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.

The late Miocene to Pliocene Silver Peak–Lone Mountain extensional complex in the western Great Basin is part of a structural stepover that links dextral transcurrent motion between the Furnace Creek fault system and northwest-striking transcurrent faults in the central Walker Lane. In the Silver Peak Range, the extensional complex is exposed as a west-northwest–trending turtleback structure that consists of a folded detachment fault separating a metamorphic lower-plate assemblage from unmetamorphosed upper-plate rocks. The upper plate preserves structurally attenuated lower Paleozoic carbonate and clastic rocks, upper Oligocene to lower Miocene volcanic rocks, and a synextensional mid-Miocene to Pliocene clastic and volcanic succession. The three-dimensional geometry of fault-bounded extensional basins formed during displacement on the detachment is preserved, and the synextensional units comprise five sequences separated by unconformities. The locus of deposition migrated spatially as dimensions of small basins changed through time. The entire extensional complex is deformed in two generations of late Cenozoic folds. North-northeast–trending folds formed first with axial traces oriented at a high-angle to upper-plate extension, and these are preferentially developed in proximity to thrusts and partially inverted extensional faults. Younger, west-northwest–trending folds parallel the axis of the turtleback structure and involve all lithologic units, several syndepositional high-angle faults, and the basal detachment. Pliocene growth of west-northwest–trending folds marked the end of slip on the exposed parts of the basal décollement and the cessation of deposition in the fault-bounded basins. Today, upper and lower plates of the extensional complex are dissected by north- to northeast-striking normal faults that cut alluvium and cross-cut and locally reactivate earlier Cenozoic structures.

Abstract Esmeralda County, Nevada, is extraordinary for the presence of Ediacaran and early Cambrian reefs at several stratigraphic positions. In this road log and field guide we present descriptions and interpretations of the most instructive exposures of three of these reef-rich intervals: (1) the Mount Dunfee section of the Middle Member of the Deep Spring Formation (Ediacaran in age), (2) the Stewart's Mill exposure of the Lower Member of the Poleta Formation (mid-early Cambrian), and (3) an exposure on the north flank of Slate Ridge of reefs near the top of the Harkless Formation (latest early Cambrian). We introduce the term “congruent ecosystems” for ecosystems of different age that occupied similar environments. The Ediacaran reefs of the Deep Spring Formation and the early Cambrian reefs of the Lower Member of the Poleta Formation occupied similar environments but exhibit distinctively different ecological structure. Thus we propose these two reef complexes as our premier example of non-congruent communities within congruent ecosystems.

Paleomagnetic and geochronologic data from mafic intrusive rocks, inferred to contain magnetizations of early Late Cretaceous age, and upper Tertiary volcanic rocks, all part of the upper plate of the Silver Peak extensional complex in the southern Silver Peak Range, add to the growing body of results suggesting that Neogene displacement transfer within the central Walker Lane involved components of modest magnitude crustal tilting and, at least locally, rotation of structural blocks. Mesozoic intrusions and upper Tertiary volcanic rocks yield paleomagnetic data that are discordant to expected field directions. The data from 49 accepted sites in mafic dikes that cut granitic rocks, 4 sites in a single Oligocene(?) ash flow tuff, 20 sites in mid-Miocene andesite flows, and 28 sites in upper Miocene to lower Pliocene pyroclastic rocks may imply a systematic progression in the magnitude of vertical axis rotation and tilting with age. At a minimum, the data are consistent with at least some 20° of clockwise rotation of upper-plate rocks in this part of the Silver Peak Range and demonstrate a greater regional extent to the area affected by clockwise rotation during Neogene displacement transfer. Eight new 40 Ar/ 39 Ar age determinations from the mafic dikes and adjacent host rocks, all somewhat disturbed age spectra, imply that these rocks cooled below ∼300 °C during the Late Cretaceous between about 90 and 80 Ma. Four mafic dike groundmass concentrates yield integrated apparent ages between 86.31 Ma ± 0.12 Ma and 80.80 Ma ± 0.11 Ma, and four age spectra from biotite from the host granite yield integrated values between 93.6 ± 0.9 Ma and 78.6 Ma ± 0.2 Ma. The mafic dikes yield in situ exclusively normal polarity results consistent with an early Late Cretaceous age of magnetization acquisition, with an overall group mean (D = 25.1°, I = 55.4°, α 95 = 3.4°) that is discordant to an early Late Cretaceous expected field (D = 337°, I = 66°). Ten of 20 sites from steeply dipping mid-Miocene andesite flows and 21 of 28 sites in gently tilted upper Miocene ash flow tuffs yield overall stratigraphically corrected group means (D = 24.4°, I = 36.7°, α 95 = 7.1°) and (D = 16.5°, I = 53.5°, α 95 = 7.6°, respectively) that are discordant in a clockwise sense to the Miocene expected direction (D = 358°, I = 55°). The paleomagnetic data support a history of tilting and vertical axis rotation of the southern Silver Peak Range, most of which occurred coincidently with latest Miocene and Pliocene exhumation of the lower-plate rocks in the extensional complex. In addition, it is possible that the paleomagnetic data from Mesozoic intrusions record an additional, modest phase of deformation that predated development of the extensional complex. The observations are consistent with a tectonic model where deformation of upper-plate rocks in this area involved a small component of west- to southwest-side-down tilting, likely related to range-scale folding during the late Miocene and Pliocene, accompanied by modest clockwise vertical axis rotation.