The Tasrirt structure (Kerdous inlier, Western Anti-Atlas, Morocco): a late Pan-African transtensive dome
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The Tasrirt structure (Kerdous inlier, Western Anti-Atlas, Morocco): a late Pan-African transtensive dome

Par A. Soulaimani, A. Pique´

 Faculté´ des Sciences Semlalia, Laboratoire de Ge´ologie Structurale, B.P. 2390 Marrakech, Morocco

Institut universitaire europe´en de la mer, Universite´ de Bretagne occidentale, 29280 Plouzané´, Brest, France

Avalable online 15 September 2004

Abstract

New geological investigations and geochronological data from the Kerdous inlier of the Moroccan Anti-Atlas suggest that an extensional event is recorded by stratigraphic and tectonic features within the Late Proterozoic–Early Cambrian cover. The Tasrirt massif is interpreted as a Late Proterozoic diapiric gneiss dome based on the pattern of metamorphic isograds, the presence of normal ductile shear bands and kinematic criteria indicative of non-coaxial deformation. The ascent of this gneiss dome was controlled by two late Pan-African shear zones with a dominant dextral wrench component. Sedimentary basins developed simultaneously with basement uplift in a NE–SW transtensional system, followed by NE–SW to E–W pure extension. 40K–40Ar mica ages of 1900–1350Ma obtained from the Palaeoproterozoic protoliths are related to a thermal rejuvenation during the late Pan-African syn-metamorphic doming events. The Tasrirt dome is thought to be the most representative of numerous domes within the Anti-Atlas and High Atlas, and is similar to related structures described along the rim of the West African craton during Late Precambrian-Lower Cambrian crustal extension.

 2004 Elsevier Ltd. All rights reserved.

1. Introduction           

The Anti-Atlas belt of southern Morocco (Fig. 1(B)) is a large Cambrian upland area where Proterozoic terrains crop out in several inliers (Choubert, 1963). The oldest rocks of the Anti-Atlas are classically known as the ‘‘PI Series’’ (Choubert, 1952), a Palaeoproterozoic low- to medium-grade metasedimentary series (Walsh et al., 2002) including intrusive granitoid orthogneisses assigned to the Eburnian orogeny (Charlot, 1978; Aı¨t Malek et al., 1998; Thomas et al., 2002). These basement rocks and their ‘‘Pll Series’’ Neoproterozoic cover were reworked during the Pan-African orogeny. In southern Morocco, the latter includes a north-dipping subduction of oceanic lithosphere (Saquaque et al., 1989) followed by the collision between the rifted margin of the West African craton and the Saghro magmatic arc to the north at ca 685Ma (Clauer, 1974). Both the Eburnian and Pan-African crystalline sequences are unconformably overlain by weakly deformed and practically unmetamorphosed

Upper Neoproterozoic clastic and volcanic rocks. Among these, the lowermost ‘‘Pll3’’ (Saghro Group) volcanoclastic sequence is interpreted as a syntectonic molasse series (Leblanc and Lancelot, 1980; Hefferan et al., 1992). The overlying Late Proterozoic ‘‘PIII’’ Ouarzazate Group (575–560Ma; (Thomas et al., 2002)), with dramatic thickness changes, is related to the onset of a new rifting phase (Pique´ et al., 1999; Soulaimani et al., 2003). Rifting continues into the Lower Cambrian with the deposition of regionally extensive clastic sediments and carbonates (Soulaimani, 1998).

The Early Cambrian transgression is well dated by interlayered flows of alkaline rocks (534 ± 10Ma, U/Pb zircon age; Ducrot and Lancelot, 1977).

In the western Anti-Atlas, the Kerdous inlier (Choubert and Faure-Muret, 1963) (Fig. 1(C)) exposes a Proterozoic crystalline basement where the development of an Eburnian magmatic event (2000–1900Ma) was established (Charlot, 1978), and confirmed by the 2058 ± 11

U/Pb age of the Anammr granite (Loughlin et al., 2002). Structural analysis has lead to the identification of two main orogenic events (Hassenforder, 1987).

The first event, which affects the older crystalline rocks, is attributed to the Eburnian orogeny. Associated structures are cut by the Tahala, Tasrirt and Anammr granitoids and lead to the domal-geometry of the Tasrirt area (Hassenforder, 1987). The second event, attributed to the Pan-African orogeny, developed new structures accompanied by retrograde metamorphism. Eburnian and Pan-African structures are both cut by the Tarc¸ ouate and Tafraoute granites. The ‘‘Kerdous slates’’ have been previously thought to represent Palaeoproterozoic rocks affected solely by the Pan-African orogeny (Charlot, 1978; Hassenforder, 1987), but the presence of Eburnian structures has been recently discussed (Nachit et al., 1996).

The Kerdous inlier is crosscut by two E–W brittleductile shear zones (Hassenforder, 1987; Fig. 1(C)): the Ameln Valley faulted zone (ZFVA) to the north, and Tasrirt-Tahala faulted zone (ZFTT) to the south, leading to three distinct domains. The northern domain is made of Neoproterozoic quartzites unconformably overlain by the post-Pan-African Tanalt conglomerates of the Ouarzazate Group. The middle area presents the juxtaposition of a Palaeoproterozoic basement (Tasrirt massif) to the east and the Anezi basin to the west.

South of the ZFTT is the Palaeoproterozoic Aı¨t Wafka Plateau and Tazerwalt sub-inlier.

Despite well-established geochrological ages of plutonic and volcanic rocks in the Kerdous massif (Charlot, 1978; Loughlin et al., 2002) and in other inliers of the western Anti-Atlas (Charlot, 1978; Aı¨t Malek et al., 1998; Thomas et al., 2002; Walsh et al., 2002), the tectonic evolution of the Kerdous inlier during the Neoproterozoic has been the subject of debate over the last years. No syn-collisional magmatic event is recorded in this area due to its tectonic setting at the rear of the Pan-African subduction system. However, Neoproterozoic magmatic activity, the tectonic significance of which is not easily established, is represented mainly by the emplacement of high level granitic plutons and bimodal volcanic rocks 100My later (583–548Ma). It is noteworthy that the Tarc¸ ouate granite in the centre of the complex is slightly foliated (Pons and Nachit, 1995) and thus critical in establishing the timing of deformation. The tectono-metamorphic timing in the Proterozoic metasedimentary rocks remains poorly constrained. Furthermore, the tectonic and magmatic events depicted during this time span within the metamorphic basement are roughly coeval with the deposition of adjacent unmetamorphosed volcanoclastics (Anezi and Tanalt series). The latter is dated in the Sirwa massif at 570 ± 7– 548 ± 3Ma (Thomas et al., 2002). The 555 ± 7Ma U– Pb age of rhyolitic ignimbrites of the Tanalt series in the Kerdous massif (Loughlin et al., 2002) is consistent with this age bracket.

It appears therefore that during the Precambrian–Cambrian transition the Kerdous massif, like the whole of the Anti-Atlas, underwent widespread magmatic and hydrothermal activity and probably a thermal rejuvenation.

The aim of the present study is to provide new observations about the relationships between the formation of sedimentary basins and the tectono-metamorphic evolution of the adjacent Proterozoic basement. Our structural data combined with new 40K–40Ar mica ages from metamorphic basement leads to a reinterpretation of the geology of the Tasrirt domain and adjacent basins.

Fig. 1. (A) Main structural domains of Morocco; (B) Simplified geological map of the Anti-Atlas showing the location of the Kerdous inlier (C), modified from the 1:1000000 geologic map of Morocco (Maroc Service Ge´ologique, 1985).

interlayered flows of alkaline rocks (534 ± 10Ma, U/Pb zircon age; Ducrot and Lancelot, 1977).

In the western Anti-Atlas, the Kerdous inlier (Choubert and Faure-Muret, 1963) (Fig. 1(C)) exposes a Proterozoic crystalline basement where the development of an Eburnian magmatic event (2000–1900Ma) was established (Charlot, 1978), and confirmed by the 2058 ± 11

U/Pb age of the Anammr granite (Loughlin et al., 2002). Structural analysis has lead to the identification of two main orogenic events (Hassenforder, 1987).

The first event, which affects the older crystalline rocks, is attributed to the Eburnian orogeny. Associated structures are cut by the Tahala, Tasrirt and Anammr granitoids and lead to the domal-geometry of the Tasrirt area (Hassenforder, 1987). The second event, attributed to the Pan-African orogeny, developed new structures accompanied by retrograde metamorphism. Eburnian and Pan-African structures are both cut by the Tarc¸ouate and Tafraoute granites. The ‘‘Kerdous slates’’ have been previously thought to represent Palaeoproterozoic rocks affected solely by the Pan-African orogeny (Charlot,1978; Hassenforder, 1987), but the presence of Eburnian structures has been recently discussed (Nachit et al., 1996).

The Kerdous inlier is crosscut by two E–W brittleductile shear zones (Hassenforder, 1987; Fig. 1(C)): the Ameln Valley faulted zone (ZFVA) to the north, and Tasrirt-Tahala faulted zone (ZFTT) to the south, leading to three distinct domains. The northern domain is made of Neoproterozoic quartzites unconformably overlain by the post-Pan-African Tanalt conglomerates of the Ouarzazate Group. The middle area presents the juxtaposition of a Palaeoproterozoic basement (Tasrirt massif) to the east and the Anezi basin to the west.

South of the ZFTT is the Palaeoproterozoic Aı¨t Wafka Plateau and Tazerwalt sub-inlier.

Despite well-established geochrological ages of plutonic and volcanic rocks in the Kerdous massif (Charlot, 1978; Loughlin et al., 2002) and in other inliers of the western Anti-Atlas (Charlot, 1978; Aı¨t Malek et al., 1998; Thomas et al., 2002; Walsh et al., 2002), the tectonic evolution of the Kerdous inlier during the Neoproterozoic has been the subject of debate over the last years. No syn-collisional magmatic event is recorded in this area due to its tectonic setting at the rear of the Pan-African subduction system. However, Neoproterozoic magmatic activity, the tectonic significance of which is not easily established, is represented mainly by the emplacement of high level granitic plutons and bimodal volcanic rocks 100My later (583–548Ma). It is noteworthy that the Tarc¸ouate granite in the centre of the complex is slightly foliated (Pons and Nachit, 1995) and thus critical in establishing the timing of deformation. The tectono-metamorphic timing in the Proterozoic metasedimentary rocks remains poorly constrained. Furthermore, the tectonic and magmatic events depicted during this time span within the metamorphic basement are roughly coeval with the deposition of adjacent unmetamorphosed volcanoclastics (Anezi and Tanalt series).

The latter is dated in the Sirwa massif at 570 ± 7–548 ± 3Ma (Thomas et al., 2002). The 555 ± 7Ma U–Pb ages of rhyolitic ignimbrites of the Tanalt series in the Kerdous massif (Loughlin et al., 2002) is consistent with this age bracket.

It appears therefore that during the Precambrian–Cambrian transition the Kerdous massif, like the whole of the Anti-Atlas, underwent widespread magmatic and hydrothermal activity and probably a thermal rejuvenation.

The aim of the present study is to provide new observations about the relationships between the formation of sedimentary basins and the tectono-metamorphic evolution of the adjacent Proterozoic basement. Our structural data combined with new 40K–40Ar mica ages from metamorphic basement leads to a reinterpretation of the geology of the Tasrirt domain and adjacent basins.

2. Tectono-sedimentary analysis of Neoproterozoic covers sequences

The Neoproterozoic cover that unconformably overlies the Kerdous basement is subdivided (from base to top) into two sequences (Hassenforder, 1987), the Anezi and Tanalt formations (informally named).

2.1. Anezi formation (Saghro group)

In the Anezi sub-rectangular basin (Fig. 1(C)), the Anezi formation is represented over a thickness of 2000m by alluvial fans and fluvial deposits. Conglomeratic horizons consist of poorly sorted, rounded or subrounded quartzitic or granitic boulders, assigned to palaeo- valley molasse fill. Within the basin, sedimentation seems have been controlled by kilometre-size NE–SW dextral strike-slip faults in a transpressional regime (Hassenforder, 1987). The siliciclastic units record many extensional structures (centimetre to metre-size syn-sedimentary normal faults, syn-sedimentary folds, etc.).

Coeval volcanic rocks and co-magmatic shallow-level granitoids occur in many locations. Most of the basin, filled with clastic and volcanic series remains undeformed, except at the basin margins.

The age of the Anezi formation has not been precisely determined. Some of its sedimentological features have been correlated with the widespread Varangian glacial event (625–580Ma) by Hassenforder (1987). However, some units of the Anezi formation overlie the southern border of the Tarcouate pluton (Hassenforder, 1987) therefore indicating an age younger than 583Ma.

2.2. Tanalt formation (Ouarzazate group)

The Tanalt formation consists of breccias and volcanic rocks that crop out discontinuously along the borders of the Kerdous inlier. Important thickness changes (0–800m) are the prominent feature of this formation. It unconformably overlies the Neoproterozoic and Palaeoproterozoic basement and in some localities large-scale palaeo-fractures separate the Tanalt Conglomerate from its substratum. This contact is examined in two localities. Along the NE flank of the Kerdous massif (Fig.2 (Ba)), at Ida Ougnidif, the Neoproterozoic quartzites of the Adrar Lkest are truncated by a NE dipping extensional fault associated with conglomerates of the Tanalt formation. These conglomerates are significantly thicker in the hanging wall, where they consist of cemented angular and badly sorted quartzitic pebbles decreasing in size progressively upwards to the NE where they are topped by thick basaltic flows (Ida Ougnidif basalt). Such flows locally display pillow and flow breccia structures indicating a subaqueous depositional setting. The conglomerate is conformably overlain by Lower Cambrian sandstones. A NE-dipping extensional fault cutting the quartzitic massif is associated with a variety of extensional structures such as vertical tension gashes and small scale grabens. Further away, syn-sedimentary folds, local intraformational unconformities and centimetre- to metre-size syn-sedimentary normal faults indicate a deposition of the sedimentary rocks during a period of NE–SW extension.

This tectonic instability persisted into the Lower Cambrian as testified by numerous extensional structures recorded along the eastern flank of the Kerdous inlier (Fig. 3(a)).

SW of Kerdous (Fig. 2(A)), the Tazeroualt Palaeoproterozoic massif is overlain westward by a thick conglomerate series of the Tanalt formation. Pebbles of variable size are here exclusively granitic, angular to sub-round in shape and cemented by quartz arenites.

Westward, the conglomerate grades progressively into Lower Cambrian sandstones and limestones interlayered with a  100m thick basaltic flow (Jbel Kerkar basalt). Numerous extensional structures such as synsedimentary breccias, local intraformational unconformities and centimetre- to metre-size synsedimentary normal faults are recorded. Regionally, the N–S striking syn-sedimentary faults (Fig. 2(Bb)), located at the present western limit of the inlier result in a thickening of the volcanoclastic deposits along the western border of the Kerdous inlier. A fast rate of Lower Cambrian subsidence is inferred for this area (Choubert, 1963; Benzaiane et al., 1983).

Both Adrar Kerkar and Ida Ougnidif basalts occupy the same lithostratigraphic position, i.e. between the Ouarzazate Group conglomerates and the Lower Cambrian sandstones. The basalts of both localities display a secondary greenschist facies paragenesis. Chemical analyses demonstrate their high-Ti composition and tholeiitic continental affinity in an anorogenic context (Soulaimani et al., 2004).

3. The crystalline substratum

In the Kerdous inlier, highly metamorphic basement rocks are exposed in two main localities on both sides of the ZFVA and TTZF shear zones (Fig. 1(C)). The most important is the Tasrirt area, a large sub-circular domain exposed to the east of the inlier associated with the Tasrirt gneiss dome of intrusive Palaeoproterozoic and Neoproterozoic granitoids.

Fig. 2. (A) Simplified geological map of Kerdous with K–Ar sample locations in the Tasrirt massif; (B) Interpretative cross-sections of Ida Ougnidif (a) and Jbel Kerkar (b) localities (respectively A–A and B–B in A); (C) Structural map of the Tasrirt dome (D) Schematic interpreted cross-section through the Tasrirt complex; (E) Kinematic model proposed for Late Proterozoic transtensional tectonics in the Kerdous inlier.

3.1. Structural geology of Tasrirt area

In the Tasrirt substratum, metamorphic grade increases towards the centre of the area, with lower greenschist rocks grading into migmatitic gneisses. Lower- greenschist facies rocks dominate the northern (Ameln Valley) and western part of the studied area, such as the Kerdous slates which contain well-preserved sedimentary structures. By contrast, the Tasrirt dome, flanked by micaschist contains gneisses and migmatites at its centre.

The most prominent features within the metamorphic basement are a regional foliation which is parallel to the axial plane of minor folds. Regionally, this foliation presents a concentric pattern around the Tasrirt dome (Fig.2(C)). Along the external rim of the dome, the foliation dips steeply towards its periphery, while it becomes nearly hornizontal towards the centre of the dome. Several indicators such as extensional crenulation cleavage, S-C shear-bands, porphyro-clasts with asymmetric strain shadows and asymmetric folds indicate highly non-coaxial strains. Along the northern border of the considered area, the ZFVA is a shear zone with several tens of meters width at the boundary between the ‘‘Kerdous slates’’ to the south and Adrar Lkest quartzite to the north (Fig.3 (b)). Highly elongated lenses of quartzite, gneiss and volcanic rocks are found along this structure. Bedding in metasedimentary rocks is transposed into a penetrative, steeply dipping foliation (S2 of Hassenforder, 1987) characterised by the predominance of recrystallisation processes. Asymmetrical and isoclinal small-scale folds plunging 25[1] to 50[1]NE, gently plunging stretching lineations (Le1) and C/S shear-bands are consistent with a global thrusting to the NE with a dextral offset (Fig.2(C)). Outside this area, the strain intensity decreases toward the south, along the Ameln Valley where the direction of shearing is interpreted to parallel down-dip mineral elongation lineation (Le1). The foliation-parallel, hundreds of metre-sized lacolith (Adrar Ouiharen augen-orthogneiss) dips to the south and presents the same northward sense of shearing provided by strain shadows around K-feldspar augen (Fig. 3(d) and (e)).

Between Adrar Ouiharen and the northern flank of Tasrirt dome, in the Ait-Oissim basin, slightly folded turbidites crop out. They are the least metamorphosed units observed in the region.

In the Tasrirt Plateau itself, the foliation (S1 of Hassenforder, 1987) is subhorizontal in migmatitic gneiss, where numerous folds with near-horizontal axial plane and curved axes are associated with normal shear-bands (Fig. 3(c)). Dislocated hinges of isoclinal folds are often seen at the microscopic scale. The deformation within various magmatic rocks is very heterogeneous, with metre-scale shear zones bounding less deformed lenticular domains. Two generations of muscovite are present in the gneissic rocks as well as in the migmatite. The largest grains are pre-tectonic while crystallisation of the secondary muscovite took place along the foliation planes. The metamorphism in the migmatized gneisses is characterised by the andalusite-sillimanite assemblage, indicative of high temperature-lower pressure conditions.

In the SW part of Tasrirt area, toward the Amanouz pass (Fig. 2(C)), the bedding is well-preserved and the foliation dips gently to the SW. Syn-metamorphic asymmetrical recumbent folds are tight to isoclinals and present a SW-vergence. Associated meso-to micro-scale normal shear bands (Fig. 3(f) and (g)) developed during later stages of shearing activity indicate a dominant SW-directed transport. To the south, the near-horizontal lineation (Le1) combined with the above kinematic criteria indicates that the southern boundary with the Adrar Ameksou quartzite is a dextral shear zone associated with a normal component (ZFTT).

In addition to Palaeoproterozoic plutons, many other late- to post-tectonic granitic bodies intrude the metamorphic rocks of this complex. The slightly foliated Tarc¸ouate granite is dated around 583 ± 11Ma and 560 ± 2Ma (Aı¨t Malek et al., 1998), thus giving a late Proterozoic age for the synchronous ductile extension, whereas the Tafraout, Agouni Yesse´ne and Tazoult (548 ± 11Ma) plutons are post-tectonic (Charlot, 1978; Loughlin et al., 2002).

3.2. Geochronology

New geochronological 40K–40Ar ages were obtained from selected minerals whose structural site was determined (Margoum, 2001). The analysed samples, whose locations are shown in (Fig. 2(A)), were extracted from: (i) ‘‘Kerdous slates’’ affected by a foliation older than the Tafraoute granite; (ii) deformed granitoids, orthogneisses, migmatites and pegmatites from the Tasrirt Plateau; (iii) the Tafraoute granite. 40K–40Ar dates are presented in Table 1.