Identification of clastic dikes as holocene seismites in the Dead Sea basin
Résumé
Off-fault seismogenic deformation in sediments (seismites) has proved useful for recovering long continuous records of earthquake activity. An important example of seismite is clastic dikes that were formed 'dynamically' by fracturing of the host rock and injection of clastic materials. However, they are easily confused with 'passive' dikes that were filled by deposition into preexisting fissures. This study demonstrates the application of anisotropy of the magnetic susceptibility (AMS) analyses to recognition of seismites. We assume that different sources of dike fill have different microfabrics, which manifest in different magnetic fabrics. To test this hypothesis we studied over 250 Holocene clastic dikes, exposed by deep incision into the lacustrine 70-15 ka Lisan Formation in the Dead Sea basin. Typically, they are vertical, up to 30-40 m high, and up to 0.4 m thick. In map view they appear in a radial arrangement spanning a sector of 60 degrees with projected strikes that converge at a structural dome above a rising salt diapir. Field relations and AMS analyses show that 'passive' dikes filled from above have vertical Kmin directions, compatible with sedimentary features. Conversely, horizontal to sub-horizontal Kmin directions occur in dikes that show segmentation typical of horizontal propagation of the fractures and lateral material transport. Vertical zoning of the clay and silt is interpreted as evidence of multiple injection events. Optical Stimulation Luminescence (OSL) ages, spanning 7-14 ka, are in agreement with dike truncation by undisturbed late Holocene alluvium. We conclude that the upward indentation of a salt diapir induced a local stress perturbation, which triggered the formation of a radial set of fractures. Subsequently the fractures were filled by horizontal injection of fine elastics pressurized by earthquake vibrations. Sedimentation from above occurred where the tops of the dikes reached the surface and remained open.