Evolution and Strain Reorganization within Late Neogene Structural Stepovers Linking the Central Walker Lane and Northern Eastern California Shear Zone, Western Great Basin

Abstract

Late Neogene deformation between the eastern Sierra Nevada and the central Basin and Range province is localized within a boundary zone that contains the Eastern California shear zone and Walker Lane, and has been characterized by dextral strike-slip to oblique-slip displacement on NNW- to northwest-trending fault systems. At the latitude of the central Sierra Nevada, structures in the northern Eastern California Shear Zone and central Walker Lane are misaligned and have been linked kinematically since the mid-Miocene by a system of extensional stepover structures. The stepovers are localized along an ancient crustal boundary and record a complex deformation history marked by a major reorganization of structural geometry and kinematics in the mid-Pliocene. Displacement transfer was initially taken up by the Silver Peak-Lone Mountain extensional complex that passed motion on the Furnace Creek fault system in the Eastern California shear zone ENE into a system of en echelon transcurrent faults of the central Walker Lane. The Miocene to Pliocene extensional complex was underlain by a northwest-dipping detachment that initiated at 12-8 Ma and continued activity into the early Pliocene. The extensional complex was bound on the west and east by northwest-trending transcurrent structures linked along the southern boundary by a northwest-dipping breakaway fault. To the north, an E-W-trending belt of extensional basins separated the extensional complex from structurally coherent tectonic blocks in the central Walker Lane. Paleomagnetic data from Cenozoic volcanic rocks and Mesozoic intrusions exposed in the upper plate of the extensional complex and sets of Miocene mafic dikes in the lower plate show uniform clockwise vertical-axis rotation of 20° to 30°. Localization of large-magnitude extension and vertical- axis rotation within the stepover formed during nonplane strain deformation characterized by vertical thinning and horizontal components of simple and pure shear operating along a northwest axis. This state of deformation persisted until the extensional complex was abandoned at about 3 Ma. In the late Pliocene, the stepover linking the Eastern California shear zone and central Walker Lane broadened and experienced kinematic reorganization with the onset of the contemporary deformation field. The active displacement transfer system, known as the Mina deflection, is underlain by an array of ENE faults with left-oblique slip that cross-cut and locally reactivated structures of the Silver Peak-Lone Mountain extensional complex. Geodetic velocities, earthquake focal mechanisms, and fault kinematic data reflect the transtensional deformation within the active boundary zone that accommodates northwestward displacement of the Sierra Nevada away from the central Great Basin. Strain is heterogeneously distributed and the eastern and western parts of the boundary zone have experienced wrench-dominated and extension-dominated transtension, respectively. The changes in geometry and kinematics of structural stepovers within the tectonic boundary zone between the Sierra Nevada and central Great Basin indicate that late Neogene deformation was not a steady-state process.