The relationship between fault zone structure and frictional heterogeneity, insight from faults in the High Zagros

Abstract

Frictional heterogeneities within fault zones depend strongly on lithology as well as shear fabric and strain localization structures. Here we investigate the impact of fault rock composition and shear fabric on friction constitutive properties for mature High Zagros (Iran) fault rocks. We present field observations along with results from friction experiments on intact and powdered fault rock from carbonate, quartzofeldspathic and schist. We measured frictional strength and rate/state dependence over normal stresses from 25 to 100 MPa and shear slip velocities from 3 to 300 μm/s. The sliding friction coefficients of intact fault rocks are lower than their powdered equivalents. The foliated quartzofeldspathic and lensoidal carbonate rocks from the Main Zagros Reverse Fault (MZRF) exhibit velocity strengthening friction and stable sliding. In contrast, the cataclasite carbonate rocks from the younger, Main Recent Fault (MRF) exhibit potentially unstable, velocity-weakening frictional behavior, consistent with regional seismicity. Our results suggest that solution seams, cleavage processes, and development of stylolite and calcite veins in the carbonate fault rocks of the MZRF continually rejuvenate and rework shear structures, which leads to dominantly aseismic shear. In contrast, younger fault rocks of the MRF form highly localized shear zones and shear fabric that result in velocity-weakening frictional behavior and seismic slip. Our field observations and laboratory data provide insights for using friction data and fault zone structure to build predictive models of the mode of fault slip in zones of tectonic collision.