Structures and microstructures of the Proto-Kern
Canyon fault (PKCF), a 130-km-long dextral strike-slip
shear zone of the southern Sierra Nevada batholith,
provide constraints on displacement, flow stress, and
strain rate during arc formation. Shear strain analyses
of S-C mylonites indicate ~5 km of ductile dextral slip
along the PKCF. But field mapping and measurements
of individual plutons and metamorphic pendants show
these bodies have much more elongated aspect ratios,
of up to 1:17, within the shear zone than outside of
it. This suggests significantly higher strain and dextral
slip of up to 15 km along the highest-strain zone
of the PKCF. Petrographic observations of high-strain
igneous rocks near Lake Isabella indicate that deformation
started at temperatures of 400-450° C and continued
through cooling to ~300° C. Based on ^(40)Ar/^(39)Ar
dating of hornblende, mica, and K-feldspar, early cooling
(~20° C/m.y.) from 88-70 Ma was followed by very
slow cooling (~1° C/m.y.). These data, combined with
cross-cutting relationships, suggest that dextral ductile
shear was active from 90-86 Ma. Grain sizes of dynamically
recrystallized pure quartz mylonites in this part
of the shear zone were used to estimate flow stresses of
20-40 MPa. Applying mylonitization temperature estimates
of 400-350° C and lithostatic pressures of 350-
400 MPa (from Al-in-hbl barometry) yields paleo-strain
rates along the PKCF of 10^(-13)-10^(-15) /s. Additional
quartzite piezometry, as well as calcite piezometry on
marble mylonites, should provide further constraints on
stress and strain rates along the length and depth exposures
of this intrabatholithic shear zone
