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Fault kinematics and basin geometries in early stages of rifting: outcrop analogues from the western High Atlas of Morocco

Abstract

The High Atlas of Morocco is a classic example of intracontinental mountain belt formed by tectonic inversion of rift troughs. The Atlas rifting initiated in the Triassic and continued into the Jurassic, when several thousands of meters of syn-rift sediments accumulated. During the Cenozoic inversion, the Atlas experienced moderate shortening strain which is least in the western segments of the chain, in spite of high topography and exhumation (associated to large-scale, mantle-driven uplift). Such conditions, combined with outstanding exposure, make the western High Atlas an ideal area to investigate the oldest features of the Atlas rift, which involve the basement and early syn-rift deposits of Triassic age, providing valuable field analogues for geometrical features of deep rifts that are usually poorly resolved by seismic profiling and analogue modelling.

A detailed field study of the western High Atlas documents fault and basin geometries of the Tirknit, Tizi n'Test and Tizi n'Tacht-Imlil Triassic basins. The Triassic basins are currently bounded by major ENE-WSW faults, traditionally interpreted as major strike slip faults (leading to pull-apart basin opening) reactivated as thrusts during the compressional stage. Our study suggests that only a few of the major Triassic faults were reactivated during the compression, and they mostly keep an original dip-slip character, with minor strike-slip component: many faults remain in net extension and preserve second-order stretching structures (e.g., extensional drag folds). Basin-bounding extensional faults were strongly segmented, linked by relay ramps. Minor syn-sedimentary normal faults are abundant at different scales, and are consistent with the large-scale kinematic pattern. During the compressional mountain building stage, the main shortening mechanisms of the western Atlas rift are a) long-wavelength folding of the Triassic basins and their basement alike, apparently with little mechanical contrast between them, and b) the neo-formation of short-cut and by-pass thrust faults, which contribute to the tilting of basement and Triassic rocks and to the rotation of the former extensional faults, with minor obliteration of the earlier rifting features.