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,1,1,2
1 Department of Geological Sciences and Cooperative Institute for Research in Environmental Sciences, CB 399, University of Colorado, Boulder, Colorado 80309–0399, USA
2 William Lettis and Associates, 1777 Bothelho Drive, Suite 262, Walnut Creek, California 94596, USA
Pliocene (ca. 3.5 Ma) removal of dense eclogitic material under the Sierra Nevada has been proposed from variations in the petrology and geochemistry of Neogene volcanic rocks and their entrained xenoliths from the southern Sierra. The replacement of eclogite by buoyant, warm asthenosphere is consistent with present-day seismologic and magnetotelluric observations made in the southern Sierra. A necessary consequence of replacing eclogite with peridotite is that mean surface elevations and gravitational potential energy both increase. An increase in potential energy should increase extensional strain rates in the area. If these forces are insufficient to significantly alter Pacific–North American plate motion, then increased extensional strain rates in the vicinity of the Sierra must be accompanied by changes in the rate and style of deformation elsewhere. Changes in deformation in California and westernmost Nevada agree well with these predictions. Existing geologic evidence indicates that a period of rapid uplift along the Sierran crest of more than 
1 km occurred between 8 and 3 Ma, most likely as a consequence of removal of lower lithosphere. About this same time, extensional deformation was initiated within 50 km of the eastern side of the Sierra (5–3 Ma), and regional shortening began to produce the California Coast Ranges (5–3 Ma). We suggest that these events were induced by the >1.2 x 1012 N/m increase of gravitational potential energy generated by the Sierran uplift. Evidence for Pliocene uplift, adjoining crustal extension, and shortening in directly opposing parts of the Coast Ranges is found along nearly the entire length of the Sierra Nevada and implies that lithosphere was removed beneath all of the present-day mountain range. The uplifted area lies between two large, upper-mantle, high-P-wave-velocity bodies under the south end of the San Joaquin Valley and the north end of the Sacramento Valley. These high-velocity bodies plausibly represent the present position of material removed from the base of the crust. Lithospheric removal may also be responsible for shifting of the distribution of transform slip from the San Andreas Fault system to the Eastern California shear zone, a prediction that awaits better-defined slip histories on both faults. Overall, the late Cenozoic deformational history of the Sierra Nevada and vicinity illustrates that locally derived forces can influence deformation kinematics within plate-boundary zones.
Key Words: Sierra Nevada • Cenozoic • tectonics • delamination • orogeny
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