James E. Conrad

A tale of 10 plutonsúRevisited: Age of granitic rocks in the White Mountains, California and Nevada

40 Ar/ 39 Ar incremental heating analysis and conventional K-Ar age determinations on plutonic rocks of the White Mountains define two stages of magmatic emplacement: Late Cretaceous, between ca. 90 Ma and 75 Ma, and Middle–Late Jurassic, between ca. 180 and 140 Ma. The Jurassic stage can be divided into two substages, 180–165 Ma and 150–140 Ma. Thermal effects of the younger plutons on the older granitoids partially to completely reset ages, making it difficult to determine the age of emplacement and cooling of several of the plutons even by 40 Ar/ 39 Ar incremental heating analyses. New data together with published ages and regional geochronological synthesis of the Sierra Nevada batholith indicate that regions within the batholith have coherent periods or episodes of magmatic activity. In the White Mountains and Sierra Nevada directly to the west there was little or no activity in Early Jurassic and Early Cretaceous time; magmatism took place during relatively short intervals of 15 m.y. or less in the Middle and Late Jurassic and Late Cretaceous periods. The new K-Ar and 40 Ar/ 39 Ar analyses of granitoids from the White Mountains help, but do not completely clarify the complex history of emplacement, cooling, and reheating of the batholith.

Constraints on the development of Proterozoic basins in central India from 40Ar/39Ar analysis of authigenic glauconitic minerals

Ages of some key stratigraphic sequences in central Indian Proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates. To help improve age constraints, single grains of glauconitic minerals taken from sandstone and limestone in two Proterozoic sequences in the Pranhita-Godavari Valley and the Chattisgarh basin were analyzed by the 40 Ar/ 39 Ar incremental heating method. Analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation, and anomalous ages that result from inherited argon or postcrystallization heating. The analyses indicate an age of 1686 ± 6 Ma for the Pandikunta Limestone and 1566 ± 6 Ma for the Ramgundam Sandstone, two units in the western belt of Proterozoic sequences in Pranhita-Godavari Valley. Glauconite from the Chanda Limestone, in the upper part of this sequence, contains inherited 40 Ar but is interpreted to reflect an age of ca. 1200 Ma. Glauconite from the Somanpalli Group in the eastern belt of the Pranhita-Godavari Valley gives an age of 1620 ± 6 Ma. In the Chattisgarh basin, glauconite from two units gives disturbed ages that suggest a period of regional heating in the Chattisgarh basin at ca. 960–1000 Ma. These new ages indicate that these sequences are 200–400 m.y. older than previously recognized, which has important implications for geochemical studies of Mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy.

Slip Rate on the San Diego Trough Fault Zone, Inner California Borderland, and the 1986 Oceanside Earthquake Swarm Revisited

The San Diego trough fault zone (SDTFZ) is part of a 90-km-wide zone of faults within the inner California Borderland that accommodates motion between the Pacific and North American plates. Along with most faults offshore southern California, the slip rate and paleoseismic history of the SDTFZ are unknown. We pre- sent new seismic reflection data that show that the fault zone steps across a 5-km-wide stepover to continue for an additional 60 km north of its previously mapped extent. The 1986 Oceanside earthquake swarm is located within the 20-km-long restraining stepover. Farther north, at the latitude of Santa Catalina Island, the SDTFZ bends 20° to the west and may be linked via a complex zone of folds with the San Pedro basin fault zone (SPBFZ). In a cooperative program between the U.S. Geological Survey (USGS) and the Monterey Bay Aquarium Research Institute (MBARI), we measure and date the coseismic offset of a submarine channel that intersects the fault zone near the SDTFZ-SPBFZ junction. We estimate a horizontal slip rate of about 1:5 0:3 mm=yr over the past 12,270 yr.

Style and age of late Oligocene‐early Miocene deformation in the southern Stillwater Range, west central Nevada: Paleomagnetism, geochronology, and field relations

Paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late Oligocene to early Miocene episode of large magnitude extension in the southern Stillwater Range and adjacent regions of west central Nevada. The southern Stillwater Range was the site of an Oligocene to early Miocene volcanic center comprising (1) 28.3 to 24.3 Ma intracaldera ash flow luffs, lava flows, and subjacent plutons associated with three calderas, (2) 24.8 to 20.7 Ma postcaldera silicic dikes and domes, and (3) unconformably overlying 15.3 to 13.() Ma dacite to basalt lava flows, plugs, and dikes. The caldera-related luffs, lava flows, and plutons were tilted 60°-70° either west or east during the initial period of Cenozoic deformation that accommodated over 100% extension. Directions of remanent magnetization obtained from these extrusive and intrusive, caldera-related rocks are strongly deflected from an expected Miocene direction in senses appropriate for their tilt. A mean direction for these rocks after tilt correction, however, suggests that they were also affected by a moderate (33.4° ± 11.8°) component of counterclockwise vertical axis rotation. Paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24.8 to 20.7 Ma postcaldera dikes and domes. In detail, an apparent decrease in rotation with decreasing age of individual, isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24.4 and 24.2 Ma. The onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked. Deformation was not driven by magma buoyancy, however, because tilting equally affected the caldera systems of different ages, including their plutonic roots. It is more likely that regional extension was focused in the southern Stillwater Range due to magmatic warming and reduction of tensile strength of the brittle crust. Faults that accommodated deformation in the southern Stillwater Range initially dipped steeply and cut deeply to expose more than 9 km of crustal section. The exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early Miocene brittle-ductile transition.

Better dates for Arctic dinosaurs

The Prince Creek Formation near Ocean Point on the Arctic coastal plain of northern Alaska contains hadrosaur (duck-billed dinosaur) bones, as well as an abundant fauna of mollusks, ostracodes, brachiopods, foraminifers, and palynomorphs. Evaluation of the marine mollusks and ostracodes suggested a Paleocene age for these strata to Marincovich et al. [1985]. A fission-track age on zircon of 50.9 +/− 7.7 Ma from an interbedded tephra [Carter et al., 1977] suggested that they could be as young as early Eocene. If the mollusk, ostracod, and fission-track ages are correct, the hadrosaurs would be of early Cenozoic age—one of the few recorded occurrences of dinosaurs younger than Cretaceous. The foraminifers, pollen, and spores, however, strongly suggest a Late Cretaceous age for their beds [Brouwers et al., 1987].

The Palos Verdes Fault offshore Southern California: Late Pleistocene to present tectonic geomorphology, seascape evolution, and slip rate estimate based on AUV and ROV surveys

The Palos Verdes Fault (PVF) is one of few active faults in Southern California that crosses the shoreline and can be studied using both terrestrial and subaqueous methodologies. To characterize the near-seafloor fault morphology, tectonic influences on continental slope sedimentary processes and late Pleistocene to present slip rate, a grid of high-resolution multibeam bathymetric data, and chirp subbottom profiles were acquired with an autonomous underwater vehicle (AUV) along the main trace of PVF in water depths between 250 and 600 m. Radiocarbon dates were obtained from vibracores collected using a remotely operated vehicle (ROV) and ship-based gravity cores. The PVF is expressed as a well-defined seafloor lineation marked by subtle along-strike bends. Right-stepping transtensional bends exert first-order control on sediment flow dynamics and the spatial distribution of Holocene depocenters; deformed strata within a small pull-apart basin record punctuated growth faulting associated with at least three Holocene surface ruptures. An upper (shallower) landslide scarp, a buried sedimentary mound, and a deeper scarp have been right-laterally offset across the PVF by 55 ± 5, 52 ± 4 , and 39 ± 8 m, respectively. The ages of the upper scarp and buried mound are approximately 31 ka; the age of the deeper scarp is bracketed to 17–24 ka. These three piercing points bracket the late Pleistocene to present slip rate to 1.3–2.8 mm/yr and provide a best estimate of 1.6–1.9 mm/yr. The deformation observed along the PVF is characteristic of strike-slip faulting and accounts for 20–30% of the total right-lateral slip budget accommodated offshore Southern California.

Chirp seismic-reflection data of field activity S-12-10-SC: Oceanside to La Jolla, offshore southern California from 2010-06-01 to 2010-06-12

This dataset includes raw and processed, high-resolution seismic-reflection data collected in 2010 to collect information on active offshore faults. The survey is area is offshore southern California between Oceanside and La Jolla. The data were collected aboard the U.S. Geological Survey R/V Parke Snavely. The seismic-reflection data were acquired using an EdgeTech 512 chirp subbottom profiler. Subbottom acoustic penetration spanned tens to about 50 meters, variable by location.

Minisparker seismic-reflection data of field activity B-1-08-SC: between Huntington Beach and San Diego, offshore southern California from 2008-04-28 to 2008-05-05

This dataset includes raw and processed, high-resolution seismic-reflection data collected in 2008 to collect information on active offshore faults. The survey area is offshore southern California between Huntington Beach and San Diego. The data were collected aboard the R/V Bold. The seismic-reflection data were acquired using a SIG 2mille minisparker. Subbottom acoustic penetration spanned tens to several hundreds of meters, variable by location.

Benthic habitat and geologic mapping of the outer continental shelf of north-central California

Point based visual observations of benthic habitat from a samples obtained with a video sled.