UPLIFT OF THE SOUTHERN SIERRA NEVADA CREST BASED ON REEVALUATION OF DATA FROM THE SAN JOAQUIN RIVER DRAINAGE

In 1981 N. King Huber proposed that the crestal elevation of the southern Sierra Nevada had increased by 2150 m (i.e., rock uplift) since ~10 Ma, based mainly on a comparison of the dip angle of lava flows in the western foothills with that of the modern channel of the San Joaquin River. This drainage is the only one in the southern Sierra with well-preserved dip indicators, thus it is crucial to the late Cenozoic tectonic history of the range. Huber’s analysis has since been criticized on two grounds: (1) Huber assumed the original dip of the flows was the same as the modern San Joaquin downstream of Friant Dam, but this is difficult to support; the current dip of the flows could be original, and (2) Huber had to project tilt from the westernmost 30 km of the drainage over 100 km to the east assuming that the range had tilted as a rigid block, but instead the range may have warped and thus, if there was any uplift, it may have been much less. We have addressed the first issue by analysis of large-scale meanders of the river preserved under the 9.192±0.015 Ma Trachyandesite of Kennedy Table. Modern meandering rivers typically do not show any dependence of the flow gradient on flow direction. However, the fossil meanders do show a very clear dependence of gradient on azimuth, with intervals perpendicular to the ‘hinge’ line of the range being steep and those parallel being flat, or even negative gradient. Removal of 1.20° of tilt restores the meanders to a uniform 0.14° gradient. This amount of tilt is very close to Huber’s estimate of 1.22°. The second criticism has been addressed by recognition that a trachyandesite vent 700 m above the headwaters of the San Joaquin River and 500 m below the Sierra Nevada crest is petrographically, chemically, and chronologically (⁴⁰Ar/³⁹Ar ages) identical to the Trachyandesite of Kennedy Table (Hildreth and Fierstein, 2016). When plotted on a river elevation profile, the vent falls very close to Huber’s reconstruction of the Miocene profile, showing that internal deformation of the Sierra block has been negligible. These new findings greatly strengthen the case for Sierra Nevada tilting. Based on our data, we estimate that the Sierra Nevada crest between the San Joaquin River and the headwaters of the Kern River has tilted upward by 2100 m since 9.2 Ma.