Field-Based Analysis of Subduction Petrofabrics

This project, carried out by a team of researchers from the University of Minnesota, determines the high-pressure petrofabrics of an exhumed subduction complex characterized by pervasive, syn-kinematic high-pressure assemblages preserved in metasedimentary (marble, quartzite) and metabasaltic (eclogite, blueschist) rocks. Field mapping in the Sivrihisar Massif, Turkey, coupled with microstructural, petrologic, and stable isotope analyses are used to determine large-scale flow, deformation mechanisms, flow laws, and fluid-rock interaction at a range of depths in a subduction zone (40-75 km). Microstructural techniques include electron back-scattered diffraction (EBSD) to determine crystallographic orientation of the rheology-controlling phases clinopyroxene in eclogite, (fibrous) calcite in marble, and quartz in quartzite; field and petrographic analysis of kinematics, including determination of kinematic vorticity using lawsonite shape and orientation; and determination of sense of shear. Petrologic investigation includes microprobe analysis of mineral compositions and zoning, and evaluation of factors influencing the degree of preservation of high-pressure assemblages at different field sites in the massif. The extent, conditions, and pathways of fluid-rock interaction is assessed via oxygen isotope analyses, including mineral-whole rock analyses and in situ ion probe analysis of compositionally and texturally zoned grains from blueschist-eclogite transition zones.

The deformation behavior of subducted crust strongly influences the geophysical and geochemical evolution of convergent plate boundaries, including the interaction of deformation, metamorphism, and fluid-rock reaction during subduction and return of deeply buried rocks to the Earth's surface, and the rates of subduction processes. This research addresses questions of the mechanisms of large-magnitude transport of high-pressure rocks in subduction zones by characterizing deformation characteristics from the map scale to the grain-scale using high-pressure minerals. The Sivrihisar Massif contains the one of the world's best-preserved and most extensive outcropping of lawsonite eclogite, a rock type predicted to be common at depth in subduction zones, but rarely exposed at the surface owing to the ease with which lawsonite is destroyed during exhumation. There have been no previous microstructural studies of this important rock type, which may be the dominant lithology in subducted oceanic crust between 45-300 km.