TRANSLATION VS. ROTATION: THE BATTLE FOR ENIGMATIC DEXTRAL SHEAR ACCOMMODATION IN THE WALKER LANE, WESTERN NEVADA, USA

The North American-Pacific plate boundary is a complex zone of distributed dextral shear accommodating ~5 cm/yr of relative plate motion. The San Andreas fault zone (SAFZ) and the Walker Lane-eastern California shear zone (WL-ECSZ) accommodate ~80% and ~20% of the plate motion, respectively. In contrast to the long (100s of km), well-organized, and through-going dextral faults of the SAFZ, the WL and ECSZ consist of discontinuous and poorly organized systems of generally short (<100 km) strike-slip faults. The discontinuous nature of faulting in the WL results in domains characterized by different patterns of faulting. West of dextral faults of the central WL is a region of N-striking normal faults and asymmetric basins, where geodetic studies define ~5mm/yr of NW-directed dextral strain. Intriguingly, this region is devoid of major strike-slip fault systems. How dextral shear is accommodated in this region is poorly understood. To elucidate the long-term tectonic development of this region, paleomagnetic data from late Oligocene ash-flow tuffs are used to determine magnitudes of vertical-axis rotation of fault blocks as a means of accommodating dextral shear. 

Characteristic remanent magnetizations of ash-flow tuffs compared to paleomagnetic reference directions define domains of low/no rotation, statistically-significant magnitudes of ~20-30° clockwise vertical-axis rotation, and >30° of clockwise rotation within regions of dextral, normal, and sinistral faulting, respectively. Boundaries between domains, although potentially gradational, are relatively discrete and support a distinction of domains by style of faulting. Comparison of ash-flow tuff rotation data to previously sampled Mio-Pliocene lavas support a late-Miocene initiation of dextral-shear. Geodetically-derived rotation-rates from previous studies are spatially consistent with paleomagnetically-defined domains but suggest a decrease in rotation rates over time. The active tectonics of the region are counterintuitive to decreases in rotation rates and may be better explained by transient strain, mechanical limitations of large, geodetically-modeled rotating blocks due to GPS station density, and/or maturing fault systems that proportionally accommodate more translation vs. rotation as they lengthen through time.