Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

of melts generated by frictional heating along brittle fault zones (Sibson, 1975). Flinty
crush rock has also been interpreted to be the result of emplacement of a fluidized gasliquid-solid
system (Reynolds, 1956; Roberts, 1966; and Taubenack, 1967). Shannon
(1988) interpreted FCR as the product of injection of phyric magma into the ring zone
during shearing. It is considered to be a hybrid mixture of magmatic material and
ultracataclastic material that was mobilized and injected into the ring zone and
surrounding wallrocks. FCR may contain a component of pseudotachylite but any
frictionally derived melts were overshadowed by phyric magma injected along the ring
zone.
Trs Mount Aetna ring shears (late Eocene to early Oligocene) – Deformation in
the Mount Aetna cauldron ring zone is characterized by both brittle and ductile fabrics
developed as a braided network of shears and faults which outline the main and northern
collapse structures (Shannon, 1988). The Mount Aetna ring shear zones (Trs) consist of
shear bands that megascopically and microscopically exhibit evidence of ductile
deformation resulting in development of protomylonite to orthomylonite fabrics and rare
ultramylonite fabrics. The ductile shear bands are surrounded by marginal zones of
deformed wallrock that behaved predominantly in a brittle manner. The brittle
deformation is characterized by megascopic and microscopic fractures that offset grain
boundaries and plagioclase twin lamellae. Other brittle deformation features include
seams of microbreccia that are irregularly developed along shear bands. On a
microscopic scale, rocks in the shear zones behaved inhomogeneously. The feldspars
typically deform in a brittle manner, while quartz and mafic minerals show evidence of
ductile flow and varying degrees of recrystallization. Local cross-cutting relationships
between ductile and brittle deformation features suggest complex alternation of
deformation mechanisms probably related to significant changes in strain rate in the
dynamic ring zone during cauldron collapse (Shannon, 1988).
The Mount Aetna ring shears cut the Mount Princeton pluton and Proterozoic
county rocks, both inside (cauldron block) and outside (extracauldron wall rocks) the ring
zone. Mount Aetna ring shears are present in a concentrated float zone between the two
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