Rheology and strain partitioning at the base of the subduction seismogenic zone: A case study from the Alps

In subduction zones, the depth of the transition from seismogenic slip to aseismic creep determines the landward extent of earthquakes and the updip extent of deep slow slip and tremor. However, the compositional, structural, and thermal controls on the depth of this transition are not as well-constrained as they are for faults in continental crust, and challenges exist in quantifying them because these properties vary within and between subduction zones. Determining the causes of the seismogenic to aseismic transition, slow slip, and tremor requires knowing which lithologies partition strain over the spectrum of strain rates that include interseismic, creep, and seismic. Here we investigate a subduction plate boundary fault exhumed from 25 km depth in the central Alps. Exposed rocks form a heterogeneous 1000 m thick tabular fault zone across which subduction deformation was accommodated. Deformation occurred at conditions slightly deeper than the extent of subduction-related pseudotachylyte reported in the region, consistent with deformation just below the seismogenic to aseismic transition. Mafic and ultramafic units include metabasalt, serpentinite, and chlorite and talc schists. Metasedimentary units include mica schists and carbonates. We summarize the current knowledge of deformation mechanisms and rheology as a function of strain rate in each of these lithologies, and reconcile them with the rock microstructures. Preliminary calculations show that while at low (creep) strain rates and hydrostatic fluid pressure, diffusive mass transfer of carbonates and quartz can accommodate a significant proportion of the total strain, at slow slip to seismic strain rates and high pore fluid pressures, frictional deformation of the talc and chlorite schists accommodate almost all strain. These results are based on very limited mechanical data and we discuss the lithologies and strain rates for which rheologic constraints are needed to better determine deformation partitioning.