Crustal thickness and velocity structure across the Moroccan Atlas from long offset wide‐angle reflection seismic data: The SIMA experiment

Abstract

The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide-angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, highresolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at 40-41 km depth in the northern part of the High Atlas. The P-wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling efforts.

New numerical and theoretical model to characterize the upper crustal structure of the Moroccan atlas from wide-angle seismic reflection data

“…Nevertheless, in this contribution, the study of the first arrivals will focus on the upper part of the crust (up to 10 km). The study of the whole crust and the Moho discontinuity has been published elsewhere [28].…”

“…As a compressional belt, the High Atlas exhibits a relatively small crustal root that contributes to the mountain topography, as revealed by gravity [7], lowresolution [8], [11] and a high resolution wide-angle reflection seismic experiment [28]. However, the total elevation of the Atlas system (where the mean altitude of the mountain belts exceeds 2000 m over large areas and some of the undeformed foreland basins stand above 1200 m) is only partially explained by shortening and crustal thickening, being the system in a state of isotactic under-compensation at the crustal scale [5], [7], [15], [29].…”

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…The crustal root contains a step in the Moho which suggests a lower crustal shortening that is comparable to the 30 km derived from the cover [280]. [281] even speculate that a planar north-dipping master thrust beneath the High Atlas connects directly down to the step in the Moho while [280] prefer a ramp-flat-ramp geometry based on their velocity model.…”

“…This thin lithosphere is in contrast to the thicknesses of more than 150 km to the north and south and may be a consequence of mantle upwelling during Neogene compression [279]. The crust/mantle boundary on the other hand forms a crustal root beneath the Atlas Mountains with a maximum thickness of 35 km beneath the Middle Atlas and reaching 40 km beneath the High Atlas [280].…”

“…[281] even speculate that a planar north-dipping master thrust beneath the High Atlas connects directly down to the step in the Moho while [280] prefer a ramp-flat-ramp geometry based on their velocity model.…”