TRANSITION FROM DUCTILE TO BRITTLE MOTION ON THE NORTHERN EXTENSION OF THE EASTERN SIERRA CREST SHEAR ZONE (ESCSZ) IN THE SADDLEBAG LAKE PENDANT, CENTRAL SIERRA NEVADA, CA

Mapping over two summers by the Undergraduate Team Research program, University of Southern California in the Saddlebag Lake Pendant, along the eastern margin of the Tuolumne batholith (TB), central Sierra Nevada has documented, in both the Virginia Canyon and Saddlebag Lake areas evidence of a wide ductile shear zone that evolved to a brittle fault. This shear zone, interpreted as the northern extension of the ESCSZ, forms the eastern margin of a package of Jurassic metasedimentary and Cretaceous metavolcanic rocks and the western margin of the Triassic metavolcanic Koip sequence.

The shear zone strikes N-NW varying in width from 1 – 2km. Shear sense indicators including S-C structures, shear bands, asymmetrical folds, sigma and delta clasts, and asymmetrical boudins occur in both areas with stretching SE plunging lineations that weaken outside the shear zone. These indicators indicate, synmagmatic, oblique, dextral shear in the 88 (Saddlebag Lake area) to 86 (Virginia Canyon area) Ma Cathedral Peak granodiorite and longer durations of shear in the sedimentary, and volcanic units. Ductile shearing in the host rocks occurred under amphibole (< 700^oC) to subgreenschist facies conditions and continued well after 83 Ma (biotite cooling ages in the Virginia Canyon area).

The steeply dipping, discreet brittle fault, in the middle of the ductile shear zone, marks the contact between the two rock packages described above as well as a surface along which dikes from the TB are truncated. Local slickenlines, steps, and offset dikes suggest dextral oblique motion mirroring the motion of the ductile shear zone. Breccias, gouge, and local pseudotachylite and large breccia-filled, quartz veins are common along the fault. Brittle motion continued after biotite closure ages and possibly significantly younger than 80 Ma during exhumation and cooling of this portion of the Sierra Nevada.

The absence of the entire Jurassic volcanic sequence across the fault in the Saddlebag Lake and Virginia Canyon areas suggests tectonic removal during faulting caused by significant displacement on this fault. The Jurassic marine sedimentary sequence and their detrital zircon histograms may provide an opportunity to better constrain the fault displacement although no matches have yet been found.