LOCALIZATION OF SHEAR ALONG A COMPOSITIONAL DISCONTINUITY: THE PROTO-KERN CANYON FAULT, SIERRA NEVADA, CALIFORNIA

Detailed field and petrologic investigations of an intra-arc, crustal-scale shear zone in the southern Sierra Nevada show that in a zone of high temperature and lithologic heterogeneity, lithologic contrast plays the greater role in localizing deformation. The shear zone – the Proto-Kern Canyon Fault – was active from ca. 100 – 80 Ma at the contact between subsolidus plutons and actively intruding magma, rather than along the axis of the youngest and therefore hottest intrusions. Metamorphic pendant rocks pinned at the contact between neighboring intrusions bear key evidence of compositionally-controlled deformation localization: Marbles and pelites are pervasively deformed with crystal plastic fabrics, while impure and pure quartzites and amphibolites are domainally sheared with vestigial lenses preserving earlier deformation, metamorphism, and primary features. Early intrusives (100 Ma) bounding the western edge of the metasediments are only rarely domainally sheared, while east-bounding intrusives (85-80 Ma) are pervasively mylonitic for up to 1 km, with the youngest phases exhibiting syn-magmatic shear fabrics. The sandwiched pendant rocks accommodated the deviatoric stress of the 0.5 - 3 kilometer-wide shear zone, their petrologic discontinuity acting as a stress buffer at the time of initiation of crustal failure.

In the Sierra Nevada batholith, discrete northwest-trending shear zones have been proposed to link together to form the Sierra Crest Shear System. Previous work has suggested that in the northern region of this shear system, movement was continuous until deformation ceased in the batholith (Tikoff, 1992). In the southern extent of the batholith, the shear zone is best expressed in the contact zone between two discrete intrusive stages, suggesting that if deformation was indeed continuous, it was continuous in this contact zone, a zone of lithologic heterogeneity, rather than in the axis of the most recent pluton disrupted by the shear zone, where temperatures would have been highest. This investigation concentrates on the edge geometry of a plutonic series, as well as the internal geometry of a ductile shear zone.