Electrical resistivity imaging of the shallow structures of an intraplate basin: The Guadiana Basin (SW Spain)

The Iberian Peninsula has suffered intense Alpine intraplate deformation giving rise to a set of sedimentary basins bounded by mountain ranges. The larger basins, such as the Duero and Tajo in the west and central Iberia, are well documented and their infill and structure are well known. However, the southern, much smaller Guadiana Basin has not been much investigated. The Guadiana Basin is an E-W to NE-SW depression developed on igneous and metamorphic rocks of the Variscan basement that overlays, through thrusting, Cenozoic rocks in its northern margin. It is infilled by no more than 200 m of continental deposits. By acquiring nine electrical tomography profiles, eight of them with an array extension ranging between 800 and 900 m and one exceeding 2000 m, we imaged basement-Cenozoic infill unconformity depth. The resistivity data obtained were inverted to 2D resistivity models from the surface down to a depth of ~ 100 m. This procedure proved useful to assess fault geometries and basin infill thickness because of the generally high resistivity contrast between the Cenozoic rocks and Variscan basement. High-angle faults control basin margins and interior structure. The larger fault displacements were detected at the northern border, where they may exceed 500 m of vertical throw. Cenozoic deposit thickness is highly variable and thicker sequences appear along the northern border related to the E-W to ENE-WSW thrusts. The basin occupies the footwall of these thrusts, which are in turn subdivided into smaller blocks limited by E-W to NE-SW faults.