Dating and geochemical tracing of paleoseismic events

Abstract

Dating of shallow faults is crucial to understanding of the mechanism of earthquake generation and future seismic risk. However, radiometric dating of low temperature authigenic minerals in shallow fault zones has been a major challenge because of the contamination of fault rocks by older mineral phases and the overprinting of the syn-tectonic isotopic signature by post-tectonic re-crystallisation events. We present a technique for a reliable dating of fault movements using a combined application of low temperature geochronology, mineralogy and isotope-trace element geochemistry. Such combined studies further allow constraining the origin and migration of seismically mobilised fluids along active fault zones. We investigated illitic clay minerals from fault gouges and late Quaternary carbonate deposits in co-seismic fissures along currently active fault zones in Turkey. Our results indicate that the North Anatolian Fault Zone has been active since the Late Paleocene ? Early Eocene that followed immediately the continental collision related to the closure of the Neotethys Ocean. Fault movement is considered to have driven deeply sourced fluids, with metamorphic fluids being supplied by the compression along the Neotethyan orogenic suture zone. Precise dating of the late Quaternary carbonate deposits by U-series geochronology provide important constraints on the late Quaternary fissure generation related to the neotectonic processes and active faulting. Isotopic and trace element data indicate that the fissure carbonate deposits precipitated from deeply hydrothermal fluids. Mobilisation of deep fluids and their surface effusion is attributed to tectonic processes such as seismic pumping. K-Ar and U-series dating of clay and carbonate minerals respectively in seismically active areas is very promising for further studies for dating of major earthquake events and their recurrent intervals, with significant implications for the Australian active fault systems.