Paleomagnetic constraints on the kinematics of inverted passive margins with salt diapiric structures. Preliminary results from syndiapiric overburden rocks of the northern margin of the Basque-Cantabrian extensional Basin.

Abstract

Salt levels deposited prior to deformation in either passive margins or orogenic settings generate a decoupling in deformation, with differential deformation in layers above and below the salt detachment. Distribution of salt detachments controls differential shortening and formation of orogenic arcs; and widespread diapirism can generate isolated minibasins which may record vertical axis rotations during their development. Our study aims to unravel the kinematics of salt diapiric structures by means of a paleomagnetic study of a portion of the Basque Arc, in the northern margin of the Basque-Cantabrian extensional Basin, where the salt overburden appears compartmentalized by several salt walls (Bakio, Bermeo, Guernica and Mungia diapirs). In this setting, 65 paleomagnetic sites from the overburden sequence have been analyzed. Predominant anticlockwise rotations with some clockwise and anticlockwise rotations in opposing flanks of the salt walls are observed. Fold tests reveal that some of the sites are clearly remagnetized, whereas some other sites show a prefolding magnetization. These observations are confirmed by magnetic hysteresis and back-field experiments in part of the studied rocks. Although further analyses will be necessary to explain the origin and distribution of the remagnetization, a preliminary kinematic model for the suprasalt deformation together with the underlying decoupled autochtonous materials can be established.