Zircon Fission Track Analysis Research Articles

A Complex Meso–Cenozoic History of Far-Field Extension and Compression: Evidence from Fission Track Analysis in the Helanshan Mountain Tectonic Belt, NW China

The Helanshan Mountain tectonic belt (HTB) is an intraplate deformation belt along the northwestern border of the Ordos Block in the North China Craton. When and why this intracontinental tectonic belt formed, its subsequent uplift and erosion, and the relationships between ranges and adjacent basins remain unclear. To better assess the connections between the temporal and structural activity in HTB, apatite fission-track (AFT) and zircon fission-track (ZFT) analyses were conducted in this study. The lack of adequate FT data from the HTB is a source of contention and dispute. This paper collected samples for AFT and ZFT techniques from the central and southern HTB, trying to improve the research. The ZFT and AFT ages could be divided into the following 7 groups: 279 Ma, 222–213 Ma, 193–169 Ma, 151–147 Ma, 130–109 Ma, 92–77 Ma, and 65–50 Ma. The inverse modeling results of AFT indicate 4 fast cooling episodes of 170–120 Ma, 120–95 Ma, 66–60 Ma, and ~10–8 Ma to the present. Combining the results of FT analysis with radial plot and inverse modeling of AFT, the following eight age groups are believed to reveal the distinct tectonic activities in HTB: the first age group of 279 Ma mainly represented the back-arc extension of the southern HTB; the age group of 222–213 Ma was bounded with NNE-SSE trending contraction between the South China block and North China Craton; the event of 193–169 Ma responded to the post-orogenic collapse followed after the second event; the 151–147 Ma group was interpreted as the eastward extrusion induced by the subduction between Qiangtang and Lhasa blocks; the Early Cretaceous (130–109 Ma) group was not only affected by the rollback of the Pacific Plate, but also denoted the collapse of the thickened lithosphere formed in the Late Jurassic; the Late Cretaceous (92–77 Ma) group was attributed to long-distance impact from the subduction of the Pacific Plate beneath the Eurasian Plate; the event during 65–50 Ma was a correspondence to far-field effect of the onset collision between the Eurasian and Indian Plates; and from 10–8 Ma to the present, the progressive collision of the Indian and Eurasian Plates have a significant impact on the HTB and the northeastern Tibetan Plateau.

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.

Relationship between the tectono-thermal events and sandstone-type uranium mineralization in the southwestern Ordos Basin, Northern China: Insights from apatite and zircon fission track analyses

Tectonic evolution significantly influenced the sandstone-type uranium mineralization in the continental basins of northern China. To better understand the relationship between the tectono-thermal evolution and uranium-ore-forming process, we selected the southwestern Ordos Basin as the research subject. Detrital apatite and zircon fission track analyses of samples from the Early Cretaceous succession were conducted in this work. The results revealed five stages of tectono-thermal events since the Late Mesozoic: (i) the Late Jurassic–Early Cretaceous (165 ∼ 140 Ma), (ii) the late Early Cretaceous (130 ∼ 100 Ma), (iii) the Late Cretaceous (100 ∼ 60 Ma), (iv) the Late Paleogene–Early Neogene (55 ∼ 20 Ma), and (v) the Late Miocene–present (<10 Ma). These activities are recognized as the results of the multiplate convergence of the Siberian, paleo-Pacific, and Indian plates since the Mesozoic. Furthermore, the Late Jurassic–Early Cretaceous uplifting of the western Ordos Basin supplied the abundant sedimentation materials and uranium materials for the Early Cretaceous succession in the basin, which acted as excellent ore-bearing layers. The regional extension in the Early Cretaceous has led to an increasing geothermal gradient, which caused the maturity of hydrocarbon source rocks and escape of oil and gas. The following tectonic revisions supplied the pathways and favorable locations for oil and gas accumulation and uranium mineralization. The latest activity has driven the reactivation and re-enrichment of the uranium mineralization since the Late Miocene.

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 (&lt;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.

Multiproxy provenance analysis of Lower to Upper Cretaceous synorogenic deposits in the Southern Andes (34–35°S): Evidence of coeval volcanism during the onset of the Andean orogeny

The combination of detrital low-temperature thermochronology with previous U–Pb geochronology, petrological and sedimentological analyses, has proven to be a valuable approach to constrain the provenance of non-marine Lower to Upper Cretaceous synorogenic deposits in the northern Neuquén Basin. This work focuses on the study of the Diamante Formation, a fluvial succession that represents the first synorogenic products of the Andean foreland basin at 34–35°S. The results indicate that the deposition of the Diamante Formation occurred simultaneously with the existence of an active western volcanic arc during the onset of the foreland basin. The facies associations evidence the transition between the backarc and the foreland basin stages as well as the inception of fluvial sedimentation in the foredeep. Petrographic analyses, together with changes in the paleocurrents and the record of limestone clasts suggest a regional detrital source shift. Apatite fission-track analyses (AFT) of a sample collected from the lower part of the Diamante Formation indicate an Albian central cooling age. This sample also evidences a remarkable presence of angular apatite and zircon crystals with subordinated rounded and subangular grains. Zircon fission-track analyses (ZFT) of a sample from the upper part of the Diamante Formation yield two discrete populations of cooling ages, both reflecting source-cooling during the Late Jurassic (∼161 Ma) and the Permian (∼265 Ma). Finally, a comparison between the AFT and the U–Pb maximum depositional zircon-age reveals a short lag time (ca. 3 ma), likely related to the rapid magmatic cooling of a coeval volcanic source at ∼110 Ma (Albian).

Multi-stage tectonic events of the Eastern Kunlun Mountains, Northern Tibetan Plateau constrained by fission track thermochronology

The present Tibetan plateau is a collage of several terranes, including the Lhasa, Qiangtang, Songpan-Ganzi and Kunlun-Qaidam terranes. Following closure of the Palaeo- and Neo-Tethyan oceans, the terranes of the Tibetan plateau migrated northward, and the deformation timing of the northern part of the Songpan-Ganze terrane between Golmud and Dulan in the Eastern Kunlun Orogenic Belt (EKOB) is still poorly understood. Fission track (FT) analysis of zircon and apatite from two localities between Golmud and Dulan has yielded new information about the timing of deformation in EKOB. Samples collected from Aikeng and Boguole yielded zircon and apatite FT central age ranges of ~193–133 Ma and ~103–29 Ma respectively. Thermal history modelling of FT data revealed five major episodes of cooling in these regions at ~200–170 Ma, ~170–100 Ma, ~100–50 Ma, ~50–18 Ma, and ~18–0 Ma. Stage one represents the collision between the Qiangtang and Kunlun tectonic blocks and the closure of the Songpan-Ganzi-Bayan Har Triassic turbidite basin. In stage two, the northward subduction of the Neo-Tethys oceanic plate accentuated sediment transport to the actively closing ocean basin between the Lhasa and Qiangtang terranes, the far-field effect of collision caused the cooling of the EKOB. Stage three is characterised by tectonic quiescence, which ended at ~50 Ma due to the collision of India and Asia. In stage four, the establishment of a strike-slip fault system and fold-thrust belt in the EKOB resulted in exhumation. Stage five saw rapid cooling in response to the most recent compressional event associated with fault development in the EKOB.

New zircon fission track analysis constraints on tectonic-mineralization epochs in the Ganzi-Litang gold belt, eastern Qinghai-Tibet plateau

The Ganzi-Litang gold belt, as a part of the Sanjiang Tethys orogen, is located between the Yidun block and Songpan-Ganzi Block/Terrane in the eastern Qinghai-Tibetan plateau and experienced oceanic slab subduction, continent-arc collision and intracontinental convergence since the Indosinian orogeny. A series of gold ore deposits have been found within the region. This paper describes the tectonic evolution and episodes of gold mineralization within this belt using zircon fission track (FT) thermochronology. A total of 15 zircon FT analysis results were obtained that can be divided into four age groups (217–213 Ma, 190–160 Ma, 155–93 Ma and 72–67 Ma) representing four distinct episodes of gold mineralization, of which the middle two episodes predominate. Eruption of the auriferous volcanic rocks that comprise the belt started at 217–213 Ma when the Ganzi-Litang Meso-Tethys ocean began to subduct. The mineralization episode at 190–160 Ma corresponds to the formation of auriferous volcanic rocks during intracontinental collisional-convergence of the Ganzi-Litang belt. Finally, the episodes at 155–93 Ma and 72–67 Ma are respectively related to the closing times of the Bangong-Nujiang and Yarlung Zangbu oceans, corresponding to tectonic quiet episodes.

Shortening of the axial zone, pyrenees: Shortening sequence, upper crustal mylonites and crustal strength

The strength of the lithosphere may be constrained qualitatively by field observations on localized vs distributed modes of deformation and by the mineral assemblages formed during deformation. The internal deformation of the Bielsa basement unit of the Pyrenean Axial zone is investigated through structural, microstructural and thermometric data. In this area, shortening is widely distributed as attested by the folded attitude of the interface between the basement and its sedimentary Triassic cover. Shortening is estimated around 1.7 km (13%) from a regional balanced cross-section and should be considered in pre-Pyrenean reconstructions. Shortening probably occurred before strain localization on crustal ramps as suggested by zircon fission-track analysis. Distributed shortening is characterized at small-scale by low-temperature mylonites and cataclasites. In thin-section, feldspar originally present in the magmatic protolith is partially to totally sericitized. This transformation led to significant weakening of the rock and took place in the 250–350 °C temperature range. Sericitization is ubiquitous, even in un-deformed granodiorites. This shows that the weakening effect of sericitization not only occurs in ultra-mylonites, ultra-cataclasites and phyllonites but also more generally in the upper crust early during the shortening history, with implications for the shortening style. Estimates of the geothermal gradient suggest that inherited thermicity may also have influenced the style of shortening. We propose that the upper crust was very weak before or at the onset of its shortening due to high-thermal gradients and fluid circulation that induced large-scale sericitization in greenschist facies conditions. This has strong implications on the rheological evolution of the upper crust submitted to metamorphic alteration in the greenschist facies and below.

Late Triassic-Cenozoic Thermochronology in the Southern Sanjiang Tethys, SW China, New Insights from Zircon Fission Track Analysis

The Sanjiang Tethys orogenic belt is located in the southeast side of the Qinghai-Tibet Plateau. It has undergone the opening and closing movements in different periods of Tethys oceans, complex accretive orogeny and strong mineralization from Paleozoic to Mesozoic. Using zircon fission track (ZFT) thermochronology, this study reveals the Sanjiang Tethys has experienced multi-stage tectonic activities during the Late Triassic-Cenozoic. The 15 ZFT ages with their decomposition components obtained from Sanjiang Tethysian region range from 212 to 19 Ma, which not only shows 6 age groups of 212, 179–172, 156–133, 121–96, 84–70 and 50–19 Ma, but also constrains the age limit of the tectonothermal events. These age groups recorded the Paleo-Tethys main and branches ocean opening/closure time. The age-elevation plot indicates the Sanjiang region had differential uplifting and exhumation and fast uplifting times of ca. 133, 116 and 80 Ma, coinciding with the age groups mentioned above. These results show new geochronological evidences and viewpoints.

Differential late-Cenozoic uplift across the Dongjiu-Milin Fault Zone in the Eastern Himalayan Syntaxis revealed by low-temperature thermochronology

The Eastern Himalayan Syntaxis (EHS), as one of the most tectonically active regions on the Earth, is one of the best places to explore the relationship between tectonics and surface exhumation process. The uplift and exhumation history are often by one of two leading models: a stationary or localized “tectonic aneurysm” or a northeasterly “migrating crustal antiform”. Previous researchers have placed great emphasis on the core zone of Namche Barwa antiform. However, the Dongjiu-Milin Fault Zone (DMFZ), which forms the northwestern boundary of the antiform and plays an important role for deformation process in the EHS, is still poorly understood. In this study, we focus on differential late-Cenozoic uplift across the DMFZ. We collected 10 bedrock samples in two vertical transects, one on the northwest side of the DMFZ on the Lhasa block and one on the southeast side on the Namche Barwa antiform, and conducted apatite fission track and zircon fission track analyses. Ten samples collected from the two vertical transects yield AFT ages between 3.2 million years ago (Ma) and 10.3 Ma and ZFT ages between 9.9 Ma and 12.9 Ma. A finite-element analysis reveals that the two sides of the DMFZ have undergone quite different uplift histories and processes in the late-Cenozoic: The Lhasa block is uniformly uplifted, while the Namche Barwa antiform is nonuniformly uplifted similar to a buckle fold. The phenomenon revealed in this study is best explained by a mechanism whereby the EHS is characterized by the northeasterly migration of an antiform.

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.

Constraining Age of Deformation Stages in the South-Western Part of Verkhoyansk Fold-and-Thrust Belt by Apatite and Zircon Fission-Track Analysis

Abstract—Zircon fission track analysis was carried out for Mesoproterozoic to Lower Paleozoic sedimentary rocks of the South-Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt. The age of thrusting stages was constrained in this region. The early stage of deformations dated as 160 Ma, the main stage dated as from 70 to 90 Ma. Thermal history modeling on apatite allowed us to establish the youngest stage of erosion from 20 to 30 Ma, which indirectly indicates the reactivation of tectonic processes in the region at the boundary of Paleogene and Neogene. The degree of heating of the rocks increases in the east direction, and if in the frontal zone the fission tracks were annealed only in apatite, then in the Sette-Daban zone fission tracks were annealed both in apatite and in zircon.

Post-rift anomalous thermal flux in the Songliao Basin, NE China, as revealed from fission track thermochronology and tectonic analysis

The Songliao Basin in northeastern China contains a nearly complete Cretaceous sedimentary record. It offers an unparalleled opportunity for studying the thermal behavior of a rifting basin adjacent to a subduction zone during various stages of its evolution. The post-rift thermal history of the Songliao Basin is here documented by systematic apatite and zircon fission track analyses on newly exhumed sandstone samples from the Upper Cretaceous Quantou and Qingshankou formations. The combination of thermochronology results and stratigraphic records indicates that there was an unambiguous post-rift thermal overprinting event in the Songliao Basin during the deposition of the Nenjiang Formation at ~85–80 Ma. This fossil thermal anomaly has not been previously recognized from organic maturity indicators and/or subsidence modeling. The zircon fission track results indicate an estimated maximum paleo-temperature as high as 170–220 °C near the Daqing Placanticline. Fission track analyses of sampled apatites further define a restricted thermal anomaly in the central part of the basin. Based on the context of local lithospheric structure and adjacent plate tectonic history, we conclude that the Late Cretaceous westward subduction of the Paleo-Pacific Plate not only produced vertical motions and directly controlled the topography of the Songliao Lake, but also dominated the mantle heat flow beneath East Asia. This finding has significant implications for the evolution of thermal flux in the basin.

Late Cretaceous–Cenozoic thermochronology in the southern Songliao Basin, NE China: New insights from apatite and zircon fission track analysis

The Late Cretaceous–Cenozoic thermochronology in the southern Songliao Basin was determined using apatite and zircon fission track analysis data. It is crucial to deciphering the evolution of the Songliao Basin and changes in the movement direction of the Pacific Plate. We present eight new apatite and eight corresponding zircon fission track data from borehole samples across the Baixingtu uplift. The apatite fission track (AFT) thermochronological modeling reveals that the basin underwent two distinct compression- and extension-related uplift stages of rapid cooling during the Late Cretaceous–Cenozoic: stage I during the Late Cretaceous–Early Paleogene (∼70–52 Ma), stage II in the Late Paleogene–Early Neogene (∼38–10 Ma). The AFT data vary from 76.0 ± 5.0 to 25.3 ± 1.9 Ma and the zircon binomial peak fitting ages ranged from 79 to 61 Ma, all younger than the stratigraphic ages (∼92–88 Ma) of their host rocks. One sample had similar fission track ages (apatite 76.0 ± 5.0 Ma and zircon 79.0 ± 3.0 Ma) within reasonable error, representing a magmatic event. The data were divided into four groups. (i) 79–76 Ma: magmatic cooling event, (ii) 69–61 Ma: uplift of the whole region and exhumation and erosion, (iii) 37–34 Ma and (iv) ∼25 Ma: various uplift events related to tectonic inversion, causing partial uplift, exhumation and erosion in the basin. This created favorable conditions for formation and storage of mineral resources, especially sandstone uranium ore deposits. Our results combined with previous research suggest that the variation in uplift and exhumation began in the basin’s SE part and propagated to the NW ∼5–8 Ma later. The tectonic inversion in the southern Songliao Basin at least began ∼37–34 Ma. This corresponds with the time of the Paleo-Pacific Plate shifting to NWW motion and the India–Asia collision. These regional-scale uplift, exhumation and inversion events were probably responses to the subduction and turning of the Paleo-Pacific Plate from the southeast and the collision of the Indian subcontinent from the southwest. It is important to study the tectonic inversion in the whole east of China since the Late Cretaceous and the change of the direction of the Pacific plate movement.

Mesozoic and Cenozoic thermal history of the Western Reguibat Shield (West African Craton)

AbstractUsing low‐temperature thermochronology on apatite and zircon crystals, we show that the western Reguibat Shield, located in the northern part of the West African Craton, experienced significant cooling and heating events between Jurassic and present times. The obtained apatite fission track ages range between 49 and 102 Ma with mean track lengths varying between 11.6 and 13.3 μm and Dpar values between 1.69 and 3.08 μm. Zircon fission track analysis yielded two ages of 159 and 118 Ma. Apatite (U–Th)/He uncorrected single‐grain ages range between 76 and 95 Ma. Thermal inverse modelling indicates that the Reguibat Shield was exhumed during the Early Cretaceous, Late Cretaceous, Palaeocene–Eocene and Quaternary. These exhumation events were coeval with regional tectonic and geodynamic events, and were probably driven by a combined effect of plate tectonics and mantle dynamics.

Sphene and Zircon Fission Track Analysis of Syenite Rocks of the Sushina Hills, Purulia-Bankura Shear Zone (TPSZ)

Sushina hills are situated in the Purulia-Bankura Shear Zone. This is tectonically disturbed narrow zone nearly 100 Km long having WNW-ESE trend. Two different rock assemblages namely Chhotanagpur Gneissic Complex and rocks of Singhbhum Group occur on the two sides of this shear zone. In this paper, I have used fission track ages of two cogenetic minerals and their corresponding closure temperatures to describe the denudation history. The difference of their fission track ages provides the rate of denudation 4 m/Ma during the period 970Ma-1458Ma due to tectonic activity. The youngest age of zircon of 970Ma represents the timing of the initiation of denudation.

Tectono‐thermal Evolution of a Distal Rifted Margin: Constraints From the Calizzano Massif (Prepiedmont‐Briançonnais Domain, Ligurian Alps)

AbstractThe thermal evolution of distal domains along rifted margins is at present poorly constrained. In this study, we show that a thermal pulse, most likely triggered by lithospheric thinning and asthenospheric rise, is recorded at upper crustal levels and may also influence the diagenetic processes in the overlying sediments, thus representing a critical aspect for the evaluation of hydrocarbon systems. The thermal history of a distal sector of the Alpine Tethys rifted margin preserved in the Ligurian Alps (Case Tuberto‐Calizzano unit) is investigated with thermochronological methods and petrologic observations. The studied unit is composed of a polymetamorphic basement and a sedimentary cover, providing a complete section through the prerift, synrift, and postrift system. Zircon fission track analyses on basement rocks samples suggest that temperatures exceeding ~240 ± 25°C were reached before ~150–160 Ma (Upper Jurassic) at few kilometer depth. Neoformation of green biotite, stable at temperatures of ~350 to 450°C, was synkinematic with this event. The tectonic setting of the studied unit suggests that the heating‐cooling cycle took place during the formation of the distal rifted margin and terminated during Late Jurassic (150–160 Ma). Major crustal and lithospheric thinning likely promoted high geothermal gradients (~60–90°C/km) and triggered the circulation of hot, deep‐seated fluids along brittle faults, causing the observed thermal anomaly. Our results suggest that rifting can generate thermal perturbations at relatively high temperatures (between ~240 and 450°C) at less than 3 km depth in the distal domains during major crustal thinning preceding breakup and onset of seafloor spreading.

Late Triassic‐Early Jurassic abnormal thermal event constrained by zircon fission track dating and vitrinite reflectance in Xishan coalfield, Qinshui Basin, central North China

Xishan coalfield, Shanxi, is located in the northwest of the Qinshui Basin, central North China. It is notable for its varieties of coal rank ranging from high volatile bituminous coals to anthracite as well as having abundant coalbed methane resources. Zircon fission track (ZFT) analyses were carried out on the zircons in 2 Upper Carboniferous and 5 Lower‐Middle Permian sandstones, and vitrinite reflectance of Late Carboniferous and Early Permian coals were measured to determine the timing of thermal events and maximum paleo‐temperatures, which were responsible for coal maturation and coalbed methane generation. Maximum paleo‐temperatures calculated from vitrinite reflectance values reached to about 232 and 223 °C in Late Carboniferous and Early Permian coals, respectively, and the estimated paleo‐temperature gradient was 11.84 °C/100 m, representing an intensive abnormal thermal event. Results of the ZFT dating indicated that 5 samples failed the χ2‐test and 2 samples passed the test. The decomposition results of the 5 samples divided their age populations into 3 periods: (a) older ages (537, 584, and 802 Ma) than sandstones ages, (b) close to or slightly older than their depositional ages (289, 301, and 331 Ma), and (c) younger than the depositional ages (181–215). The 2 samples that passed χ2‐test yield the central ages of 168 ± 7 Ma and 190 ± 8 Ma, respectively, younger than the deposition age. The close to or older ages than the sandstones depositional ages represent the tectonothermal events occurring in their source areas; the younger ages indicate the existence of the postdepositional tectonothermal event. The agreement of the partly annealing temperature zone (210–300 °C) of zircon fission tracks with the calculated maximum paleo‐temperatures from vitrinite reflectance suggests a Late Triassic‐Early Jurassic abnormal thermal event with the formation time of the present coal rank being 181–215 Ma, rather than a unique intrusion at 95–135 Ma on the western margin of coalfield as previously believed. Combined with other ZFT ages regionally, this abnormal event also occurred in the southern as well as the northern parts of the Qinshui Basin. The Late Triassic‐Early Jurassic intensive extension in the North China Craton is the geodynamic setting of this tectonothermal event.

Late Cenozoic thrusting of major faults along the central segment of Longmen Shan, eastern Tibet: Evidence from low-temperature thermochronology

The Cenozoic orogenic process of the Longmen Shan (LMS) and the kinematics of major faults along the LMS are crucial for understanding the growth history and mechanism of the eastern Tibetan Plateau. Three major faults, from west to east, are present in the central segment of the LMS: the Wenchuan-Maoxian Fault (WMF), the Beichuan-Yingxiu Fault (BYF), and the Jiangyou-Guanxian Fault (JGF). Previous researchers have placed great impetus on the Pengguan Massif, between the WMF and BYF. However, limited low-temperature thermochronology data coverage in other areas prevents us from fully delineating the tectonic history of the LMS. In this study, we collect 22 samples from vertical profiles in the Xuelongbao Massif and the range frontal area located at the hanging walls of the WMF and JGF respectively, and conduct apatite and zircon fission track analyses. New fission track data reveal that the Xuelongbao Massif has been undergoing rapid exhumation with an average rate of ~0.7–0.9mm/yr since ~11Ma, and the range frontal area began rapid exhumation at ~7.5Ma with total exhumation of ~2.5–4.5km. The exhumation histories indicate that the three major faults (WMF, BYF and JGF) in the central LMS are all reverse faults, and show a basinward in-sequence propagation from middle Miocene to present-day. Such a pattern further implies that upper crustal shortening is the primary driver for the LMS' uplift during the Late Cenozoic. Nevertheless, middle-lower crustal deformation is difficult to be constrained by the exhumation histories, and its contribution to LMS' uplift cannot be ruled out.

Late Cretaceous-Cenozoic exhumation of the southeastern margin of Coastal Mountains, SE China, revealed by fission-track thermochronology: Implications for the topographic evolution

Apatite and zircon fission-track (FT) analyses were used to reveal the Late Cretaceous-Cenozoic exhumation of the Southeastern margin of Coastal Mountains (SCM), Southeast (SE) China. Zircon FT ages ranging from 92 ± 5 to 98 ± 6 Ma, and apatite from 54 ± 4 to 68 ± 6 Ma with mean measured track lengths between 12.2 ± 1.6 and 12.9 ± 2.0 μm, were obtained for the granitoids sampled along an east to west transect across the SCM. The three-stage, involving the first rapid stage, following by slow stage, and finally accelerated cooling stage, and inferred exhumation since the Late Cretaceous have been identified in the study region based on the FT data and modeling results. Assuming a steady-state paleogeothermal gradient of 25 °C/km founded on geological setting, the exhumations of 4000 m, 2000 m and 1600 m, and the paleoelevation decreases of 727 m, 364 m and 291 m, were achieved in the SCM, during 94–60 Ma, 60–25 Ma and 25–0 Ma, respectively. This hence reveals that the paleoelevation in the SCM was decreased rapidly from ∼1400 m a.s.l. in the early Late Cretaceous to ∼650 m a.s.l. in the early Paleogene, subsequently slowly to ∼300 m a.s.l. in the Late Oligocene, and finally to ∼0 m a.s.l. in the present at an accelerated rate. Our results indicate that the SCM experienced an episodic exhumation, and successive decreases in paleoelevation, predominantly induced by complex geodynamic processes.