Low-temperature Thermochronometric Data Research Articles

Tracing short-lived hydrothermal circulation systems and water–rock interactions around small-scale intrusions

Hydrothermal circulation around magmatic intrusions affects hazards, critical metal deposition, and heat transfer, however it is often difficult to directly observe. We combine stable isotope and thermochronologic measurements, hydrothermal circulation modeling, and forward thermal history modeling to understand the short-lived hydrothermal circulation system that developed around the Little Devils Postpile, a small-scale mafic intrusion in the eastern Sierra Nevada Mountains of western North America. Bedrock samples collected in transects orthogonal to the intrusion reveal reset and partially reset apatite (U-Th-Sm)/He and zircon (U-Th)/He ages up to ∼13m from the contact. Measurements of hydrogen isotopes in apatite crystals from the same samples show deuterium depletion near the contact; δD values vary between −120‰ and −132‰ within 3m of the contact, −112‰ to −104‰ at 4–6m from the contact, and −85‰ to −95‰ at distances of 12m to 40m from the contact. In contrast, oxygen isotopes do not vary spatially with a δ18O of +6.4 ± 0.4‰. This depletion of δD and consistency of δ18O suggests that short-lived water–rock interaction occurred following the intrusion emplacement, facilitating the exchange of hydrogen isotopes via diffusion rather than dissolution-precipitation processes. Conductive cooling calculations predict an alteration aureole ten times larger than is observed by the low-temperature thermochronometric ages. Numerical models of hydrothermal circulation, for a range of host bedrock permeabilities and magmatic emplacement temperatures, reveal that the geometry of alteration is most consistent with buoyancy-driven hydrothermal circulation and a relatively high crustal permeability. With this case study, we show that combining thermochronology and hydrothermal modeling allows for the opportunity to model hydrogen isotope exchange and predict plausible apatite-water hydrogen fractionation. These new constraints on the geometry of water–rock interactions coupled with hydrothermal circulation modeling provide (1) a rare window into the pressure–temperature evolution of a small magmatic intrusion, (2) constraints on crustal permeability, and (3) a useful tool for interpreting low-temperature thermochronometric data in volcanic settings.

Quantifying drainage-divide migration from orographic rainfall over geologic timescales: Sierra de Aconquija, southern Central Andes

Drainage-divide migration, controlled by rock-uplift and rainfall patterns, may play a major role in the geomorphic evolution of mountain ranges. However, divide-migration rates over geologic timescales have only been estimated by theoretical studies and remain empirically poorly constrained. Geomorphological evidence suggests that the Sierra de Aconquija, on the eastern side of the southern Central Andes, northwest Argentina, is undergoing active westward drainage-divide migration. The mountain range has been subjected to steep rock trajectories and pronounced orographic rainfall for the last several million years, presenting an ideal setting for using low-temperature thermochronometric data to explore its topographic evolution. We perform three-dimensional thermal-kinematic modeling of previously published thermochronometric data spanning the windward and leeward sides of the range to explore the most likely structural and topographic evolution of the range. We find that the data can be explained by scenarios involving drainage-divide migration alone, or by scenarios that also involve changes in the structures that have accommodated deformation through time. By combining new 10Be-derived catchment-average denudation rates with geomorphic constraints on probable fault activity, we conclude that the evolution of the range was likely dominated by west-vergent faulting on a high-angle reverse fault underlying the range, together with westward drainage-divide migration at a rate of several km per million years. Our findings place new constraints on the magnitudes and rates of drainage-divide migration in real landscapes, quantify the effects of orographic rainfall and erosion on the topographic evolution of a mountain range, and highlight the importance of considering drainage-divide migration when interpreting thermochronometer age patterns.

Proterozoic to Phanerozoic Tectonism in Southwestern Montana Basement Ranges Constrained by Low Temperature Thermochronometric Data

AbstractCrystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ∼3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt‐Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier‐Laramide orogeny. We investigated the long‐term (>1 Ga), low‐temperature (erosion/burial within 10 km of the surface) thermal histories of these tectonic events with zircon and apatite (U‐Th)/He thermochronology. Data were collected across nine sample localities (n = 55 zircon and n = 26 apatite aliquots) in the northern and southern Madison ranges, the Blacktail‐Snowcrest arch, and the Tobacco Root uplift. Our zircon (U‐Th)/He data show negative trends between single aliquot date and effective uranium (a radiation damage proxy), which we interpreted with a thermal history model that considers the damage‐He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures above 400°C to near surface conditions between 800 and 510 Ma. Subsequent Phanerozoic exhumation culminated by ∼75 Ma. Late Phanerozoic cooling is coincident with along‐strike Sevier belt thin‐skinned thrusting in southeastern Idaho, and older than exhumation in basement‐involved uplifts of the Wyoming Laramide province. Our long‐term, low‐temperature thermal record for these southwestern Montana basement ranges shows that: (a) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, possibly influencing Phanerozoic thrust architecture and (b) the late Phanerozoic thick‐skinned thrusting recorded by these rocks is among the earliest thermochronologic records of Laramide basement‐involved shortening and was concomitant with Sevier belt thin‐skinned thrusting.

(U-Th)/He Thermochronology of the Indus Group, Ladakh, Northwest India: Is Neogene Cooling a Continental-Scale Thermal Event in the India-Asia Collision Zone?

Abstract The India-Asia continental collision zone archives a sedimentary record of the tectonic, geodynamic, and erosional processes that control the thermal history of the Himalayan orogenic interior since the onset of collision in early Paleogene time. In this paper, we present new (U-Th)/He thermochronometric cooling age data from 18 detrital zircons (ZHe) and 19 detrital apatites (AHe) of the early Eocene–early Miocene (ca. 50–23 Ma) continental facies of the Indus Group along the India-Asia collision zone in Ladakh, northwest (NW) India. This along-strike regional-scale low-temperature thermochronometric data set from the Indus basin is the first report of ZHe and AHe cooling ages from western and eastern Ladakh. Thermal modeling of our ZHe and AHe cooling ages indicates a postdepositional Neogene cooling signal in the Indus Group. Cooling initiated at ca. 21–19 Ma, was operational along the ~300 km strike of the collision zone in NW India by ca. 11 Ma, and continued until ca. 3 Ma. The Miocene cooling signal, also present along the India-Asia collision zone in south Tibet, is a continental-scale cooling event likely linked to increased erosional efficiency by the Indus and Yarlung Rivers across an elevated region resulting from the subduction dynamics of the underthrusting Indian plate.

The burial and exhumation history of the Liuqu Conglomerate in the Yarlung Zangbo suture zone, southern Tibet: Insights from clumped isotope thermometry

The Liuqu Conglomerate, exposed along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet, archives not only the process of the India-Asia collision, but also the exhumation history of the India-Asia collision zone. The burial and exhumation history of the Liuqu Conglomerate thus could potentially provide clues to the India-Asia collision and orogeny. Low-temperature thermochronometric data indicate that maximum burial temperatures of the Liuqu Conglomerate ranged from 80 to 110 °C; however, the exact burial history is not well constrained. Here, we examine the burial conditions of the Liuqu Conglomerate by applying clumped isotope thermometry to paleosol carbonates. Extensive microcrystalline recrystallization indicates that the samples have been diagenetically altered during burial. Moreover, the clumped isotope temperatures (ranging from ∼48 to ∼97 °C) are clearly above Earth surface conditions, indicating the original climatic information was overwritten by alteration at higher burial temperatures. Nevertheless, the calculated diagenetic water δ18O values represent maximum meteoric water values that can be used to broadly constrain the paleoelevation. The highest clumped isotope temperature implies that the maximum burial temperature of the Liuqu Conglomerate was at least ∼97 °C. We conclude that the Liuqu Conglomerate was buried as deeply as 3.7–4.3 km during the latest Paleocene to Eocene, and then exhumed during the Miocene (∼10–12 Ma). Incision of the paleo-Yarlung River was likely responsible for erosion and exhumation of the Liuqu Conglomerate, which suggests that the drainage system of Paleo-Yarlung River was similar to that of today and supports the model of outward growth of the Tibetan Plateau.

Pliocene erosional pulse and glacier-landscape feedbacks in the western Alaska Range

Pliocene–Pleistocene glaciation modified the topography and erosion of most middle- and high-latitude mountain belts, because the evolution of catchment topography controls long-term glacier mass balance and erosion. Hence, characterizing how erosion rates change during repeated glaciations can help test hypothesized glacier erosion-landscape feedbacks across a range of settings. To better understand how glaciations and landscapes coevolve on geologic timescales, I quantify erosion rates in the glaciated western Alaska Range with low-temperature thermochronometric data and modeling. Zircon (U–Th)/He and apatite fission track data suggest mountain-building was underway by the early Miocene. In contrast, lower-temperature apatite (U–Th)/He age-elevation and grain age-kinetic data indicate that erosion accelerated coincident with regional Pliocene glaciation ca. 4 Ma. Furthermore, erosion rates calculated within an eroding half-space indicate slow erosion at a rate ≤0.3 km/m.y. before 4.2 Ma, an initial pulse of rapid erosion at a rate of 1.0–1.6 km/m.y. during 4.2–2.9 Ma, and more moderate erosion at a rate of 0.4–0.7 km/m.y. since 2.9 Ma. The initial erosion pulse suggests a significant transient landscape adjustment to the introduction of efficient glacial erosion. The subsequent decrease in Pleistocene erosion rates is consistent with a negative feedback between continuing glaciation and glacier size/erosivity: If glacial erosion outpaces rock uplift, glacier erosion decreases over time as topography, mass balance, valley gradients, and ice flux are reduced. These findings imply that in areas of moderate rock uplift rates, the onset of local Plio–Pleistocene glaciation may have been punctuated by an initial pulse of rapid landscape change, after which change became more gradual.

Oligocene-Miocene burial and exhumation of the southernmost Gangdese mountains from sedimentary and thermochronological evidence

The Kailas conglomerates crop out ubiquitously along the southernmost boundary of the Gangdese batholith. They unconformably overlie the Gangdese batholith and are displaced by the Great Counter thrust (GCT) fault, forming a fault contact with the Xigaze forearc basin, the associated subduction complex and the Tethyan Himalayan sequence. These strata furnish a record of uplift and paleoenvironmental change in the Indus-Yarlung suture zone during the Oligocene-Miocene. Our new and previously published low-temperature thermochronometric data from the Gangdese batholith and the Kailas conglomerates indicate a period of rapid exhumation beginning approximately 17–15Ma centered on the southern margin of the Gangdese batholith, whereas regional uplift commenced significantly earlier during the deposition of the Kailas conglomerates, based on the presence of an abrupt facies transition from deep-water lacustrine deposits to red alluvial fan or fluvial deposits. The period of rapid exhumation probably lagged behind the initiation of faster uplift, while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy. Subsequently, the Kailas conglomerates were buried in association with the development of the north-directed Great Counter thrust, while rapid exhumation was facilitated by efficient incision by the paleo-Yarlung river at approximately 17–15Ma.

Meso-Cenozoic tectonic evolution of the Talas-Fergana region of the Kyrgyz Tien Shan revealed by low-temperature basement and detrital thermochronology

This study provides new low-temperature thermochronometric data, mainly apatite fission track data on the basement rocks in and adjacent to the Talas-Fergana Fault, in the Kyrgyz Tien Shan in the first place. In the second place, we also present new detrital apatite fission track data on the Meso-Cenozoic sediments from fault related basins and surrounding intramontane basins. Our results confirm multi-staged Meso-Cenozoic tectonic activity, possibly induced by the accretion of the so-called Cimmerian blocks to the Eurasian margin. New evidence for this multi-staged thermo-tectonic activity is found in the data of both basement and Meso-Cenozoic sediment samples in or close to the Talas-Fergana Fault. Zircon (U–Th)/He and apatite fission track data constrain rapid Late Triassic–Early Jurassic and Late Jurassic–Early Cretaceous basement cooling in the Kyrgyz Tien Shan around 200 Ma and 130–100 Ma respectively. Detrital apatite fission track results indicate a different burial history on both sides of the Talas-Fergana Fault. The apatite fission track system of the Jurassic sediments in the Middle Tien Shan unit east of the Talas-Fergana Fault is not reset, while the Jurassic sediments in the Fergana Basin and Yarkand-Fergana Basin, west of the fault zone, are partially and in some cases even totally reset. The totally reset samples exhibit Oligocene and Miocene ages and evidence the Cenozoic reactivation of the western Kyrgyz Tien Shan as a consequence of the India-Eurasia convergence.

A helium-based model for the effects of radiation damage annealing on helium diffusion kinetics in apatite

Widely used to study surface processes and the development of topography through geologic time, (U–Th)/He thermochronometry in apatite depends on a quantitative description of the kinetics of 4He diffusion across a range of temperatures, timescales, and geologic scenarios. Empirical observations demonstrate that He diffusivity in apatite is not solely a function of temperature, but also depends on damage to the crystal structure from radioactive decay processes. Commonly-used models accounting for the influence of thermal annealing of radiation damage on He diffusivity assume the net effects evolve in proportion to the rate of fission track annealing, although the majority of radiation damage results from α-recoil. While existing models adequately quantify the net effects of damage annealing in many geologic scenarios, experimental work suggests different annealing rates for the two damage types. Here, we introduce an alpha-damage annealing model (ADAM) that is independent of fission track annealing kinetics, and directly quantifies the influence of thermal annealing on He diffusivity in apatite. We present an empirical fit to diffusion kinetics data and incorporate this fit into a model that tracks the competing effects of radiation damage accumulation and annealing on He diffusivity in apatite through geologic time. Using time–temperature paths to illustrate differences between models, we highlight the influence of damage annealing on data interpretation. In certain, but not all, geologic scenarios, the interpretation of low-temperature thermochronometric data can be strongly influenced by which model of radiation damage annealing is assumed. In particular, geologic scenarios involving 1–2 km of sedimentary burial are especially sensitive to the assumed rate of annealing and its influence on He diffusivity. In cases such as basement rocks in Grand Canyon and the Canadian Shield, (U–Th)/He ages predicted from the ADAM can differ by hundreds of Ma from those predicted by other models for a given thermal path involving extended residence between ∼40–80 °C.

Miocene development of alpine glacial relief in the Patagonian Andes, as revealed by low-temperature thermochronometry

Apatite thermochronometry and synthetic maps of ages and rates for thermochronometric data are used to estimate the timing of incision of valley relief in the Andes. Central Patagonia offers a unique location to study the feedbacks between long-term climate, topography, and erosion due to the high relief and well-resolved mid-latitude glacial history. New apatite (U–Th)/He ages from two vertical transects and two 4He/3He release spectra in the fjord network around 47°S reveal fast cooling (15–30 °C/Ma) from ∼10 to 5 Ma. Samples currently at the surface cooled below ∼35 °C by ∼5 Ma, indicating slow cooling and little erosion in those regions since 5 Ma. We show that these very low-temperature thermochronometric data are useful indicators of changes in topography, and insensitive to deep thermal processes, such as migration of the Chile triple junction. Map-based predictions of the thermochronometric signatures of disparate topographic scenarios show the distribution of sample data necessary to resolve the timing of relief change. Comparisons to predicted cooling ages and rates indicate that our new apatite He data are most consistent with a pulse of early glacial incision, with much of the observed valley relief in Patagonia carved between 10 and 5 Ma. Early onset of glaciation in Patagonia is supported by glacial till with bracketing ages of 7.4 and 5 Ma. We therefore conclude that the observed thermochronometric signal of fast cooling from 10 to 5 Ma is likely due to an increase in valley relief coinciding with these early glaciations in the Andes. In other glaciated areas at lower latitudes, studies have found a dramatic increase in valley relief at ∼1 Ma. This timing has generated the idea that incision of glacial valleys may be related to the mid-Pleistocene transition, when the global glacial cycle changed from 40 to 100 ka periods. Our results from a higher latitude indicate an alternative, that glacial valleys incised rapidly after the onset of alpine glaciation.

Compilation of low-temperature thermochronometric data of bedrocks in the Japanese Islands

Compilation of low-temperature thermochronometric data of bedrocks in the Japanese Islands

Spatial and temporal pattern of erosion in the Three Rivers Region, southeastern Tibet

Convergence and collision between India and Eurasia have produced the Tibetan Plateau, which stands 5 km high over a region of 3 million km2. Its southeastern margin lies in the restraining bend between the Sichuan basin and the Eastern Himalayan Syntaxis. In this region three parallel rivers, the Salween, the Mekong and the Yangtze, carve gorges up to 3 km deep. Along the longitudinal profiles, large-scale knickzones, defined by very high steepness, correspond to the gorges of the Salween and Mekong. The Yangtze, instead, has a nearly linear profile upstream from its first big bend. New low-temperature thermochronometric data reveal a complex pattern of erosion in the Three Rivers Region. From the Salween in the west to the Yangtze in the east the magnitude and rate of erosion decrease. From south-to-north erosion rates exhibit variable gradients in space and time. Along the Salween and the Mekong a northward increase of erosion rate is followed by a decrease with additional distance to the north. Variations of erosion rate in time are characterized by a deceleration along the Salween and a general deceleration with local acceleration along the Mekong. This pattern, together with river profile analysis, is best explained by active coupling between tectonics and river incision related to the indentation and northward migration of the corner of the Indian continent.

Constraining the age of Liuqu Conglomerate, southern Tibet: Implications for evolution of the India–Asia collision zone

Controversy over the depositional age and provenance of the Liuqu Conglomerate along the major structural Indus–Yarlung suture zone in South Tibet clouds our understanding of the process of the India/Asia collision. Here, we report low-temperature thermochronometric data (apatite fission track, apatite and zircon (U–Th)/He for the Liuqu Conglomerate in the Xigaze area). Our new data constrain its depositional age to latest Oligocene–Early Miocene time, indicating that rather than having formed immediately following Paleogene India–Asia collision or collision between India and an intra-oceanic arc as previously proposed, the Conglomerate was probably deposited in an intermontane basin, at a slightly later time than the Gangrinboche Group to the north. The Liuqu Conglomerate should therefore not be used as a key horizon in models constraining the early stages of India/Asia collision. Our data together with previous studies suggest that the Liuqu Conglomerate was sourced from the Xigaze forearc basin, Indus–Yarlung suture zone, as well as the Tethyan Himalaya. Furthermore, our data indicate that exhumation of the Liuqu Conglomerate commenced at ∼<10–12 Ma, suggesting significant erosion in the Indus–Yarlung suture zone attributable to incision of the Yarlung Zangbo in Mid to Late Miocene time.

Rock uplift and erosion rate history of the Bergell intrusion from the inversion of low temperature thermochronometric data

The Bergell intrusion (European Alps) was one of the first locations where thermochronometry was used to resolve changes in erosion rate, yet, relating these changes to variations in climate or in local tectonics remains challenging. One approach that enables changes in erosion rate to be related to changes in climate or rock uplift rate is to utilize landscape evolution models, as topographic response to these forcing parameters is unique. Furthermore, low temperature thermochronometric systems have the potential to resolve topography through time and thus topographic response. We present new (U-Th)/He data for samples collected across 2 km of relief from the Bergell. The ages range from ∼2 to ∼16 Ma and define an age elevation with an apparent exhumation rate of 0.1 km/Myr. In order to infer erosion rates, we use a thermokinematic model to solve the heat equation in the crust, track material points through time and predict thermochronometric data. Paleo-topography and erosion rate are parameterized using the stream power model. We find that rock uplift rates were 0.4 km/Myr from ∼25 to ∼20 Ma and subsequently decreased to 0.05 km/Myr. This results in a gradual decrease in erosion rate from rates of 0.4 to 0.1 km/Myr. A recent increase in rock uplift rate at ∼4 Ma to ∼0.6 km/Myr is required to explain the youngest ages and high topographic relief.

A linear inversion method to infer exhumation rates in space and time from thermochronometric data

Abstract. We present a formal inverse procedure to extract exhumation rates from spatially distributed low temperature thermochronometric data. Our method is based on a Gaussian linear inversion approach in which we define a linear problem relating exhumation rate to thermochronometric age with rates being parameterized as variable in both space and time. The basis of our linear forward model is the fact that the depth to the "closure isotherm" can be described as the integral of exhumation rate, ..., from the cooling age to the present day. For each age, a one-dimensional thermal model is used to calculate a characteristic closure temperature, and is combined with a spectral method to estimate the conductive effects of topography on the underlying isotherms. This approximation to the four-dimensional thermal problem allows us to calculate closure depths for data sets that span large spatial regions. By discretizing the integral expressions into time intervals we express the problem as a single linear system of equations. In addition, we assume that exhumation rates vary smoothly in space, and so can be described through a spatial correlation function. Therefore, exhumation rate history is discretized over a set of time intervals, but is spatially correlated over each time interval. We use an a priori estimate of the model parameters in order to invert this linear system and obtain the maximum likelihood solution for the exhumation rate history. An estimate of the resolving power of the data is also obtained by computing the a posteriori variance of the parameters and by analyzing the resolution matrix. The method is applicable when data from multiple thermochronometers and elevations/depths are available. However, it is not applicable when there has been burial and reheating. We illustrate our inversion procedure using examples from the literature.

Doing more with less: Bayesian estimation of erosion models with detrital thermochronometric data

Detrital low temperature thermochronometric data provides spatial and temporal information on catchment erosion, which is relevant to problems in climate, tectonics and geomorphology. However, direct inference of erosion rates from such data is not trivial and only the simplest inverse problems have been addressed previously. In this paper, we present a new approach that relies on the Bayesian interpretation of probability and uses a Markov chain Monte Carlo algorithm for inversion, which affords flexibility in the choice of specific model parametrization and transparent assessment of model uncertainty. We demonstrate how a single detrital sample sourced from a high relief catchment can constrain long-term (>106years) changes in erosion rate that are in good agreement with published bedrock age–elevation profiles. Furthermore, we use detrital data to jointly invert for long-term exhumation history and spatial variability in short-term (<103years) sediment supply, information relevant to many geomorphic studies. Where cooling histories are simple, we show that even small sample sizes (<20grains) reliably estimate long-term rates of exhumation. We suggest that the presented approach to modeling detrital low-temperature thermochronometric data is both a powerful and efficient tool for solving tectonic and geomorphic problems.