THE ROLE OF A CRUSTAL BOUNDARY IN UNROOFING THE SOUTHERN SIERRA NEVADA BATHOLITH

Structural and geochemical data from the southern Sierra Nevada batholith, California, suggest a major crustal boundary controlled unroofing and played an integral part in exposing an oblique crustal section. In the Lake Isabella region, the PKCF is nearly vertical for ~50 km north of the lake, but to the south it shallows progressively over a 70-km stretch, finally rooting into lower crustal tectonites of the Rand fault system. Reverse-dextral slip between 100-70 Ma coincided with emplacement of the Rand schist beneath the Tehachapi range. The 1-2 km wide ductile to brittle damage zone of the PKCF has a principal slip line with a ~10°N pitch in a ~80°E dipping plane. Dextral offset is difficult to document because it occurred during, and bounds the western margin of, the last pulses of voluminous Sierran arc magmatism. But this slip is potentially substantial, as ~75 km of the PKCF actually defines the sharp Sr_i 0.706 line south of Lake Isabella. The PKCF shallows to typical ~30°E dips in high-pressure rocks of the eastern Tehachapis, where its mylonitic fabric merges with upper plate tectonites above the Rand schist.

Plutons at the latitude of Lake Isabella were emplaced at 2-4 kbars, while emplacement pressures in the Tehachapi range reach ~10 kbar. The PKCF sharply disrupts the barometric gradient, suggesting that it controlled differential unroofing of the oblique crustal section. Geochemical evidence also indicates the PKCF was a major channel for lower crustal aqueous fluid flow. Coupled d¹⁸O zircon and whole rock analyses indicate that the deep-level rocks of the Tehachapis underwent sub-solidus exchange with a high-d¹⁸O source like the Rand Schist. The whole rock data define a zone of anomalously high values that “fingers” northward from the Tehachapi range along the PKCF for ~75 km, traversing a pressure gradient of ~4 kb. We propose that aqueous metamorphic fluids generated during schist underthrusting penetrated the batholithic upper plate and moved upwards along the PKCF. Preliminary evidence points to the PKCF as a zone of localized fluid flow, which, combined with a high thermal state during final stages of pluton emplacement, led to a zone of weakness that controlled differential unroofing along this major crustal boundary and displaced barometric and chemical contours.