AN ALTERNATIVE MODEL FOR LATE NEOGENE UPLIFT OF THE SIERRA NEVADA, CALIFORNIA

The eastern escarpment of the Sierra Nevada has long been considered a classic example of basin-and-range block faulting. Considerable evidence indicates that formation of this escarpment dates to 4.0 to 3.5 Ma, which coincides with the initiation of accelerated tilting of the Sierra Nevada block. These events have been proposed to be linked through delamination and foundering of a dense eclogitic lithosphere. I propose that an alternative model of tectonic displacement provides a more direct linkage. Geodetic (GPS) measurements indicate that the Sierra Nevada and White Mountain blocks are currently diverging at a rate of 4 mm/yr, with displacement oriented N55W. This displacement produces transtensional movement on the fault system, which is oriented more nearly due north-south. Projection of this displacement backwards in time for 3.7 Myr closes the current topographic gaps between bedrock bodies. Such movement requires displacement along low-angle detachment faults rather than steeply dipping normal faults. Movement along detachment faults explains several otherwise-enigmatic aspects of the tectonics of the Eastern Sierra. It also suggests that accelerated late Neogene tilting of the Sierra Nevada may be largely due to tectonic denudation.