EOCENE TO MODERN TILTING OF THE SIERRA NEVADA: INSIGHTS FROM CENTRAL VALLEY STRATIGRAPHY

The Cenozoic tilting history of the Sierra Nevada has remained a controversial subject from the time of Josiah Whitney, 150 years ago, right up to the present day. In 1977 T.A. Grant suggested that if the Sierra Nevada was progressively tilting, then the tilt of alluvial strata should increase with unit age. This idea was pursued by J.R. Unruh in 1991, and others, who demonstrated that information on tilt history could be obtained from stratigraphic dips of sedimentary units.

In this study I have expanded on the work of these earlier investigators by constructing detailed cross sections from the middle of the Central Valley eastward across the crest of the Sierra Nevada over an area from the Kaweah River in the south to the Feather River in the north (~35.5° to ~39.5° N), using a combination of petroleum literature, hydrogeology studies, and geomorphology and Quaternary geology. Corrections were made for the depositional dips of the units and for sediment compaction. The dips of geomorphic markers in the mountain range, such as lava flows, were compared with the dips of subsurface stratigraphic units of the same age and the two were in good agreement, demonstrating that the stratigraphic dips do provide accurate information on changes in dip of the Sierra Nevada land surface.

The ages of units included in the study range from Eocene (~37 Ma) to modern river channels. The inferred amounts of tilt since the Eocene vary systematically from south to north, going from 4.0° at the Kaweah River to 0.75° at the Feather River. The southern and northern sections are internally fairly consistent, with the change observed as a step between the San Joaquin and Tuolumne River drainages (between 37.5° and 38.5° N). Inferred Eocene crest paleoelevations in the north are similar to modern ones, whereas those in the south are much lower (1 to 2 km compared to ~4 km). In the north, the relatively minor tilting is observed only between 7 and 1.7 Ma, with little tilt since 1.7 Ma. In contrast, in the south tilting has been continuous since the Eocene, but has accelerated with time. Packages of middle Pleistocene to modern glaciofluvial sediments from both the Kaweah and San Joaquin Rivers demonstrate that in the south the tilting is continuing up to the present day at a rate of 0.4 to 0.6 km of crest uplift/Ma. These rates are similar to those inferred from modern geodetical data.