Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Junction Fault Zone, Sierra Nevada, CA, May Transfer Displacement Between the Kern Canyon and Owens Valley Faults

John M. Bartley¹, Drew S. Coleman², and Allen F. Glazner^2
1 Geology and Geophysics, Univ of Utah, 135 S 1460 E, 717 WBB, Salt Lake City, UT 84112-0111, [email protected]
? ² Department of Geological Sciences, Univ of North Carolina, CB# 3315, Chapel Hill, NC 27599

Mapping in the Sierra Nevada north of Mount Whitney defines a previously unrecognized zone of normal faults, herein called the Junction fault zone (JFZ) for prominent exposures on Junction Peak. Myriad closely spaced (1-20 m) faults that strike 057±6° and dip 55±5° SE form a zone at least 3 km wide across strike. Within the JFZ, erosion localized by the faults accounts for most, if not all, cols and passes (e.g., Forester, Junction, Shepherd). The faults appear to reflect slip across pre-existing joints and contain ubiquitous quartz-epidote slip fibers and frictional-wear slickenlines, the majority of which rake more than 60° and thus indicate predominant dip slip. Some faults contain a second set of gently raking frictional-wear slickenlines. Unslipped joints measured adjacent to the fault zone on Mt. Tyndall have similar spacing but consistently steeper dips (75±4°SE) and more northerly strikes (048±3°). The JFZ was mapped only in the large Late Cretaceous Paradise and Whitney plutons and, therefore, offset markers are rare. However, normal slip is consistently indicated by Riedel shears and is locally confirmed by offset dikes and contacts. If domino-style faulting accounts for the 20° dip difference between the joints and faults, 18% extension across the JFZ is implied. Any individual fault surface would have accommodated only a few tens of centimeters to a few meters of slip but, owing to pervasive faulting, stretching across the JFZ would sum to 0.5 ± 0.1 km. The ~10° clockwise difference between the mean strikes of the joints and faults and the gently plunging slickenlines suggest a significant component of strike slip as well. The age of faulting is poorly known because the JFZ only cuts rocks older than 83 Ma. However, the JFZ passes westward into an intricate zone of intersecting E-striking normal and N-striking dextral faults and then abruptly terminates at the north-striking Kern Canyon fault zone. The JFZ appears to continue northeastward to the NNW-striking Owens Valley fault zone, but this extremely rugged part of the eastern flank of the Sierra Nevada is not yet mapped in detail. The location, orientation and kinematics of the JFZ are ideal for transfer of dextral shear between the active Kern Canyon and Owens Valley faults, and thus the JFZ may be quite young and may even be presently active.

Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_84930.htm). © Copyright 2005 The Geological Society of America (GSA).