MICROROUGHNESS AND FRICTIONAL POWER DISSIPATION ON PSEUDOTACHYLYTE-BEARING FAULT SURFACES

Experimental simulations of earthquake conditions reveal a notable decrease in friction during slip. Unlike static friction, the steady-state dynamic friction coefficient has a clear dependence on applied normal stress. In the case of melt generation it has been hypothesized that the steady state represents a balance between melt generation, extrusion, and surface roughness. In particular, the amplitude of fault surface microroughness should decrease with increasing normal stress. As a consequence, the melt-wall rock boundary should be smoother in contractional than extensional bends. We have therefore undertaken a study of a natural (solidified) melt-bearing fault zone to explore the relationship between microroughness and inferred fault-normal stress during seismogenic slip.

The Gole Larghe Fault Zone, Italy, contains many pseudotachlyte-bearing faults with 0-20 m of slip. We have examined the 3D geometry of a single fault with 30-100 cm of slip. At the outcrop scale, this fault is distinctly wavy with contractional and extensional bends as well as relatively straight sections. We quantified the micro-scale (< mm - cm) roughness for 6 samples from a range of positions along the fault. Intact sample cores (2-3.5 cm diameter, 4-6 cm length) have been imaged using high-resolution x-ray computed tomography (CT). The surfaces of the fault zone were extracted from the CT volume and the microroughness of these surfaces was quantified using a Fourier spectral and spatial analysis.

Fault surfaces from all samples are smoother at the mm-scale in the slip parallel than slip perpendicular direction, an effect we attribute to frictional wear. Smoothing is least for extensional fault sections and greater for neutral and contractional sections. Many surfaces also exhibit recessed biotite grains, an effect we attribute to preferential melting. This effect is again greatest for neutral and contractional fault sections. Samples from natural faults thus show an evolution of microroughness in response to changing geometry (and thus fault-normal stress and frictional power) along a single fault. Quantification of microroughness will eventually allow for estimation of frictional power dissipated during earthquakes, a parameter that is unavailable from seismology.