ESTIMATES OF CHEMICAL MASS TRANSFER IN ANCIENT ACCRETIONARY COMPLEXES: IMPLICATION FOR STRAIN ASSOCIATED WITH INTERSEISMIC DEFORMATION ALONG THE PLATE INTERFACE

Deformation recorded in exhumed accretionary complexes has the potential to reveal the amount of strain accumulating along the subduction interface. Field studies in the Kodiak accretionary complex and the Shimanto belt characterize two distinct structural domains- a 10-100 m wide of tectonic mélange zone and a narrow zone of cataclasite at the top of the mélange. It is likely that the mélange zone accumulates non-coaxial strain accommodated by diffusive mass transfer during the interseismic period. Fluid-mobile elements dissolved along microfaults in mudstones of the mélange and reprecipitated in tensile cracks of sandstones as mineral veins, leaving fluid-immobile elements concentrated along pressure solutions seams to form an anastomosing scaly fabric in mudstones. The magnitude of diffusive mass transfer can be estimated by a mass balance approach, which provides us an alternative method to quantify strain in accretionary complexes where geometric strain markers are scarce. In this study, we first characterized the compositions of the three phases- scaly fabrics, wall-rocks, and mineral veins in the mélange samples collected from the Kodiak accretionary complex and the Shimanto belt using LA-ICPMS. We then used the immobile element, Ti, and the mobile element, Si, as chemical reference frames respectively to determine the volume loss in scaly fabrics and volume gain in mineral veins. The comparison between the two values allows us to determine whether the dissolution along scaly microfaults serves as a sole source for the precipitation in cracks of sandstones. By integrating the amount of volume loss along individual slip surfaces across the network using image analysis, we were able to determine the total volume strain within an area of interest. We specifically targeted veins precipitated in opening bends of curved slip surfaces and determined whether the amount of slip, i.e., shear strain, correlates with volumetric strain. Our application of this geochemical approach demonstrates the potential to fully quantify the strain related to underthrusting along the subduction interface, with important implications for the factors that control plate boundary slip behavior along in active subduction systems.