Untitled

Abstract

The Adobe Hills region (California and Nevada, USA) is a faulted volcanic fi eld located within the western Mina defl ection, a right-stepping zone of faults that connects the northern Eastern California shear zone (ECSZ) to the south with the Walker Lane belt (WLB) to the north. New detailed geologic mapping, structural studies, and 40 Ar/ 39 Ar geochronology in the Adobe Hills allow us to calculate fault slip rates and test predictions for the kinematics of fault slip transfer into the Mina defl ection. The Adobe Hills are dominated by Pliocene tuffaceous sandstone, basaltic lavas that yield 40 Ar/ 39 Ar ages between 3.13 ± 0.02 and 3.43 ± 0.01 Ma, and basaltic cinder cones. These Pliocene units unconformably overlie Middle Miocene latite ignimbrite that yields an 40 Ar/ 39 Ar age of 11.17 ± 0.04 Ma, and Quaternary tuffaceous sands, alluvium, and lacustrine deposits cap the sequence. Northwest-striking normal faults, west-northwest-striking dextral faults, and northeast-striking sinistral faults cut all units; the northeast-striking sinistral faults are the youngest and most well developed fault set. We calculate ~0.1 mm/yr of approximately east-west horizontal extension and northwest dextral shear since the Pliocene. The prominent northeast-striking sinistral faults offset basalt ridgelines, normal fault-hanging-wall intersections, a channelized basalt fl ow, a basalt fl ow edge, and a basalt fl ow contact a net minimum of 921 ± 184 to 1318 ± 264 m across the Adobe Hills. These measured sinistral offsets yield a minimum Pliocene sinistral fault slip rate of 0.2-0.5 mm/yr; our preferred minimum slip rate is 0.4-0.5 mm/yr. The geometry and orientation of the prominent sinistral faults are consistent with simple shear/couple clockwise block rotation within a broad dextral shear zone. Vertical axis block rotation data are needed to test this interpretation. We propose that a set of faults subparallel to Sierra Nevada-North America motion and associated releasing steps, located west of the White Mountains fault zone and east of the Long Valley Caldera, transfer a portion of dextral Owens Valley fault slip northwestward onto the sinistral faults in the Adobe Hills. Dextral slip distributed across faults between the White Mountains fault zone and the Sierra Nevada and east of the Fish LakeValley fault zone may account for the apparent discrepancy between summed long-term geologic slip rates and present-day geodetic rates across the northern ECSZ. Fault slip in the Adobe Hills is part of a regional pattern of initiation and renewal of dextral, sinistral, and normal fault slip during the Pliocene that extends from lat ~40°N to ~36°N within the ECSZ-WLB and along the western margin of the Basin and Range Province. This regional deformation episode may be related to changes in gravitational potential energy.

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

“…Model 2 applies to the transfer zone mapped herein (Fig. 4B, panel 3), but with faults that dip more steeply (80°-90°) than those portrayed in model 2 of Nagorsen- Rinke et al (2013). We predict that future paleomagnetic work along the NE-striking fault zone in the Sierra Crest-Little Walker volcanic center will show evidence for clockwise block rotations, similar to those determined in the Carson domain and Mina deflection (described earlier herein).…”

“…1). Three models have been proposed to explain fault-slip transfer across the Mina deflection, summarized by Nagorsen- Rinke et al (2013): (1) normal faults at a dextral strike-slip fault stepover (e.g., see panel 2, Fig. 4B), (2) oblique-slip (sinistral and normal) faults, or (3) clockwise block rotations between sinistral faults (e.g., see Fig.…”

“…4B), (2) oblique-slip (sinistral and normal) faults, or (3) clockwise block rotations between sinistral faults (e.g., see Fig. 3; Nagorsen- Rinke et al, 2013). Model 2 applies to the transfer zone mapped herein (Fig.…”

“…1) has more abundant Cenozoic volcanic rocks than the northern and southern Walker Lane, probably because it lies at a higher rift angle relative to the overall plate margin (Busby, 2013). The abundant volcanic rocks make the central Walker Lane ideal for detailed reconstructions of transtensional rifting and magmatism in a continental setting, because the volcanic strata are dateable by the 40 Ar/ 39 Ar method (e.g., Busby et al, 2008aBusby et al, , 2008bBusby et al, , 2013aBusby et al, , 2013bBusby et al, , 2016Hagan et al, 2009;Nagorsen-Rinke et al, 2013;Fleck et al, 2015). Our studies have focused on the western margin of the central Walker Lane along the central Sierra Nevada range front and range crest (Fig.…”

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

“…As summarized in Rood et al (2011), mismatches might arise due to the clustering of past earthquakes (e.g., Rockwell et al, 2000), resulting in an overestimate of geologic strain release rates; geologic deformation accommodated by undetected structures (e.g., Sheehan, 2007), resulting in an underestimate of geologic strain release rates; or geodetic sampling early in the interseismic cycle (e.g., Peltzer et al, 2001;Dixon et al, 2003), resulting in geodetic strain rates that are higher than the average interseismic rate (e.g., when geodetic sampling measures viscoelastic relaxation). Importantly, some slip rate variations have been shown to record coordination of strain between neighboring fault systems (e.g., Lee et al, 2009;Petronis et al, 2009;Nagorsen-Rinke et al, 2013) as opposed to the timing of data collection relative to seismic events. Thus, when evaluating temporal and spatial changes in vertical displacement rates for the Wassuk Range fault, we will need to account for the potential impacts of one or more of the above factors in our interpretation of deformation in the context of the Walker Lane -ECSZ system.…”