Deformation Partitioning in the Rif Fold-Thrust Belt (Northern Morocco):
Implications for Paleogeography and New Play Types in Petroleum Exploration
Oral presentation at American Association of Petroleum Geologists (AAPG) Annual Convention and Exhibition (ACE) 2012, Theme 2: Emerging Oil and Gas Plays in Africa.
The Rif fold-thrust belt of Morocco has been described in terms of large-scale tectonostratigraphic units; a sub-division based on legacy biostratigraphic mapping, much of it model-driven by the notion that the units contain far-travelled gravity nappes. These putative nappes include: an olithsostrome in the Mésorifaines of supposed Tortonian-age comprising km3-scale Jurassic carbonate blocks; the Nappe Prérifaines; the Ouezzanne nappe and the Numidian nappe (Nappe des Flysch). All were purportedly emplaced by gravity sliding very large distances from an unknown, non-metamorphic hinterland. Reconstruction of paleogeography from remnant outcrops of such nappes will be impossible if the hypothesis is correct.
On the other hand, if the Rif has a more conventional structure, then restoration of paleogeography, with important implications for the petroleum system, becomes a realistic goal. The arcuate form of the Rif is due to systematic arrangement of linked contractional and transpressional segments. Deformation was partitioned into NW-trending segments, characterized by across-strike contraction with transport to the SW, and SW-trending segments characterized by transpression. Within this framework, valid cross sections show that deformation in the Mesozoic rocks of the Mésorifaines conforms to conventional ideas of thrust belt geometry controlled by mechanical stratigraphy. In the external Prérifaines, Paleogene to Miocene rocks were deposited in a series of southward-younging foreland basins and do not comprise allochthonous units of gravity nappes.
Progressive southward inversion of these basins demonstrates in-sequence thrust propagation. Thrust imbrication of Triassic and Tertiary rocks implies that deposition of the foreland basin sequences led to emplacement of a partial salt canopy prior to imbrication by the advancing thrust belt. Later reactivation of thrusts in extension/transtension provided accommodation space for Late Miocene sediments and permitted further flow of Triassic evaporates along reactivated faults. We conclude that Rif fold-thrust belt architecture is conventional and was controlled by stratigraphic thickness and facies variations during southward-propagation of a strongly-partitioned thrust belt through its own foreland basin deposits. This calls for a reappraisal of petroleum system play-types in the light of paleogeography determined by restoration of regional cross sections.
Keywords : deformation, fold and thrust belt, inversion