Low-temperature Thermochronometry Research Articles

Assessing the long-term low-temperature thermal evolution of the central Indian Bundelkhand craton with a complex apatite and zircon (U-Th)/He dataset

Modern approaches in low-temperature thermochronometry are capable of extracting long-term thermal histories from cratonic settings that may elucidate potential drivers of deep-time phases of intracontinental burial and erosion. Here, we assess the utilization of the Radiation Damage Accumulation and Annealing Model for apatite (RDAAM) and zircon (ZRDAAM) to track the long-term low-temperature thermal evolution of the Archean Bundelkhand craton and the surrounding undeformed strata of the ∼1.7–0.9 Ga Vindhyan successions in central India. We correspondingly interpret a complex basement and detrital zircon and apatite (U-Th)/He (ZHe and AHe, respectively) dataset in light of observed model limitations and known geologic context. ZHe and AHe dates from across the craton reveal a significant (>300 Myr) date inversion between the two systems within grains with moderate to high effective uranium (eU) concentrations. Inverse thermal models utilizing current ZRDAAM and RDAAM parameters are not capable of reproducing observed coupled basement ZHe and AHe data for the same thermal history. However, meaningful thermal information can be extracted from AHe inverse models coupled with a forward modeling approach applied to detrital ZHe data from Vindhyan deposits, which have notably lower eU concentrations and yield significantly older ZHe dates (between ∼1,475 and 575 Ma) than basement zircon. Resulting thermal models indicate that the Bundelkhand craton experienced peak burial temperatures of ∼150°C between 850 and 475 Ma, followed by a major crustal cooling event at ∼350–310 Ma, possibly driven by late Paleozoic glaciations and/or epeirorogenic uplift. Inverse models including AHe data require a Deccan Traps related thermal perturbation between ∼66 and 65 Ma, and we suspect that this event overprinted basement zircon with moderate to high eU concentrations. Although the effects of zonation, grain morphology, and/or uncertainties in damage-annealing parameters contribute to disparities between predicted and observed AHe and ZHe dates, these factors alone cannot account for the major ZHe and AHe date inversion observed from the Bundelkhand craton. Instead, it is likely the case that current damage-dependent models for ⁴He diffusion are not adequately calibrated at the resolution necessary to predict short-lived thermal perturbations that occurred in a late phase relative to a prolonged period of extensive damage accumulation.

Seismicity recorded in hematite fault mirrors in the Rio Grande rift

AbstractExhumed fault rocks provide a textural and chemical record of how fault zone composition and architecture control coseismic temperature rise and earthquake mechanics. We integrated field, microstructural, and hematite (U-Th)/He (He) thermochronometry analyses of exhumed minor (square-centimeter-scale surface area) hematite fault mirrors that crosscut the ca. 1400 Ma Sandia granite in two localities along the eastern flank of the central Rio Grande rift, New Mexico. We used these data to characterize fault slip textures; evaluate relationships among fault zone composition, thickness, and inferred magnitude of friction-generated heat; and document the timing of fault slip. Hematite fault mirrors are collocated with and crosscut specular hematite veins and hematite-cemented cataclasite. Observed fault mirror microstructures reflect fault reactivation and strain localization within the comparatively weaker hematite relative to the granite. The fault mirror volume of some slip surfaces exhibits polygonal, sintered hematite nanoparticles likely created during coseismic temperature rise. Individual fault mirror hematite He dates range from ca. 97 to 5 Ma, and ~80% of dates from fault mirror volume aliquots with high-temperature crystal morphologies are ca. 25–10 Ma. These aliquots have grain-size–dependent closure temperatures of ~75–108 °C. A new mean apatite He date of 13.6 ± 2.6 Ma from the Sandia granite is consistent with prior low-temperature thermochronometry data and reflects rapid, Miocene rift flank exhumation. Comparisons of thermal history models and hematite He data patterns, together with field and microstructural observations, indicate that seismicity along the fault mirrors at ~2–4 km depth was coeval with rift flank exhumation. The prevalence and distribution of high-temperature hematite grain morphologies on different slip surfaces correspond with thinner deforming zones and higher proportions of quartz and feldspar derived from the granite that impacted the bulk strength of the deforming zone. Thus, these exhumed fault mirrors illustrate how evolving fault material properties reflect but also govern coseismic temperature rise and associated dynamic weakening mechanisms on minor faults at the upper end of the seismogenic zone.

Thermal history of the East European Platform margin in Poland based on apatite and zircon low-temperature thermochronology

Abstract. The Phanerozoic tectonothermal evolution of the SW slope of the East European Platform (EEP) in Poland is reconstructed by means of thermal maturity, low-temperature thermochronometry, and thermal modelling. We provide a set of new thermochronometric data and integrate stratigraphic and thermal maturity information to constrain the burial and thermal history of sediments. Apatite fission track (AFT) analysis and zircon (U-Th)/He (ZHe) thermochronology have been carried out on samples of sandstones, bentonites, diabase, and crystalline basement rocks collected from 17 boreholes located in central and NE Poland. They penetrated sedimentary cover of the EEP subdivided from the north to south into the Baltic, Podlasie, and Lublin basins. The average ZHe ages from Proterozoic basement rocks as well as Ordovician to Silurian bentonites and Cambrian to lower Carboniferous sandstones range from 848 ± 81 to 255 ± 22 Ma with a single early Permian age of 288 Ma, corresponding to cooling after a thermal event. The remaining ZHe ages represent partial reset or source ages. The AFT ages of samples are dispersed in the range of 235.8 ± 17.3 Ma (Middle Triassic) to 42.1 ± 11.1 Ma (Paleogene) providing a record of Mesozoic and Cenozoic cooling. The highest frequency of the AFT ages is in the Jurassic and Early Cretaceous prior to Alpine basin inversion. Thermal maturity results are consistent with the SW-ward increase of the Paleozoic and Mesozoic sediments thickness. An important break in a thermal maturity profile exists across the base Permian–Mesozoic unconformity. Thermal modelling showed that significant heating of Ediacaran to Carboniferous sedimentary successions occurred before the Permian with maximum paleotemperatures in the earliest and latest Carboniferous for Baltic–Podlasie and Lublin basins, respectively. The results obtained suggest an important role of early Carboniferous uplift and exhumation at the SW margin of the EEP. The SW slope of the latter was afterward overridden in the Lublin Basin by the Variscan orogenic wedge. Its tectonic loading interrupted Carboniferous uplift and caused resumption of sedimentation in the late Viséan. Consequently, a thermal history of the Lublin Basin is different from that in the Podlasie and Baltic basins but similar to other sections of the Variscan foreland, characterized by maximum burial at the end of Carboniferous. The Mesozoic thermal history was characterized by gradual cooling from peak temperatures at the transition from Triassic to Jurassic due to decreasing heat flow. Burial caused maximum paleotemperatures in the SW part of the study area, where the EEP was covered by an extensive sedimentary pile. However, further NE, due to low temperatures caused by shallow burial, the impact of fluids can be detected by vitrinite reflectance, illite/smectite, and thermochronological data. Our new results emphasize the importance of using multiple low-temperature thermochronometers and thermal modelling in connection with thermal maturity analysis to elucidate the near-surface evolution of platform margins.

Tectonic Evolution of the SE West Siberian Basin (Russia): Evidence from Apatite Fission Track Thermochronology of Its Exposed Crystalline Basement

The West Siberian Basin (WSB) is one of the largest intracratonic Meso-Cenozoic basins in the world. Its evolution has been studied over the recent decades; however, some fundamental questions regarding the tectonic evolution of the WSB remain unresolved or unconfirmed by analytical data. A complete understanding of the evolution of the WSB during the Mesozoic and Cenozoic eras requires insights into the cooling history of the basement rocks as determined by low-temperature thermochronometry. We presented an apatite fission track (AFT) thermochronology study on the exposed parts of the WSB basement in order to distinguish tectonic activation episodes in an absolute timeframe. AFT dating of thirteen basement samples mainly yielded Cretaceous cooling ages and mean track lengths varied between 12.8 and 14.5 μm. Thermal history modeling based on the AFT data demonstrates several Mesozoic and Cenozoic intracontinental tectonic reactivation episodes affected the WSB basement. We interpreted the episodes of tectonic activity accompanied by the WSB basement exhumation as a far-field effect from tectonic processes acting on the southern and eastern boundaries of Eurasia during the Mesozoic–Cenozoic eras.

Carbonate clumped isotope constraints on burial, uplift and exhumation histories of the Colorado Plateau

The timing, extent and driving mechanisms of burial, uplift and exhumation of the Colorado Plateau, and the carving of the Grand Canyon have been at the center of a long-standing controversy. Here, we develop a new approach to constrain burial and exhumation histories of sedimentary basins from the distributions of ‘clumped’ isotope compositions in carbonate minerals. We apply this approach to constrain peak-burial temperatures and infer the extent of peak sediment overburden and total exhumation of the Colorado Plateau at several sites extending from the Plateau's southwestern margin to its interior. Our constraints are broadly consistent with independent reconstructions of thermal history from low-temperature thermochronometry and oil-maturation indices. Together these data suggest that ∼3 km of rocks have been removed by erosion across the Plateau. This inferred prior extent of sediment overburden is consistent with the presence of a ∼1 km deep canyon at the Upper Gorge of the Grand Canyon during the Early Eocene, and therefore with topography at that time similar to today. Combined with the published record of partial exhumation—based on apatite-thermochronometry and stratigraphic constraints—we find that 63-83% of the total exhumation of the southwestern Plateau occurred prior to the Neogene. This is consistent with Sevier-Laramide contractional tectonism and/or pre-Neogene modification of the Colorado Plateau lithosphere driving uplift there. In contrast, apatite thermochronometry suggests that in the interior plateau, ∼50% of the total exhumation has occurred after the integration of the Colorado River – indicating a much younger source for uplift and the exhumation there. Considering recent suggestions that efficient exhumation of the Plateau by escarpment-retreat is associated with the incision of the drainage system, early exhumation of the Lower Gorge and Grand Wash segments of the Grand Canyon is consistent with the pre-Neogene carving of these segments, followed by relative stability of the base level.

Constraints on rock uplift in the eastern Transverse Ranges and northern Peninsular Ranges and implications for kinematics of the San Andreas fault in the Coachella Valley, California, USA

AbstractThe nexus of plate-boundary deformation at the northern end of the Coachella Valley in southern California (USA) is complex on multiple levels, including rupture dynamics, slip transfer, and three-dimensional strain partitioning on nonvertical faults (including the San Andreas fault). We quantify uplift of mountain blocks in this region using geomorphology and low-temperature thermochronometry to constrain the role of long-term vertical deformation in this tectonic system. New apatite (U-Th)/He (AHe) ages confirm that the rugged San Jacinto Mountains (SJM) do not exhibit a record of rapid Neogene exhumation. In contrast, in the Little San Bernardino Mountains (LSBM), rapid exhumation over the past 5 m.y. is apparent beneath a tilted AHe partial retention zone, based on new and previously published data. Both ranges tilt away from the Coachella Valley and have experienced minimal denudation from their upper surface, based on preservation of weathered granitic erosion surfaces. We interpret rapid exhumation at 5 Ma and the gentle tilt of the erosion surface and AHe isochrons in the LSBM to have resulted from rift shoulder uplift associated with extension prior to onset of transpression in the Coachella Valley. We hypothesize that the SJM have experienced similar rift shoulder uplift, but an additional mechanism must be called upon to explain the pinnacle-like form, rugged escarpment, and topographic disequilibrium of the northernmost SJM massif. We propose that this form stems from erosional resistance of the Peninsular Ranges batholith relative to more-erodible foliated metamorphic rocks that wrap around it. Our interpretations suggest that neither the LSBM nor SJM have been significantly uplifted under the present transpressive configuration of the San Andreas fault system, but instead represent relict highs due to previous tectonic and erosional forcing.

Cretaceous and late Cenozoic uplift of a Variscan Massif: The case of the French Massif Central studied through low-temperature thermochronometry

Abstract Located in the foreland domain of the Alpine and Pyrenean mountain belts, the French Massif Central presents enigmatic topographic features—reaching elevations of ∼1700 m above sea level and ∼1000 m of relief—that did not originate from Alpine compressional nor from extensional tectonics. Similar to other Variscan domains in Europe, such as the Bohemian, Rhenish, and Vosges/Black Forest Massifs, a Cenozoic uplift has been postulated, although its timing and quantification remain largely unconstrained. With respect to the other Variscan Massifs, the French Massif Central is wider and higher and shows a more intense late Cenozoic volcanism, suggesting that deep-seated processes have been more intense. In this study, apatite fission-track and (U-Th)/He thermochronometry were applied to investigate the long-term topographic evolution of the Massif Central. Our new thermochronological data come from the eastern flank of the massif, where sampling profiles ran from the high-elevation region down to the Rhône River valley floor with a total elevation profile of 1200 m. Age-elevation relationships, mean track-length distributions, and thermal modeling indicate a two-step cooling history: (1) a first exhumation event, already detected through previously published thermochronology data, with an onset time during the Cretaceous, and (2) a more recent Cenozoic phase that is resolved from our data, with a likely post-Eocene onset. This second erosional event is associated with relief formation and valley incision possibly induced by a long-wavelength domal uplift supported by mantle upwelling.

Late-stage tectonic evolution of the Al-Hajar Mountains, Oman: new constraints from Palaeogene sedimentary units and low-temperature thermochronometry

AbstractMountain building in the Al-Hajar Mountains (NE Oman) occurred during two major shortening stages, related to the convergence between Africa–Arabia and Eurasia, separated by nearly 30 Ma of tectonic quiescence. Most of the shortening was accommodated during the Late Cretaceous, when northward subduction of the Neo-Tethys Ocean was followed by the ophiolites obduction on top of the former Mesozoic margin. This shortening event lasted until the latest Santonian – early Campanian. Maastrichtian to Eocene carbonates unconformably overlie the eroded nappes and seal the Cretaceous foredeep. These neo-autochthonous post-nappe sedimentary rocks were deformed, along with the underlying Cretaceous tectonic pile, during the second shortening event, itself including two main exhumation stages. In this study we combine remotely sensed structural data, seismic interpretation, field-based structural investigations and apatite (U–Th)/He (AHe) cooling ages to obtain new insights into the Cenozoic deformation stage. Seismic interpretation indicates the occurrence of a late Eocene flexural basin, later deformed by an Oligocene thrusting event, during which the post-nappe succession and the underlying Cretaceous nappes of the internal foredeep were uplifted. This stage was followed by folding of the post-nappe succession during the Miocene. AHe data from detrital siliciclastic deposits in the frontal area of the mountain chain provide cooling ages spanning from 17.3 to 42 Ma, consistent with available data for the structural culminations of Oman. Our work points out how renewal of flexural subsidence in the foredeep and uplift of the mountain belt were coeval processes, followed by layer-parallel shortening preceding final fold amplification.

Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

Abstract. We present a study of pressure and temperature evolution in the passive continental margin under the Oman Ophiolite using numerical basin models calibrated with thermal maturity data, fluid-inclusion thermometry, and low-temperature thermochronometry and building on the results of recent work on the tectonic evolution. Because the Oman mountains experienced only weak post-obduction overprint, they offer a unique natural laboratory for this study. Thermal maturity data from the Adam Foothills constrain burial in the basin in front of the advancing nappes to at least 4 km. Peak temperature evolution in the carbonate platform under the ophiolite depends on the burial depth and only weakly on the temperature of the overriding nappes, which have cooled during transport from the oceanic subduction zone to emplacement. Fluid-inclusion thermometry yields pressure-corrected homogenization temperatures of 225 to 266 ∘C for veins formed during progressive burial, 296–364 ∘C for veins related to peak burial, and 184 to 213 ∘C for veins associated with late-stage strike-slip faulting. In contrast, the overlying Hawasina nappes have not been heated above 130–170 ∘C, as witnessed by only partial resetting of the zircon (U-Th)/He thermochronometer. In combination with independently determined temperatures from solid bitumen reflectance, we infer that the fluid inclusions of peak-burial-related veins formed at minimum pressures of 225–285 MPa. This implies that the rocks of the future Jebel Akhdar Dome were buried under 8–10 km of ophiolite on top of 2 km of sedimentary nappes, in agreement with thermal maturity data from solid bitumen reflectance and Raman spectroscopy. Rapid burial of the passive margin under the ophiolite results in sub-lithostatic pore pressures, as indicated by veins formed in dilatant fractures in the carbonates. We infer that overpressure is induced by rapid burial under the ophiolite. Tilting of the carbonate platform in combination with overpressure in the passive margin caused fluid migration towards the south in front of the advancing nappes. Exhumation of the Jebel Akhdar, as indicated by our zircon (U-Th)/He data and in agreement with existing work on the tectonic evolution, started as early as the Late Cretaceous to early Cenozoic, linked with extension above a major listric shear zone with top-to-NNE shear sense. In a second exhumation phase the carbonate platform and obducted nappes of the Jebel Akhdar Dome cooled together below ca. 170 ∘C between 50 and 40 Ma before the final stage of anticline formation.

How local crustal thermal properties influence the amount of denudation derived from low-temperature thermochronometry: REPLY

How local crustal thermal properties influence the amount of denudation derived from low-temperature thermochronometry: REPLY

Exhumation and topographic evolution of the Namche Barwa Syntaxis, eastern Himalaya

The Namche Barwa Syntaxis, as one of the most tectonically active regions, remains an appropriate place to explore the relationship between tectonics, surface processes, and landscape evolution. Two leading models have been proposed for the formation and evolution of this syntaxis, including the tectonic aneurysm model and the syntaxis expansion model. Here we use a multi-disciplinary approach based on low-temperature thermochronometry, numerical modeling, river profile and topographic analyses to investigate the interactions between tectonics, erosion, and landscape evolution and to test these models. Our results emphasize the presence of young cooling ages (i.e., <1Ma) along the Parlung River, to the north of the syntaxis. Using numerical modeling we argue that a recent increase in exhumation rate is required to expose these young ages. Our river analysis reveals spatial variations in channel steepness, which we interpret to reflect the rock uplift pattern. By establishing the relationship between erosion rates and topographic features, we find that erosion rates are poorly to weakly correlated with topographic features, suggesting that the landscape is still evolving. Altogether, these results seem better explained by a mechanism that involves a northward expansion of the syntaxis, which causes high rock uplift rates to the north of the syntaxis and a transient state of topography adjusting to an evolving tectonic setting.

Current strain accumulation in the hinterland of the northwest Himalaya constrained by landscape analyses, basin-wide denudation rates, and low temperature thermochronology

Rupture associated with the 25 April 2015 Mw 7.8 Gorkha (Nepal) earthquake highlighted our incomplete understanding of the structural architecture and seismic cycle processes that lead to Himalayan mountain building in Central Nepal. In this paper we investigate the style and kinematics of active mountain building in the Himalayan hinterland of Northwest India, approximately 400km to the west of the hypocenter of the Nepal earthquake, via a combination of landscape metrics and long- (Ma) and short-term (ka) erosion rate estimates (from low temperature thermochronometry and basin-wide denudation rate estimates from 10Be concentrations). We focus our analysis on the area straddling the PT2, the physiographic transition between the Lesser and High Himalaya that has yielded important insights into the nature of hinterland deformation across much of the Himalaya. Our results from Northwest India reveal a distinctive PT2 that separates a Lesser Himalaya region with moderate relief (∼1000m) and relatively slow erosion (<1mm/yr) from a High Himalaya with extreme relief (∼2500m), steep channels, and erosion rates that approach or exceed 1mm/yr. The close spatial similarity in relative rates of long- and short-term erosion suggests that the gradient in rock uplift rates inferred from the landscape metrics across the PT2 has persisted in the same relative position since at least the past 1.5Ma. We interpret these observations to suggest that strain accumulation in this hinterland region throughout at least the past 1.5Ma has been accomplished both by crustal thickening via duplexing and overthrusting along transient emergent faults. Despite the >400km distance between them, similar spatiotemporal patterns of erosion and deformation observed in Northwest India and Central Nepal suggest both regions experience similar styles of active strain accumulation and both are susceptible to large seismic events.

Tectonics drives rapid exhumation of the western Himalayan syntaxis: Evidence from low-temperature thermochronometry of the Neelum valley region, Pakistan

Tectonics drives rapid exhumation of the western Himalayan syntaxis: Evidence from low-temperature thermochronometry of the Neelum valley region, Pakistan

(U–Th)/He thermochronometry reveals Pleistocene punctuated deformation and synkinematic hematite mineralization in the Mecca Hills, southernmost San Andreas Fault zone

The timing, tempo, and processes of punctuated deformation in strike-slip fault systems are challenging to resolve in the rock record. Faults in the Mecca Hills, adjacent to the southernmost San Andreas Fault, California, accommodate active deformation and exhumation in the Plio-Pleistocene sedimentary rocks and underlying crystalline basement. We document the spatiotemporal patterns of San Andreas Fault-related deformation as recorded in crystalline basement rocks of the Mecca Hills using fault microstructural observations, geochemical data, and hematite (n=24) and apatite (n=44) (U–Th)/He (hematite He, apatite He) thermochronometry data. Reproducible mean hematite He dates from minor hematite-coated fault surfaces in the Painted Canyon Fault damage zone range from ∼0.7–0.4 Ma and are younger than ∼1.2 Ma apatite He dates from adjacent crystalline basement host rock. These data reveal concomitant Pleistocene pulses of fault slip, fluid flow, and synkinematic hematite mineralization. Hematite textures, crystal morphology, and hematite He data patterns imply some damage zone deformation occurred via cyclic crack-seal and creep processes. Apatite He data from crystalline basement define distinct date-eU patterns and indicate cooling across discrete fault blocks in the Mecca Hills. Uniform ∼1.2 Ma apatite He dates regardless of eU are located exclusively between the Painted Canyon and Platform faults. Outside of this fault block, samples yield individual apatite He dates from ∼30–1 Ma that define a positive apatite He date-eU correlation. These patterns reveal focused exhumation away from the main trace of the San Andreas Fault at ∼1.2 Ma. Low-temperature thermochronometry of fault-related rocks provides an unprecedented window into the 105–106-yr record of San Andreas Fault-related deformation in the Mecca Hills and documents hematite deformation mechanisms that may be operative in other strike-slip faults world-wide.

How local crustal thermal properties influence the amount of denudation derived from low-temperature thermochronometry

ranslating amounts and rates of rock cooling derived from low-temperature thermochronometry into denudation requires assumptions about the local geothermal gradient. The temperature gradient in the crust depends on many factors, including basal heat flow, crustal heat production, and thermal conductivity. Consequently, geothermal gradients may be variable on time scales over which rock cooling is tracked by thermochronometry. Using one-dimensional numerical modeling of heat transfer in rocks of varying thermal characteristics, we show that the geothermal gradient of the eroded layer is the most important factor for accurate estimation of denudation amounts. Using a three-dimensional numerical model (Pecube), we demonstrate the impact of crustal heat production and thermal conductivity on estimates of total denudation derived from apatite fission track data from central west Britain. We show that the regional variation in cooling ages measured in Caledonian granites can be explained by geothermal gradient variation due to the presence of a heat-producing granite batholith and removal of insulating sedimentary rocks, and does not require variable denudation. Neglecting the blanketing effect leads to twofold overestimation of the amount of denudation. The occurrence of heat-producing basement that was once covered by a sedimentary blanket is common, in particular in the core of mountain belts. Accurate determination of the amount and rate of denudation from thermochronometric studies in these situations must take into account the composition of the eroded rocks.

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.

The general a posteriori truncation method and its application to radiogenic source identification for the Helium production–diffusion equation

The purpose of the present paper is twofold: (i) to formulate the truncation method with a posteriori parameter choice rule in a uniform framework that can be adapted to solve a kind of linear inverse problems for which the solution can be represented by the eigensystem; (ii) to apply the a posterioritruncation method to the radiogenic source identification for the Helium production–diffusion equation, which is closely connected to the Helium isotopes dating as one method of the low temperature thermochronometry, the specific rate of convergence between the regularized solution and the exact solution is calculated, and numerical examples shed light on the efficiency and accuracy of the proposed method.

Progress in Methodology of Low Temperature Thermochronometry

Acta Geologica Sinica - English EditionVolume 90, Issue 5 p. 1927-1928 Research Advances Progress in Methodology of Low Temperature Thermochronometry Gelian GONG, Corresponding Author Gelian GONG ggl@gig.ac.cn State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, ChinaCorresponding author. E-mail: ggl@gig.ac.cnSearch for more papers by this authorJibin ZHOU, Jibin ZHOU State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, ChinaSearch for more papers by this authorWeidong SUN, Weidong SUN Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, ChinaSearch for more papers by this author Gelian GONG, Corresponding Author Gelian GONG ggl@gig.ac.cn State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, ChinaCorresponding author. E-mail: ggl@gig.ac.cnSearch for more papers by this authorJibin ZHOU, Jibin ZHOU State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, ChinaSearch for more papers by this authorWeidong SUN, Weidong SUN Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, ChinaSearch for more papers by this author First published: 16 November 2016 https://doi.org/10.1111/1755-6724.12837AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume90, Issue5October 2016Pages 1927-1928 RelatedInformation

Exploring IRSL50 fading variability in bedrock feldspars and implications for OSL thermochronometry

Optically Stimulated Luminescence (OSL) is a well-established Quaternary dating method, which has recently been adapted to application in low-temperature thermochronometry. The Infra-Red Stimulated Luminescence (IRSL) of feldspar, which so far is the most promising target signal in thermochronometry, is unfortunately prone to anomalous fading. The fading of feldspar IRSL is at times not only challenging to measure, but also laborious to incorporate within luminescence growth models. Quantification of IRSL fading is therefore a crucial step in OSL thermochronometry, raising questions regarding (i) reproducibility and reliability of laboratory measurements of fading, as well as (ii) the applicability of existing fading models to quantitatively predict the level of IRSL field saturation in nature. Here we investigate the natural luminescence signal and anomalous fading of IRSL measured at 50 °C (IRSL50) in 32 bedrock samples collected from a variety of lithologies and exhumation settings (Alaska and Norway). We report a large span of IRSL50 fading rates between samples (g2days ranging from ∼0.5 to ∼45%/decade), which further demonstrates (i) a good reproducibility between two common fading measurement protocols, and (ii) the ability of tunnelling models to predict the level of feldspar IRSL50 field saturation in nature. We observe higher IRSL50 fading in feldspar with increasing Ca content, although other factors cannot be dismissed at present. Finally, our dataset confirms that the applicability of feldspar IRSL50 in OSL thermochronometry is limited to rapidly-exhuming settings or warm subsurface environments.

Record of paleofluid circulation in faults revealed by hematite (U-Th)/He and apatite fission-track dating: An example from Gower Peninsula fault fissures, Wales

Fault rock low-temperature thermochronometry can inform the timing, temperature, and significance of hydrothermal fluid circulation in fault systems. We demonstrate this with combined hematite (U-Th)/He (He) dating, and sandstone apatite fission-track (AFT) and apatite and zircon (U-Th)/He (He) thermochronometry from fault-related fissures on the Gower Peninsula, Wales. Hematite He dates from 141 ± 5.1 Ma to 120 ± 5.0 Ma overlap with a 131 ± 20 Ma sandstone infill AFT date. Individual zircon He dates are 402–260 Ma, reflecting source material erosion, and imply a maximum Late Permian infill depositional age. Burial history reconstruction reveals modern exposures were not buried sufficiently in the Triassic–Early Cretaceous to have caused reheating to temperatures necessary to reset the AFT or hematite He systems, and thus these dates cannot reflect cooling due to erosion alone. Hot fluids circulating through fissures in the Early Cretaceous reset the AFT system. Hematite was either also reset by fluids or precipitated from these fluids. Similar hematite He dates from fault-related mineralization in south Glamorgan (Wales) and Cumbria (England) imply concomitant regional hot groundwater flow along faults. In this example, hydrothermal fluid circulation, coeval with North Atlantic rifting, occurred in higher-permeability fissures and fault veins long after they initially formed, directly influencing local and regional geothermal gradients.