Complete Strain Record of a Highly Asymmetric Shear Zone: From Fault Core Gouges to Surface Rupture of Historical Earthquakes in the Alhama de Murcia Fault (SE Iberian Peninsula)

Abstract

Classical models of fault rock architecture point to a relatively simple and symmetric architecture of a fault zone, where the fault core is a narrow foliated cataclasite (few centimeters) bounded by a thick damage zone developed cutting through host rocks. Those models are far from the studied fault rocks developed in the Alhama de Murcia Fault (AMF), SE Spain, where fault core is in contact with an almost no deformed hosting rock at one side and to a wide damage zone towards the opposite boundary. The AMF is an active shear zone and the source of destructive recent and historical earthquakes. It has more than 10 km accumulated slip, and it develops a more than 100 meters wide shear zone with fault rocks that have been continuously sampled and analyzed combining a drill core from a 174 m deep vertical borehole, six trenches excavation, and outcrop surfaces cleaning. Hand specimen and microanalyses were used to classify the fault rock in deformation domains in a strongly heterogeneous shear zone according to its lithologies and structural features. It ranges from 10 to 30 meters wide fault core, where steady strain occurs, to an intensely deformed damage zone where strain is concentrated along discrete gouge bands. Trenching also showed a surface rupture that offsets Arabic archaeological remains related to the 1674 catastrophic event occurred in Lorca (Murcia). Steady homogeneous deformation was found in the areas closest to the hanging wall, in the fault core, where Lower Paleozoic Schists are the potolith of ultrafault gouges. As deformation increased, the shear zone involved Permian-Triasic basement rocks and Miocene sedimentary rocks in heterogeneous deformation domains. In the lower domains, strain is located in anastomosing shear bands which are spatially related with a surface seismic rupture of the 1674 destructive earthquake.