A PRESERVED RECORD OF FAULT BARRIERS FROM PSEUDOTACHYLYTE BEARING FAULTS IN THE CARTER LAKE PSEUDOTACHYLYTE SYSTEM, NORTHERN SAWATCH RANGE, CENTRAL COLORADO

A newly discovered system of pseudotachylyte bearing faults has been mapped in the northern Sawatch Range of central Colorado. It is here in referred to as the Carter Lake pseudotachylyte system. The fault zone strikes roughly east-west (~240-260°) and dips steeply to the north (~70° N) and exhibits a right lateral strike-slip offset. The pseudotachylyte bearing faults appear to cut through intact basement rock, including Paleoproterozoic migmatites, gneisses and schists, and Paleoproterozoic and Mesoproterozoic granitic igneous rocks of the Holy Cross City and St. Kevin(?) granites. The pseudotachylytes are black to brown in color, weathering to a rusty or light gray color, and include survivor clasts of quartz, breccias zones, and melt pods or reservoirs. Average vein thicknesses range from 0.2-0.3 cm to 1.0-1.5 cm but can locally reach thicknesses of 5-6 cm.

Detailed outcrop-scale mapping of selected outcrops has revealed that the pseudotachylyte bearing faults will locally deviate from the general east-west trend. Careful study suggests that these abrupt changes in fault geometries are related to heterogeneities in the preexisting metamorphic fabrics, and that these heterogeneities acted as barriers to either arrest or redirect fault propagation. In particular, folds in the metamorphic fabric can produce changes in the strike of faults of up to 40-50° or cause the faults to splay and die-out within several meters.