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Abstract: We present new data for the age of the Mono Lake Excursion at its type locality. Using the (U-Th)/He system on allanite, we dated Wilson Creek Ash 15 (Lajoie, 1968) to 38.7±1.2ka (2 SE). The new age for this ash supports the hypothesis (Kent et al., 2002; Zimmerman et al., 2006) that the Mono Lake Excursion is coincident with, and probably the same event as, the Laschamp Geomagnetic Excursion (40.4±2ka), an event that shares similar magnetic characteristics with the excursion identified at Mono Lake. We also estimate an allanite magma residence time of slightly less than 30ka based on 230Th/238U disequilibrium and the (U-Th)/He-based eruption age. [Copyright &y& Elsevier]

Presents information on a study which provided new constraints on the evolution of transpression over the last few Myr and on the role played by the San Andreas fault during the uplift of elements of the San Bernardino Mountains. Geological background; Materials and methods; Measurement of replicate helium ages; Conclusions.

Abstract: This study calibrates the production rate of cosmogenic 3He in pyroxene, olivine, garnet, zircon and apatite as well as 21Ne in quartz and pyroxene against the known production rate of 10Be in quartz. The Devil''s Kitchen rhyolite from the Coso volcanic field in southeastern California (elev. ~1300 m) was chosen for this study due to its young age (~610 ka) and diverse mineral assemblage. Based on 10Be, our two rhyolite samples have apparent exposure ages of ~49 and 93 ka, indicating substantial erosion after eruption. Combining data from the two samples, we estimate sea level high latitude 3He spallation production rates of 145±11, 141±16, and 144±30 at g−1 a−1 (2σ) for pyroxene, olivine and spessartine garnet respectively. For zircon and apatite, we estimate apparent 3He spallation production rates of 114±8 and 149±28 at g−1 a−1 (2σ) respectively. The rates for zircon and apatite are reported as apparent production rates because we do not explicitly address the redistribution of spallation produced 3He from adjacent minerals. These estimates quantitatively account for production of 3He from both cosmogenic and radiogenic neutron reactions on 6Li within the analyzed phases and also implanted from nuclear reactions in neighboring minerals; the high U, Th and Li content of this rhyolite provides a particularly rigorous test of this correction. We estimate 21Ne production rates of 17.7±1.6 and 34.1±3.2 at g−1 a−1 (2σ) in quartz and pyroxene (Fe/Mg=0.7 by mass) respectively. Although high U and Th contents create the potential for significant production of nucleogenic 21Ne, this component is small due to the young eruption age of the rhyolite. [Copyright &y& Elsevier]

We present a new method for tracing sediment using detrital apatite (U-Th)/He (AHe) thermochronometry, and use this to quantify the spatial distribution of catchment erosion in the eastern Sierra Nevada, California. Well-developed age-elevation relationships permit detrital AHe ages to track the elevations where sediment grains were shed from bedrock. We analyzed sediment exiting nonglaciated Inyo Creek and adjacent (formerly) glaciated Lone Pine Creek. Statistical comparison of measured AHe age probability density functions (PDFs) with predicted PDFs based on catchment hypsometries suggests that Inyo Creek is eroding uniformly, consistent with field observations of weathered hillslopes tightly coupled to the fluvial system. In contrast, significant mismatch between measured and predicted PDFs from Lone Pine Creek reveals that sediment derives primarily from the lower half of the catchment. The dearth of older ages is likely due to sediment storage in cirques and moraines and/or focused erosion at intermediate elevations, both potential consequences of glacial modification. Measured PDFs can also improve cosmogenic fluclide-based erosion rates by more accurately scaling nuclide production rates. Our results demonstrate the utility of detrital AHe thermochronometry for quantifying erosion in fluvially and glacially sculpted catchments. [ABSTRACT FROM AUTHOR]

Reports apatite/helium age data from the central Sierra Nevada in California, that place limits on the morphology and evolution of longitudinal profiles of major river drainages developed in the Late Cretaceous period. Helium thermochronology and topographic relief; Valley and ridge denudation rates through the Cenozoic era; Elevation of a Cretaceous interior Cordilleran plateau.

U-Th and (U-Th)/He zircon geochronology redefines the timing of volcanic activity in the Salton Trough (Southern California, USA), the subaerial extension of the incipiently oceanic Gulf of California. U-series disequilibrium corrected (U-Th)/He zircon analyses for a granophyre ejecta clast from the Red Island rhyolite dome indicate an eruption age of 2480 ± 470 a (calendric dates between 0 and 940 Before Common Era, BCE; error at 95% confidence). This eruption age is supported by U-Th zircon crystallization ages for two obsidian-bearing lavas: Red Island (the host for the granophyre) and Obsidian Butte, a pre- historic quarry for obsidian that is widely distributed in southern California and northern Mexico archaeological sites. Lavas and granophyre display overlapping zircon crystallization age distributions that support field and compositional evidence that they are cogenetic and contemporaneous. The (U-Th)/He eruption age is younger and significantly more precise than previous ages for these volcanoes, and is the first indication that the eruption of obsidian flows coincided with human presence in the region. A late prehistoric eruption age agrees with the absence of the Obsidian Butte lithic source among early prehistoric cultural artifacts, previously attributed to submergence of the quarry location during hypothesized persistent flooding by ancient Lake Cahuilla. [ABSTRACT FROM AUTHOR]

The Sierra Nevada batholith is an ∼600-km-long, NNW-trending composite arc assemblage consisting of a myriad of plutons exhibiting a distinct transverse zonation in structural, petrologic, geochronologic, and isotopic patterns. This zonation is most clearly expressed by a west-to-east variation from mafic to felsic plutonic assemblages. South of 35.5°N, the depth of exposure increases markedly, and fragments of shallow-level eastern Sierra Nevada batholith affinity rocks overlie deeper-level western zone rocks and subjacent subduction accretion assemblages along a major Late Cretaceous detachment system. The magnitude of displacement along this detachment system is assessed here by palinspastic reconstruction of vertical piercing points provided by batholithic and metamorphic pendant structure and stratigraphy. Integration of new and published U-Pb zircon geochronologic, thermobarometric, (U-Th)/He thermochronometric, and geochemical data from plutonic and metamorphic framework assemblages in the southern Sierra Nevada batholith reveal seven potential correlations between dispersed crustal fragments and the Sierra Nevada batholith autochthon. Each correlation suggests at least 50 km of south- to southwest-directed transport and tectonic excision of ∼5-10 km of crust along the Late Cretaceous detachment system. The timing and pattern of regional dispersion of crustal fragments in the southern Sierra Nevada batholith is most consistent with Late Cretaceous collapse above the underplated accretionary complex. We infer, from data presented herein (1) a high degree of coupling between the shallow and deep crust during extension, and (2) that the development of modern landscape in southern California was greatly preconditioned by Late Cretaceous tectonics. [ABSTRACT FROM AUTHOR]

The White Mountains represent the westernmost range of the central northern Basin and Range province. They are situated to the east of the unextended Sierra Nevada and represent a crustal block that is bounded along its western flank by the high-angle White Mountains fault zone. The fault zone accommodates up to ∼8 km of total dip-slip displacement. Investigation of the structural and thermal history of the White Mountains indicates a two-stage Cenozoic tectonic evolution. Preextensional Miocene volcanic rocks preserved along the eastern side of the range unconformably overlie Mesozoic granitic basement and currently dip up to 25° to the east, recording the total Cenozoic tilt of the crustal block. Apatite fission-track and (U-Th/He) thermochronological data indicate that the White Mountains underwent rapid exhumation and eastward tilting in the middle Miocene, starting at ca. 12 Ma. Geologic mapping (1: 10,000), fault kinematic analysis, and dating of younger volcanic sequences show that following middle Miocene east-west extension, the White Mountains have been dominated by right-lateral transtensional deformation related to the Walker Lane belt. The eruption of late Miocene and Pliocene volcanic sequences in the eastern White Mountains postdates the majority of the uplift of the range, as evidenced by infilling of paleodrainages and the presence of east-directed flow fabrics. Fault kinematic indicators from the White Mountains fault zone are characterized by apparent overprinting of dip-slip fault-motion indicators by right-lateral slickenfibers and fault striations, demonstrating that the range-bounding fault system along the western side of the White Mountains was reactivated as a dextral strike-slip fault system. At the northern and southern ends of the range, Pliocene right-lateral transtension along this northwest-southeast-trending fault systems resulted in the formation of northeast-trending pull-apart basins that truncate the mountain range... [ABSTRACT FROM AUTHOR]