Paleo- stress and strain rates in an intra-arc strike-slip fault, Sierra Nevada, California

Abstract

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.

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