The Sierra Nevada of California, USA, is an ancient mountain range, formed from the batholithic core of a continental volcanic arc during the Triassic through Late Cretaceous. Nevertheless, beginning with the earliest geological research in the mid−nineteenth century, many researchers have maintained that the range has been rejuvenated by crest uplift in the latest Cenozoic (Pliocene−Quaternary). Other researchers have maintained that the range has been stable since at least the Eocene. One archive of information on the tilting history of the range that has been only lightly investigated is the inclination of formation boundaries in the subsurface of the Central Valley, which lies to the west of the Sierra Nevada. The Central Valley sediments lie on top of the quite rigid Sierra Nevada crustal block. Tilting of the block should both deepen the Central Valley and raise the crest of the range. Although this approach has been previously used at a limited number of locations, it has not been employed systematically along the entire extent of the Sierra Nevada block.
In order to improve the spatial and temporal resolution of data on Sierra Nevada block tilting, I have constructed cross sections from the west side of the Central Valley to east of the Sierra Nevada crest at seven locations, using a combination of published petroleum and hydrogeology data sources. Dips were measured from the cross sections on formation tops ranging in age from 61 Ma to 15 ka. These dips were converted to tectonic tilt estimates by subtracting estimates of depositional dip and sediment compaction. Using the width of the range at each cross section, these tilt values were used to calculate rock uplift with time. Using estimates of landscape erosion rate, range-crest elevations at specific times were reconstructed. From these reconstructions, a coherent tilting history emerged. The northern Sierra was virtually quiescent until after 5 Ma. Between 5 Ma and 1.7 Ma, a brief episode of tilting resulted in 1.0−1.5 km uplift of the range crest. Since 1.7 Ma, tilting has slowed significantly. In contrast, the southern Sierra has been tilting throughout the Cenozoic, although quite slowly for most of it. At ca. 15 Ma, it began to tilt more rapidly, and after 5 Ma, quite rapidly, producing 2−4 km of crest uplift since 12 Ma. Data from Quaternary units indicate that this tilting is ongoing.
The cause of early and middle Cenozoic slow tilting of the southern Sierra is unclear, but it may be related to isostatic response to mountain erosion and valley deposition and/or plate-boundary interactions along the western side of the block. The more rapid tilting since the middle Miocene is most plausibly attributed to changes in the distribution of mantle density, and therefore crustal buoyancy, as a result of foundering (sometimes described as “delamination”) and downward convection of the arclogite root of the Mesozoic volcanic arc. The location and timing of the foundering process inferred from geophysical and geochemical data correspond well with the tilt history inferred from formation dips in this study. The nature of lithospheric foundering of the northern Sierra, if any, is poorly understood, but the synchrony of timing between the major tilting events in the northern and southern Sierra (ca. 5 Ma) points to foundering as the likely cause of tilt in the north as well.
