TECTONIC GEOMORPHOLOGY OF THE SOUTHERN SIERRA NEVADA MOUNTAINS (CALIFORNIA): EVIDENCE FOR UPLIFT AND BASIN FORMATION

There are a number of hypotheses regarding Late Cenozoic uplift of the Sierra Nevada Mountains (Sierra), California, U.S.A.:,a single, slowly tilting, homogenous block driven by normal faulting on the eastern margin; rapid Pliocene uplift generated by lower crust delamination; erosion driven in part by late Pleistocene glaciation; uplift related to the Mendocino triple-junction's northward migration. Tectonic geomorphic studies have focused on the eastern margin and the historically active Owens Valley frontal fault; however, crustal delamination is along the western margin and adjoining valley. To test the various late Cenozoic uplift hypotheses, we examined geomorphic indices of the western Sierra such as longitudinal profiles of the larger westerly flowing rivers, mountain front sinuosity, valley floor width to height ratio, range-crest profile and basin relief ratio. We chose these morphometrics and the western rivers to avoid the influence of glaciation and to better investigate the western margin of the range. In some areas, application of certain morphometrics is limited due to the presence of dams along the rivers. In the southernmost area, the mountain front is more linear and valley narrower and the Kern River has a convex-up profile. The morphometrics indicate slightly greater tectonic activity near the Kings River as well, but not a strongly as the south. Our results show that relative tectonic activity is greatest in the southern Sierra near the Kern River Gorge fault and the highest range elevations. In addition, the region centered near the Kings River, where crustal delamination is suspected shows greater tectonic activity, but at a lower rate, compared to other areas. We interpret these data to indicate that the southern Sierra is presently experiencing relative base level changes (basin formation and range uplift) related to proximity to the San Andreas and related fault zones and that the Kings River geomorphology records a rapid uplift in the past, probably related to Pliocene delamination.