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

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