Christopher J. Pluhar

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

The Walker Lane currently accommodates ∼20% of the dextral motion between the Pacific 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-west–extending 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 defined 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°–60° 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 significant contribution of crustal block vertical-axis rotations in other transtensional regions of the world.

Monitoring thermal springs to improve land management decision-making, Sierra Nevada, California

The Sierra National Forest administers Mono Hot Springs and other nearby geothermal features, a concentration of more than a dozen springs, pools, and seeps in the high Sierra Nevada, California. The Native American Mono Tribe traditionally uses Mono Hot Springs for spiritual purposes, while simultaneously, the Mono Hot Springs Resort holds a special use permit for some of the geothermal waters. To support environmental assessments for area management, the Sierra National Forest studied thermal spring chemistry and temperature, evaluating potential use conflicts. An initial multi-year monitoring of eleven representative thermal springs was followed a decade later by another multi-year sampling of the same springs, providing insight into the geothermal character of Mono Hot Springs. Measured water temperatures ranged from 44.5° C to 24.3° C and pH 8.0 to 7.03, depending upon the thermal spring, higher pH values correlating with lower temperatures. Thermal spring temperatures varied seasonally with higher temperatures in springtime and lower ones in autumn. pH did not exhibit a coherent seasonal variation. Mono Hot Spring temperature decreased and pH increased during the decade-long study period, with even greater longer-term temperature change evidenced at nearby Mono Crossing. Silica and cation geothermometry at Mono Hot Springs suggests that the geothermal waters reached equilibrium with 74-79° C rock at depth at estimated pH 5-6. The spatial distribution of neighboring thermal springs, regional seismicity, and mapped faults suggests that Mono Hot Springs rises along faults running nearly north-south, connecting to Mammoth Mountain and Long Valley, California, 30 km to the north.