Rheology of Metasedimentary Rocks at the Base of the Subduction Seismogenic Zone

Abstract

AbstractThe Arosa Zone, Central Alps, represents a Cretaceous to Paleogene fossil subduction interface from paleodepths of 10 to >35 km. Metasedimentary schists directly below a preserved seismogenic zone exhibit viscous (foliations, folds, and lineations) and brittle (crack‐seal quartz ± calcite veins) deformation features. Si‐in‐phengite barometry and published temperature estimates yield deformation and metamorphism conditions of 0.8–0.9 GPa and 300–350°C for an albite‐rich pelitic schist and 0.9 GPa and 420°C for a quartz and calcite‐rich calcareous schist. Petrography, microprobe element maps, and electron backscattered diffraction data constrain the modal mineralogies, microtextures, and active deformation mechanisms of these schists. There is evidence of minor crystal plasticity of quartz in both rocks and calcite in the calcareous schist. However, the most prominent deformation microstructures formed during pressure solution creep of quartz and albite. Pressure solution creep was activated due to pinning by ubiquitous fine grained phengite. Rheologic modeling indicates that pressure solution of quartz and albite can accommodate tectonic strain rates at stresses of <30 of MPa and is weaker than dislocation creep at these same conditions; results that are consistent with our microstructural observations. However, to accommodate slow slip strain rates by either pressure solution or dislocation creep, stresses would need to be >>100 MPa. Triple oxygen isotope data from veins in these rocks combined with published data suggest a metamorphic mafic source for these vein filling and pressure solution accommodating fluids, indicating that dehydration from subducting oceanic lithosphere likely provided the fluids that facilitated viscous deformation in these rocks.