Syn- to post-rift diapirism and minibasins of the Central High Atlas (Morocco): the changing face of a mountain belt

The Atlas Mountains are classically regarded as a failed Mesozoic rift arm subject to Alpine inversion, folding and thrusting. Here, we present new integrated structural and sedimentological studies that have revealed numerous Early–Middle Jurassic diapiric ridges and minibasins, characterized by distinctive halokinetic structures. Diachroneity in halokinesis is observed across the Central High Atlas, waning first in the SW during the Early–Middle Jurassic (Jbel Azourki and Tazoult ridges) and continuing to late Middle Jurassic towards the NE (Imilchil region). The halokinetic structures are readily differentiated from the effects of later Alpine deformation, allowing a new picture of the Central High Atlas to emerge. The most pervasive deformation in the Central High Atlas is associated with Early–Middle Jurassic diapirism, whereas the impact of Alpine inversion is mostly focused at the basin margins. This new understanding helps explain previously problematic aspects of the Atlas Mountains, which we now recognize as an exceptionally well exposed natural laboratory for understanding the interactions between halokinesis, tectonics and sedimentation.

Synrift and post-rift salt basins are common in continental passive margins characterized by multiple diapiric structures and associated halokinetic strata (Trusheim 1960;Sannemann 1968;Jackson & Seni 1983;Hudec & Jackson 2007). These margins are mostly investigated by means of seismic data. When exhumed in mountain belts shortening commonly obscures their diapiric architecture (Letouzey et al. 1995).
The Atlas Mountains are classically regarded as a failed rift arm subject to Alpine inversion (Mattauer et al. 1977;Vially et al. 1994;Ziegler et al. 1995). The Jurassic basin fill is superbly exposed in the Central High Atlas (Fig. 1), an area typically considered to be dominated by Alpine tectonism (Frizon de Lamotte et al. 2000). Recent structural reconstructions indicate shortening of 10-20%, and portray a classical mountain belt structure (Poisson et al. 1998;Teixell et al. 2003;Michard et al. 2011). However, there is surprisingly inconsistency between sections published through the Central High Atlas. Many of the exposed structural culminations identified in this range (Jossen 1990;Fadile 2003) are difficult to reconcile with the classical structural style expected in folded and thrust mountain belts (McClay & Price 1981).
Here we present new integrated structural and sedimentary studies that reveal a very different picture of deformation in the Central High Atlas. Study of structures in shallow water (platform) and basin settings has been undertaken over an area of c. 14000 km 2 (Fig. 1b), and demonstrates that deformation in the Central High Atlas is dominated by syn-to post-rift diapirism, with concomitant minibasin formation. Singular diapiric structures were already described in the area (Bouchouata et al. 1995;Ettaki et al. 2007;Michard et al. 2011); they were, however, never addressed from a basin-scale perspective. In this paper, we focus on structures that exemplify the relationships typical of a platform-basin transect controlled by diapirism (Jbel Azourki-Tazoult area and associated Amezraï minibasin) and of diapirs and minibasins developed in basinal settings (Imilchil area). In these areas, the effects of Alpine inversion are readily differentiated (Fig. 1).
A comprehensive understanding of kalokinetic processes on passive margins can be gained through the study of 2D and 3D seismic data. In contrast, some of the most significant advances in understanding the structural and sedimentological responses to diapirism have been derived through analogue modelling and outcrop studies (Ge et al. 1997;Rowan & Vendeville 2006;Rowan et al. 2012). This paper presents detailed studies of a region of the Central High Atlas characterized by well-preserved diapiric, tectonic and sedimentary relationships of Early and Middle Jurassic age by means of fieldwork, remote sensing mapping and balanced cross-sections, which constrain the sequential restoration of coupled diapiric ridges and minibasins and the distinction of diapiric structures from later shortening to the present structure. What differentiates this study from many others is the scale. To our knowledge this is the first outcropbased study on a scale comparable with an offshore seismic survey across a complete transition from shallow water to basinal environments including multiple minibasins and diapiric structures.

The Central High Atlas Jurassic diapiric province
Geological setting and regional stratigraphy The ENE-WSW-trending, 2000 km long Atlas basin extending from Morocco to Tunisia formed coevally with Late Permian-Early Jurassic Central Atlantic opening within a large-scale leftlateral transtensive rift system (Fig. 2;Piqué et al. 2000). In this scenario, Triassic sediments were unconformably deposited on top of the folded Hercynian basement in half-graben basins bounded by ENE-WSW faults (Piqué et al. 2000;Frizon de Lamotte et al. 2008). These basins were filled by more than 1 km of reddish siltstones and evaporite-bearing shales including irregularly distributed halite and gypsum during the Late Triassic as proved by seismic lines and borehole data (Benaouiss et al. 1996;Courel et al. 2003). These deposits were unconformably overlain by upper Triassic to lowermost Jurassic tholeiitic basalts of the Central Atlantic Magmatic Province ( Fig. 2; Marzoli et al. 2004). The Late Triassic schizohaline environment in the Central High Atlas graded eastwards to hypersaline and open Tethys marine deposition in Tunisia (oujidi et al. 2000;Courel et al. 2003). These evaporitebearing shales and associated evaporites became the main source layer for the diapiric structures of the Central High Atlas described here but also in the Sahara Atlas (Vially et al. 1994;Frizon de Lamotte et al. 2000;Bracene et al. 2003) and Tunisian Atlas (Hlaiem 1999;Zouaghi et al. 2005;Zouaghi et al. 2013). The scarcity of evaporites along some of the studied ridges as well as the frequent diapir inclusions reveals a complex diapiric evolution as discussed below.
The Jurassic succession is composed of Hettangian-Pliensbachian platform carbonates that are developed across the Central High Atlas, interfingering with basin deposits characterized by hemipelagic and slope sedimentation, and covered by a Pliensbachian-Aalenian eastward prograding, mixed clastic-carbonate platform with source areas around the western closure of the Tethys realm ( Fig. 1; Souhel et al. 2000). These sediments are overlain by a shallowing upwards succession, from oolitic limestones and corals to continental red beds, deposited across the Atlas basin from Aalenian to Callovian times ( Fig. 1; Bouchouata et al. 1995;Fadile 2003).
The final stage in the evolution of the Central High Atlas corresponds to the Eocene to Quaternary episode of compressional deformation and uplift (Monbaron 1982;Tesón & Teixell 2008), the sedimentary record of which is found essentially in the plains bordering the High Atlas.

Halokinetic structures of the Central High Atlas
The Central High Atlas is formed by 15-80 km long ENE-WSWtrending ridges, slightly oblique to the main tectonic boundaries (Fig. 1b). Intersecting these ridges is a subsidiary NW-SE ridge set, separating elliptical to subcircular basins <30 km wide. These ridges expose upper Triassic shales, evaporites and basalts, with metre-to hectometre-scale slivers of Hettangian carbonates and Middle Jurassic gabbro. Lower to Middle Jurassic deposits filling these basins have typical thicknesses of 3-4 km, and comprise halokinetic sequences sensu Giles & Lawton (2002) (Fig. 2) that are diachronous from one basin to the next. This well-exposed polygonal array of interrelated ridges and basins forms an intricate system similar to that described in other salt basins (Mart & Ross 1987;Rowan & Vendeville 2006). Here we describe structures from the Central High Atlas in the context of a platform-basin transition.
In the SW of the study area the Jbel Azourki ridge (Fig. 1) is a key structure separating Early Jurassic shallow platform environments to the south from basinal environments to the north. This c. 80 km long ENE-WSW-trending highly segmented structure contains numerous elongate Triassic outcrops, interpreted as diapir pedestals localized along an inferred basement normal fault system at depth. The diapir pedestal outcrops include Central Atlantic Magmatic Province basalts and very limited Triassic shales. Hettangian slivers and Middle Jurassic gabbro intrusions are very rare along this structure. The architecture of the platform margin localized along the Jbel Azourki ridge is complex, with both erosional features and narrow minibasins subparallel to the fault system at depth. on the Jbel Azourki platform, the exposed carbonate succession is 450 m thick, and displays halokinetic hooks and syndepositional faults. NW of the Jbel Azourki ridge <1700 m of platform and basinal facies fill the Amezraï minibasin (Fig. 3a). Younger Toarcian-Aalenian mixed platform strata have a maximum thickness of c. 4 km. They onlap and finally truncate the older platform system, but display a fanning of beds on both flanks of the diapiric ridge. In contrast, the overlying Late Aalenian-Early Bajocian platform carbonates onlap and finally overlap the ridge, recording Middle Aalenian waning of diapirism along the eastern part of the Jbel Azourki ridge. However, diapirism faded earlier in its central part, as suggested by two isolated outcrops of Liassic carbonates lying on top of the diapiric materials of the Taghia pedestal (Fig. 4).
The Tazoult ridge is located about 20 km to the north of the Jbel Azourki ridge and in a more basinal position in the early Jurassic Central High Atlas rift system (Fig. 1). The Tazoult ridge is c. 25 km long and shows a narrow and irregular geometry in map view (Fig. 5). The ridge core is 0.6 km wide in the lower parts and defines a southward leaning canopy <2 km wide at its crest. It is formed by Triassic shales with many slivers of Hettangian carbonates and Central Atlantic Magmatic Province basalts and Middle Jurassic gabbro. The lower Jurassic units around the diapir show truncations that are typical of halokinetic strata and are age equivalent to those described on the Jbel Azourki ridge. The lower Sinemurian-Pliensbachian succession onlaps the diapir at a very low angle and defines halokinetic hooks on a scale of hundreds of metres, with observed thicknesses of 900 m on both flanks of the ridge. The overlying Toarcian-Aalenian mixed platform succession also onlaps the diapir, defining stacked halokinetic sequences with thicknesses of tens of metres reaching up to c. 730 m and c. 2500 m in thickness on the northern and southern flanks, respectively (Fig. 5b). The southern succession with subvertical to overturned attitude is located below the salt canopy and represents an exceptional field example of such features, which are rarely described in the literature (Davison et al. 1996;Ringenbach et al. 2013). The Tazoult ridge plunges both to the NE and SW at its terminations. A diapiric weld forms the NE termination of the ridge, which is overstepped by Late Aalenian-Early Bajocian platform carbonates. These carbonates, however, fossilize the entire Tazoult ridge, as also observed along the cross-section in Figure 5b and in its SW termination. The proposed basement normal fault at depth is inferred by the variations in thickness of the lower Pliensbachian-Early Aalenian depositional units in both flanks of the Tazoult ridge in agreement with the regional rift setting ( Further to the basin centre is the Imilchil diapiric system. It consists of interrelated diapir walls and elongated minibasins mildly deformed during Cenozoic shortening, from which the Tassent and Ikkou ridges and adjacent minibasins are shown here (Fig. 6a). The cores of the ridges in this area contain a significant component of Middle Jurassic gabbro (Michard et al. 2011) and scattered slivers of Hettangian limestones. The Middle Jurassic gabbro often forms more than 50% of the exposed lithologies and in some ridges it may constitute the whole diapiric core. However, these rocks are always younger than the enclosing materials as indicated by the radiometric ages of the intruded bodies as well as the cross-cutting relationships of the associated subvolcanic and volcanic bodies with the Jurassic succession (Hailwood & Mitchell, 1971;Fadile 2003). Thick Pliensbachian-Bajocian mixed carbonate-siliciclastic strata comprise unambiguous halokinetic sequences with diagnostic structural and stratal relationships along the northern limb of the Tassent ridge (Fig. 6b). Middle Jurassic strata (Bathonian and Callovian in age) seal the Ikkou ridge and thus record the end of its diapiric activity (Fig. 5c). The most outstanding characteristic of the Imilchil diapiric region is the diachroneity in the subsidence of the minibasins. up to 2.5 km thick Toarcian-Lower Bajocian strata comprise the main fill of the Ikassene basin, <1.9 km of upper Bajocian-Bathonian strata dominate the Lake Plateau minibasin fill, and c. 2 km of Bathonian-Lower Callovian sediments are the main Ikkou basin fill (Fig. 6). SE of the Amagmag Ridge, the Almghou minibasin (Fig. 1b) contains a Toarcian-Lower Bajocian halokinetic succession >1 km thicker than the time-equivalent fill of the Lake Plateau and Ikkou minibasins. Thickness variations within minibasins record shifts of the basin depocentres and lateral    movements of their substrate. Maximum subsidence becomes younger from NW to SE through the Ikassene, Lake Plateau and Ikkou minibasins (Fig. 6d). Sedimentation rates of about 2 km Ma −1 occurred during periods of maximum minibasin subsidence.

Discussion: evolution of the Central High Atlas diapiric basin
Previously, Jurassic ridges in the Central High Atlas have been related to transtensive tectonics and Middle Jurassic plutonism (Gonzague Dubar 1938;Laville & Harmand 1982), preceded by Early Jurassic diapirism (Bouchouata et al. 1995;Michard et al. 2011). Evaporites are, however, observed in only a few ridges (e.g. Ikerzi and Toumliline), around the southwestern closure of the Central High Atlas Jurassic basin and in scarce relics within the remaining ridges and diapirs. In addition, large salt pedestals are inferred from gravity modelling below the Toumliline ridge and beneath the Tassent ridge (Ayarza et al. 2005). Scarcity of Triassic evaporites at outcrop could result from the original configuration of the Late Triassic basin associated with a variable distribution of evaporites or/and from post-Triassic processes that could result from a combination of the following factors: (1) extrusion to the sea floor and seawater dissolution during passive diapir growth; (2) extrusion and hydrothermal evaporite dissolution favoured by Middle Jurassic plutonism; (3) potential extrusion during Cenozoic diapir rejuvenation; (4) surficial dissolution during Tertiary uplift and exhumation of the Central High Atlas. In this sense, it is interesting to highlight that c. 61% of the Tassent ridge outcropping core is formed by subvolcanic bodies with emplacement ages >10 myr younger than the enclosing strata (Hailwood & Mitchell 1971;Armando 1999;Bensalah et al. 2013). This entails a large amount of replaced material, which in our opinion could correspond to the missing evaporite rocks. Nevertheless, overpressured shale could also have played a significant role in the diapiric evolution of the Central High Atlas.
The angular relationship between halokinetic strata and diapiric ridge walls in the Jbel Azourki and Tazoult area shifts from subparallel to high angle at the top of the Hettangian-Pliensbachian strata (Fig. 7). Syndepositional faulting and hooks on a scale of hundreds of metres associated with the Lower Jurassic beds are interpreted here as the result of initial pillow growth followed by reactive and active diapirism over 18 myr during the Hettangian-Pliensbachian. After this period, thick Toarcian-Aalenian mixed carbonate-siliciclastic eastward prograding platforms stacked in halokinetic sequences against the wall of the growing diapirs. These tens of metres thick sequences might record the passage to passive diapirism, which lasted over 12 myr ending in the Late Aalenian in the Jbel Azourki-Tazoult area. We interpret this shift to passive and more extrusive diapir evolution as triggered by sediment loading as described in other similar scenarios (Demercian et al. 1993;Diegel 1995). Additionally, the changing nature of the stratigraphic succession may also control the variation of the cut-off angle on the diapir flank (Alsop et al. 2000). The diapir activity shift towards the Imilchil area is indicated by the age of the minibasins, with observed halokinetic strata of Pliensbachian to Callovian age (Fadile 2003). Based on stratal patterns and available biostratigraphic data (Fadile 2003) minibasin subsidence occurred over 13.7 myr for the Ikassene minibasin and 3 myr for the Lake Plateau and Ikkou minibasins (Figs 6 and 7).

Conclusions
Field and remote sensing mapping and structural analysis of the study area in the Central High Atlas Jurassic System have been used to identify ten diapiric ridges and eight minibasins. Detailed study of the Jbel Azourki, Tazoult, Tassent and Ikkou ridges and associated minibasins provide multiple examples of halokinetic strata. This analysis showed that diapiric activity migrated from the margin to the centre of the basin and from the west to the east from Lias to Dogger times.
This new work revolutionizes understanding of the Central High Atlas, the structure of which is best interpreted as a diapiric basin, characterized by intense syn-and post-rift diapirism and minibasin formation. The Jurassic Central High Atlas basin shares many features with other basin-fills deposited on a mobile substrate (Cobbold & Szatmari, 1991;Vially et al. 1994;Rowan & Vendeville, 2006;Callot et al. 2012;Davison et al. 2012).
Probably the strongest analogy is, however, to the Moroccan passive margin (Tari et al. 2003;Hafid et al. 2006) and Tertiary diapirism of the Red Sea (Mart & Ross 1987;Davison et al. 1996;Bosworth et al. 2005). Alpine inversion of the Central High Atlas is here interpreted as overprinting the well-recognized Jurassic diapiric basin and mostly mainly focused at the basin margins.  The Geofacets-GSL Millennium Edition Geofacets from Elsevier is an innovative web-based research tool designed by geoscientists for geoscientists. Elsevier and the Geological Society of London (GSL) have partnered together to provide GSL members with a unique opportunity to gain individual access to thousands of geological maps from the renowned Lyell Collection through the Geofacets platform. Email membership@geolsoc.org.uk to sign up today!