The Crustal Architecture of an Inverted Rift: constraints from the SIMA (Seismic Imaging of the Moroccan Atlas) Project

Antonio Teixell, Pui Ayarza, Ramón Carbonell, M. Harnafi, A. Kchikach, Alan Levander, Josep Gallart, Maria Luisa Arboleya, I. Palomeras, J. Marti, M. Charroud, and M. Amrhar

The Atlas Mountains derive from the inversion of a Triassic-Jurassic continental rift that connected the Atlantic and Tethys oceans in the NW of the African plate. Tertiary compression, with a main phase from the Late Oligocene-Miocene to recent, reactivated older extensional faults and created thick-skinned fold and thrust belts with moderate degrees of shortening (<25%). Models for the structure of the Atlas include a supracrustal belt soled in a detachment in the middle crust (based on low-resolution seismic refraction) and a deep-rooted thrust system offsetting the lower crust and the Moho (based on gravity). All previous models show moderate crustal thickness (<40 km) and a state of isostatic undercompensation. If topography does not respond to isostatic equilibrium at a crustal level, the mantle must be involved in the uplift process. Potential field modeling suggested an asthenospheric upwelling placing the base of the lithosphere as shallow as 70 km. Mantle-sourced uplift exceeded the Atlas deformed belts and raised the pre-Rifean marine corridor, thus reducing the Atlantic-Mediterranean connections in the Messinian. To picture the Moho topography and depth, and to constrain the seismic velocity structure, a 700 km long, seismic wide-angle reflection and refraction transect was acquired in 2010, extending from the Sahara to the Gibraltar arc, across the High and Middle Atlas and the Rif. Seismic energy released at 6 shot points was generated by the detonation of 1 TN explosives and recorded by 900 Reftek-125a recorders. The data allow the identification of crustal (Pg and PiP) and mantle phases (PmP and Pn). Forward modeling pictures the Moho as an asymmetric feature that defines a crustal root characteristic of young orogens. Modeling supports a deep-reaching system where upper-crustal thrusts converge in a north-dipping main thrust that offsets the lower crust and uppermost mantle beneath the northern High Atlas.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013