Detrital Zircon Fission Track Research Articles

Paleozoic-Mesozoic multistage tectonic evolution of the North Qilian Shan revealed by detrital zircon U-Pb and fission-track double dating

Knowledge of the tectonic history of the North Qilian Shan contributes to our understanding of the formation of the Tibetan Plateau. However, when the North Qilian Shan were formed has been controversial. We report new detrital zircon U-Pb (n = 899) and double dating fission track and U-Pb (n = 450) results from nine samples of Lower Cretaceous and Oligocene sandstone units, integrating with published geochronology and thermochronology data in order to reconstruct the evolution history of the North Qilian Shan. Nine samples from this unit show detrital zircon fission track age populations that center at: 636–603, 495, 406–331, 290–266, 247–224, 193–151, and 144–102 Ma. These fission track data are older than their corresponding depositional ages, suggesting that the fission track ages are unreset and record magmatic-related cooling or exhumation stages of the North Qilian Shan since the Late Neoproterozoic. Deconvolved fission track peak ages document major cooling events associated with the opening of the North Qilian Ocean at 636–603 Ma. Late Cambrian peak age (495 Ma) is attributed to the early phase of the subduction of the North Qilian Ocean. The Devonian period exhibited a cooling phase between 406 and 381 Ma, corresponding to the collision event between the Qaidam terrane, Central Qilian Shan, and Alxa Block. The peak age of 331 Ma signifies the cooling event associated with the subduction of the Paleo-Asian Ocean. The peak ages during the Early-Middle Permian period (290–266 Ma) provide evidence of significant exhumation events that occurred during the Late Paleozoic era. These events were linked to the subduction of the Paleo-Tethys Ocean to the south and the Paleo-Asian Ocean to the north. The Triassic peak ages (247–224 Ma) unveil exhumation signals in the North Qilian Shan region that were generated by the far-field effect of the collision between the Qiangtang and Kunlun terranes. During the Jurassic (193–151 Ma) and the Early Cretaceous periods (144–102 Ma), the exhumation of the North Qilian Shan region was a consequence of the subduction of the Neo-Tethys Ocean and the collision between the Lhasa and Qiangtang blocks, respectively. The peak ages of the Xinminpu Formation in the Early Cretaceous (123–121 Ma) are in close proximity to their depositional age (125–105 Ma), indicating that the North Qilian Shan underwent rapid exhumation during this period. The peak ages observed in the Oligocene Baiyanghe Formation (31–24 Ma) from 144 to 102 Ma, indicate that it has been influenced by the recycled sediments originating from the Cretaceous Xinminpu Formation. The North Qilian Shan gradually emerged during the late Paleozoic and Mesozoic eras, providing the foundation for its subsequent reactivation in the Cenozoic era. The multi-stage cooling or exhumation events of North Qilian Shan since Late Neoproterozoic reflect the complex formation process of the northeastern margin of Tibetan Plateau.

Polyphase Exhumation of the East Kunlun Orogenic Belt: Evidence from Modern River Detrital Zircon and Apatite Fission Track Dating

Abstract The East Kunlun Orogenic Belt (E-KOB) stands out as one of the most prominent basin-mountain geomorphic features in the northern interior of the Tibetan Plateau. It records a series of accretion-collision events from the Mesozoic to the Cenozoic. In particular, with the uplifting of the Tibetan Plateau, the E-KOB experienced intracontinental deformation and exhumation in the Cenozoic. Clarifying the exhumation history of the E-KOB is crucial to define the growth time and mechanism of the Tibetan Plateau. In this study, we apply detrital zircon fission-track (ZFT) and apatite fission-track (AFT) analyses on modern river sands in order to constrain the regional exhumation history of the eastern E-KOB. Four peak ages have been identified and interpreted as results of rapid exhumation correlated with intracontinental deformation. Two older peak ages at 144.7–141.0 and 114.6–82.1 Ma are in good accordance with the collision time of the north-south Lhasa-Qiangtang Block along the Bangong-Nujiang suture zone and the subsequent progressive deformation stage toward the north. Peak age at 60.9–45.3 Ma is coeval with the initial timing of the India-Asia collision. The youngest peak age at 25.1–18.3 Ma matches well with the extensive outward and upward growth of the Tibetan Plateau during the Oligocene to Miocene time. The Cretaceous and early Cenozoic rapid exhumations suggest that the E-KOB has been involved in the intracontinental deformation induced by collisions of the Lhasa-Qiangtang and India-Asia from the south. It implies that the northern Tibetan Plateau likely has been elevated or was a structural high before the Eocene. In addition, some of our detrital samples show a younger ZFT peak age than the AFT peak age. We attributed this data bias to the contribution of hydrodynamic sorting and/or lithological difference. The combination of ZFT and AFT dating has advantages in eliminating interfering age signals in detrital thermochronology.

The Formation of the North Qilian Shan through Time: Clues from Detrital Zircon Fission-Track Data from Modern River Sediments

Understanding the formation of the North Qilian Shan in the NE Tibetan Plateau provides insights into the growth mechanisms of the northern region of the plateau across time. Detrital zircon fission-track (ZFT) analyses of river sediments can provide a comprehensive understanding of the exhumation history during prolonged orogenesis. Here, we applied the detrital thermochronology approach to the Qilian Shan orogenic belt. This work presents the first single-grain detrital ZFT data from river-bed sediments of the upper Hei River catchment in North Qilian Shan. The single ZFT ages are widely distributed between about 1200 Ma and about 40 Ma. These data record the protracted history of the Qilian Shan region from the Neoproterozoic evolution of Rodinia and late Paleozoic amalgamation of Central Asia to the accretion of the Gondwanian blocks during the Meso-Cenozoic era. Strong post-magmatic cooling events occurred in North Qilian Shan at 1200~1000 Ma, corresponding to the assembly of the Rodinia supercontinent. The age population at 800 Ma documents the oceanic spreading in the late Neoproterozoic dismantling of Rodinia. ZFT ages ranging from about 750 Ma to 550 Ma (with age peaks at 723 Ma and 588 Ma) are consistent with the timing of the opening and spreading of the Qilian Ocean. The age peaks at 523 Ma and 450 Ma mark the progressive closure of that ocean ending with the collision of the Qilian block with the Alxa block—North China craton in the Devonian. The Qilian Ocean finally closed in Late Devonian (age peak at 375 Ma). In the late Paleozoic (275 Ma), the subduction of the Paleotethys Ocean led to extensive magmatic activity in the North Qilian Shan. During the Lower Cretaceous (145 Ma), the accretion of the Lhasa block to the south (and potentially the closure of the Mongol-Okhotsk Ocean to the northeast) triggered a renewed tectonic activity in the Qilian Shan. Finally, a poorly defined early Eocene exhumation event (50 Ma) suggests that the NE Tibetan Plateau started to deform nearly synchronously with the onset of the India-Asia collision. This study demonstrates the usefulness of combining modern-river detrital thermo-/geochronological ages and bedrock geochronological ages to understand large-scale orogenic evolution processes.

Thermal Annealing Characteristics of Detrital Zircon Fission Track Obtained from Natural Borehole Samples

Thermal Annealing Characteristics of Detrital Zircon Fission Track Obtained from Natural Borehole Samples

Provenance Shifts During Neogene Brahmaputra Delta Progradation Tied to Coupled Climate and Tectonic Change in the Eastern Himalaya

AbstractThe Bengal Basin preserves the erosional signals of coupled tectonic‐climatic change during late Cenozoic development of the Himalayan orogen, yet regional correlation and interpretation of these signals remains incomplete. We present a new geologic map of fluvial‐deltaic deposits of the Indo‐Burman Ranges (IBR), five detrital zircon fission track analyses, and twelve high‐n detrital zircon U‐Pb age distributions (dzUPb) from the Barail (late Eocene–early Miocene), Surma (early–late Miocene), and Tipam (late Miocene–Pliocene) Groups of the ancestral Brahmaputra delta. We use dzUPb statistical tests to correlate the IBR units with equivalent age strata throughout the Bengal Basin. An influx of trans‐Himalayan sediment and the first appearance of ∼50 Ma grains of the Gangdese batholith in the lower Surma Group (∼18–15 Ma) records the early Miocene arrival of the ancestral Brahmaputra delta to the Bengal Basin. Contributions from Himalayan sources systematically decrease up section through the late Miocene as the contribution of Trans‐Himalayan Arc sources increases. The Miocene (∼18–8 Ma) deposition of the Surma Group records upstream expansion of the ancestral Brahmaputra River into southeastern Tibet. Late Miocene (<8 Ma) progradation of the fluvial part of the delta (Tipam Group) routed trans‐Himalayan sediment over the shelf edge to the Nicobar Fan. We propose that Miocene progradation of the ancestral Brahmaputra delta reflects increasing rates of erosion and sea level fall during intensification of the South Asian Monsoon after the Miocene Climate Optimum, contemporaneous with a pulse of tectonic uplift of the Himalayan hinterland and Tibet.

Determining the tempo of exhumation in the eastern Himalaya: Part 2. Integrating bedrock and detrital cooling ages through thermokinematic modelling

AbstractThe exhumation record of a fold‐thrust belt is preserved by thermochronologic minerals, such as zircon and apatite, both in exposed bedrock and in synorogenic sedimentary rocks in the foreland basin. Treating these as separate records can lead to potentially contrasting interpretations of a single exhumation history. Integrating the bedrock and detrital records with thermokinematic models of sequential deformation of a fold‐thrust belt can identify viable exhumation pathways of the bedrock and elucidate both how bedrock exhumation varies in space and time and how accurately the basin records exhumation changes in the source region. Predicted bedrock cooling ages and modelled basin thickness are used to estimate the amount and source of sediments supplied to the foreland basin during each increment of deformation to predict the detrital cooling signal over time. Applying this integrated bedrock‐detrital model to a cross‐section in Arunachal Pradesh, NE India demonstrates spatial and temporal variability in exhumation, with a background exhumation rate of <2 mm/yr, periods of rapid exhumation at rates of 3–7 mm/yr, and short pulses of 10–12 mm/yr rates during out‐of‐sequence thrusting. Our results predict that the detrital apatite fission track (DAFT) system records a constant lag time of 0.5–1 Myr. Although the response of the detrital zircon fission track (DZFT) system is more complex, the system records changes in lag time (1–5 Myr) as a function of the kinematics, deformation rates and thermal profile of the crust in the hinterland. However, the DZFT cooling signal is delayed by 1–2 Myr relative to the age of marked shifts in location, magnitude and rate of exhumation in the source region. Our models also highlight the importance of recycled foreland deposits in matching the ca. 20 Ma static peak in the DZFT record and ca. 14 Ma static peak in the DAFT record.

Determining the tempo of exhumation in the eastern Himalaya: Part 1. Geometry, kinematics and predicted cooling ages

AbstractQuantitative integration of cross‐section geometry, kinematics and cooling ages requires notably more complicated kinematic and exhumation pathways than are typically assumed with a simple in‐sequence model of cross‐section deformation. Incorporating measured basin thickness and depositional ages, determined from magnetostratigraphy or young detrital zircon fission track cooling ages, provide further constraints on the timing of fault motion, as changes in shortening rate that may not alter bedrock cooling ages can affect the depositional age of foreland basin strata. Thermokinematic models of balanced cross‐sections in Arunachal Pradesh, NE India demonstrate that the kinematic sequence and shortening rate exert the largest control over the pattern of predicted cooling ages for the region, by dictating the location and timing of rock uplift and exhumation (cooling) over ramps. The best fit to the measured bedrock cooling ages and basin constraints is achieved with a kinematic sequence involving early foreland propagation of four Lesser Himalayan faults combined with variable shortening rates. Fast rates (25–30 mm/yr) are required to accompany early foreland propagation at ca. 14–13 Ma followed by slower rates (18–20 mm/yr) until 10 Ma. Shortening rates increase to ca. 25–35 mm/yr at ca. 10 Ma until ca. 5–7 Ma. A decrease in shortening rate occurs between 7 and 5 Ma, with rates of 9–15 mm/yr until the present. Although non‐unique, the updated cross‐section geometry and kinematics highlight the components of geometry, deformation and exhumation that must be included in any valid cross‐section model for this portion of the eastern Himalaya such as the location of active ramps, and location and age of two key fault systems, the Bomdila imbricate zone and the thrust faults that form the Lumla duplex. Less unique are the specific geometries of faults, thickness of strata they carry, shortening rates, particularly between 14–8 Ma, and model parameters such as topography, heat production and flexure.

Bedrock and detrital zircon thermochronology to unravel exhumation histories of accreted tectonic blocks: An example from the Western Colombian Andes

The Northern Andes are the result of multiple tectonic phases, which include extensional and compressional volcanic arcs, strike-slip fragmentation, and accretion of exotic terranes. The alternations of these tectonic settings have overprinted and fragmented the geological record, which has hindered precise paleogeographic and tectonic reconstructions. In the western segment of the Colombian Andes, the oceanic-cored Western Cordillera (WC) and the continental Central Cordillera (CC) are separated by the Romeral Fault Zone (RFZ). This segment of the Andes preserves the record of Cretaceous back-arc extension, the onset of compression, and the accretion of the Caribbean plateau during the Late Cretaceous-Paleocene. To refine the tectonic evolution of the Western Colombian Andes, this study documents new detrital zircon fission-track (ZFT) data from the Meso-Cenozoic sedimentary cover in the CC, the RFZ, and the WC; new ZFT and zircon helium (ZHe) bedrock data from the CC and the RFZ; and new U–Pb detrital data from the Miocene sedimentary cover of the RFZ. Within the RFZ, we obtained bedrock ZFT ages of 239.0 ± 11.0 Ma and 111.1 ± 4.3 Ma, and detrital ZFT data from the Abejorral Formation are interpreted as the result of post-magmatic cooling and Cretaceous rifting. Late Cretaceous to Eocene ZFT and ZHe ages (~61–50 Ma) in the CC and detrital ZFT data in the WC record exhumation and deformation of the CC during and after the collision of the Caribbean plateau with the continental margin. Finally, detrital U–Pb and ZFT data from the Amagá Formation record the formation of a Miocene intermountain basin. This study reconstructs the basin geometries and the deformation patterns before, during, and after the collision of the Caribbean plateau with the South American margin. We highlight that in accretionary orogens, in which prolonged deformation and erosion have removed the sedimentary cover, the quantification of differential basement exhumation is key to reconstruct thick-skin deformation and to define major tectonic boundaries.

Detrital Zircon Fission-Track Thermochronology of the Present-Day River Drainage System in the Mt. Kailas Area, Western Tibet: Implications for Multiple Cooling Stages of the Gangdese Magmatic Arc

It is still controversial how the high elevation of the Tibetan Plateau established after the Indian-Asian collision during the Cenozoic. The timing of Gangdese magmatic arc exhumation and uplift history would provide useful message for this disputation. We present six zircon fission-track (ZFT) data from modern river sand in the western Tibet, around the Mt. Kailas, to decipher the long-term exhumation histories of the Gangdese magmatic arc. The data suggests that all the Gangdese magmatic arc rocks experienced rapid cooling during the Eocene (∼46’35 Ma) and Oligocene (∼31’26 Ma). The movement along the north-south trending extensional fault and dextral strike-slip Karakoram fault induced the adjacent rocks exhumed at the Middle Miocene (∼15’16 Ma) and Late Miocene (∼10’11 Ma), respectively. According to the minimum and central AFT ages for each sample, the fastest exhumation rate is about 0.4 km/Ma, with average long-term exhumation rates on the order of ∼0.3 km/Ma since the Oligocene. This result supports the outward growth model for plateau forming, indicating the southern margin of the Gangdese magmatic arc attained high elevation after the Oligocene.

Tracking the Growth of the Himalayan Fold‐and‐Thrust Belt From Lower Miocene Foreland Basin Strata: Dumri Formation, Western Nepal

AbstractNew data from the lower Miocene Dumri Formation of western Nepal document exhumation of the Himalayan fold‐thrust belt and provenance of the Neogene foreland basin system. We employ U‐Pb zircon, Th‐Pb monazite, 40Ar/39Ar white mica, and zircon fission track chronometers to detrital minerals to constrain provenance, timing, and rate of exhumation of Himalayan source regions. Clusters of Proterozoic–early Paleozoic (900–400 Ma) Th‐Pb monazite and 40Ar/39Ar white mica detrital ages provide evidence for erosion of a Greater Himalayan sequence protolith unaffected by high‐grade Eohimalayan metamorphism. A small population of ~40 Ma cooling ages in detrital white mica grains shows exhumation of low‐grade metamorphic Tethyan Himalayan sequence through the ~350 °C closure temperature along the Tethyan Frontal thrust (proto‐South Tibetan detachment) during the late Eocene. Dumri Formation detritus shows a ~12 Myr time difference between cooling of its source rocks through the ~350 and ~240 °C closure temperatures as recorded by ~40–38 Ma youngest peak cooling ages in 40Ar/39Ar detrital white mica and ~28–24 Ma youngest populations in detrital zircon fission track. Exhumation between circa 40 and 28 Ma is consistent with slip and exhumation along the Main Central Thrust. Combined with similar data from northwestern India, our study suggests west‐to‐east spatially variable exhumation rates along strike of the Main Central Thrust. Our data also show an increase in exhumation during middle Miocene–Pliocene time, which is consistent with growth of the Lesser Himalaya duplex.

Combined detrital zircon fission track and U-Pb dating of the Late Paleozoic to Early Mesozoic sandstones in the Helanshan, western Ordos fold-thrust belt: Constraints for provenance and exhumation history

In order to unravel the Mesozoic sedimentation in response to multistage deformation of the western Ordos fold-thrust belt, combined zircon U-Pb and fission track analyses of single zircon grain are conducted for seven sandstone samples in the Helanshan area. Zircon FT ages are calculated using absolute uranium concentration determined by LA-ICP-MS during the procedure of zircon U-Pb dating. The newly-obtained zircon FT data show age features that are comparable with the previous zircon FT ages obtained by conventional FT methods (by which uranium contents are determined through an external detector to map induced fission tracks). The newly-obtained zircon FT ages have four subordinate FT age peaks (i.e., 225 Ma, 165 Ma, 125 Ma and 80 Ma) which are well matched with the regional geodynamic events occurred in the Late Triassic, Late Jurassic, Late Cretaceous and Early Cenozoic, respectively; and the zircon U-Pb ages have two main peaks at the Paleoproterozoic and the Late Paleozoic. The zircon U-Pb ages, integrating with the FT ages and available paleocurrents, suggest that the Paleoproterozoic basement rocks and Late Paleozoic plutonic rocks of the Alxa Block are the source rocks for the Triassic sandstones, and the Jurassic sandstones are associated with a multi-cycling sedimentation. The Jurassic sandstones contain recycled zircons that firstly deposited in the Triassic strata and subsequently exhumed during the Late Triassic-Early Jurassic.

Evolution and provenance of the Xuefeng intracontinental tectonic system in South China: Constraints from detrital zircon fission track thermochronology

The Xuefeng Intracontinental Tectonic System (XITS) is a large-scale compressional tectonic system in the central South China block. It is characterized by northeast-trending fold-thrust belts involving the Xuefeng zone, the Chevron syncline fold-thrust zone, the Chevron anticline fold-thrust zone and the Central Sichuan basin. This paper reports detrital zircon fission track (ZFT) analyses of the sandstones sampled from Neoproterozoic to Middle Jurassic sequences, and further discusses their relation with low-temperature tectonic evolution and sediment provenance. The totally annealed ZFT data of 452–377 Ma from the Neoproterozoic and Devonian sandstones were related to an Early Paleozoic low-temperature exhumation of the Jiangnan orogen due to intracontinental orogeny in the South China block. Those annealed ZFT components from Triassic to Jurassic sandstones, grouped 236–189 Ma, 189–142 Ma, and 142–124 Ma decreasingly from east to west structurally imply a northwestward progressively squeezing by the Xuefeng intracontinental orogeny as a result of the Paleo-Pacific subduction. The unannealed age components (411 Ma, 362–377 Ma, 270–216 Ma, and 209–183 Ma) of the Upper Triassic to Middle Jurassic sandstones, are comparable to the multistage tectonic evolution of the Qinling orogen through subduction, accretion and collision and are therefore indicative of sample potential provenance.

Downstream evolution of the thermochronologic age signal in the Brahmaputra catchment (eastern Himalaya): Implications for the detrital record of erosion

The Namche Barwa massif in the eastern Himalayan syntaxis is characterized by very rapid exhumation and provides a significant proportion of the sediment flux carried by the Brahmaputra River. We present new detrital zircon fission-track (ZFT) and muscovite 40Ar/39Ar (MAr) data from modern sediments of rivers draining the eastern Himalaya. The cooling-age populations for both thermochronometers contain a characteristic <2 Ma signature related to the rapid exhumation of Namche Barwa, which can be traced hundreds of kilometers downstream from their source into the Brahmaputra foreland, despite dilution from downstream tributary catchments. To estimate present-day erosion in the catchments, we apply a mixing model based on linear inversion of the binned age distributions. The inversion predicts relative erosion rates in the syntaxial region that are an order of magnitude higher than those in upstream catchments, and about twice as high as those in the southern Himalayan catchments, consistent to first order with previous estimates of erosion-rate patterns in the eastern Himalaya. A comparison of the observed downstream evolution of age distributions with a sediment-flux model suggests that the ZFT signal can be explained by dilution from Himalayan tributaries only, whereas the MAr signal is also affected by selective sequestering (possibly through winnowing) of micas as they are transported downstream. Nevertheless, thermochronological ages <2 Ma provide a diagnostic signal of syntaxial exhumation in the sedimentary record of the eastern Himalaya; this study suggests the most robust signal to be recorded in the most proximal deposits with respect to the syntaxis.

Detrital zircon fission-track thermochronology and magnetic fabric of the Amagá Formation (Colombia): Intracontinental deformation and exhumation events in the northwestern Andes

New detrital zircon-fission track (ZFT) and magnetic fabric data are presented to constrain the time of deposition, provenance and deformation of the of Lower and Upper members of the Amagá Formation in the Amagá Basin. The Amagá Basin is located in the northern Andes, between the Western and Central Cordilleras of Colombia. The Amagá Formation was deposited in a transpressive geodynamic context and is allegedly synchronous with tectonic events such as the Andean orogeny and the Panama-Choco Block collision with the northwestern South American Plate. Detrital ZFT data confirm an Oligocene age for the Lower Member and a middle-Miocene age for the Upper Member of the Amagá Formation. In addition to constraining the depositional age, the ZFT data presented in this study also reflect Paleocene-Eocene, late to early Oligocene and late to middle Miocene cooling in sediment source areas mainly located in the Central and Western Cordilleras of Colombia. These ages can be associated with regional exhumation events in the central and northern Andes of South America. Collisional stages of the Panama-Choco Block against northwestern South America, subduction of the Farallon-Nazca Plate and strike-slip reactivation periods of the Cauca-Romeral fault system, caused NW-SE compression and NE-SW simple shear in the Amagá Basin. This deformational regime, identified by magnetic fabric data, induces syn- and post-depositional deformation over the Amagá Formation.

Paleocene‐Early Eocene uplift of the Altyn Tagh Mountain: Evidence from detrital zircon fission track analysis and seismic sections in the northwestern Qaidam basin

AbstractMost existing tectonic models suggest that the deformation and uplift of the northern Tibetan Plateau is the latest crustal response to the collision of the India Plate and Eurasian Plate. The tectonic evolution of Altyn Tagh Mountain (hereafter called simply the “Altyn Tagh”), on the northern margin of the Tibetan Plateau, has attracted considerable scientific attention. In this study, we use fission track age dates of detrital zircons from the northwestern Qaidam basin together with sedimentary observations to understand more fully the Cenozoic tectonic uplift of the Altyn Tagh. Detrital zircons from five borehole samples distributed in different folds in the northwestern Qaidam basin yielded ages mainly ranging from 88.5 to 49.2 Ma, older than their sedimentary deposition ages (43.8–22 Ma). The binomial distribution in grain age fitted peaks was generally dominated by one young peak, P1, which varied from 73.6 to 47.2 Ma. A thinning of the Cenozoic Lulehe Formation (53.5–43.8 Ma) stretched from the inner Qaidam basin to the slopes of the Altyn Tagh in the seismic sections of the northwestern Qaidam basin. Based on magnetostratigraphic dating, there was a hiatus in sedimentation in the Qaidam basin between 65 Ma and 54 Ma; this was confirmed by seismic profiles and borehole data, which show an unconformity between the Mesozoic Quanyagou Formation and the Lulehe Formation. Combined with an analysis of provenance, the detrital zircon young peak age and the sedimentary record revealed that the most significant regional uplift of the Altyn Tagh occurred during the Paleogene‐Early Eocene, almost coinciding with the collision of the Indian and Eurasian plates between 65 Ma and 44 Ma.

Thermal imprint of rift-related processes in orogens as recorded in the Pyrenees

The extent to which heat recorded in orogens reflects thermal conditions inherited from previous rift-related processes is still debated and poorly documented. As a case study, we examine the Mauléon basin in the north-western Pyrenees that experienced both extreme crustal thinning and tectonic inversion within a period of ∼30 Myrs. To constrain the time–temperature history of the basin in such a scenario, we provide new detrital zircon fission-track and (U–Th–Sm)/He thermochronology data. The role of rift-related processes in subsequent collision is captured by inverse modeling of our thermochronological data, using relationships between zircon (U–Th–Sm)/He ages and uranium content, combined with thermo-kinematic models of a rift-orogen cycle. We show that the basin recorded significant heating at about 100 Ma characterized by high geothermal gradients (∼80 °C/km). Our thermo-kinematic modeling and geological constraints support the view that subcontinental lithospheric mantle was exhumed at that time below the Mauléon basin. Such a high geothermal gradient lasted 30 Myr after onset of convergence at ∼83 Ma and was relaxed during the collision phase from ∼50 Ma. This study suggests that heat needed for ductile shortening during convergence, is primarily inherited from extension rather than being only related to tectonic and/or sedimentary burial. This should have strong implications on tectonic reconstructions in many collision belts that resulted from inversion of hyper-extended rift basins.

Bias in detrital zircon geochronology and thermochronometry

Detrital studies that utilize zircon U–Pb geochronology and fission-track (FT) thermochronometry are subject to a range of potential sources of bias that should be properly evaluated and minimized. Some of them are common to any single-grain mineral analysis (e.g., variable bedrock mineral fertility, hydraulic sorting during transport, selective grain loss during sample processing), whereas others are intrinsic to zircon, and are related to radiation damage and age discordance. In this article, we quantify the impact of intrinsic bias on detrital studies thanks to the analysis of modern detritus shed from the European Alps, and illustrate the general implications on geological interpretations.We show that detrital zircon U–Pb age distributions based on statistically robust datasets are highly reproducible and representative of the parent bedrock ages in the catchment. Arbitrary or selective removal of discordant grain ages can be minimized by using the Kolmogorov–Smirnov test to identify an appropriate cutoff level. Loss of metamict (α-damaged) zircon has a minor impact on data representativeness, and is mainly controlled by regional metamorphism rather than by mechanical abrasion during river transport. Zircon FT grain-age distributions were found to have poor reproducibility, although age spectra are consistent with bedrock data. However, unlike the U–Pb datasets, U-rich zircon grains (>1000ppm) are systematically missed, and undatable grains may exceed 50%. We identify two major sources of distribution bias specific to zircon FT datasets: (i) sediment sources dominated by U-rich zircon grains are markedly underrepresented in the detrital record, because such grains often have uncountable high densities of fission tracks (“U concentration bias”); (ii) sediment sources that shed zircon grains with high levels of α-damage are underrepresented, because these grains are lost during chemical etching for FT revelation (“etching bias”). In the case of multimethod dating on the same grains (e.g., FT and U–Pb double dating), bias affecting detrital zircon FT dating propagates to the entire dataset. These effects may not impact on exhumation-rate studies that utilize the youngest grain ages (i.e., lag-time approach). However, they represent a limiting factor for conventional provenance studies, and generally preclude application of zircon FT dating to sediment budget calculations.

Spatial extent of rapid denudation in the glaciated St. Elias syntaxis region, SE Alaska

AbstractThe ongoing collision of the Yakutat Terrane with the North American Plate formed the St. Elias Range in southeastern Alaska. Previously published detrital zircon fission track dating of sand‐size material detected very rapid rock denudation within the syntaxis region of the St. Elias Range, but the size and spatial extent of the rapid exhumation are elusive due to the glacial cover that inhibits direct bedrock studies. Two possible mechanisms were suggested to explain the rapid denudation occurring underneath the Seward‐Malaspina Glacier, which differ in their spatial extent of rapidly exhumed rocks. This study evaluates the spatial extent of rapid denudation at the St. Elias syntaxis region. We investigate the cobble‐sized material of the Malaspina Glacier. Petrological classifications are used as a provenance tool and combined with zircon and titanite (U‐Th)/He analysis to reveal cooling rates of the glacial detritus. In total, 1998 cobbles are classified in the field. Petrographic characterization is conducted on 91 thin sections and the (U‐Th)/He ages of 59 cobbles are measured. Fourteen cobbles reveal young cooling ages of 3–2 Ma originating from various lithologies of undeformed and deformed magmatic and metamorphic rocks. Older age populations peak at 12, 27, and 36 Ma. The cooling ages and the mapped known extent of the different lithologies suggest that a larger then previously assumed area of the St. Elias syntaxis region experienced rapid exhumation. Rapid rock exhumation extends beyond the Contact Fault and comprises most of the Seward‐Malaspina Glacier catchment.

Detrital Zircon Fission-Track Thermochronology of the Present-Day Isère River Drainage System in the Western Alps: No Evidence for Increasing Erosion Rates at 5 Ma

The Isère River system drains parts of the Western Alps in south-eastern France. Zircon fission-track data of the Isère River and its tributaries show a range of apparent cooling ages from about 7 to 150 Ma. Zircons with Jurassic to early Tertiary cooling ages are derived from partially reset or non-reset sedimentary cover units of the internal and external Alps, while grains belonging to the minimum age fraction are derived from areas of active river incision in the external crystalline massifs or from the Penninic front. With the absence of major normal faults, upper crustal exhumation in the Western Alps is driven by erosion. First-order long-term exhumation rate estimates based on minimum ages are about 0.5–0.6 km/Myr for the fastest exhuming areas, while drainage basin average rates based on central ages are about 0.2–0.4 km/Myr. These rates are slower than published short-term erosion rates determined from detrital quartz 10Be analyses in the Pelvoux massif. While present-day erosion is faster than the long-term average exhumation rates, the Isère River drainage zircon fission-track data do not show evidence for increasing erosion rates at 5 Ma. Exhumation has not been sufficient in this area to expose rocks with &lt;5 Ma cooling ages today. The increase in erosion may have happened only in glaciated areas between 1 and 2 Ma.

The Tectonic and Uplift History of the Kuruketage Area in the North-East Edge of the Tarim Basin, China: Constraints from Detrital Zircon and Apatite Fission Track Data

This study uses the application of zircon fission track (ZFT) and apatite fission track (AFT) thermochronometry technique to investigate the tectonic and uplift history of the Kuruketage area, north-east edge of the Tarim Basin. Based on measured ZFT, AFT and equivalent vitrinite reflectance measurements of samples in sedimentary rocks in Kuruketage area, the temperature time evolution history from early Paleozoic strata was modeled. The results show that the youngest peaks of ZFT at 371-392Ma and 328 - 305.7Ma record Hercynian tectonic and uplift event; the AFT peaks at 134.5 - 164Ma, 73 - 100Ma and 35.4Ma mainly represent the Late-Cretaceous tectonic and uplift event in Kuruketage area. The AFT thermal modeling results from the early Paleozoic strata indicate that the maximum paleo-temperature (at 140 215°C) experienced in late Silurian to early Devonian, and the strata temperature decreased to about 120°C before the Late-Cretaceous.