Three definitions of ductile shear zones resulting from grain-size evolution below a frictional fault

Abstract:

Ductile shear zones act as the continuation of faults into the lower crust, and their structure depends in part on the faults’ frictional properties in the brittle-ductile transition (BDT) region. We explore how fault and ductile shear zone structure are coupled in a two-dimensional steady-state model of a continental strike-slip fault zone. In the upper crust, deformation takes the form of localized slip on a fault described with rate-and-state friction. In the lower crust, represented with feldspar, 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.

The frictional properties of lower-crustal faults are not well-understood, and we focus specifically on the effects of various possible extrapolations of laboratory friction data to lower-crustal conditions. We also consider the effects of varying rheology and background geotherm. We define three aspects of shear zone structure: (1) the structural shear zone, the region in which grain size is significantly reduced (at or below 50 mm); (2) the kinematic shear zone, the region in which the viscous strain rate is elevated at least 100 times above the background strain rate; (3) the deformation zone, the region in which 90% of the total deformation occurs. In general, the structural shear zone is a broad region extending 20 ± 8 km below the BDT, with a halfwidth of 22 to 32 km. In contrast, the kinematic shear zone is a narrow elliptical region extending 10 ± 2 km below the BDT with a narrow halfwidth of 2.5 km at most. The deformation zone width broadens approximately linearly with depth with a slope of 2.5. The deformation zone is relatively insensitive to changes in frictional properties, geotherm, and rheology. The structural and kinematic shear zones, on the other hand, are substantially sensitive to these parameters. For example, a simulation with diffusion and dislocation creep operating in parallel and linearly increasing velocity-strengthening behavior below 20 km depth produces the narrowest structural shear zone but the widest kinematic shear zone.