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.

Abstract The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km 3 ) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.