LOCALIZATION OF PSEUDOTACHYLYTE IN MAFIC-INTERMEDIATE DIKES DURING MESOPROTEROZOIC REACTIVATION OF THE HOMESTAKE SHEAR ZONE, COLORADO

The Homestake shear zone (HSZ) is a subvertical structure that originated during Paleoproterozoic continental assembly and reactivated under lower temperatures during Mesoproterozoic intracratonic transpression in a mid-crustal environment. Reactivation was recorded by development of a characteristic assemblage of tectonites, including mylonite, ultramylonite, and a 25-km-long pseudotachylyte system. Most pseudotachylytes are clustered into 5- to 20-m-wide fault zones that are concordant to a predominantly NE-striking composite foliation in migmatitic biotite gneisses. In several locations outside of these fault zones, fault rocks with an outcrop form characteristic of pseudotachylyte are present within 0.3- to 1-m-thick melanocratic dikes of mafic to intermediate composition (52.8-59.6% SiO₂; 2.1-4.7% Na₂O+K₂O). The dikes are concordant to the HSZ fabric and predominantly consist of fine-grained biotite and calcic plagioclase, with discontinuous folded bands of quartz and alkali feldspar derived from the surrounding migmatitic gneiss. The inferred pseudotachylyte is exposed in mm- to cm-width fault veins exhibiting sharp boundaries with the host dike, branching injection veins, and rough-weathering, mm-scale clusters of the biotite-rich dike suspended within a very fine-grained crystalline matrix. In thin section and electron microprobe BSE images, the matrix appears as a much finer-grained variety of the host dike and includes biotite and finely recrystallized quartz. Whole-rock XRF data show the matrix and host dike are similar, with a slight enrichment in CaO, Ba, and Sr in the matrix. In one dike, the inferred pseudotachylyte can be traced for more than 100 m along strike; local fault splays and injection veins penetrate into the adjacent migmatic gneiss. If the fault rock is indeed pseudotachylyte generated by friction-induced melting during seismic slip, these observations indicate that decimeter- to meter-scale dikes can localize earthquake ruptures.