Lithification processes in subduction zones and their influence on the structural style of accretionary prisms

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Megathrust earthquakes with moment magnitudes Mw>9 occur mainly on accretionary margins such as the Sumatra event in Dec. 2004 (Indonesia) which had devastating consequences. The catastrophic slip along the plate interface during earthquakes initiates in the so-called seismogenic zone. The cause for the onset of unstable slip on the subduction thrust is still subject to debate, because the seismogenic zone of an active megathrust zone has not been sampled so far. One of the most accepted hypotheses for the onset of earthquake nucleaction is the lithification of the accretionary prism at depth. Lithification also affects the structural style of the prism and causes a decrease in taper angle, which is supposed to be linked to the occurrence of the seismogenic zone. A suite of diagenetic to low-temperature (T) metamorphic reactions is believed to cause the lithification but the identity and spatio-termporal distribution of these processes in active accretionary prisms is largely unknown. The proposed study aims to identify key diagenetic to low-T metamorphic processes in the active Nankai accretionary prism. For the research a unique sample set exits that comprise 1) samples from the Integrated Ocean Drilling Program (IODP) up to 2 km depth below the seafloor, 2) clastic ejecta from mud volcanoes that originate from the interior of the inner wedge recovered in summer 2012 with R/V Sonne and 3) onshore samples from the Shimanto Belt (Japan), a fossil accretionary prism in the hinterland of the accretionary prism. The samples experienced temperatures up to the downdip limit of the seismogenic zone and cover the different tectonic segments of pre-subduction, outer and inner wedge. A comprehensive suite of petrograhic analysis is proposed to identify and characterize the diagenetic to low-T metamorphic processes that cause lithifiation. The research is complemented with laboratory physical property measurements to assess the impact of lithification on mechanical and hydrogeological properties at depth. Utilization of logging-while-drillling data will constrain the spatio-temporal occurrence of the lithification processes in the wedge. Coulomb wedge analysis will be employed to assess the implications of the processes on the structural style of the subduction zone forearc and the occurrence of seismogenesis.