We speculate that at a min- imum, this seismicity indicates one or more faults that permit deep circulation of thermal water to Mono Hot. In a trans-tensional setting, this faulting and seismicity would likely produce porosity and geothermal fluid-flow pathways and might also include dike emplacement. Such faults are consistent with formation under dextral trans-tension in this part of the eastern Sierra Nevada ( Unruh et al., 2003). The proposed feature, composed of mapped faults, seismicity, and thermal springs, is subparallel to known left-stepping north- south-oriented normal faults, such as the Hilton Creek Fault ( Figure 1), that accommodate NNW-oriented dextral shear in the area ( Unruh et al., 2003). This low-magnitude (M 2- 3) seismicity is mostly shallow (<10 km in depth) and exhibits dominantly normal fault focal mechanisms mostly oriented subparallel to this hypothesized lin- eament (Figures 1 and 2A). The seismicity trend between Long Valley and Mono Hot Springs passes through Mam- moth Mountain, the Fish Creek/Iva Bel Hot spring, and unnamed Quaternary active normal faults near Mammoth (USGS, 2017). A concentration of epicenters forms a cluster between Long Valley and Mono Hot Springs, as well as a large concentration north of Blayney Meadow (Figure 2A). small-magnitude seismicity occurs within the Sierra Nevada around the Long Valley Caldera (Figure 2A and ANSS Comprehensive Earthquake Catalog). The study area lies within 30 km of the Microplate's eastern edge, where faults mark the Walker Lane, Sierra Nevada frontal fault, and the edge of the Long Valley Caldera (Figure. The Sierra Nevada Microplate moves northwestward rel- ative to stable North America ( Gordon, 1991, 2001 Unruh et al., 2003 and Unruh and Hauksson, 2009), separated from it by the extensional Basin and Range and dextral Walker Lane tectonic provinces ( Unruh et al., 2003 Unruh and Hauksson, 2009 Dixon et al., 1995Dixon et al.,, 2000and Kreemer et al., 2009). ![]() The mi- croplate is caught up between motion of the Pacific and North American plates, with most displacement occurring on faults of the San Andreas system and Coast Range faults (Argus and Gordon, 2001) and the remaining 20-25 percent occurring in the Walker Lane and Basin and Range ( Dixon et al., 1995Dixon et al.,, 2000). Despite being in a region of seemingly low deformation rate, one or more factors have or could have contributed to the existence of the Mono Hot Springs geothermal area: 1) tectonic activ- ity of the Walker Lane and Sierra Nevada frontal fault zone located 30 km to the north, northeast, and east along the margin of the Sierra Nevada microplate 2) volcanic activity of the Long Valley, 30 km to the north- east, and Mammoth Mountain, 30 km to the north ( The Sierra Nevada Microplate consists of the Sierra Nevada and the Great Valley physiographic provinces as a single coherent piece of lithosphere Gordon, 1991, 2001 Kreemer et al., 2009). study area lies in the upper San Joaquin River watershed at the eastern-central edge of the Sierra Nevada Microplate, a portion of relatively unfaulted lithosphere between the California Coast Ranges and the Walker Lane ( Figure 1 Gordon, 1991, 2001 Unruh et al., 2003).
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