INVESTIGATION OF THE DAMAGE ZONE AROUND A MEGASPLAY FAULT, WITH IMPLICATIONS FOR FLUID FLOW AND SEISMIC ACTIVITY

Deformation on any given fault not only affects the main fault surface (e.g. the megasplay surface observed and cored at site C0010, Nankai Accretionary Prism, Japan) but an area of damage on either side of the fault surface. Microstructures within damage zones range from pressure solution features, and healed and un-healed fractures, to alignment of platy minerals perpendicular to the imposed stress, all of which can be observed in thin-section.

This study presents preliminary results from coring at site C0010, part of IODP Expedition 365 and the NanTroSEIZE program, aiming to assess the nature of the damage zone around the megasplay fault surface, and relate that damage to distance from the fault, lithological variation (that is, clay content) or both. Core samples were taken every 10m away from the fault surface, within the seismically defined thrust wedge, based on work by the Expedition 365 science party. Thin sections were made and each section was analyzed for microscale evidence of damage, including microfractures and disaggregation structures, whether by grain boundary fracturing or transgranular fracturing. The number of microfractures per length of the field of view, or percentage disaggregation per field of view were recorded for each thin section (that is, the damage intensity). Lastly, samples were analysed for bulk rock chemistry and then in detail for clay content and clay types by a commercial laboratory.

Results indicate that a significant portion of the thrust wedge at site C0010 is affected by microscale damage, with demonstrable decrease in damage away from the megasplay zone. Damage intensity is in turn affected by illite/smectite content; where there is a high smectite content, the microfractures are more prone to healing than where bulk clay content and relative smectite content is low. These results have implications for the behavior of faults as baffles or as conduits, and potentially for the seismogenic nature of megasplay structures.