MAPPING AND ANALYSIS OF TECTONIC SIGNALS FROM NEOGENE COVER STRATA IN THE CENTRAL AND SOUTHERN SIERRA NEVADA, CALIFORNIA

The existence of Neogene uplift and deformation of the Sierra Nevada Batholith (SNB) has been debated for two decades in the context of a hypothesized high elevation pre-Oligocene plateau east of the modern range (DeCelles, 2004). One line of argument employed towards a more recent SNB uplift episode uses apparently-tilted Neogene volcanic flows emplaced on the batholith surface as markers of tilt and uplift occurring after eruption of these flows (Wakabayashi and Sawyer, 2000, 2001). While such markers provide local evidence of east-up Sierran tilting during the Neogene, they have not yet been thoroughly analyzed for evidence of syntectonic deformation within the batholith.

Two late Miocene potassic volcanic units in the central Sierra Nevada, the Stanislaus Group and the Trachyandesite of Kennedy Table, filled in the paleochannels of the Stanislaus and San Joaquin Rivers respectively at the time of eruption and now act as cover strata recording stream paleogeometries. From these recorded geometries it is possible to reconstruct the late Miocene stream thalwegs and gradients and then calculate both the range slope and the elevation of the channel at the modern range crest at the time of eruption, giving an estimate of rock uplift since emplacement as well as indications of flexure or faulting within the batholith. We carried out geologic mapping and paleomagnetic analysis to update the existing inaccurate mapping of the Stanislaus River region (Ransome, 1898; Turner and Ransome, 1898) and correlate exposed remnants of the flows over erosional gaps. We then employed the backtilt-calculation methods of Wakabayashi and Sawyer (2000, 2001) to calculate a rock uplift estimate at the modern range crest upslope of the North Fork Stanislaus River headwaters of approximately 1,880 m since eruption. Geospatial analysis performed on the mapped flows to partition the total post-Miocene uplift and slope steepening between the batholith proper and the metamorphic foothills revealed zones of deformation within the basement rocks which developed after eruption. We conclude that within the Stanislaus River watershed, uplift-related deformation within the basement rock is accommodated by east-down normal faulting in the metamorphic foothills which does not continue into the batholith proper.