DEFORMATION PARTITIONING ALONG AN IDEALIZED SUBDUCTION PLATE BOUNDARY AT DEEP SLOW SLIP CONDITIONS

Near the base of many subduction seismogenic zones, the plate interface periodically slips at rates 1-2 orders of magnitude faster than plate velocities. A number of competing hypotheses for the mechanisms of this slow slip exist, but testing them requires that we know how deformation is partitioned across the lithologically complex plate boundary interface. We use the Arosa zone, a ~520 m thick exhumed subduction interface, as a case study to evaluate one potential distribution of lithologies at the base of the seismogenic zone and the partitioning of strain amongst these lithologies throughout the slow slip cycle.

In this presentation, we synthesize and discuss published constitutive relations as a function of deformation rate for 5 primary lithologic units. We use the laboratory-derived relations to predict (1) the shear stress across the plate boundary as a function of slip velocity and (2) the partitioning of deformation among the different lithologic units for slow slip and aseismic creep velocities. This analysis is conducted for pore fluid pressures from hydrostatic to near-lithostatic. Our results show that, at pore fluid pressure close to hydrostatic, aseismic creep and slow slip velocities occur by viscous deformation of calcareous and quartzose units. However, once the pore fluid pressure increases above 80% of lithostatic, plate boundary slip migrates from the calcareous and quartzose rocks during aseismic creep to frictional deformation of phyllosilicate-rich phases, particularly talc schist, during slow slip. To a first order, this result is insensitive to variations in the thicknesses of metasedimentary units that may be present along subduction plate boundaries.