The uplift of the Sierra Nevada and implications for late Cenozoic epeirogeny in the western Cordillera
Tilted Cenozoic strata in the eastern Great Valley, California, record progressive late Cenozoic uplift of the Sierra Nevada. The magnitude of tilting is combined with other geologic data, using a technique pioneered by Grant and others, to infer the rate and timing of post-late Miocene uplift between the Kings River and the Feather River. Angular unconformities of late Neogene age have been reported throughout the east-central Great Valley. Typically, stratigraphic units of late Oligocene through Miocene-Pliocene age beneath the unconformities dip approximately 1.2°-1.6° southwest. Middle to late Pliocene units above the unconformities dip approximately 0.6°-0.9° southwest and progressively overlap younger units from east to west. These angular unconformities are interpreted to represent the onset of late Cenozoic uplift of the Sierra Nevada due to westward tilting. Dated volcaniclastic units in the east-central Sacramento Valley bracket the onset of tilting there between 3.4 and 8.4 Ma. Stratigraphic and geologic relations from the Stanislaus River area, eastern San Joa-quin Valley, suggest that the late Cenozoic tilting probably began approximately 5 Ma. Previously published tilt data from the San Joaquin Valley combined with new work in the eastern Sacramento Valley indicate that the rate of post-late Miocene tilting has been approximately uniform in the east-central Great Valley. Early to middle Pleistocene units typically dip 0.5°-0.7° southwest; middle Pleistocene units dip 0.2°-0.4°; middle to late Pleistocene units dip 0.1°-0.15°; late Pleistocene units dip 0.05°-0.1°. If it is assumed that major westward tilting began 5 Ma, these data indicate an approximately uniform tilting rate of 0.28° per million years. These data have important implications for models of the uplift. Simultaneous inception of tilting in the east-central Great Valley at approximately 5 Ma does not support models that link uplift of the Sierra Nevada to the migration of the Mendocino triple junction. Late Cenozoic tilting of the Sierra Nevada block does not appear to coincide temporally with the onset of major Basin and Range extension. The uplift may be due to thinning of the mantle lithosphere beneath the Sierra Nevada, but the tilt data do not afford a direct test of this hypothesis. Post-late Miocene tilting of the Sierra Nevada is temporally associated with late Cenozoic uplift of the northern Basin and Range, uplift of the Cascades, uplift of the Colorado Plateau, and uplift of the southern Rocky Mountains, supporting the contention that tilting of the Sierra Nevada may have occurred as part of a Cordillera-wide uplift event.