PARTITIONED DEXTRAL TRANSPRESSIVE AND CONTRACTIONAL STRAIN FIELDS AND SHEAR ZONES AROUND THE TUOLUMNE INTRUSIVE SUITE, CALIFORNIA: IMPLICATIONS FOR THE CRETACEOUS TECTONIC EVOLUTION OF THE SIERRA NEVADA BATHOLITH

During the Cretaceous regional strain fields in the Sierra Nevada batholith have been described as extensional, transtensional, neutral, transpressional, and contractional. Furthermore the magnitude of displacements and the role of regional shear zones during Cretaceous magmatism remain unclear.

Our field work to date in host rock pendants along the eastern and northern margins of the Tuolumne Intrusive Suite (TIS), California and in internal pendants have identified a number of structural domains. Along the eastern margin of the TIS the Saddlebag Lake pendant shows domains of dextral transpressive shear alternating with domains of pure shear. In the northern part of this pendant, we have mapped out a large dextral transpressive shear zone separating zones of metavolcanic and metasedimentary rocks and structural domains dominated by SW-vergent and NE-vergent thrust faults.

To the north of the TIS we find a major change in structural style from east to west. In the east, WNW-striking metavolcanic rocks are bounded by primary contacts. In the west NNW-striking units including several granitoid bodies are separated by SW-vergent reverse faults. Ongoing work will examine whether the structural boundary between these domains coincides with the proposed Mojave- Snow Lake fault. In pendants within the TIS we see also see evidence of strong transposition of older structures during dextral transpression separating zones dominated by folding.

Our findings present widespread evidence for strongly partitioned dextral transpression during the Cretaceous: (1) reverse faults indicate dip-slip displacement without a component of lateral shear; and (2) dextral transpressive shear zones show both horizontal and vertical displacement. Despite further support for oblique convergence between the Farallon and the North American plates, our study has several regional implications, including the paleogeography of western Nevada and California and the degree to which terranes have been transported along the Cordilleran margin.