Intracontinental Rifting and Inversion: Missour Basin and Atlas Mountains, Morocco1 Weldon Beauchamp,2 Muawia Barazangi,2 Ahmed Demnati,3 and Mohamed El Alji3 ABSTRACT The Atlas paleorift system is one of the largest rift systems in Africa. Little hydrocarbon explo- The intracontinental High and Middle Atlas ration has occurred within the Atlas Mountains and mountain belts in Morocco intersect to form the the margins of the paleo-Atlas rift system. Inversion southern and western margins of the Missour of synrift structures can lead to both the destruc- basin, an intermontane basin formed as a result of tion and preservation of synrift traps and the cre- the uplift and inversion of the Mesozoic Atlas paleo- ation of new hydrocarbon traps. The study of the rifts. These rifts were areas where the crust was effects of inversion in the Missour basin may lead to greatly attenuated and more subject to deformation the discovery of footwall subthrust hydrocarbon in response to nearby plate boundary tectonics. traps in the Mesozoic sedimentary sequence of the Data from observations based on seismic reflection Atlas Mountains. profiles and wells over the Missour basin for hydro- carbon exploration and field mapping were used to understand the basin evolution, structural styles, INTRODUCTION and inversion timing of the nearby Atlas Mountains. Hercynian and Mesozoic normal faults were reacti- Mountain belts located along convergent plate vated into high-angle reverse and thrust faults in boundaries, such as the Andes or the Himalayas, the Mesozoic during the Jurassic, Early Cretaceous have been and still are the focus of intense geologi- (early Alpine phase), and the Paleogene (late Alpine cal and geophysical studies. In contrast, intraconti- phase). The reactivation of synrift normal faults of nental mountain belts, including the Atlas system in the paleo-Atlas rifts inverted previous half grabens Morocco, lack even an agreed-upon first-order con- into anticlinal structures, with the axis of the half ceptual model of their deep structure and active graben centered below the axis of the inverted deformation. Geological evidence suggests such anticline. The resulting inverted fold geometries intraplate belts have significantly contributed to are controlled by the geometries of the extensional the evolution of the continental lithosphere since planar or listric faults. the Precambrian. Rifting during the Triassic and Jurassic was widespread around the world. The Atlas rift sys- tem of north Africa, the North Sea rift, the Andean rift system of Colombia and Venezuela, ©Copyright 1996. The American Association of Petroleum Geologists. All rights reserved. and the Palmyride rift of Syria are just a few of the 1Manuscript received June 5, 1995; revised manuscript received January intracontinental rift systems active during the 16, 1996; final acceptance April 12, 1996. 2Institute for the Study of the Continents and Department of Geological Triassic and Jurassic. Some of these same rift sys- Sciences, Cornell University, Snee Hall, Ithaca, New York 14853-1504. tems were inverted into intracontinental moun- 3Office National de Recherches et d’Exploitations Petrolieres, B.P. 8030, tain belts (i.e., the Atlas Mountains, Palmyride Rabat, Morocco. This study would not have been possible without the assistance of mountains, and the northern Andes). These rift ONAREP (Office National de Recherches et d’Exploitations Petrolieres) and systems were the focus of sedimentation during the Geological Survey of Morocco. We greatly appreciate input from our the synrift and postrift phases of rifting. Rift colleagues at Cornell, Francisco Gomez, Dogan Seber, Bob Litak, Alex Calvert, William Philpot, and Rick Allmendinger, in the preparation of this basins contain approximately 5% of the world’s manuscript. We also thank the donors of the Petroleum Research Fund, sedimentary volume, but they also contain administered by the American Chemical Society (ACS-PRF 29505-AC2), and to a grant from AASERT(DAAH04-94-G-0361) for support of this research. 10–29% of the known hydrocarbon reserve base We thank Richard G. Stanley, Z. R. Beydoun, and John A. Best for their (∼275 billion bbl) (Katz, 1995). This high concen- constructive reviews of this manuscript. In addition, we would like to thank tration of reserves is partially due to the limited Harold A. Illich and Geomark Research, Incorporated, for their geochemical analysis and help in evaluating potential source rocks in Morocco. INSTOC migration distance allowed by the geometries of contribution 223. rift systems. AAPG Bulletin, V. 80, No. 9 (September 1996), P. 1459–1482. 1459 1460 Intracontinental Rifting and Inversion Figure 1—Location map 10 ° Tangiers 5° MEDITERRANEAN SEA of the Atlas mountains and Missour basin of Morocco. ° Rif 35 Atlas The Missour basin is Tell ATLANTIC A bounded by the Middle Pre-Rif Atlas and High Atlas OCEAN Study Atlas mountains. Area Rabat N Meknes Middle Casablanca High Moroccan Plateau Meseta Missour Saharan Atlas High Atlas Marrakech Ar Rachidia A' Atlas ° ° ° High 10 0 10 France Western 40° Agadir Spain Location ° 30 Anti Atlas Map Atlas Mountains Morocco & Paleorift 30° Algeria Libya Rif Mountains ° 0 km 200 20 The uplift and inversion of hydrocarbon-bearing shaped the present architecture of the four major rifts can result in the remigration and redistribution geological structures of Morocco: the Rif fold-thrust of hydrocarbons into structures generated by the mountain belt in the north, and the Middle Atlas, reactivation of preexisting faults formed during rift- the High Atlas, and the Anti-Atlas mountain belts of ing. One must have a good understanding of the central Morocco (Figure 1). The Rif belt is funda- geometry of structures formed by the reactivation mentally different than the Atlas system. The Rif is of synrift faults, because these structures have the an asymmetric, Alpine-type, fold-thrust belt with potential to trap significant amounts of hydrocar- numerous, well-mapped thrusts and complex bons. Additionally, one must generate models to nappe structures (Loomis, 1975; Leblanc and understand the development of individual inverted Olivier, 1984; Morley, 1987; Doblas and Oyarzun, structures, as well as the uplift and inversion of 1989; Ait Brahim and Chotin, 1990; Leblanc, 1990; entire rift systems such as the Atlas Mountains, to Miranda et al., 1991), whereas the Atlas system is an better understand the potential of unexplored intracontinental, largely symmetrical mountain belt. intracontinental rifts and mountain belts around the world. Our research in Morocco is a step toward understanding and resolving the history Regional Tectonics and Rifting and present architecture of such belts. The Atlas system evolved within the stable plat- form of North Africa. Two major events shaped the GEOLOGIC SETTING geological evolution of the system: early Mesozoic extension and rifting, and Mesozoic–Cenozoic The Mesozoic and Cenozoic geological evolution compressional-transpressional phases that resulted of Morocco can be viewed as a response to two in the inversion of the rift systems (Figure 2). The major geological events: (1) the opening of the Atlas system is thus an intracontinental orogene North Atlantic and the western Tethys in the early “sandwiched” within the Proterozoic–Paleozoic Mesozoic, and (2) the Africa-Europe continental col- northern African platform, and is fundamentally dif- lision in the middle Cenozoic (Michard, 1976; ferent from orogenes located along convergent/col- Mattauer et al., 1977; Bensaid et al., 1985; Pique et lisional plate boundaries. Thrusts, strike-slip faults, al., 1987; Jacobshagen et al., 1988; Dewey et al., and block uplift tectonics characterize the Cenozoic 1989; Westaway, 1990). These two major events deformation of the Atlas system (e.g., Schaer and Beauchamp et al. 1461 (A) (A') Figure 2—Conceptual model for Northwest Southeast the development of the Missour basin and the Atlas Mountains, Middle Atlas Rift High Atlas Rift Morocco. See Figure 1 for location of AA′. ? Synrift Phase- Extension (Triassic-Upper Jurassic) Middle Atlas Rift High Atlas Rift ? Postrift Phase-Transgression (Cretaceous-E. Tertiary) Middle Atlas High Atlas Mountains Mountains Missour Basin Alpine-Rif Thrust Sequence ? Compressional Phase - Alpine Orogeny-Inversion (Oligocene) (NOT TO SCALE) Precambrian Triassic Middle-Upper Jurassic Tertiary-Quaternary Cretaceous Rif-Allochthonous Paleozoic Lower Jurassic Rodgers, 1987; du Dresnay, 1988; Fraissinet et al., existing weaknesses and faults formed during the 1988; Jacobshagen et al., 1988; Medina, 1988; Giese Hercynian orogeny. The High Atlas rift extends to and Jacobshagen, 1992; Jacobshagen, 1992). the Atlantic margin where it forms a failed rift or The Missour basin and the High and Middle Atlas aulacogen, and eastward (High Atlas/Saharan rift) mountain belts that form its boundaries (Figure 1) across Morocco, Algeria, and Tunisia (Figure 1). are examples of how large stresses can be transmit- The Middle Atlas rift and mountains trend north- ted to intraplate zones of weakness from the colli- east, where they extend beneath the thrusted sion zones along the nearby plate margins. High Alpine Rif allochthonous sedimentary rocks. The strain rates created by the thinning of the continen- intersection of the Middle and High Atlas rifts/ tal lithosphere resulted in the deformation of the mountains may represent a failed triple junction, or crust by extension and rifting in the North African