Stress, strain, and B‐type olivine fabric in the fore‐arc mantle: Sensitivity tests using high‐resolution steady‐state subduction zone models

Abstract

Shear‐wave splitting observations at several subduction systems show trench‐parallel fast directions in the fore‐arc mantle. The presence of B‐type olivine fabric in the mantle wedge may provide an explanation for this pattern of anisotropy under low‐temperature and hydrated conditions. Sensitivity tests are shown that provide insights into the distribution and magnitude of B‐type fabric using two‐dimensional, high‐resolution, kinematic‐dynamic subduction zone models. These models include a wet olivine rheology and a simulated aseismic creep zone in the fore‐arc mantle with parameterized viscous coupling between the slab and mantle wedge. The calculated thermal structure is in reasonable agreement with heat flow observations and Q tomography for small amounts of coupling and moderate amounts of shear heating in the seismogenic and aseismic creep zones. These small amounts of viscous coupling give rise to a slowly flowing fore‐arc mantle with thermal, stress, and strain conditions suitable for significant B‐type fabric development. The minimum shear stress necessary to drive geologically significant flow in the predicted B‐type region is sensitive to the magnitude of shear heating along the aseismic creep zone and ranges from less than 100 to 300 MPa. The minimum time required to generate sufficient finite strain for fabric development in the B‐type region is around 10 Myr depending on the amount of coupling and shear heating. The predicted distribution of olivine fabric is consistent with a pattern of shear wave anisotropy with trench‐parallel fast polarization directions above the fore arc and a rapid rotation to trench‐perpendicular in the arc and back arc.