Structure and evolution of the Neogene Guercif basin at the junction of the Middle Atlas mountains and the Rif thrust belt, Morocco

The Guercif basin of northern Morocco occupies a 50 x 60 km area where the transpressional Middle Atlas mountains terminate and abut the Rif thrust be lt. Analysis of over 800 km of 2-D (two-dimensional) seismic reflection pro files 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 therein referred to as the Guercif graben), which contrasts with the wider physiogr aphic expression of the basin today. Geohistory analysis indicates that tec tonic subsidence persisted until the Messinian, and sediment loading contin ued to drive subsidence even after extension stopped, Timing constraints de monstrate the contemporaneity of the Guercif graben and west-southwest-verg ent 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 nort hern Middle Atlas region adjacent to the Rif, may represent the distal effe cts of a broad lateral shear zone bounding the thrust belt, The Neogene Guercif basin is superimposed on the Mesozoic Middle Atlas rift , which experienced basin inversion during the Cenozoic, and seismic reflec tion interpretations in the southern Guercif basin depict old Mesozoic rift faults reactivated as reverse faults, Unconformities illustrate that the u plift of the Middle Atlas appears to be primarily a late Cenozoic phenomeno n. The Guercif basin offers a special opportunity for petroleum exploration within an aborted rift basin such as the Middle Atlas. Mesozoic source roc ks in the Middle Atlas may have been sufficiently buried beneath Neogene ba sin sediments to reach maturity, and the late Cenozoic timing of contractio n can produce suitable structural traps.