THE DAMAGE IS DONE: LOW FAULT FRICTION RECORDED IN THE JAPAN TRENCH DéCOLLEMENT DAMAGE ZONE

Structures that define fault damage zones form when the stresses in the volume of rock surrounding a fault exceed the strength of the rock. They form throughout the deformation on the fault, in response to processes associated with fault propagation, linkage, and repeated slip. As off fault stresses are in part dependent on fault friction, fracture attitudes and kinematics should be useful for constraining friction. We used drill core recovered during Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project (JFAST)) to characterize the damage zone close to the plate boundary fault. Ten styles of induced structures were used to develop a new set of criteria for identifying induced structures in drill cores containing fine-grained sedimentary rocks, and distinguishing them from tectonic structures. Damage zone structure density has a maximum adjacent to the fault. Core- and micro-scale structures decrease in density sharply with distance away from the fault. The total damage zone width is interpreted to be of the order of tens of metres. Additionally, tectonic structure density locally increases where cores contain faults with significant displacements inferred from stratigraphic information indicating that tectonic structure density can be used to identify secondary faults. At all distances from the fault, core-scale structures dip between ~10° and ~70° and are mutually crosscutting. Compared to similar displacement faults, the fault in this study has an anomolously narrow damage zone. Motivated by independent constraints demonstrating that the plate boundary is weak, we tested if the observed damage zone characteristics could be consistent with low-friction fault. Quasi-static models of off-fault stresses and deformation due to slip on a wavy, frictional fault under conditions similar to the JFAST site predict that low-friction fault produces narrow damage zones with no preferred orientations of structures. These results are consistent with long-term frictional weakness on the décollement at the JFAST site.