Structure and Evolution of the Neogene Guercif Basin at the Junction of the Middle Atlas Mountains and the Rif Thrust Belt, Morocco

Abstract

The Guercif basin of northern Morocco occupies a 50 ´ 60 km area where the transpressional Middle Atlas mountains terminate and abut the Rif thrust belt. Analysis of over 800 km of 2-D (two-dimensional) seismic reflection profiles and eight exploratory wells, in combination with existing geological data, suggests a late Miocene episode of extension (4%, or 1.7 km, maximum) and a subsequent episode of contraction since the end of the Miocene. Most of the late Miocene deposition was concentrated in a narrow graben (herein referred to as the Guercif graben), which contrasts with the wider physiographic expression of the basin today. Geohistory analysis indicates that tectonic subsidence persisted until the Messinian, and sediment loading continued to drive subsidence even after extension stopped. Timing constraints demonstrate the contemporaneity of the Guercif graben and west-southwest-vergent thrust tectonics of the Rif thrust belt. Similar timing and proximity with the Rif, as well as the graben geometry, suggest that extension in the Guercif basin, in addition to other smaller extensional basins in the northern Middle Atlas region adjacent to the Rif, may represent the distal effects of a broad lateral shear zone bounding the thrust belt.