Multidimensional magnetotelluric imaging of crustal and uppermost mantle structures of the Atlas mountains of Morocco

Abstract

The primary goal of the PICASSO (Program to Investigate Convective Alboran Sea System Overturn) project, and the concomitant TopoMed (Plate reorganization in the western Mediterranean: Lithospheric causes and topographic consequences - an ESF EUROSCORES TOPO-EUROPE Collaborative Research Project) project, is to provide new constraints on the crustal and lithospheric structures of the Alboran Sea, Betics, Rif, Atlas Mountain belt and surrounding areas in the western Mediterranean. A component of both PICASSO and TopoMed is electromagnetic imaging using the magnetotelluric method. The land-based magnetotelluric survey¿s main objectives are to image the electrical conductivity structure of the Atlas Mountains of Morocco, as well as to test the hypotheses for explaining the observation of a "missing" mantle root inferred from the integrated geophysical-petrological forward modelling that combines elevation, gravity, geoid, surface heat flow and seismic data. Two-dimensional (2-D) and three-dimensional (3-D) inversion of magnetotelluric data, acquired along two profiles (the so-called MEK and MAR profile), are conducted to obtain enhanced insight into the subsurface geology of the Atlas Mountains. 2-D and 3-D isotropic modelling results from the approximately 220 km long MEK profile, crossing the Tabular (TMA) and the Folded (FMA) Middle Atlas, the High Moulouya Plain (HMP), the High Atlas (HA) and the eastern Anti-Atlas (AA), indicate that the TMA, FMA and the HMP are characterized by a low conductivity (3 - 10 Wm) lower crust underlain by relatively normal ( 150 Wm) upper mantle structure. In order to account for the observed low resistivities in the lower crust 3.3 - 10% of partial melt fraction is required. The interpreted lower crustal partial melt beneath the Middle Atlas and the HMP may have been formed by mantle-derived CO2-H2O rich supercritical fluids, which originate from the metasomatic alteration of the lherzolitic Atlas lithosphere during the partial melting process. On the other hand, 3-D isotropic inversion results of the southernmost MT data sites located on Paleozoic outcrops of the eastern Anti-Atlas system showed a homogeneous and high electrical resistivity crustal and lithospheric structure. Investigation of the approximately 225 km long MAR profile, crossing the Haouz Basin, the Western High Atlas, the Souss Basin and the Anti-Atlas, shows that the conductive Western High Atlas is confined by two resistive (>500 - 750 Wm) basins, the Souss basin to the south and the Haouz basin to the north. The enhanced conductivity structure is mainly parallel to the early Mesozoic rifting structures and can be attributed to the presence of fluids, the presence of clay ions or possible mineralization along the fault zones. During the 3-D inverse modelling of the MEK profile MT data, new 3-D inversion results confirm the high conductive zones of the 2-D inversion results except for one, which is found at the middle to lower crustal depth beneath the Anti-Atlas. Based on a synthetic data set from a single MT profile, in the presence of a regional, elongated 2-D conductive structure, the importance of including the xxi diagonal components of the impedance tensor in 3-D inversion is demonstrated. Moreover, in the case of using only the off-diagonal components of the impedance tensor, the model mesh and the data set must be aligned with quasi 2-D geo-electrical strike in order to map the right geometry and shape of the oblique conductive structures.