ACTIVE FAULTING IN THE SOUTHERN SIERRA NEVADA: INITIATION OF A NEW BASIN-AND-RANGE NORMAL FAULT?

Field mapping and analysis of high-resolution digital topographic data reveal the existence of several extensional basins within the southern Sierra Nevada. These basins are bounded by faults or topographic lineaments, typically offset in an en echelon pattern across the basin, suggesting a pull-apart origin. Basin fills are comprised of Quaternary lacustrine and fluvial sediments. The age of basin fill, combined with pronounced geomorphic expression of faulting, is indicative of active tectonism. Basin bounding faults disrupt a Pliocene-age erosion surface cut across the southern Sierra Nevada, indicating initiation of faulting in late Pliocene or Pleistocene time. Geographically, the basins, faults, and lineaments extend over 60 km within the southern Sierra Nevada, from the southwestern corner of the range, near the San Andreas-Garlock junction, northward through the core of the Sierra Nevada crustal block. The southernmost basin along this trend is Walker Basin, which is bounded on the west by the Breckenridge fault, a normal or dextral oblique-slip fault with at least 1 km of structural relief. A topographic lineament trending northeastward from Walker Basin transects the upper end of the lower Kern Gorge and connects basins developed north of Lake Isabella, including Cannell Meadow, Long Meadow, and Big Meadow. All of these apparently evolving basins lie astride a band of seismicity that trends from the 1952 Arvin-Tehachapi earthquake to the Durrwood Meadows earthquake swarm of 1983-1984, perhaps the most prominent seismic trend in the southern Sierra Nevada.

The observed basins, faults, and lineaments indicate a nearly continuous fault zone over 60 km long within the southern Sierra Nevada. The spatial correlation of these tectonic features with observed seismicity, as well as the relative youth of these features, presents the possibility that these features are part of a nascent Basin-and-Range style normal fault initiating within the Sierra Nevada. The location and timing of observed faulting are consistent with models of continual westward migration of tectonism along the margin of the Basin and Range, and thus may provide an unprecedented opportunity to study mechanisms of fault initiation, growth, and propagation.