KINEMATICS AND CUMULATIVE DISPLACEMENT ACROSS THE NORTHERN WALKER LANE, AN INCIPIENT TRANSFORM FAULT, NORTHWEST NEVADA AND NORTHEAST CALIFORNIA

The northern Walker Lane (NWL) consists of an overlapping, left-stepping system of dextral strike-slip faults in NW Nevada and NE California. It currently accommodates ~20% of the dextral motion between the North American and Pacific plates. Onset of strike-slip faulting probably occurred ~3-5 Ma coincident with northward passage of the Mendocino triple junction. Because the NWL may be the youngest, least evolved part of the transform plate boundary, it affords an opportunity to analyze incipient transform faults in transtensional settings.

The NWL can be divided into two broad overlapping domains. The eastern domain consists of the NW-striking, right-lateral Pyramid Lake fault (PLF), which probably continues southeastward along the SW margin of the Carson Sink and links with major dextral faults in the central Walker Lane. The western domain includes several overlapping NW-striking dextral faults (Mohawk Valley, Honey Lake, and Warm Springs Valley faults). Slip on the PLF appears to transfer northward to a system of W-dipping normal faults, including major range-front faults of the Lake and Nightingale Ranges, whereas dextral slip in the western domain transfers southward to a broad system of mainly E-dipping normal faults that coalesces near Reno to form the Sierra Nevada frontal fault system. Major dextral faults essentially terminate in extensional duplexes. Offset segments of an ~E-trending Oligocene paleovalley in the Nightingale Range, Virginia Mts, Dogskin Mountain, and possibly Diamond Mts suggest ~10, 8, and 10-15 km of dextral displacement on the PLF, Warm Springs Valley, and Honey Lake faults, respectively. Cumulative offset across the NWL is therefore estimated at 25-30 km, suggesting long-term slip rates of ~5-10 mm/yr.

The left-stepping dextral faults in the NWL may represent Riedel shears developing above a nascent lithospheric-scale transform. Although small left steps along individual faults are associated with local shortening, the broad left steps between major dextral faults generally accommodate little shortening and do not correspond to typical restraining bends. A Riedel shear geometry can account for a lack of shortening in the broad left steps but curiously does not explain the near parallelism of NWL dextral faults with the current N37W North America-Pacific relative plate motion.