A Structural Modeling Approach on Timing & Evolution of Mesozoic Anticlines in the Western High Atlas, Morocco

Abstract

The Moroccan passive margin has been the objective of a variety of studies to investigate the petroleum potential and to locate play systems in the offshore and partly onshore Agadir-Essaouira basin. These basins are largely dependent on the structural development of Atlas mountains, being characterised by different phases of tectonic rifting, regional shortening and differential exhumation and subsidence patterns. The evolution of salt-cored anticlines in the Western High Atlas are possibly a key in understanding the tectonic history of the area especially in the context of the observed vertical movements. This study has the aim to provide additional information on timing and evolution of salt-cored anticlines in the Western High Atlas, to get a better understating of folding mechanisms related to vertical movements in the hinterland. Their role in controlling sedimentation pathways is investigated, since the basin links the paleozoic massifs as a sediment source and the offshore as sediment sink system. A new modeling technique was applied to two salt-cored anticlines, the Jbel Amsittene and the Imouzzer anticline, combining remote sensing geological and structural data, integrated in a 3D structural modeling environment (Gocad). This allows for the extraction of the thickness distribution for the identification of pre-Alpine locations of sediment depocenters. Further, the tectonic history of folding, using 2D section balancing (Move) and 3D unfolding and strain analysis (Gocad), is investigated. In a small field study, the large scale modeling results are compared to small scale field observations. Salt and folding mechanisms in the context of the geological setting and history are discussed and their impact on Jurassic to Early Cretaceous sediment pathways are outlined. The expression of folding on the sea-floor and the impact of the presence of pre-orogenic anticlines on the Alpine folding processes are debated. The results suggest Early to Middle Jurassic salt activities potentially initiated by pre-orogenic exhumation in the hinterland and subsidence towards the offshore, resulting in a hydraulic head gradient causing salt flow. Another scenario are pre-orogenic phases of tectonic shortening or salt flow by increased thermal activities. The topography that was created certainly controlled the sediment distribution of passive margin sands into the offshore basin. Jurassic salt mobilisation also had an impact on the Alpine-folding process which might have resulted in a connection of smaller isolated salt diapirs leading to a variety of strike orientations of folds in the basins.