Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U-th)/He thermochronology

High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperature-time paths inferred from 40Ar/39Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar40Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (<1°C/m.y.) from Jurassic times until the late Miocene or early Pliocene. The regional extent and consistency of thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rates were low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30°-50°C/m.y.) coincident with exhumation from inferred depths of ~8-10 km, at denudation rates of 1-2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period (~3°C/m.y.), suggesting limited exhumation (1-2 km). However, these samples record a slight increase in cooling rate (from <1 to ~3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene (~5-12 Ma)

A revised alpha-ejection correction calculation for (U-Th)/He thermochronology dates of broken apatite crystals

Accurate corrections for the effects of alpha ejection (the loss of daughter He near grain or crystal surfaces due to long alpha stopping distances) are central to (U-Th)/He thermochronometry. In the case of apatite (U-Th)/He dating, alpha-ejection correction is complicated by the fact that crystals are often broken perpendicular to the c axis. In such cases, the correction should account for the fact that only some parts of the crystal are affected by alpha ejection. A common current practice to account for such broken crystals is to modify measured lengths of broken crystals missing one termination by a factor of 1.5, and those missing both terminations by a factor of 2. This alpha-ejection "correction correction"systematically overestimates the actual fraction of helium lost to alpha ejection, and thus overcorrects the measured date relative to that determined for an otherwise equivalent unbroken crystal. The ratio of the alpha-ejection-affected surface area to the volume of a fragmented crystal is equivalent to the surface-area-to-volume ratio of an unbroken crystal that is either twice as long (for fragments with one termination) or infinitely long (for fragments with no termination). We suggest that it is appropriate to revise the fragmentation correction to multiply the lengths of crystals missing one c-axis termination by 2, and those missing both c-axis terminations by some large number 320. We examine the effect of this revised correction and demonstrate the accuracy of the new method using synthetic datasets. Taking into account alpha ejection, the rounding of the He concentration profile due to diffusive loss, and the accumulation of radiation damage over a range of thermal histories, we show that the revised fragmentation alpha-ejection correction proposed here accurately approximates the corrected date of an unbroken crystal ("true"date) to within <0.7% on average (±4.2%, 1σ), whereas the former method overcorrects dates to be g1/43% older than the "true"date on average. For individual grains, the former method can result in dates that are older by a few percent in most cases, and by as much as 12% for grains with aspect ratios of up to 1:1. The revised alpha-ejection correction proposed here is both more accurate and more precise than the previous correction, and does not introduce any significant systematic bias into the apparent dates from a sample. © 2022 John J. Y. He.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Characteristics and Consequences of Red Bed Bleaching by Hydrocarbon Migration: A Natural Example From the Entrada Sandstone, Southern Utah

Extensive regions of yellow and white (“bleached”) sandstones within the terrestrial Jurassic red bed deposits of the Colorado Plateau reflect widespread interaction with subsurface reduced fluids which resulted in the dissolution of iron-oxide grain coatings. Reduced fluids such as hydrocarbons, CO2, and organic acids have been proposed as bleaching agents. In this study, we characterize an altered section of the Slick Rock member of the Jurassic Entrada Sandstone that exposes bleached sandstone with bitumen-saturated pore spaces. We observe differences in texture, porosity, mineralogy, and geochemistry between red, pink, yellow, and gray facies. In the bleached yellow facies we observe quartz overgrowths, partially dissolved K-feldspar, calcite cement, fine-grained illite, TiO2-minerals, and pyrite concretions. Clay mineral content is highest at the margins of the bleached section. Fe2O3 concentrations are reduced up to 3× from the red to gray facies but enriched up to 50× in iron-oxide concretions. Metals such as Zn, Pb, and rare-earth elements are significantly enriched in the concretions. Supported by a batch geochemical model, we conclude the interaction of red sandstones with reduced hydrocarbon-bearing fluids caused iron-oxide and K-feldspar dissolution, and precipitation of quartz, calcite, clay, and pyrite. Localized redistribution of iron into concretions can account for most of the iron removed during bleaching. Pyrite and carbonate stable isotopic data suggest the hydrocarbons were sourced from the Pennsylvanian Paradox Formation. Bitumen in pore spaces and pyrite precipitation formed a reductant trap required to produce Cu, U, and V enrichment in all altered facies by younger, oxidized saline brines. © 2022. The Authors.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Thermal-topographic modeling of the Northern Apennines

Geomorphology can have a strong influence on thermochronometer. In particular, the closure isotherm will tend to form a smoothed version of the surface topography, with greater topographic relief producing larger deflections of the isotherm. Also, erosion rate will influence the mean closure depth given that the faster erosion causes isotherms to migrate to shallower depths. We report on modeling here using the PECUBE software (Braun, 2003), in which we solved the heat transport equation in three dimensions by conduction and advection together with an evolving surface topography, to constrain the evolution of surface relief in the Northern Apennines. In particular, we wish to constrain the decrease in surface topography that may have occurred during the emergence of the orogen as it started interacting with the Apulian margin some 5-10 Myr ago. The Neighbourhood Algorithm (or NA) (Sambridge, 1999) is used to constrain the values of free parameters (i.e. time at which the relief starts to decrease, amount of topography decrease, crustal elastic thickness) by minimizing a misfit function derived from observed and modeled ages. We used a 128-node cluster of PCs to sample parameter space in a meaningful way. Dataset comprises zircon and apatite fission-track and zircon and apatite (U-Th)/He ages obtained through the Northern Apennine chain. The spatial distribution of ages shows a decrease eastwards, with the oldest apatite fisison-track ages of about 11 Ma to the west and the youngest of about 3 Ma along the central range. Results obtained by NA inversion indicate that the dataset can be equally explained with low values of the relief loss and elastic plate thickness or higher values of the relief loss and elastic plate thickness. However, beacuse many of He ages and the general pattern of younger ages near the central axis of the orogen are not well reproduced in the predictions, a detailed investigation of age-elevation relationships suggests that a model in which the denudation is solely driven by surface erosion and isostasy is not appropriate for this chain. The thermochronological dataset is better explained by a model in which relief reduction associated with the emergence of the wedge is accompanied by ongoing tectonic uplift

Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. The Alpi Apuane massif in the northern Apennines exposes synorogenic metamorphic rocks, and provides information about exhumation processes associated with accretion and retreat. (U-Th)/He and fission-track ages on zircon and apatite are used to resolve exhumational histories for the Apuane metamorphic rocks and the structurally overlying, very low grade Macigno Formation. Stratigraphic, metamorphic, and thermochronologic data indicate that the Apuane rocks were structurally buried to 15–30 km and 400C at about 20 Ma. Exhumation to 240C and 9 km depth (below sea level) occurred at 10–13 Ma. By 5 Ma the Apuane rocks were exhumed to 70C and 2 km. The Macigno and associated Tuscan nappe were also structurally buried and the Macigno reached its maximum depth of 7 km at 15 to 20 Ma. Stratigraphic evidence indicates that the Apennine wedge was submarine at this time. Thus we infer that initial exhumation of the Apuane was coeval with tectonic thickening higher in the wedge, as indicated by synchronous structural burial of the Tuscan nappe. From 6 to 4 Ma, thinning at shallow depth is indicated by continued differential exhumation between the Apuane and the Tuscan nappe at high rates. After 4 Ma, differential exhumation ceased and the Apuane and the Tuscan nappe were exhumed at similar rates (0.8 km/Ma), which we attribute to erosion of the Apennines, following their emergence above sea level