USING PALEOTHALWEGS TO LEARN ABOUT THE SIERRA NEVADA: MAPPING AND GIS ANALYSIS OF THE MIOCENE STANISLAUS GROUP, CALIFORNIA

The presence or absence of post-Miocene uplift and tilting of the Sierra Nevada Batholith (SNB) has been hotly debated for the last two decades. One school of thought uses apparently-tilted volcanic rocks as markers of tilt and uplift. However, no one has used such markers to look in detail at possible flexure or other deformation within the Sierra Nevada batholith. The exposed Miocene volcanic flows filling the paleochannel of the Stanislaus River could be used for this, but much of the geologic mapping of these flows is more than a century old and is based on vastly erroneous topographic basemaps (Ransome, 1898; Turner and Ransome, 1898). To quantify the change in slope that could result from central Sierran flexure, we have mapped exposures of the 9-11.5 Ma Stanislaus Group lavas and ignimbrites in the Miocene paleochannel of the Stanislaus River along the ENE-WSW traverse between Grizzly Meadow and Tuttletown. Mapping was executed with an emphasis on capturing internal contacts between members of the Stanislaus Group to gain a sense of uneroded paleohorizontal markers during a relatively short timeframe (<2 My) prior to uplift. Derivation of regional paleoslope from the mapped flows displays the expected SW regional dip, but a convex upwards profile, with a shift from approximately 0.3 degrees of dip in the northeastern portion of the study area to approximately 2.5 degrees of dip in the southwestern portion; this gives an increase in slope of approximately 0.06 degrees per kilometer trending SSW. This is not an expected equilibrium stream profile in a situation of homogeneous lithology. The leveling off of the contact slope at higher elevations strongly suggests that the batholith is experiencing regional flexure in this area to accommodate the uplifted mass of the Sierra Crest. Another possible explanation is that this profile reflects the existing amount of tilt and flex of the batholith surface at the time of eruption; it is also possible that the paleothalweg profile was simply convex upwards, though this would be in contrast to all previous studies of the region. Interpolation between flow exposures in a GIS environment does not indicate the existence of significant post-Stanislaus-Group-emplacement faulting within the study area, lending weight to the findings of Wakabayashi and Sawyer (2000, 2001).