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[1] New structural, paleomagnetic, and apatite (U-Th)/ He results from the continental margin inboard of the Queen Charlotte fault (~54 [degrees]N) delineate patterns of brittle faulting linked to transform development since ~50 Ma. In the core of the orogen, ~250 km from the transform, north striking, dip-slip brittle faults and vertical axis rotation of large crustal domains occurred after ~50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until

In this study we integrate the apatite (U-Th)/He thermochronometric technique with geomorphic, structural, and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late Cenozoic faulting in the southern Sierra Nevada. The geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low-relief landscape, little affected by internal deformation, to a more complex landscape affected by numerous faults. Regionally, the relict landscape surface is readily resolved by age-elevation relationships of apatite He ages coupled to geomorphology. These relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting. On the basis of 63 new apatite He ages and stratigraphic data from proximal parts of the San Joaquin basin we resolve two sets of normal faults oriented approximately N S and approximately NW. Quaternary west-side-up normal faulting along the N-S Breckenridge--Kern Canyon zone has resulted in a southwest step over from the Owens Valley system in the controlling structure on the regional west tilt of Sierran basement. This zone has also served as a transfer structure partitioning Neogene-Quaternary extension resulting from normal displacements on the NW fault set. This fault system for the most part nucleated along Late Cretaceous structures with late Cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region. Citation: Maheo, G., J. Saleeby, Z. Saleeby, and K. A. Farley (2009), Tectonic control on southern Sierra Nevada topography, California, Tectonics, 28, TC6006, doi:10.1029/2008TC002340.

[1] The Taiwan mountain belt is composed of a Cenozoic slate belt (Hsuehshan Range units, HR, and Backbone Slates, BS) and of accreted polymetamorphic basement rocks (Tananao Complex, TC). Ongoing crustal shortening has resulted from the collision between the Chinese continental margin and the Luzon volcanic arc, which initiated ~6.5 Ma ago. The grade and age of metamorphism and exhumation are a key record of the development of the orogenic wedge. Because the Taiwan mountain belt is mostly composed by accreted sediments lacking metamorphic index minerals, quantitative constraints on metamorphism are sparse. By contrast, these rocks are rich in carbonaceaous material (CM) and are therefore particularly appropriate for RSCM (Raman Spectroscopy of CM) thermometry. We apply this technique in addition to (U-Th)/He thermochronology on detrital zircons to assess peak metamorphic temperatures (T) and the late exhumational history respectively, along different transects in central and southern Taiwan. In the case of the HR units, we find evidence for high metamorphic T of at least 340[degrees]350 [degrees]C and locally up to 475[degrees]C, and for relative rapid exhumation with zircon (U-Th)/He ages in the range of 1.5-2 Ma. Farther east, the BS were only slightly metamorphosed (T < 330 [degrees]C), and zircons are not reset for (U-Th)/He. From the eastern BS to the inner TC schists, T gradually increases from ~350[degrees]C up to ~500[degrees]C following an inverted metamorphic gradient. Available geochronological constraints and the continuous thermal gradient from the BS to the basement rocks of the TC suggest that the high RSCM T of the TC were most probably acquired during the last orogeny, and were not inherited from a previous thermal event. Zircons yield (U-Th)/He ages of ~0.5-1.2 Ma. Peak metamorphic T and the timing of exhumation do not show along-strike variations over the TC in the studied area. In contrast, exhumation is laterally diachronous and decreases southward in the case of the HR units. In particular, our data imply that the HR units have been exhumed by a minimum of 15 km over the last few Ma. In the case of the BS, they show far less cumulated exhumation and much slower cooling rates. We propose that most of the deformation and exhumation of the Taiwan mountain belt is sustained through two underplating windows located beneath the Hsuehshan Range and the TC. Our data show significant departures from the predictions of the prevailing model in Taiwan, which assumes a homogeneous critical wedge with dominant frontal accretion. Our study sheds new light on how the mountain belt has grown as a possible result of underplating mostly. doi: 10.1029/2006TC002064.

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 nuclide-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. Keywords: detrital thermochronometry, apatite (U-Th)/He, cosmogenic nuclides, erosion, Sierra Nevada.

Alpine glaciers are efficient agents of erosion and capable of significantly modifying topography. Despite recent advances in theoretical and field studies that quantify glacial erosion processes, few studies have documented glacial erosion rates over long (>[10.sup.6] yr) or large (more than tens of kilometers) scales. We use apatite (U-Th)/He (AHe) and apatite fission track (AFT) cooling ages to address the late Miocene to Holocene erosion history across two 60-km-long transects of the heavily glaciated southern Coast Mountains, British Columbia. Observed AHe cooling ages from equal elevation samples range between 1.5 and 8 Ma and suggest that thick alpine glaciers resulted in a 16 km shift of the highest point in the topography in the past 1.5-4.0 m.y. We evaluated temporal and spatial variations in erosion rates using a three-dimensional thermal-kinematic model that predicted AHe and AFT ages at the surface for different erosion histories. Comparison of model predicted and observed cooling ages suggests an increase in erosion rates of as much as 300% over the past 1.5-7 m.y., coincident with the onset of glaciation of this range. Keywords: glacial erosion, apatite (U-Th)/He, numerical modeling, Coast Mountains, paleotopography.

(U-Th)/He dating of late-stage authigenic goethite, combined with corrections for diffusive loss of [sup.4]He by the [sup.4]He/[sup.3]He methodology, reveals strong correlation between a sample's age and its depth in ferruginized channel sediments from the Yandicoogina deposit, Western Australia. Corrected ages, ranging from ca. 18 Ma near the surface to ca. 5 Ma at the bottom of the profile, indicate that ferruginization of the aggraded channels becomes progressively younger with depth. This trend is consistent with goethite precipitation at the groundwater-atmosphere interface during water table drawdown driven by the aridification of Western Australia during the Neogene. The results demonstrate that the (U-Th)/He system is ideal for dating goethite if diffusive loss corrections are applied. The approach is suitable for dating weathering reactions on Earth and should also be suitable for dating Fe oxyhydroxides in the Martian regolith. Keywords: (U-Th)/He dating, goethite, weathering geochronology, channel iron deposits, iron ore.

The end-Permian mass extinction (251 Ma) was the largest in Earth's history, and the great extent of biospheric perturbation is recorded as dramatic shifts in carbon isotope ratios of sedimentary materials. Both terrestrial and extraterrestrial events are commonly invoked as causative mechanisms for the crisis, and the primary reason for the event remains the subject of controversy. Geochemical indicators sensitive to the influence of extraterrestrial material involve platinum group elements and osmium and helium isotope ratios. Analyses of extinction levels in two sections from Austria and Italy reveal no evidence of an extraterrestrial impact. The end-Permian crisis, it appears, was a homegrown catastrophe. Keywords: Permian-Triassic boundary, impacts, mass extinctions, Os isotopes, He isotopes.

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 representa 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[degrees] to the east, recording the total Cenozoic tilt of the crustal block. Apatite fission-track and (U-Th/He) thermochronoiogical 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 and transfer strike-slip displacement eastward from the Owens Valley fault zone to the Fish Lake Valley fault zone. The inception of strikeslip faulting in Fish Lake Valley occurred at ca. 6 Ma as constrained by late Miocene volcanic units. Right-lateral faulting on the western side of the White Mountains occurred at ca. 3 Ma and is distinctly younger than the faulting in the Fish Lake Valley area, indicating a westward migration of transcurrent deformation through time. Keywords: White Mountains, Basin and Range, Walker Lane belt, tectonics, thermochronology.

Uranium, thorium and helium age profiles are presented for Late Cretaceous transverse river drainages in the Sierra Nevada Mountains of California. The profiles place certain elevations at slightly older ages than previous studies. Calculations from these findings indicate a Cordilleran plateau lay at a 3,000 m elevation during the Cretaceous era.

A technique to estimate the age of topographic relief using the ages of apatites determined from uranium, thorium and helium content is described. The results from the sierra Nevada, California, suggest that the sierra developed an Andean size topography between 185 Myr and 70-80 Myr. This coincided with crustal thickening. A gradual reduction of mean crestal elevation is inferred.

A study was conducted on the uplift and erosion of the San Bernardino Mountains (SBMs) in southern California. The low-temperature cooling history of the SBMs was analyzed using radiogenic helium thermochronometry. Results showed that the helium ages of the uplift predate the present uplift of the range and that exhumation was only several kilometers since the Late Cretaceous. These results suggest that the uplift and exhumation is associated with transpression along the San Andreas fault.

Researchers have characterized extraterrestrial (ET) helium, likely carried by interplanetary dust particles (IDPs), in deep‐sea sediments spanning more than the last 100 Myr. Here we complement those low resolution and deep time studies by measuring He in modern Antarctic air and recent ice. We analyzed 180 air filter samples collected in 2017 and 2018 at the South Pole and detected 3He above blank levels in 178. The filters collected during the austral springs had elevated 3He in multiple subsamples indicating the presence of many individual IDPs and potentially, a temporal variation in the ET small particle flux. Our calculated mean 3He flux of 1.4 ± 1.2 × 10−12 cc STP cm−2 ka−1 is the first such measurement from air samples. We also melted, filtered, and analyzed one hundred and forty‐one 1 m‐long ice sections from a ˜2000‐yr‐old South Pole core. We detected 3He above blank levels in 139 of the 141 ice core samples and calculated an average flux of 1.2 ± 0.3 × 10−12 cc STP cm−2 ka−1. Our two flux values are within a factor of two of those calculated from stratospheric IDP concentrations, those previously measured for sections of the GISP2 and Vostok ice cores, and from sediment cores from different locations and ages. The similarity of these flux values over disparate time scales (1–108 yr) and geographic locations (90° S to equator) indicates modest temporal variability and remarkable agreement among diverse IDP archives. These data provide a compelling link from IDPs collected in the stratosphere to those recorded in deep time sedimentary archives. [ABSTRACT FROM AUTHOR]

A study of oxygen isotopes suggests that sediment does not survive subduction zone conditions. An oxygen isotope equilibrium exists between olivine phenocrysts and plagioclase as revealed by the studies of phenocrysts from basalts from Pitcairn Island, in the Southeast Pacific Ocean. Possible causes for the differences in the results of the study and analysis of Pitcairn seamount glasses are discussed.