Applying magnetic techniques to determine the evolution of reactive diapirs: A case study of the Lusitanian basin

The western Iberian margin underwent complex evolution during the Mesozoic and Cenozoic. It was subjected to successive extensional and compressional phases, where salt tectonics played a significant role in both. The role of onshore structures during basin formation and inversion is not fully understood, and different chronological relationships between structures have been proposed. To better understand the history of salt body emplacement, we present paleomagnetic and magnetic fabric data from Jurassic carbonates, comprising cover rocks of the diapiric system. Magnetic fabrics show two types of behavior. Type 1 fabrics show magnetic foliation parallel to bedding-the magnetic lineation, after bedding correction, is horizontal and parallel to the diapiric contacts. This fabric is a consequence of overpressure during salt migration, whose effect is limited to the area near the diapirs. Type 2 fabric is characterized by a magnetic lineation parallel to the strike of bedding and-the intermediate and minimum magnetic axes are dispersed on a vertical girdle parallel to the shortening direction. This is interpreted as a compressional fabric that developed during basin inversion. Paleomagnetic results reveal a stable paleomagnetic component carry by magnetite with characteristic unblocking temperatures (175-450 degrees C) and coercivities (30-100 mT). Twelve of the 14 calculated mean directions show homogeneous behavior in their magnetic properties and directions, with positive inclinations, northward directions, with synfolding to postfolding behavior. We calculated the remagnetization direction (Dec: 347.5 degrees, Inc.: 51.3 degrees) and compare it to the APWP. It was likely acquired during the Cretaceous Normal Superchron, following the extensional deformation phase and prior to basin inversion. The paleodips (i.e., the bedding at remagnetization times) indicate that tilting occurred mainly during the extensional stage, meaning that the diapirs had already been developed. This indicates a strong Mesozoic salt tectonic component in the present-day structuring of this sector of the Lusitanian basin.