Keywords:Subduction zone processes, Shimanto Belt , Silica redistribution, Oxygen isotope thermometry
Silica redistribution along plate interfaces is a fault zone aging process likely to affect the slip behavior of subduction interfaces. Numerical modeling for slip according to the rate law based on silica kinetics reproduces the characteristics of real-world subduction zones, such as an updip limit to the seismogenic zone, earthquake clustering, and a power-law size distribution of earthquake magnitudes. In the field, the structure of subduction fault zones is typically marked by veined sandstone blocks within mudstones with scaly fabric, with mudstones providing a local source of silica and other elements that precipitate as minerals veins, which act as a sink for mobile elements. The silica kinetics related to diffusion from shearing mudstones to cracking sandstone blocks in mélange provides a temperature dependent mechanism for fault zone aging whereby the contact area across the fault zone increases in the time between slip instabilities. In the Shimanto belt of Japan, scaly fabric and vein microstructures show textural variability as a function of temperature. There is a gradient in the age of paleosubducting oceanic crust from north to south within the inner Shimanto belt, from the Yokonami mélange, with a thick pelagic chert section and older basalt and younger mélanges in the south (Okitsu, Hyuga, and Mugi) with little or no chert section, suggesting younger subducting crust. Thus, the mélanges of the Shimanto belt were accreted at different temperatures and depths along the interface, and there also may have been differences in the subduction geotherm. We suggest that these variations in temperature are likely to lead to different silica kinetics and rates of mineral redistribution, recorded in microstructural variations that may reflect different frictional properties along the plate interface. The slip behavior of the plate interface depends on temperature as evidenced by down dip variations in slip mode and differences in earthquake size distributions for subduction zones with different age crust. We therefore have undertaken a study of oxygen isotope thermometry using coexisting mineral pairs of quartz-albite-calcite observed in the tectonic mélanges from the Shimanto belt at a variety of estimated temperatures. This data provides an independent method for evaluating the temperature during mineral redistribution that can be compared with current estimates based on vitrinite reflectance and fluid inclusion analyses.