Neotectonic fault detection and lithosphere structure beneath SW of High Atlas (Morocco)

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The High Atlas is a 100 km wide zone defined by E-W to NE-SW trending folds nearly orthogonal to the Atlantic coastline. The major compressional structures in the High Atlas consist of large-scale fold systems which affect Mesozoic and Cainozoic formations. The extreme West of the High Atlas including the region of Agadir is defined as an earthquake Zone. Historical seismicity data shows that the Agadir region was hit by two destructive earthquakes in 1731 and 1960 with magnitude 6.4 and 6.0, respectively. The present study has two main goals: 1) to use remote sensing techniques to detect and map the surface geological structures including faults; 2) to use the local earthquake tomography for imaging the lithosphere (subsurface) and detect deep structures. For the remote sensing techniques we used ETM + Landsat7 images and the SRTM 90 m image as a Digital Terrane Elevation Model. This study focuses on the computerized identification, feature extraction and quantitative interpretation of lineaments over the SW High Atlas. The analysis developed here is based on the numerical enhancement of a Landsat image and on the statistical processing of data generated through enhancement. The results generated by the numerical enhancement and statistical analysis are presented on fault maps, lineament maps, polar diagrams and lineament density maps. The lineaments have a high concentration of orientations around the directions N40E, N80W and N-S. For the subsurface study, seismic data sets were used to define the 3-D velocity structures. We also used local earthquake tomography to obtain the velocity map and crustal structure of the SW High Atlas region. The tomography results show a new and detailed lithosphere structure defined by a high velocity body in the northern of SW High Atlas from 15 to 45 Km depth, dipping to the north beneath the Essaouira basin in the western Meseta with P velocity variations from 6.5 to 7.8 km/s. This anomaly can be interpreted as an old subduction zone marking the limit between SW of High Atlas and the western Meseta. A second high velocity body, dipping north beneath the Hercynian Tichka Massif is detected. This positive velocity anomaly can be interpreted as an old subduction zone marking the limit between Meseta Domain and West African Craton. The occurrence of tholeiitic and alkaline magmatism respectively in the Essaouira basin zone and in the Hercynian Ticka Massif zone leads us to conclude that remains of subduction exist in these zones. The negative lithospheric anomalies are interpreted as heat or hot asthenospheric material upwelling from depth crossing the main crustal structures in SW High Atlas. The negative lithospheric anomalies are interpreted as hot asthenospheric material or heat coming from depth to replace the detached crust. Based on our analysis of local P wave velocities and profiles, the crustal thickness beneath SW of High Atlas region varies from 30 km to 45 km in central and western part.