A multi-kilometer pseudotachylyte system as an exhumed record of earthquake rupture geometry at hypocentral depths (Colorado, USA)

Abstract

A system of pseudotachylyte-bearing fault zones preserved along the Proterozoic Homestake shear zone in the southern Rocky Mountains provides an avenue for investigating earthquake processes at the hypocenter. The results of detailed field mapping suggest that pseudotachylyte may serve as a dynamic indicator of rupture directivity and yield general estimates of some earthquake source parameters when examined at the multi-kilometer, fault-system scale. Pseudotachylyte fault veins are primarily exposed within eight NE-striking, sub-vertical fault zones that have a cumulative length of more than 21 km. The fault zones are mapped for 7.3 km along strike and fan to the northeast from a 170-m-wide outcrop belt to a maximum cross-strike width of 2.3 km. Pre-existing structural control on rupture geometry is indicated by concordance between foliation and fault veins, as well as spatial coincidence between the limbs of map-scale, rootless isoclinal folds and the location of most fault zones. The central portion of the longest fault zone exhibits evidence for dextral oblique slip that involved more than 2.1 m of strike-slip offset between four parallel fault veins that are interpreted to have formed in response to a single rupture event. In addition, an along-strike continuity and systematic distribution of fault zones, a progressive northeastward decrease in pseudotachylyte volume and maximum vein thickness, and a relative scarcity of cross-cutting relationships further suggests that the majority of the frictional melt in the system may have developed in response to one (or several) multi-kilometer ruptures, as opposed to hundreds of shorter ruptures. The similarity of kilometer-scale relationships observed along the Homestake pseudotachylyte system with the subsurface slip distribution and surface geometry of present-day, strike-slip earthquakes is interpreted to indicate that frictional melting occurred within a concentrated zone of moment release or an earthquake hypocenter during one or more M w ≥ 6.3 earthquakes ( M 0 ≥ 6.9 × 10 25 dyn cm) that involved northeastward rupture propagation.