Fault Zone Processes Recorded within Ancient Subduction Plate Boundaries

Abstract:

We analyze the microstructures of nine ancient plate boundary faults from two subduction complexes: the Kodiak accretionary complex in Alaska and the Shimanto Belt of Japan. Based on work by many researchers on fluid inclusion analyses and vitrinite reflectance, activity on these fault zones likely spans the inferred temperature range associated with the seismogenic zone from 150˚C to 350˚C. Fault zones record deformation associated with a compactive strain path, with shear largely occurring on anastomosing arrays of scaly slip surfaces. Several of the fault zones include occurrences of pseudotachylite. One of the features common to all the examples is the pervasive evidence for mineralized hydrofractures, with a general transition from calcite to quartz with increasing depth. Veins are associated with simple shear, in some cases with evidence for recurrent events of simple shear. Microstructures indicate mode I cracking, with anistropic crystal growth related to competition within open fractures. There is also evidence in many of the cases for repeated cracking and sealing. Silica depletion zones and dissolution fabrics adjacent to veins are consistent with local redistribution of silica/calcite, and models for the kinetics of fracture filling are consistent with sealing of the fracture network within secular time scales. Some microstructures indicate only partial sealing between fracturing events. Finally, the orientation of veins at high angles to the shear zone is consistent with hydrofracturing adjacent to a weak fault, possibly due to rotation of stresses after slip events. These results emphasize the need to consider the feedbacks between fluid sources, hydrofractures, silica redistribution and crack sealing in the evolution of subduction megathrusts. We propose that there is a state variable related to the fracture porosity or thickness of the hydrofractured zone that varies over the seismic cycle and leads to a range of fault slip behavior.