TECTONIC GEOMORPHOLOGY ALONG THE WESTERN PIEDMONT OF THE SIERRA NEVADA MOUNTAINS, CALIFORNIA

Hypotheses regarding uplift of the Sierra Nevada Mountains (SNM), California vary from a single tilting block to more rapid Pliocene and/or Quaternary uplift in the south. To test these hypotheses, we examined the geomorphology of large westerly flowing rivers and the piedmont along the western SNM. Selected river reaches are unglaciated and trend roughly orthogonal to the mountain front/piedmont intersection. Mountain front sinuosity is 1.4 -1.5 near the Kern, Tule and Kaweah Rivers, increases northward near the Mokelumne River (2.7) and then decreases again further north near the Feather River (1.3). Valley floor width to height ratios increase from the Kern River (0.1) northward to the Tule (2.4) and Kaweah (8.6) Rivers. The southernmost river, the Kern, has a convex up longitudinal profile near the Kern Gorge Fault. The longitudinal profiles of the Tule and Kaweah Rivers, north of the Kern, have the steepest gradients and relief ratios for the basins are 0.073 and 0.062, respectively. Relief ratios of the Kings, Merced and Mokelumne Rivers decrease northward from 0.037 to 0.030. The mountain front/piedmont profile rises near the Kern (Bakersfield Arch) and north of the Kaweah, with intervening lower elevations. Our interpretation of these morphometric indices is that the SNM, near the Kern River, are actively uplifting. The Kaweah/Tule region experienced a pulse of uplift in the past, but has not been active in the Quaternary(?). The Kaweah/Tule morphology is consistent with rapid Pliocene uplift (Stock, 2003) and delamination of the lower crust in this area (Foster and Saleeby, 2003).