QUANTIFICATION OF STRAIN RELATED TO INTERSEISMIC DEFORMATION ALONG SUBDUCTION INTERFACES USING CHEMICAL MASS-BALANCE ANALYSIS

Exhumed subduction mélanges preserve a record of deformation and fluid-rock interaction under seismogenic pressure-temperature conditions (150-350˚C). Such rocks have the potential to reveal the processes at the grain scale that control the complex plate boundary slip behaviors of active boundaries. Diffusive solute transfer down potential gradients is inferred as an important aseismic deformation mechanism along the subduction interface. The magnitude of diffusive mass transfer strain, likely in the interseismic period, is difficult to quantify because the magnitudes are large and spatially heterogeneous and geometric strain markers are rare; consequently, we explore the feasibility of a chemical approach. Field observations from the Shimanto belt and the Kodiak accretionary complex demonstrate that underthrusting of rocks along the subduction interfaces is characterized by veining of sandstone blocks in conjunction with development of scaly fabric in mudstones that records noncoaxial strain and dissolution. Microprobe maps show that scaly fabrics are depleted of fluid-mobile elements (e.g., Li, Sr, K, Ca), and enriched in fluid-immobile elements (e.g., Ti, HFSE, HREE), whereas veins show an opposite pattern. Here, we present results from a comprehensive geochemical characterization of the scaly microfaults and wall-rocks of the mélange samples collected from the Shimanto belt and the Kodiak accretionary complex using LA-ICPMS. Isocon analysis shows that Ti is a suitable chemical reference frame for estimates of bulk mass change and volume strain. Using Ti as the reference element, our calculations indicate a large difference in average mass loss in scaly folia between the mélanges that have experienced the paleotemperatures corresponding to the updip and downdip limit of seismogenic zones. The result suggests that the strain related to diffusive mass transfer varies as a function of temperature and can be quantified through geochemical mapping of scaly fabric domains. Given typical plate tectonic convergence rates and the thermal structure of active subduction zones, underthrusting along the plate interface is limited in time, so strain measurements allow estimates of the maximum interseismic diffusive mass transfer strain rate along ancient plate boundaries.