USING FRICTIONAL MELT TO CALCULATE THE RELATIVE SLIP VELOCITY IN AN ANCIENT FAULT SYSTEM, HOMESTAKE SHEAR ZONE, COLORADO

Past studies indicate that pseudotachylyte can be used to determine the relative seismic slip velocity on faults. We test these models in the Homestake shear zone (HSZ), Colorado. The HSZ is a 25-km-long, multi-fault pseudotachylyte system hosted by amphibolite-facies, semi-pelitic gneiss and schist. In the HSZ, pseudotachylyte occurs in interlayered semipelitic gneiss and schist as dextral and subordinate sinistral strike-slip faults that are typically parallel to foliation but locally crosscut fabric. Rock deformation experiments show that rock is weaker along foliation and thus stronger oblique to foliation; this suggests there may be differences in slip velocity between the two orientations. We sampled pseudotachylyte from both structural domains to estimate slip velocity. A published study proposed that the relationship between slip velocity and frictional melt can be given as ω/ω₀ ≈ (1/a), where ω is defined as half of the average thickness of the vein, ω₀ isdefined as the RMS (root mean square) of the distribution and the radius of spheres fitting individual asperities quantifying vein topography, and ais a characteristic velocity for a given rock type, which can be found in a laboratory setting and is a material characteristic. Adobe Illustrator and an online RMS calculator was used to quantify the ω and ω₀ values from polished slabs, thin sections, and outcrop photos. The velocity of fault slip can be relatively determined by the ω/ω₀ratio. Another model using the angle of injection veins relative to fault veins concludes that the higher the angle from the fault vein, the faster the rupture. The angle of injection veins are often steep for higher ω:ω₀ratios, while lower ω:ω₀ ratios often have low-angle injections. The average ω:ω₀ ratio for foliation-parallel samples at HSZ is 8.55 +/- 2.21. When this method was repeated at the outcrop scale, in an area where frictional melts crosscut foliation, an average ω:ω₀ ratio yielded 4.25 +/- 7.46. The results provide preliminary evidence for the thesis that concordant samples of frictional melt form at a higher slip velocity, while discordant samples form at a lower slip rate. This also makes apparent the variance in slip velocities that may occur in one given fault system at depth.