Letter
Anti-Atlas (Morocco) role in Neoproterozoic Western
Gondwana reconstruction
Kevin P. Hefferan
a,*, Hassan Admou
b, Jeffrey A. Karson
c,1, Ali Saquaque
daDepartment of Geography and Geology,Uni6ersity of Wisconsin–Ste6ens Point,Ste6ens Point,WI 54481, USA bFaculty of Science,Uni6ersity of Cadi Ayyad,Marrakech, Morocco
cDi6ision of Earth and Ocean Sciences,Box90230,Duke Uni6ersity,Durham,NC 27708, USA dReminex,Marrakech, Morocco
Received 10 January 2000; accepted 31 March 2000
Abstract
Reconstruction of latest Neoproterozoic Gondwana hinges on the interpretation of the subduction and collision kinematics of Pan-African orogenic belts that rim the West African craton. The Anti-Atlas suture zone of southern Morocco has presented an enigma in this reconstruction as the inferred subduction zone polarity and age of suturing appear to be incongruous with better known West African orogens to the west (Mauretanian, Bassaride and Rokelide) and Transaharan orogens to the east (Ougarta, Tuareg, Gourma and Dahomeyan). Contrary to previous interpretations, new data from the Anti-Atlas indicate a history of late Neoproterozoic (750 – 600) north-dipping subduction culminating in the (600 Ma) collision of the Saghro magmatic arc to the north with the north-facing rifted margin of the West African craton. Thus, the Anti – Atlas suture links a 6000-km long chain of Pan-African suture zones that essentially encircle the West African craton. The suture zones demarcate the former position of subduction zones that in all cases dipped away from the West African craton. The Anti-Atlas suture links the western and eastern segments of the Pan-African orogenic belts associated with the amalgamation of Western Gondwana. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Pan-African; Anti-Atlas mountains; Gondwana; Neoproterozoic; Ophiolite
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1. Introduction
Pan-African orogenic belts (Fig. 1A) have figured prominently in recent reconstructions of latest Neoproterozoic supercontinents (Bond et al., 1984; Hoffman, 1991; Nance et al., 1991; Rogers et al., 1995; Dalziel, 1997; Trompette, * Corresponding author. Fax: +1-715-3463372.
E-mail addresses:kheffera@uwsp.edu (K.P. Hefferan)., ad-mou@ucam.ac.ma (H. Admou)., jkarson@eos.duke.edu (J.A. Karson)., saquaque@re.managem.co.ma (A. Saquaque).
1Fax: +1-919-6845833.
(WAC) suture zones. Most previous models (Leblanc and Lancelot, 1980; Bodinier et al., 1984; Villeneuve and Cornee, 1994) of the Anti-Atlas belt interpreted this region as the result of one or more collisions across rifted margins or marginal basins that formed above a south-dip-ping subduction zone, descending beneath the WAC and culminating in a collision at 685 – 623 Ma. Despite the apparent continuity of Pan-African orogenic belts around the rim of the WAC, these interpretations suggested two major anomalies. First, the Anti-Atlas belt appeared to be the only Neoproterozoic West African orogen in which the subduction zone dipped towards the WAC; in all other late Neoproterozoic West
determined to be 600 Ma (Caby et al., 1989; Villeneuve and Cornee, 1994). Thus, the Anti-At-las belt appeared to represent a temporal and geometric discontinuity in an otherwise consistent regional pattern of Pan-African subduction and collision in the assembly of Western Gondwana.
Our studies in the Anti-Atlas region provide strong evidence for north-dipping subduction (away from the WAC) and subsequent collision at
600 Ma that reconcile both of these apparent problems. The 1000-km long Anti-Atlas suture zone links the 2000-km long Transaharan suture to the east and the 3000-km long Mauretanian-Rokelide suture to the west. Collectively, these delineate a broadly contemporaneous series of
Fig. 2. Generalized tectonic correlation chart for Pan African orogens of the WAC and Avalon. See text for references and discussion.
sutures extending clockwise from Liberia to the Gulf of Benin.
2. Pan-African orogenic belts of northwest Africa
Pan-African (725 – 500 Ma) orogenic belts ex-tend from northwest Africa to east Africa, and the Arabian – Nubian shield, southward to Brazil, and also occur in North America and Europe (Schenk, 1971; O’Brien et al., 1983; Rast and Skehan, 1983; Nance et al., 1991; Kro¨ner, 1993; Rogers et al., 1995; Trompette, 1997). In north-west Africa, these belts include: the Anti-Atlas, Ougarta, Pharusian – Tuareg, Gourma, and Da-homeyan orogenic belts to the east, and the Mauretanian, Bassaride and Rokelide belts to the west (Fig. 1B). In all cases, these orogenic belts appear to have developed as a result of successive collisions of magmatic arcs, accretionary melanges and/or previously amalgamated terranes (e.g., Saquaque et al., 1989; Black et al., 1994) with rifted margins of the WAC.
3. West African craton rifting
Neoproterozoic Pan-African passive margin de-velopment around the WAC is marked by 1100 – 700 Ma siliciclastic sedimentation and the deposition of carbonate shelf sequences, followed by a 900 – 700 Ma rifted margin sequence and the opening of ocean basins (Fig. 2). In the Anti-At-las region, on the northern edge of the WAC, a rifting age of 788910 Ma (Rb/Sr, Clauer, 1976) is inferred from gabbro intrusions into a shelf sequence. In the Transaharan region to the east, gabbroic and ultramafic intrusions into the base of the continental crust suggest that rifting oc-curred at 900 – 800 Ma (Black et al., 1979). Rifting may have begun as early as 1100 Ma in the Dahomeyans, with the development of a
simi-4. WAC suture zones
4.1. Anti-Atlas suture
The Anti-Atlas suture (Fig. 1C), well exposed in erosional inliers at Bou Azzer and Siroua, formed by the collision of the northern rifted margin of the 2000 Ma WAC and the Neoproterozoic Saghro magmatic arc to the north (Saquaque et al., 1989). North-dipping subduction of oceanic lithosphere generated calc-alkaline plutons, north-dipping, dextral, oblique-slip thrust faults and greenschist to amphibolite facies metamorphism. Calc-alkaline plutonism in Saghro reportedly ranged in age from 778944 to 58095 Ma, U/Pb zircon ages; Z. Mrini, personal communication). The significance of the apparently prolonged pe-riod of Saghro plutonism is a matter of debate; multiple working hypotheses include: accretion of discrete allochthonous blocks with disparate plu-tonic histories, multiple episodes of subduction, or erroneous geochronologic dates. Precise age deter-minations of the Saghro plutonism are critical to a thorough understanding of the Anti-Atlas evolution.
Saghro plutonism overlapped in time with the development of blueschist-bearing accretionary melanges and the emplacement of ophiolitic slices in the Bou Azzer region. Kinematic indicators within the blueschists display southward-vergent thrust movement, which appears to have been the dominant sense of motion during melange devel-opment The Bou Azzer ophiolite was emplaced in an accretionary melange prior to 650 Ma, based on preliminary U/Pb zircon age of a cross-cutting quartz diorite pluton (S.D. Samson, personal communication).
Oblique suturing (615 – 565) of the WAC to the Saghro arc (Fig. 3A) produced sinistral-oblique, north-dipping thrust faults and the
devel-tion (580915 Ma; 565920 Ma, U/Pb zircon; Juery, 1976) within the suture zone. A widespread neoautochthonous blanket of post-tectonic, silicic pyroclastic material (Ouarzazate Formation) is overlain by carbonate, clastic and minor alkaline volcanic rocks of the early Cambrian Adoudou-nien Formation (534910 Ma, U/Pb zircon; Ducrot and Lancelot, 1977), which marks a grad-ual return to continental platform sedimentation (Fig. 2). No autochthonous arc intrusions oc-curred south of the Anti-Atlas suture zone in the 778 – 565 Ma interval. North-dipping thrust faults, the occurrence of blueschist-bearing ophiolitic melange, the absence of calc-alkalic plutons south of the suture zone, and the Saghro arc north of the suture are all consistent with a north-dipping subduction zone away from the WAC.
4.2. Pharusian–Tuareg suture
Southeast of the Anti-Atlas belt, two Neoproterozoic collisional events occurred in the Pharusian – Tuareg region. West-dipping subduc-tion beneath the eastern border of the Tuareg shield (730 – 670 Ma) resulted in the closure of an intervening ocean basin and the East Sahara craton to the east (Black et al., 1979, 1994). The collision between the Tuareg shield and the East Sahara craton (700 – 670 Ma) produced green-schist to amphibolite facies metamorphism, ob-duction of ophiolites and late- to post-tectonic (660 Ma) molasse deposits (Black et al., 1994; Liegeois et al., 1994).
dextral and sinistral offset. Final suturing (600 Ma) produced dextral, oblique-slip along the Raghane shear zone. Post-tectonic calc-alkalic in-trusions continued to 524 Ma, and molasse (530 Ma) deposits overlie the deformed se-quences (Black et al., 1979; Caby et al., 1981; Liegeois et al., 1994). Dual gravity anomalies, paired metamorphic belts containing eclogite, blue amphibole-bearing metamorphic assemblages overprinted by a regional greenschist facies meta-morphism, dismembered ophiolites, the absence of calc-alkalic plutons west of the suture zone, and the Tilemsi arc east of the suture are consis-tent with an east-dipping subduction zone away from the WAC (Caby et al., 1981; Black et al., 1994; Liegeois et al., 1994).
4.3. Dahomeyan suture
Southeast of the WAC, east-dipping subduction beneath the Benin – Niger shield (700 – 600 Ma) produced calc-alkalic plutons (Sonoumon arc), east-dipping thrust faults, and the emplacement of ophiolitic slices (Affaton et al., 1991; Trompette, 1994). The collision (640 – 560 Ma) of the Benin – Niger shield with the WAC (Fig. 3C) was accom-panied by the intrusion of syn- to late-tectonic plutons (Affaton et al., 1991; Villeneuve and Cornee, 1994) and the development of north-striking, predominantly dextral-shear zones. These shear zones link with shear zones in the Pharusian – Tuareg region to the north (Caby et al., 1981; Affaton et al., 1991). Late- to
of the suture zone are consistent with an east-dip-ping subduction zone, inclined away from the WAC (Trompette, 1994).
4.4. Mauretanian–Bassaride–Rokelide suture
West-dipping subduction produced two colli-sional events along the western border of the WAC in the latest Neoproterozoic to earliest Pa-leozoic. In the Bassaride – Mauretanian region, west-dipping subduction resulted in the closure of an ocean basin between the Senegalese microcon-tinent to the west and the WAC rifted margin to the east (Villeneuve and Dallmeyer, 1987). Calc-alkalic arc intrusions (680 – 660 Ma), west-dip-ping thrust faults, greenschist to amphibolite metamorphism, and ophiolite emplacement oc-curred prior to the collision (660 – 640 Ma) between the WAC and the Senegalese microconti-nent, with late molasse deposition (Villeneuve and Cornee, 1994). A subsequent metamorphic event (550 Ma) in the Rokelides is related to west-dipping subduction and the collision between the Guyana craton and the southwest part of the WAC (Villeneuve and Dallmeyer, 1987; Vil-leneuve and Cornee, 1994). Gravity data and the absence of calc-alkalic plutons east of the Pan-African suture zone are consistent with a west-dipping subduction zone, inclined away from the WAC (Villeneuve and Dallmeyer, 1987).
5. Pan-African sutures
Fig. 2 provides a generalized summary of Pan-African suture zones and related Neoproterozoic orogenic events along the northern margin of Western Gondwana. Despite the limited age con-straints, these Pan-African belts display remark-ably similar histories of allochthonous terrane accretion, calc-alkalic intrusions, metamorphism, ophiolite emplacement, accretionary melange gen-eration, and molasse deposition. Currently
avail-that suturing in the Anti-Atlas region (615 – 565 Ma) was broadly contemporaneous with collision between the WAC and the Benin – Nigeria shield (640 – 560 Ma) in the Dahomeyan Orogen (Af-faton et al., 1991; Villeneuve and Cornee, 1994) and collision (645 – 580 Ma) of the Tuareg/East Sahara assemblage with the WAC (Black et al., 1994; Liegeois et al., 1994) in the Pharusian oro-gen (Fig. 2). The WAC tectonostratigraphic rela-tions indicate a consistent latest Neoproterozoic link between the Anti-Atlas suture and the Transaharan suture and are further linked to Pan-African sutures in the Mauretanian – Bassaride – Rokelide orogens to the west (although diachronous at 660 and 550 Ma) and the Amazon Brazilian orogens (600 Ma) to the south (Trompette, 1994, 1997; Rogers et al., 1995; Un-rug, 1997). Together, these orogenic belts define a broadly synchronous belt of latest Neoproterozoic sutures, produced by subduction zones dipping away from the rifted margins of the WAC.
6. The Avalonian – Cadomian connection
sinistral-oblique shearing, the development of mo-lasse basins and post-tectonic (540 Ma) calc-al-kalic intrusions (Murphy and Nance, 1989; Nance et al., 1991).
Recent work on Cadomian terranes of the En-glish Channel Islands confirmed the presence of 206192 Ma basement (Icart Gneiss, Guernsey). The Icart Gneiss bears a similar crystallization age and Nd isotopic compositions to 2.1 Ga WAC gneissic basement (Boher et al., 1992; Sam-son and D’Lemos, 1998). This Paleoproterozoic gneissic basement has been intruded by a syn-tec-tonic 61192 Ma (U/Pb zircon) quartz diorite (Perelle) pluton (Samson and D’Lemos, 1999), estimated previously to be 700 Ma (Dallmeyer et al., 1991). On the island of Sark (Channel Islands), an orthogneiss interpreted previously as Paleoproterozoic in age has been dated at 616 Ma
+4/
−2 (U/Pb zircon, Samson and D’Lemos, 1998). The 611 – 616 calc-alkalic intrusions (Channel Islands and Bou Azzer), coupled with similar Nd isotopic compositions of the Guernsey and WAC (Birimian terrane) gneissic basement, suggest a fundamental link between the Cadomian and WAC Neoproterozoic belts (Samson and D’Lemos, 1998).
7. Conclusions
We suggest that the latest Neoproterozoic WAC was encircled by a series of curvilinear, outward-dipping, subduction zones extending from the Rokelides to the Dahomeyans — a 6000-km long Pan-African ‘ring of fire’. The WAC evolved from a continental nucleus which grew by the progressive accretion of microconti-nents, magmatic arcs and subduction melanges to form the core of the latest Neoproterozoic West-ern Gondwana supercontinent 600 Ma. The Anti-Atlas Neoproterozoic suture zone constitutes the missing link uniting a concentric ring of Pan-African belts that ultimately formed the core of Western Gondwana.
Collision and suturing in the Anti-Atlas region (615 – 565 Ma) were broadly contemporaneous with suture zone development throughout the WAC as well as the Avalonian – Cadomian belts
(635 – 555 Ma) to the north. Currently, radio-metric and isotopic data are not available to firmly establish the Cadomian – WAC link. Much of the available radiometric data for the WAC consist of Rb/Sr whole-rock or U/Pb zircon bulk analyses that are inherently unreliable, as demon-strated in the Channel Islands studies (Samson and D’Lemos, 1998, 1999).
Acknowledgements
Thoughtful reviews by A. Kro¨ner, K.A. Eriksson and J.B. Murphy improved this manuscript. Research support was provided by the University of Wisconsin – Stevens Point, Duke University and the University of Cadi Ayyad. Reminex (Marrakech, Morocco) provided logistic support in the field. Special thanks to S.D. Sam-son and J.B. Murphy.
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