IMPACT OF THE REMOVAL OF SUB-SIERRA NEVADA ECLOGITIC ROOT UPON THE BASIN AND RANGE, COAST RANGES, AND SAN ANDREAS FAULT, CALIFORNIA

Pliocene (~3.5 Ma) removal of dense eclogitic material under the Sierra Nevada has been proposed on the basis of xenolith and volcanic rock petrology, geochemistry, and seismological and magnetotelluric observations. A necessary consequence of replacing eclogite with peridotite is that elevations and gravitational potential energy both increase, which in turn should increase extensional strain rates in the area. If Pacific-North America plate motion is constant, 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 are consistent with these predictions. Uplift along the Sierran crest of >~1 km is dated to 3-8 Ma, and probably was accompanied by an increase in gravitational potential energy of 1.2 x 10¹² N/m or more. Extensional deformation within ~50 km of the eastern side of the modern Sierra initiated about 3 Ma, and shortening that produced the California Coast Ranges is estimated to have begun about 3-5 Ma. The Pliocene uplift, and initiation of extension and contraction along the margins of the Sierra, all suggest that this delamination event extended the entire length of the range. The uplifted area lies between two large, upper-mantle, high-P-wave-velocity bodies at each end of the Great Valley that plausibly represent the material removed from the base of the crust. Finally, reduction of the east-west width of the rigid Sierra Nevada-Great Valley block should transfer slip to the east side (the Eastern California Shear Zone) and reduce the San Andreas system’s slip rate near 3.5 Ma. This too is compatible with available reconstructions of these systems, underscoring the variety of implications of body force changes within a plate boundary zone.