Abstract

The timing, magnitude, style, and kinematics of deformation in the Eastern California shear zone and Walker Lane belt are important in defining interactions between shear-dominated plate-boundary tectonics and extension-dominated plate-interior tectonics. Geologic studies of middle Miocene strata in a 50-km-long north-south belt along the west margin of the Wassuk Range, west-central Walker Lane belt, reveal a pattern of folding on subhorizontal axes associated with displacements on convex-upward faults, a pattern that greatly reduces estimates of extension. We report an array of previously unrecognized shortening structures, including east-west folds, reverse faults, and steep-axis folds that formed at high angle to, and synchronous with, regional extension in the Coal Valley portion of the Walker Lane belt west of the Wassuk Range. We also report new radiometric age data and field relationships that support a reinterpretation of the history, magnitude, and distribution of extensional deformation. Instead of extreme (150%–200%) extension at 15–13 Ma, the new data support moderate (perhaps 30%) extension beginning ca. 10 Ma, coincident with the development of an inboard component of plate-boundary transtensional deformation. Also, new age data from moderately tilted pre-extension volcanic rocks forming the lower part of the Tertiary stratigraphic section in the southern Singatse Range, west of the northern Wassuk Range, show a lack of the extreme extension reported for coeval strata in the central part of that range. Published tectonic models that assume that extreme middle Miocene extension was uniformly distributed from the central Singatse Range through the Coal Valley area and southward to the Mina Deflection are invalid, as are models of 100 km of westerly extensional strain migration from the Basin and Range into the Sierra Nevada. Our reinterpretation reflects a return to a Miocene history, developed almost four decades ago, of formation of a volcanic highland, followed by sedimentation in broad basins controlled partly by east-west structures, followed in turn by extensional deformation that formed the existing ranges and basins after ca. 10 Ma. Following these early studies, the history was revised based on geologic mapping and thermochronologic studies in the Gray Hills–Wassuk Range directly north of the Coal Valley area.

The published thermochronologic studies of a transect across the Wassuk Range show approximate invariance across 4 km of the central range. Previously, these data had been modeled as recording rigid, whole-block west tilting of ∼50° at ca. 15 Ma, and the approximate invariance resulted in geologically instantaneous uplift of ∼6 km. However, middle Miocene strata lack evidence for such rapid large uplift; that is, large volumes of proximal coarse clastic rocks are not found, and the strata do not exhibit a pattern of growth-fault fanning expected in the tilted fault-block model. The invariance is more consistent with arching during and following magmatism than with fault-related whole-block tilting. During main-phase extension, which began at ca. 10 Ma, the range was again flexed during uplift, similar to smaller structural blocks directly to the west.

How or whether the extension-normal shortening structures we describe accommodated plate-boundary strain is not clear. Northwest-striking dextral faults are not reported in the Coal Valley area or to the northwest, so there is no clear association between the shortening structures and strain accommodation at terminations, left bends, or stepovers of such faults. We speculate that the complex heterogeneous Miocene strain of the Coal Valley area records coupling of approximately east-west regional extension with extension-normal shortening. The shortening could record midcrustal flow, possibly responding to lateral gravity gradients.

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