EFFECT OF CRUST-MANTLE BOUNDARY ON THE GEOMETRY OF DUCTILE SHEAR ZONES

Ductile shear zones act as the continuation of faults into the lower crust and upper mantle, and their structure depends in part on the rheology of these two regions. At a given temperature, olivine is much stronger than feldspar, leading to a strength contrast across the crust-mantle boundary. We use a two-dimensional viscoelastic steady-state model of a continental strike-slip fault zone to explore the effects of this transition on ductile shear zone structure. In the upper crust, deformation takes the form of localized slip on a fault described with rate-and-state friction. In the lower crust and upper mantle, distributed viscous flow occurs through two deformation mechanisms: grain-size sensitive diffusion creep and grain-size insensitive dislocation creep. Grain size is determined using either a wattmeter or a piezometer. This model makes it possible to self-consistently simulate the variations of strain rate, grain size, and stress in the vicinity of a strike-slip fault.

We compare monomineralic simulations (only feldspar or olivine) with layered simulations (feldspar over olivine). We define three aspects of shear zone structure: (1) the deformation zone, where 90% of the total deformation occurs (2) the kinematic shear zone, where viscous strain rate is elevated at least 100 times above the background strain rate; (3) the structural shear zone, where grain size less than 50 μm. In monomineralic simulations, the structural shear zone is a broad region (27±5 km wide) extending 20 ± 8 km below the brittle-ductile transition (BDT). In contrast, the kinematic shear zone is a narrow elliptical region extending 10 ± 2 km below the BDT with a width of 2.5 km at most. The deformation zone broadens approximately linearly with depth with a slope of 2.5. In monomineralic models, the structure of these zones is linked to stress enhancements at the tip of the frictional fault, which dramatically increase strain rate. In layered models, the uppermost mantle acts an additional source of high stress. As a result, layered models have narrower kinematic shear zones and broader structural shear zones. These results highlight that shear zone structure is strongly dependent not just on local conditions (rheology, composition) but also upon the rest of the lithospheric fault zone system. This work was funded by NSF-1807051 and EAR-1629356.