Kinematics of the west-central Walker Lane: Spatially and temporally variable rotations evident in the Late Miocene Stanislaus Group

Abstract

The Walker Lane currently accommodates ~20% of the dextral motion between the Pacifi c and North American plates. This accommodation occurs on regional-scale systems of strike-slip and normal faults located between the northwestward-translating Sierra Nevada microplate and the east-westextending Basin and Range. At the western edge of the central Walker Lane (lat ~38°-39°N) is a region of crustal blocks bounded by asymmetric basins and normal faults, here defi ned as the west-central Walker Lane. Although this region is apparently devoid of major active strike-slip faults, the presence of Neogene clockwise vertical-axis tectonic block rotations indicates the accommodation of dextral shear. We measured vertical-axis rotation by comparing remanence directions of widespread members of the Eureka Valley Tuff of the Late Miocene Stanislaus Group within the west-central Walker Lane to the same units on the Sierra Nevada microplate.Results show that the study area is organized into discrete domains with heterogeneous regional distribution of clockwise vertical-axis rotation, ranging from ~10° to 60°, since ca. 9.5 Ma. The highest measured magnitudes of vertical-axis rotation (~50°-clockwise) are interpreted as a region of high deformation that includes the asymmetric Bridgeport Valley. Previous work underestimated vertical-axis rotation magnitudes in the region because published reference directions for two of the three members of the Eureka Valley Tuff (By-Day Member and Upper member) derive from the rotated region. We recalculate a reference direction for the By-Day Member of declination 353.2°, inclination 43.7°; α 95 = 10.8°. This corroborates a reference direction for the By-Day Member from the Stanislaus Group type section, situated on the relatively stable Sierra Nevada microplate, providing a robust reference direction for paleomagnetic studies. We present a kinematic model in which dextral shear in the west-central Walker Lane is accommodated by ~30° of clockwise rotation in the Sweetwater Mountains and Bodie Hills since the Late Miocene. This model incorporates rotation magnitudes, paleostress orientations, edge effects, and bounding faults of rotating tectonic blocks to reveal timing, patterns, and mechanisms of crustal deformation . The results and models presented here elucidate the complex and evolving nature of the west-central Walker Lane. The rotational history of dextral shear accommodation demonstrates that the west-central Walker Lane should be included as an important part of the Walker Lane transtensional zone. The results presented in this study not only improve understanding of deformation in the Walker Lane, but illuminate the potentially signifi cant contribution of crustal block vertical-axis rotations in other transtensional regions of the world.

A tale of two Walker Lane pull-apart basins in the ancestral Cascades arc, central Sierra Nevada, California

“…This includes clockwise rotations of ~18° in volcanic rocks on the Sierran frontal fault zone just north of the Little Walker caldera (Figs. 1 and 2A), described herein and by Pluhar et al (2009) and Carlson et al (2013). Geodetically determined rates of rotation (Bormann et al, 2016) appear to underestimate the contribution of vertical-axis rotation determined by paleomagnetic studies, perhaps because fault-bounded, rotated blocks are significantly smaller than the regional-scale blocks presented in geodetic models (Carlson, 2017).…”

“…Like the Carson domain, the Mina deflection shows paleomagnetic evidence of clockwise vertical-axis block rotations (Petronis et al, 2004(Petronis et al, , 2007(Petronis et al, , 2009Gledhill et al, 2016). Clockwise block rotations of up to 55° also occur in the region between the Carson domain and the Mina deflection (Carlson et al, 2013;Carlson andFaulds, 2015, 2016). This includes clockwise rotations of ~18° in volcanic rocks on the Sierran frontal fault zone just north of the Little Walker caldera (Figs.…”

The unusual temporal and spatial slip history of the Wassuk Range normal fault, western Nevada (USA): Implications for seismic hazard and Walker Lane deformation

“…1; Ramelli et al, 1999). However, our results, when combined with new studies of the northern ECSZ -Walker Lane system (e.g., Hammond et al, 2011;Rood et al, 2011;Wesnousky et al, 2012;Carlson et al, 2013;Dong et al, 2014), support a revised model for how the system has evolved over time. This model suggests spatially separate zones which alternate between areas dominated by clockwise crustal block rotations accommodated by NE-to E-trending faults to areas dominated by strain partitioning, with normal faults on the west and dextral faults on the east (Fig.…”

“…North of Mono Lake, rates of extension rapidly decrease and remain low as far north as at the southern termination of the Genoa fault, the northern extent of their study. Rood et al (2011) posit that this extensional deformation is transferred eastward from the Sierran rangefront across the same region in which Carlson et al (2013) showed clockwise block rotations. Although not discussed explicitly in Rood et al (2011), this eastward shift in post-20 ka extension implies that the Wassuk Range fault system since ∼20 ka accommodates a significant portion of total extension at this latitude of the Walker Lane.…”

“…Similarly, Carlson et al (2013) studied the evolution of faults and crustal blocks to the west of the southern Wassuk Range, within the Sierra Nevada -Walker Lane transition zone north of Mono Lake, where they documented significant (∼30°)…”