Abstract This one-day field trip will examine Proterozoic pseudotachylyte and ultramylonite in the Homestake shear zone (HSZ) in the northeastern Sawatch Range of central Colorado. Our ongoing research and geologic mapping shows that the HSZ incorporates a 25-km-long, partitioned system of dip-slip mylonites and ultramylonites and strike-slip to oblique-slip pseudotachylytes and uniquely preserves details of earthquake rupture at the fault-system scale. The HSZ originated as a high-temperature structure during continental assembly at ~1.7 Ga, and was reactivated as a subvertical, transpressional system at ~1.4 Ga under lower temperatures in a mid-crustal, intracontinental setting. The shear zone was seismogenic in this later deformation cycle. The HSZ shows a lateral frictional-plastic strain gradient across a width of 3–4 km, from mylonite and ultramylonite with mutually crosscutting pseudotachylyte, to mylonitic and recrystallized pseudotachylyte, to a system of dispersed pseudotachylyte-bearing fault strands. The broad width of the shear zone and delocalization of seismogenic fault strands suggests the HSZ is an example of a strong-type seismogenic fault. This trip will examine outcrops demonstrating this frictional-plastic strain gradient and discuss implications for coeval plastic flow and earthquake rupture near the base of the seismogenic zone. We also review the petrology and geochemistry of pseudotachylytes as well as the depth and environment of their generation.