ABSTRACT We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.

Paleomagnetic data from three regionally extensive Oligocene ignimbrite sheets, two sequences of Miocene andesite flows, and ten sequences of Upper Miocene to Pliocene basaltic andesite flows in the Candelaria Hills and adjacent areas, west-central Nevada, provide further evidence that, since the late Miocene, and possibly between latest Miocene and earliest Pliocene time, the broad region that initially facilitated Neogene displacement transfer between the Furnace Creek and central Walker Lane fault systems experienced some 20° to 30° of clockwise vertical-axis rotation. The observed sense and magnitude of rotation are similar to those previously inferred from paleo-magnetic data from different parts of the Silver Peak Range to the south. We propose that clockwise rotation within the transfer zone formed in response to horizontal components of simple and pure shear distributed between early-formed, northwest-striking right-lateral structures that initiated in mid- to late Miocene time. Notably, the spatial distribution of the early-formed transfer zone is larger and centered south of the presently active stepover, which initiated in the late Pliocene and is characterized by a trans-tensional deformation field and slip on east-northeast–oriented left-oblique structures that define the Mina deflection. The sense and magnitude of rotation during this phase of deformation, which we infer to be of pre–latest Pliocene age, are inconsistent with the geodetically determined regional velocity field and seismologically determined strain field for this area. As a consequence, the longer-term kinematic evolution of the stepover system, and the adjoining parts of the Furnace Creek and Walker Lane fault systems, cannot be considered as a steady-state process through the Neogene.

The Queen Valley pull-apart basin is located at the northern extent of the White Mountains in western Nevada. The basin is bounded to the south by the NE-trending Queen Valley fault zone and to the north by the E-W–trending Coaldale fault zone. The curvilinear trace of the Queen Valley normal fault extends ~16 km northeast from the northern termination of the Owens Valley–White Mountain fault zone to the western Coaldale fault system. Using new (U-Th)/He and 40 Ar/ 39 Ar geochronology, fault kinematic data, and detailed geologic mapping (1:10,000), this study documents a three-stage late Tertiary tectonic evolution of the eastern Queen Valley area and defines the role of the Queen Valley fault system as an integral part of the right-lateral transtensional Walker Lane belt. The Queen Valley area was affected by an ignim-brite flare-up in Utah, Nevada, and California, as recorded by late Oligocene rhyolites (ca. 26 Ma). The eruption of these widespread ash flows was accompanied locally by extension, creating a series of ENE-trending half grabens. The faults are sealed by Miocene andesite (ca. 12 Ma), constraining the timing of extension to late Oligo-cene or early Miocene. Mid-Miocene Basin and Range extension produced E-dipping normal fault systems in the Yerington area to the north and W-dipping normal faults in the White Mountains to the south. Displacement between these fault systems with opposite polarity was accommodated by a series of right-lateral faults in the Queen Valley area. A change in extension direction from E-W extension to NW-SE during the Pliocene resulted in a transition to transcurrent and transtensional structures in the central Walker Lane belt. The beginning of transtension on the east side of the White Mountains was marked by the opening of the Fish Lake Valley pull-apart basin at ca. 6 Ma, as constrained by Upper Miocene volcanic units. Similarly, the Queen Valley pull-apart basin was a product of the reactivation of the White Mountain–Owens Valley fault zone as a right-lateral fault ca. 3 Ma, based on thermochronological data and offset Pliocene basaltic andesite (ca. 3.1 Ma) along the Queen Valley fault.