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