Seismic reflection profiles amplify our understanding of the geologic evolution of Morocco. Examples have been selected to illustrate structural styles and to discuss selected aspects of hydrocarbon exploration.

The Paleozoic of Morocco

The Paleozoic Tindouf foredeep is related to the Hercynian folded belt of the Anti-Atlas. We lack reflection seismic information across the Anti-Atlas, but excellent geologic maps suggest the presence in the subsurface of a frontal triangle zone as multiple decollement levels within the Paleozoic and the Precambrian. A definitive evaluation of the prospectivity of the Tindouf basin and its margins could be helped by new regional seismic profiles. The position of such profiles should be preceded by a careful structural analysis of the Anti-Atlas. Seismic profiles across segments of the eastern Anti-Atlas reveal deep decollement levels in the middle crust. North of this same area, i.e., in the Boudnib-Errachidia region, a thick upper Paleozoic "intramontane basin" occurs which has been penetrated by a limited number of wells.

The Hercynian folded belt underlies the remainder of Morocco and outcrops in the Moroccan Meseta and the Meseta of Oran. Seismic profiles across the Jebilet (north of Marrakech) reveal Paleozoic overthrusts that have been cut at right angles by later Alpine basement-involved overthrusts.

The Triassic and Jurassic Evolution of Morocco

By the end of the Paleozoic, Morocco was consolidated as part of Pangea. The opening of the Atlantic Ocean was preceded by Triassic-Lower Jurassic rifting which was followed by massive regional subsidence during the Jurassic and much of the Lower Cretaceous. This is illustrated on seismic profiles of the Tarfaya region and the northward continuation of the Atlantic shelf. Onshore Morocco Tethys-related rifting occurs mostly during the Jurassic to be followed by more localized rift-related subsidence during much of the Cretaceous.

Tethys-related and Atlantic-related rifts are often discordant to each other with northerly-trending Atlantic Triassic rifts contrasting northeast-trending Tethys-Jurassic rifts. The latter system was inverted during the alpine orogeny to form the mountains of the High and Middle Atlas. However, during the Mesozoic much of onshore Morocco was a relatively stable platform superposed on Jurassic halfgrabens. Only mildly inverted rift systems are seen on seismic profiles of the Guercif area in eastern Morocco and in the Rides Prerifaines which is an area that has seen a measure of exploration success.

The Neogene compression in Morocco

During the Neogene the early Mesozoic halfgrabens of onshore Morocco were inverted to form the High and Middle Atlas. These inversions involve decollement levels within the middle and lower crust and are the expression of the late alpine collision on Africa with Iberia which also led to the formation of the Rif Mountains and the Betic Cordillera. Associated with the inversions are also minor decollement folded belts involving only sediments and that were formed whenever the amount of Cenozoic shortening exceeded the amount of early Mesozoic extension. Seismic examples are known from the north flank of the Jebilet, from both flanks of the Middle and High Atlas, from the Guercif basin and the Rides Prerifaines.

The Rif Mountains of northern Morocco form a complex folded belt involving numerous allochthonous sedimentary thrust sheets. The external units of the Prerif are most promising for exploration. The Mesozoic platform of the Foreland extends well below the external Rif, with a relatively thin platform to the west and northwest but, farther east, with inverted transtensional Jurassic halfgraben systems that underlie the Rides Prerifaines. Significant new hydrocarbon discoveries are most likely to occur in the underlying platform, with only smaller amounts from within the nappe Prerifaine and satellite basins superposed on that unit. The Nappe Prerifain itself on reflection profiles resembles an "accretionary wedge," with chaotic internal structures, shale diapirs, etc. This unit was emplaced in a short time interval during the Upper Messinian.

The more internal zones of the Rif form a complex folded belt that has not seen much exploration perhaps because earlier seismic surveys yielded poor results. New regional seismic profiles are needed to adequately evaluate this area.

Synorogenic Neogene extension in Northern Morocco

The highest and most internal units of the Rif folded belt are characterized by extensional tectonics, which can be observed in the outcrops of the high mountains of the Internal zone in the Chefchaouen complex, were extended following their emplacement as thrust sheets.

Spectacular normal faults separated by northeast-striking transfer faults may be observed north to the shores of the Alboran sea. Similar coeval Neogene extension and compression has also been observed on the Tyrrhenian side of the Appenines, in the Pannonian Basin of Hungary and in the Aegean Sea.

Conclusion

The regional structural evolution of Morocco is important for an understanding of the maturation and migration of hydrocarbons. Good quality seismic data are available from many areas of Morocco. However, in some areas, particularly the Rif Mountains and the front of the Anti-Atlas, new seismic is required to make an adequate assessment of the hydrocarbon potential.

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