NEOGENE TO QUATERNARY KINEMATICS OF THE CENTRAL SIERRAN FRONTAL FAULT SYSTEM IN THE SONORA PASS REGION: PRELIMINARY STRUCTURAL, PALEOMAGNETIC, AND NEOTECTONIC RESULTS

We document syntectonic volcanism and fault evolution in a zone of active continental transtension in the Sonora Pass area, Sierra Nevada frontal fault, from its initiation ~10.3 Ma to the present. The onset of high-potassium volcanism was synchronous with transtensional deformation and initial Sierran range-front faulting. Syndepositional faults that cut proximal volcanic facies occur within 0-25 km of the range front, suggest the major Sierran frontal faults initiated within the late Miocene arc axis. Volcanic facies analysis shows that the Tertiary stratigraphy established west of the crest can be correlated in detail into highly extended terrane east of the crest. EW-trending cross-faults and highly-tilted fault blocks suggest a major right-stepping accommodation zone transfers active slip to the Walker Lane here. Previously recognized NS-trending scarps cut both the late Pleistocene and last glacial maximum moraines, along with EW-trending scarps and mole tracks identified in this study indicate ongoing deformation.

Distinctive Late Tertiary and Quaternary strain markers include: (1) 6 unconformity surfaces--both erosional and angular--in the Tertiary; (2) regionally correlated stratigraphic markers (the Stanislaus Group) from Sonora Pass to Mono Basin with high resolution 40Ar/39Ar dates; (3) well-exposed paleocanyons with up to 300 m of relief and distinctive stratigraphy that can be used as kinematic indicators across faults; and (4) well-preserved glacial deposits of 6 different ages ranging from >550 ka (perhaps 800-900 ka) to ~20 ka cut by late Quaternary faults (Clark, 2003).

The Sierran frontal fault system is dominated by five major faults. Three of these are subparallel to the modern range front (NNW-SSE to N-S) and dip steeply with significant down-to-the-east throw. Two cross faults trend NE-SW to ENE-WSW. The field data suggests that the range-front faults may have accommodated vertical axis rotations. We have collected samples at 15 sites from 7 different fault blocks, mostly of the Stanislaus Group (Table Mountain Latite flows and Eureka Valley Tuff) to quantify the rotational component using paleomagnetic data. Thus, we are developing stratigraphic, structural and chronologic constraints for the rate and style of deformation along a part of the Sierra frontal fault system since the late Miocene.