Toarcian unconformity, which descends down- direction is significantly oblique with respect to the NE– section to the NE (e.g., Medina 1995). SW general trend of older, Middle Triassic to Early A wedge geometrically similar to the one just described Jurassic syn-rift faults mapped or inferred in the Argana is formed by the post-rift part of the succession, that is, by Middle Jurassic to Lower Cretaceous sediments which SW NE overly the break-up unconformity (e.g., Stets 1992). These 0km sediments show a fan geometry with dips to the NW in the S, becoming northward in the N (Figs. 5, 6). The corre- 144Ma -2 sponding formations are basically continuous over the a 160M entire area and show a pattern of decreasing thicknesses 176Ma from the SW to the NE (Stets 1992;Zu¨hlke et al. 2004). -4

The Imi-n-Tanout wedge in Cretaceous times -6 basement The geometry of the post-rift portion of the Imi-n-Tanout 234Ma 50km wedge prior to Alpine shortening can be reconstructed using thickness measurements available from the literature Fig. 7 The Imi-n-Tanout wedge at the end of the Early Cretaceous. (e.g., Zu¨hlke et al. 2004; Bouaouda 2004) integrated with Greatly simplified from Zu¨hlke et al. (2004). The thick dashed line our own geological sections. Thicknesses for basically all corresponds to the syn- to post-rift transition post-rift formations increase gradually from NE to SW reaching [3,000 m toward the coastal areas (Fig. 7). (a)NN (b) Subsidence rates seem to be very high during the Late Jurassic. No fault is inferred along the section which could provide a significant contribution to the observed thickness changes (Hafid et al. 2008). The direction of the profile of Fig. 7 is conditioned by the present outcrops distribution along and in the neigh- borhood of the Argana valley. We derive the original trend of the wedge by plotting strikes and dips of Middle Jurassic to Lower Cretaceous layers and compensating for Alpine deformation. The result (Fig. 8) shows that the Imi-n- Middle Jurassic Upper Jurassic Middle Jurassic Tanout wedge opened along a NNW–SSE to NW–SE trending axis. This axis must have roughly corresponded to Fig. 8 Geometry of the Imi-n-Tanout wedge. a Present-day position of sedimentary layers; b position of the same layers prior to Alpine the trend of boundary between the subsiding area in the W folding. Poles have been rotated in a counter-clockwise sense looking and the exhuming domain in the E. The NNW–SSE W, around a E–W trending axis by 15°

Fig. 6 Geological map of the 9°10'0"W 9°0'0"W 8°50'0"W NE part of the Imi-n-Tanout wedge. Simplified from sheet Marrakech of the 1:500,000 geological map of Morocco 18 Imi n’Tanout 31°10'0"N Figure 9 JEBEL LEMGO 15 20 18 15 15 15 15 15 Tertiary 28 12 25 Upper Cretaceous 29 20 25 Lower Cretaceous 23 Upper Jurassic L-M Jurassic 25 Triassic Permian 4 km 31°0'0"N

123 2158 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 area (e.g., Medina 1995; Hafid 2000) discarding a possible sedimentary structures are not uncommon in the Imi-n- reactivation (inversion) of Triassic to Early Jurassic normal Tanout wedge and typically show folds and thrusts with a faults as a mechanism contributing to Middle Jurassic to NW–SE trending axis (Fig. 10). All these structures doc- Early Cretaceous vertical movements. ument Late Jurassic to Early Cretaceous NE–SW shorten- ing approximately perpendicular to the axis of the Imi-n- Tanout wedge (see Sect. ‘‘The Imi-n-Tanout wedge in Syn-sedimentary deformations in the Essaouira basin Cretaceous times’’), suggesting that they developed within the same deformation regime. To constrain the tectonics driving the Middle Jurassic to Early Cretaceous upward and downward vertical move- Tidsi ments, which cannot be explained by models of rifted margins, we have investigated structures cropping out in The Tidsi anticline (Figs. 1, 11), exposed in the western the Essaouira-Agadir basin, which is the subsiding domain High Atlas ca. 20 km east of the Atlantic coast, is a flanking the region experiencing exhumation (Fig. 5). complex structure characterized by a main fold trending Jurassic to Cretaceous sediments are present in these areas, NNE–SSW, which interferes in the south with a shorter and syn-sedimentary deformations can be identified and E–W to WSW–ENE trending anticline. The regional stra- interpreted. Jurassic and Cretaceous have been tigraphy, the geometry of the fold and its predominantly described but their growth generally associated with dia- Mesozoic age have been documented in previous studies pirism rather than with regional tectonics (e.g., Hafid (e.g., Amrhar 1995; Pique´ et al. 1998; Hafid 2000) and 2000). interpreted as driven by halokinesis. No evaporites crop out in the Tidsi anticline, but a diapir-like body is imaged in Imi-n-Tanout seismic lines across the area (Hafid 2000).

Middle Jurassic to Lower Cretaceous sediments to the Growth of the Tidsi anticline South of the Imi-n-Tanout line are deformed by syn-sedi- mentary structures documenting the style of deformation The Mesozoic age of the growth of the Tidsi structure is during this times span. At the km-scale, fairly symmetric, based on (1) an unconformity separating Triassic from NW–SE trending folds are observed in the northern part of Portlandian (i.e., uppermost Jurassic) strata, (2) a signifi- the wedge, which are sealed by Lower Cretaceous strata cant increase in the thickness of the Lower Cretaceous (Fig. 9). At a smaller, meters to tens of meters scale, syn- from the eastern to the western side of the Tidsi anticline

Fig. 9 Geological map of the 9°10'0"W 9°5'0"W NW part of the Imi-n-Tanout n wedge showing syn- c1 4-6 sedimentary folds sealed by Upper Cretaceous rocks. Simplified from sheet Tamnar of the 1:100,000 geological map fold III n fold II fold I of Morocco 4-6

n4-6

n3c

j5-6

31°5'0"N n3a-b

j7

Quaternary n3c U. Hauterivian c Oxfordian- 1 Cenomanian j n3a-b Hauterivian 5-6 n Barremian- Kimmeridgian 4-5 j 2 km Albian 7 Berriasian

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Fig. 10 Outcrop-scale syn-sedimentary structures in the Middle Jurassic to Lower Cretaceous formations of the INT wedge

WESTERN FLANK

Albian EASTERN FLANK Fig. 12a

Aptian Albian Baremian Aptian Baremian

Hauterivian

Hauterivian 100 m Portlandian- Berriasian Fig.12b Triassic

Valanginian Portlandian- Berriasian Triassic

2 km

Fig. 14 Upper Cretaceous Lower Cretaceous Portlandian-Berriasian Triassic basalt

Fig. 11 Map of the Tidsi anticline. Representative stratigraphic columns for the western and eastern sides of the structure are shown in the insets, after the 1:100,000 geological map of Morocco (Tamnar sheet) (Fig. 11) and, (3) the sedimentary wedges exposed along erosional surface separating sub-vertical Triassic layers the N–S trending flanks of the structures (Fig. 12). from Lower Cretaceous strata, which dip by 40–50° to the The unconformity at the base of the Portlandian is well E. The base of Lower Cretaceous layers is generally par- exposed along the foot of the relief forming the eastern side allel to the unconformity surface, showing that the relief of the Tidsi structure (Fig. 12) and is formed by an formed during pre-Late Jurassic folding had been 123 2160 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165

Fig. 12 Syn-sedimentary wedges along the N–S trending flanks of the Tidsi anticline. See Fig. 11 for location effectively peneplained and that the entire area experienced perpendicular to the fold (Fig. 12). The growth of the Tidsi regional subsidence during the Early Cretaceous (Fig. 13a). anticline took place during generalized, long wavelength Lower Cretaceous rocks, however, are thinner in the E than subsidence (Fig. 13b), affecting the entire region W of the in the W of the Tidsi structure recording the beginning of MAM and allowing for continuous sedimentation. differential vertical movements possibly related to the onset of west-vergent folding-thrusting. A re-activation of Tectonics during Tidsi folding a syn-rift normal fault is unlikely because the main extensional fault in the area is located several kilometers to Notwithstanding the possible role of diapirism in control- the E of the Tidsi structure and no significant inversion can ling the growth of the Tidsi anticline, other structures be identified in seimic sections [see Fig. 14 in Hafid observed in the area document the co-existence of tectonic (2000)]. deformations hitherto neglected. Notwithstanding the poor constraints on the initial The nicest structures are visible along a [1-km-long stages of the Tidsi structure, anticlinal folding was ongoing cliff in the southern part of the Tidsi anticline where Tri- in the Late Cretaceous causing the formation of the well- assic to Aptian sediments crop out (Fig. 14). Two wedges known sedimentary wedges seen along sections formed by Lower Cretaceous rocks are visible, opening

WEWE Upper Cretaceous

Lower Cretaceous

Upper Jurassic Permo-Triassic (with volcanics) (a) Early Cretaceous (b) Late Cretaceous

Fig. 13 Evolutionary scheme of the Tidsi anticline in pre-Alpine times 123 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 2161 toward the south and bounded on the southern side by Cretaceous to the Early Jurassic, which is the approximate steeper layers. Strata geometry points to a NNE-ward age of the opening of the Central Atlantic. The section has vergence of the structure (Fig. 14). Aptian strata in the a composite character and includes information from the uppermost part of the outcrop are sub-horizontal thereby northern and southern side of the High Atlas. documenting the pre-Aptian age of deformation. The tec- The geometry of Middle Jurassic to Lower Cretaceous tonic nature of these structures is not only suggested by sediments in the western part of the section is obtained their overall geometry but is proven by their vergence from the literature data (Zu¨hlke et al. 2004; Pique´ et al. toward the core of the Tidsi anticline, which is incompat- 1998; Bouatmani et al. 2007; Le Roy and Pique´ 2001; ible with a halokinetic origin. Hafid 2000). Sediment thicknesses were not corrected for compaction during retrodeformation. Vertical movements in the MAM have been derived from the t-T paths obtained The Middle Jurassic to Early Cretaceous tectonic by Ghorbal (2009) for samples from the northern and evolution of central Western Morocco southern foothills of the High Atlas (e.g., Fig. 2), translated into depths adopting a constant gradient of 25 °C/km. Kinematics and tectonics of a section from the MAM to the Essaouira basin Kinematic evolution

Data and methods The upper panel of Fig. 15 shows a simplified geological transect along the western High Atlas toward the end of the To provide a kinematic frame for the evolution of western Early Cretaceous. The section is characterized by an up to Morocco, we construct a *100-km-long, E–W trending 6-km-thick succession of Triassic to Lower Cretaceous section from the MAM in the E to the coastal domains of sediments thinning toward the E and essentially pinching- the Essaouira-Agadir basin in the W depicting the situation out against the MAM. The post-rift (post-Toarcian) part of at the end of the Early Cretaceous before the Cenomanian the succession which accumulated following the end of transgression. We then retrodeform the section compen- normal faulting reaches thicknesses of [3 km. No signifi- sating for vertical movements from the end of the Early cant normal fault is present at the scale of the transect.

Fig. 14 Line-drawing of the structures exposed in the southern part of the Tidsi anticline (see Fig. 11) for the position. The cliff is ca. 200 m high 123 2162 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165

Early Cretaceous (100Ma) section, and it is during this time frame that syn-sedi- Essaouira-Agadir imi-n-tanout wedge MAM mentary contractional structures developed, which have basin 0km been described in chapter 4. Coeval subsidence in the W 144 -1 and exhumation in the E ended probably toward the end of the Early Cretaceous, beginning of the Late Cretaceous 160 pre-Liassic -2 when the entire region, as well as North Africa, was cov- 195 -3 ered by Cenomanian marine sediments (e.g., Luning et al. 2004) and followed by generalized subsidence. The profiles of Fig. 15 show that no major fault existed Late Jurassic (150Ma) during Middle Jurassic to Early Cretaceous times accom- shortening 0km modating the differential movement between the subsiding -1 160 ? Essaouira-Agadir basin and the exhuming MAM. These -2 movements, on the contrary, were associated with gen- Triassic eralized westward tilting of the pre- and syn-rift succes- -3 sions. The timing and geometry of such tilting is documented in the Imi-n-Tanout wedge. Because of the excessive subsidence and, more importantly, exhumation, this tilting cannot be the expression of post-rift thermal Middle Jurassic (170Ma) 0km subsidence. 195 Our, admittedly speculative, working hypothesis is that -1 Triassic ? the excessive subsidence in the Essaouira basin and -2 exhumation in the MAM were caused by roughly E–W -3 trending shortening active during Middle Jurassic to Early

assumes that sampled rocks were Cretaceous times. A contractional regime is compatible ca. 50km at the surface at the end of the Hercynian orogeny with the widespread occurrence of syn-sedimentary con- tractional structure in Upper Jurassic to Lower Cretaceous Fig. 15 Middle Jurassic to Early Cretaceous vertical movements successions. The E–W shortening could be accommodated along a section from the Essaouira basin in the W to the MAM in the E. The section trends roughly E–W and includes information from either by deep W-vergent thrusting in the MAM or by both sides of the High Atlas folding, or a combination of the two (e.g., Cloetingh and Burov 2011). The eastern part of the section was probably occupied by a peneplain close to sea level resulting from the erosion of Extrapolation to surrounding areas the mountain belt which must have formed during earlier exhumation stages. The coexistence of subsidence in the W and exhumation in The oldest snapshot of our kinematic scheme (at 170 Ma) the E, as well as the occurrence of syn-sedimentary depicts the situation at the end of the Liassic shortly after the structures documenting shortening during Jurassic to cessation of normal faulting in the Essaouira basin. In the W, Lower Cretaceous times, are not confined to the western a ca. 3-km-thick succession was present, which was pinch- High Atlas but are is observed also to the North and to the ing-out toward the E. Toward the W, the Triassic wedge is South of the chain. interpreted to be controlled by a major, E-dipping listric fault presently covered by younger sediments (Medina The region N of the High Atlas 1995). In the E, the presently cropping out basement rocks of the MAM were at the end of the Liassic at depths of ca. The exhumation we have reported in the MAM can be 3 km. If the assumption is justified that they were close to the traced to the N of the High Atlas. Indeed, Mesozoic ages surface at the end of the Hercynian orogeny then one must and t-T paths comparable to the ones discussed above have envisage the presence of 2–3 km of sediments which were been obtained from rocks from the Jebilet Hills N of removed prior to the Late Cretaceous. Marrakech to the Rabat area (e.g., Ghorbal et al. 2008; Starting from 170 to 150 Ma, the rocks of the MAM Saddiqi et al. 2009; Barbero et al. 2011). Subsidence along began moving upward, thereby probably generating a relief the Central Atlantic margin to the N of the Essaouira- which was the source area for the thick terrigenous suc- Agadir basin has been quantified and modeled by Gouiza cessions deposited in the Moroccan offshore (Price 1981; et al. (2010). The curves these authors produced document Le Roy and Pique´ 2001). This upward movement was persisting post-rift subsidence compatible with what we coeval with increased subsidence in the western parts of the have shown for the Eassouira-Agadir basin. Gouiza et al. 123 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165 2163

(2010) could only fit the data from the best constrained part Conclusions of the section by envisaging intense sub-crustal thinning during Middle Jurassic times. The Middle Jurassic to Early Cretaceous time interval is of Jurassic contractional structures similar to those particular interest for the evolution of Morocco and, indi- described above in the Essaouira basin are observed in the rectly, also for surrounding regions because of the coex- Abda basin, located to the N of the High Atlas. Here, istence of subsidence and exhumation along an E–W seismic data document gentle folded Triassic to Middle trending profile. The upward movements have allowed for Jurassic layers sealed by Upper Jurassic(?)-Cretaceous the erosion of substantial amount of rocks, part of which strata (Echarfaoui et al. 2002). was then transported toward the Central Atlantic Ocean and deposited on the Moroccan passive continental margin The regions to the S of the High Atlas (e.g., Lancelot and Winterer 1976; von Rad and Sarti 1986; Price 1981). The zone of exhumation inferred from the MAM continued A significant part of these movements, ca. 50 % for the also to the S of the High Atlas. Low-T geochronology subsiding domain and [70 % for the one experiencing results from the westernmost part of the Anti-Atlas have exhumation, is incompatible with processes generally very recently become available (Ruiz et al. 2011). expected to take place during the post-rift of continental According to this study, rocks of the area have experienced margins, namely thermal subsidence. Even more complex continuous exhumation (in the order of 3–4 km) during mechanisms indirectly related to rifted margin formation, Triassic to Early Cretaceous time followed by more limited such shoulder uplift and secondary convection cells are subsidence and exhumation during Tertiary times. This unable to explain the observed movements. pattern is not substantially different from that documented As an alternative, we tentatively propose a scenario in from the Meseta and the MAM. These results are com- which vertical movements are driven roughly E–W trend- patible with previous, very limited geochronology data ing shortening superimposed on the thermal subsidence of from Malusa` et al. (2007) and illite maturity studies (Ruiz the margin. This picture is compatible with the widespread et al. 2008) and by maturity and geochronology work in the occurrence of contractional structures developed in Juras- Tindouf basin (Zu¨hlke et al. 2010), south of the Anti-Atlas. sic to Early Cretaceous times in the western parts of the In both cases, rocks were found to be too mature/hot with High Atlas and surrounding areas. respect to their geological evolution and present position. Indeed, our findings are not unique, and larger-than- Subsidence patterns have been analyzed by Gouiza expected upward movements have been documented in (2011) in the Tarfaya basin which lies to the W of the Anti- other passive margins such as Brazil (Bonow et al. 2009), Atlas region. Similarly to what is shown above, these the Dhofar margin (e.g., Bache et al. 2011 and references curves show strong subsidence persisting through Middle therein) and Antarctica (Storti et al. 2008). to Late Jurassic times, that is, coeval with the subsidence reported for the Essaouira-Agadir basin. Acknowledgments A significant part of the work at the base of this contribution has been conducted by Bachelor and Master students (K. Brautigam, J. Klaver, D. Fernandez-Blanco) of the VU University The regions to the E of the High Atlas of Amsterdam during the last few years. N. Froitzheim (Bonn) is thanked for sharing his publications and knowledge. M. Hafid is At the regional scale, the vertical movements and defor- thanked for support and discussions. The Editor of the International mations documented in Morocco are partly coeval with the Journal of Earth Sciences and the two reviewers of the journal are thanked for their constructive criticisms. Austrian orogeny (Barremian to Aptian), which resulted in a W–E directed shortening episode well-documented in the Open Access This article is distributed under the terms of the entire West Sahara domain in Algeria, Tunisia and Libya Creative Commons Attribution License which permits any use, dis- (Acheche et al. 2001; Mitra and Leslie 2003; Underdown tribution, and reproduction in any medium, provided the original author(s) and the source are credited. and Redfern 2008). The origin of this tectonic stage remains enigmatic, a possible explanation could be the change in the rate and direction of opening of the Atlantic following the separation between West Africa and South References America and the initiation of the South Atlantic drifting (Mitra and Leslie 2003). Shortening resulted in N–S ori- Acheche M, M’rabet A, Ghariani H, Ouahchi A, Montgomery S ented folding, reactivation and uplift of the already existent (2001) Ghadames basin, southern Tunisia: a reappraisal of Triassic reservoirs and future prospectivity. AAPG Bull fault blocks (Mitra and Leslie 2003), and the Austrian 85:765–780 unconformity which truncated the Barremian-Neocomian Amrhar M (1995) E´ volution structural du Haut Atlas occidental dans deposits in the region. le cadre de l’overture de l’Atlantique central et de la collision 123 2164 Int J Earth Sci (Geol Rundsch) (2012) 101:2151–2165

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