Fission Track Analysis Research Articles

Tectonic control over exhumation in the Arunachal Himalaya: new constraints from Apatite Fission Track Analysis

Abstract New apatite fission track (AFT) ages have been obtained from a synformal nappe of the Higher Himalayan Crystallines emplaced over the Lesser Himalayan metasedimentary zone of the Arunachal Himalaya, India. The AFT cooling ages within the nappe range between 5.0 ± 0.8 and 14.4 ± 1.3 Ma. Modelled exhumation rates calculated from these cooling ages vary from 0.25 ± 0.12 to 0.69 ± 0.25 mm a −1 , which indicates slow exhumation since the Middle to Late Miocene. The AFT cooling ages are younging on both the northern and southern flanks of the synform and the oldest ages are confined to the core. The close mimicking of a shallow crustal exhumation pattern with the synformal structure suggests a strong control of the development of the synform on the exhumation path of the rocks and hence a tectonics–exhumation linkage in the central Arunachal Himalaya. Comparison of these AFT ages with the regional thermochronological record of the Eastern Himalaya reflects a variation in exhumation rates with strike. The AFT age pattern in the central Arunachal Himalaya does not match the pattern of precipitation, which suggests an absence of climate-driven tectonic deformation via focused erosion.

Petroleum system modeling of the East Coast Basin, Hawke Bay, New Zealand

This study evaluates the petroleum prospectivity of the East Coast Basin, an element of the Hikurangi convergent margin of New Zealand. The basin tectonic environment changed from a dormant convergent margin to convergent margin around the early Miocene, likely causing a decrease in the basin thermal regime. Two end-member paleoheat flow scenarios were used to evaluate thermal maturation of two key source rocks, the Waipawa Black Shale and Whangai Formation, based on calibration to six wells across the basin using vitrinite–inertinite reflectance and fluorescence analysis, as well as Rock-Eval Tmax (the temperature at which the maximum rate of hydrocarbon generation occurs during pyrolysis analysis), thermal alteration index, apatite fission track analysis, and present-day temperatures. This study finds that paleobasal heat flow ranged between 40 and 55 mW/m−2 and decreased to 30 mW/m−2 between 15 and 5 Ma, as bracketed by the two end-member heat flow scenarios. The heat flow scenarios were applied to a palinspastically reconstructed basin and petroleum system model across Hawke Bay. We find that the two distinct heat flow scenarios cause a 3–6.5 m.y. difference in the relative timing of hydrocarbon generation across the basin. Structural events in the basin associated with convergent margin tectonics act as the main control on the timing of generation. Modeling results show transformation of kerogen to hydrocarbons began between early and late Miocene for both source rocks depending on the structural regime in the basin. The structural control on source rock maturation highlights the need for robust palinspastic reconstructions in addition to paleothermometric data to evaluate the petroleum prospectivity of convergent margin basins.

Cooling, exhumation, and deformation in the Hindu Kush, NW Pakistan: New constraints from preliminary 40Ar/39Ar and fission track analyses

Asian crust in the Hindu Kush region in northern Pakistan records a protracted history of rifting, subduction and collision not commonly preserved within the Himalaya. Because of this, it is key to understanding the development of the southern Eurasian margin both prior to and after collision with India. New mica 40Ar/39Ar and apatite fission track geochronologic data from this region provide constraints on the kinematics of the Hindu Kush. 40Ar/39Ar muscovite and biotite ages from the late Cambrian Kafiristan pluton are 379.7 ± 1.7 Ma and 47.2 ± 0.3 Ma, respectively. The muscovite age may record cooling or partial resetting, while the biotite age is interpreted to record a thermal disruption associated with the early stages of continental collision in the Himalayan system. A 111.0 ± 0.6 Ma muscovite age from the northern part of the Tirich Mir pluton (∼123 Ma old; U-Pb) is interpreted to indicate a recrystallization event ∼12 Myrs after its intrusion. In addition, a younger muscovite age of 47.5 ± 0.2 Ma was derived from the opposite side of the same pluton in the immediate hanging wall of the Tirich Mir fault. This Eocene age is interpreted to represent the time of recrystallization during fault (re)activation in the early stages of India-Asia continent-continent collision. 40Ar/39Ar biotite analysis from the Buni-Zom pluton yields an age of 61.6 ± 1.1 Ma and is interpreted to reflect cooling at mid-upper crustal levels subsequent to the pluton’s emplacement in the middle Cretaceous. Finally, 17.1–21.3 Ma 40Ar/39Ar ages from the Garam Chasma pluton and surrounding metapelites indicate cooling immediately following crystallization of the leucogranite body in the earliest Miocene/latest Oligocene.The younger cooling history is resolved by fission track dating of apatite (AFT). In the vicinity of the bounding Tirich Mir fault, the Tirich Mir pluton yields an AFT age of 1.4 ± 0.3 Ma, which is consistent with active exhumation associated with the surface uplift of the 7700+ m Tirich Mir peak. The Garam Chasma pluton has a young age of 3.5 ± 0.2 Ma, which also records rapid rock uplift and exhumation in the area. Finally, an AFT age of 9.1 ± 2.1 Ma was extracted from a metapelite in the footwall of an east verging thrust fault separating it from the Garam Chasma pluton to the west. The difference in ages, Pliocene vs. late Miocene, reflect differential cooling/exhumation paths across that structure.

Quantitative Identification of the Annealing Degree of Apatite Fission Tracks Using Terahertz Time Domain Spectroscopy (THz-TDS).

Apatite fission-track (AFT) analysis, a widely used low-temperature thermochronology method, can provide details of the hydrocarbon generation history of source rocks for use in hydrocarbon exploration. The AFT method is based on the annealing behavior of fission tracks generated by 238U fission in apatite particles during geological history. Due to the cumbersome experimental steps and high expense, it is imperative to find an efficient and inexpensive technique to determinate the annealing degree of AFT. In this study, on the basis of the ellipsoid configuration of tracks, the track volume fraction model (TVFM) is established and the fission-track volume index is proposed. Furthermore, terahertz time domain spectroscopy (THz-TDS) is used for the first time to identify the variation of the AFT annealing degree of Durango apatite particles heated at 20, 275, 300, 325, 450, and 500 ℃ for 10 h. The THz absorbance of the sample increases with the degree of annealing. In addition, the THz absorption index is exponentially related to annealing temperature and can be used to characterize the fission-track volume index. Terahertz time domain spectroscopy can be an ancillary technique for AFT thermochronological research. More work is urgently needed to extrapolate experimental data to geological conditions.

Mesozoic-Cenozoic exhumation history of the Qimen Tagh Range, northeastern margins of the Tibetan Plateau: Evidence from apatite fission track analysis

The Tibetan Plateau (TP) is the highest plateau in the world. It is composed of a series of E-W stretched blocks/terranes originating from the Gondwana super-continent. As such, the TP has become one of the world's critical foci of Cenozoic geological studies. The Paleozoic and Mesozoic tectonic histories of the Proto-Tibetan region played an important role in the Cenozoic evolution of the TP. Understanding the pre-Cenozoic building of the Qimen Tagh Range located along the northeastern margins of the TP will help to constrain the development of the northern boundaries of the Proto-Tibetan region. Here, we present apatite fission track data for six granite samples along two vertical transects in the North Qimen Tagh Range. These thermochronometric data indicate that the North Qimen Tagh Range experienced a four-stage cooling history, i.e., during the periods ~262–240Ma, ~165–160Ma, 105–80Ma and ~14/10–0Ma. They reveal that the initial uplift of the northern margins of the TP (i.e., the Qimen Tagh and East Kunlun mountain ranges) during the Late Permian-Middle Triassic separated the paleo-Qaidam Basin from the paleo-Hoh Xil Basin. We therefore suggest that these North Qimen Tagh Range cooling events most probably correspond with the diachronous collisions of the Qiangtang and Lhasa terranes during the Mesozoic, as well as the Indo-Eurasian continental collision which occurred during the Cenozoic.

Early Cenozoic Denudation of Central West Britain in Response to Transient and Permanent Uplift Above a Mantle Plume

AbstractUpwelling mantle plumes beneath continental crust are predicted to produce difficult to quantify, modest uplift and denudation. The contribution of permanent and transient components to the uplift is also difficult to distinguish. A pulse of denudation in Britain in the Early Paleogene has been linked, although with some controversy, with the arrival of the proto‐Iceland mantle plume. In this contribution we show that combining apatite and zircon (U‐Th‐Sm)/He and apatite fission track analyses from central west Britain with numerical modeling clearly identifies a pulse of early Cenozoic denudation. The data indicate that rock uplift and denudation were centered on the northern East Irish Sea Basin and 1.0–2.4 km of rocks were removed during the latest Cretaceous‐early Paleogene. Uplift and erosion appears to have started a few million years before the earliest magmatism in the region. The regional denudation pattern mirrors the distribution of low‐density magmatic rocks that has been imaged in the deep crust. However, the injection of the underplating melt is not enough to account for the total denudation. An additional regional uplift of at least 300 m is required, which is consistent with a transient thermal effect from the hot mantle plume. The rapid exhumation event ceased by ~40 Ma and the data do not require significant Neogene exhumation.

Late Paleozoic – Mesozoic tectonic evolution of the Eastern Taimyr-Severnaya Zemlya Fold and Thrust Belt and adjoining Yenisey-Khatanga Depression

Combined structural, thermochronological and geochronological studies were carried out to unravel the complex late Paleozoic – Mesozoic tectonic evolution of the eastern Taimyr – Severnaya Zemlya Fold and Thrust Belt, along with the adjoining Yenisey-Khatanga Depression and Olenek Fold Zone. New detailed field mapping, a fault and fold geometry and kinematic study, and paleostress reconstruction were undertaken for key areas within the Central and Southern Taimyr domains and the Olenek Fold Zone. Thirty seven samples were used for apatite fission track (AFT) analysis along with two samples for U-Pb zircon granite dating, and three samples for Ar-Ar metamorphic muscovite dating.The late Paleozoic (Early Carboniferous – Permian) tectonic event is recognized only in a relatively narrow zone including the Northern Taimyr Domain and part of the Central Taimyr Domain. It started in the Early Carboniferous (Visean) and is synchronous with the earliest stages of collision in the Uralian Orogen. Our results reveal a widespread distribution of latest Triassic – earliest Jurassic and Cretaceous deformations. The latest Triassic – earliest Jurassic tectonic event significantly overprinted the Northern and Central Taimyr domains and original unaltered late Paleozoic AFT ages are identified only locally. New completely reset AFT cooling ages group at 195-180 Ma, in reasonable agreement with structural data. Older Neoproterozoic and late Paleozoic Ar-Ar ages of metamorphic muscovite are retained, demonstrating that the dated rocks were not heated above 300 °C. An Early Cretaceous tectonic event is inferred from both the structural study and ca. 115–125 Ma AFT cooling ages, and is most widespread in the Southern Taimyr Domain, Olenek Fold Zone and adjoining parts of the Yenisey-Khatanga Depression, modifying older geological structures. A younger Late Cretaceous tectonic event at ca. 100-60 Ma is also inferred from AFT cooling ages, but its distribution is poorly constrained.Our study reveals that the distribution of late Mesozoic tectonic events has been significantly underestimated in previous structural studies of the region. The latest Triassic – earliest Jurassic and Cretaceous tectonic events are easily correlated with those in the Pai-Khoi – Novaya Zemlya and northern Verkhoyansk fold and thrust belts.

Using slope-area and apatite fission track analysis to decipher the rock uplift pattern of the Yumu Shan: New insights into the growth of the NE Tibetan Plateau

Studies have shown that the growth of the Qilian Shan, the northeastern margin of the Tibetan Plateau, started ~10 Ma ago. However, when and how it expanded northwards is still under debate. Here we focus on the rock uplift pattern of the Yumu Shan, an active fault-related fold in the Hexi Corridor north to the Qilian Shan. Normalized channel steepness achieved from the analysis of river longitudinal profiles shows a spatially variant rock uplift pattern, with higher rates in the middle part and lower rates towards the west and east tips. The compression of the mountain is typically accommodated by fault-fold related shortening and vertical thickening. Apatite fission track thermochronology reveals that the growth of the Yumu Shan started ~4 Ma ago, similar to the work on active tectonics. Combining the onset ages of the growth of the Qilian Shan (10 Ma), Laojunmiao anticline (3–4 Ma), Baiyanghe anticline (3–4 Ma), Wenshu Shan (4.5 Ma) and Heli Shan (2 Ma), we draw an conclusion that the NE margin of the Tibetan Plateau initiated growth in the mid-Miocene and expanded to the Hexi Corridor and to the south of the Alxa block in the early Pleistocene.

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.

Petroleum source rocks of Western Australia

Petroleum geochemical analysis of samples from the Canning, Carnarvon, Officer and Perth basins identified several formations with source potential, the: • Triassic Locker Shale and Jurassic Dingo Claystone of the Northern Carnarvon Basin; • Permian Irwin River Coal Measures and Carynginia Formation, Triassic Kockatea Shale and Jurassic Cattamarra Coal Measures of the Perth Basin; • Ordovician Goldwyer and Bongabinni formations, Devonian Gogo Formation and Lower Carboniferous Laurel Formation of the Canning Basin; • Devonian Gneudna Formation of the Gascoyne Platform and the Lower Permian Wooramel and Byro groups of the Merlinleigh Sub-basin of the Southern Carnarvon Basin; and • Neoproterozoic Brown, Hussar, Kanpa and Steptoe formations of the Officer Basin. Burial history and geothermal basin modelling was undertaken using input parameters from geochemical analyses of rock samples, produced oil, organic petrology, apatite fission track analysis (AFTA), heat flows, subsurface temperatures and other exploration data compiled by the Geological Survey of Western Australia (GSWA). Of these basins, the Canning, Carnarvon, and Perth basins are currently producing oil and gas, whereas the Southern Carnarvon and Officer basins have no commercial petroleum discovery yet, but they do have source, reservoir, seal and petroleum shows indicating the presence of petroleum systems. The Carnarvon Basin contains the richest identified petroleum source rocks, followed by the Perth and Canning basins. Production in the Carnarvon Basin is predominantly gas and oil, the Perth Basin is gas-condensate and the Canning Basin is oil dominated, demonstrating the variations in source rock type and maturity across the state. GSWA is continuously adding new data to assess petroleum systems and prospectivity of these and other basins in Western Australia.

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.

Identification of microscopic uranium particles using fission tracks in nuclear solid-state detectors

Improved environmental-particle analysis for safeguards is a continuing research goal. In this paper, we propose a modified method of fission-track analysis, employing the focused ion-beam (FIB) technique and using a single-crystal silicon substrate to carry dust particles. The use of a silicon substrate may eliminate the need to relocate identified particles of interest from the particle carrier to another substrate for mass-spectrometric analysis. FIB–SEM and SIMS were used to evaluate the efficiency of the proposed FTA method.

Exhumation of the Mesozoic Guojialing Granodiorite: Implication for the Preservation of Gold Deposits in the Jiaobei Terrane, China

AbstractPreservation conditions are very important for mineral systems and a suitable exhumation process is critical for endogenetic deposits, especially for those deposits formed in orogenic settings, where deposits are inclined to erode away due to strong uplift. TheGuojialing batholith, intruding into theLinglong granites and theJiaodongGroup right before regional gold mineralization, is one of the most important gold ore‐hostingMesozoic intrusions in theJiaobei terrane. Gold deposits and the intrusion together underwent similar tectonothermal evolutionary processes. Exhumation and denudation process of theGuojialing granodiorite was constrained by biotite geobarometry and apatite fission track (FT) analysis. Biotite geobarometric data yields an emplacement depth of 3.0 km, while denudation since 110Ma was calculated from theFTdata at about 2.7 km. FT inverse modeling revealed a rapid uplift sinceca100 Ma. Compared with the gold ore‐forming depth which is confined between 2.5 and 9.5 km by fluid inclusion studies, great gold potential in the depths is inferred in theJiaobei terrane. Our result is consistent, to some extent, with the hypothesis of aMesozoic paleoplateau inEastChina.

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.

Plutonium Bioassay Testing of United States Atmospheric Nuclear Test Participants and Occupation Forces of Hiroshima and Nagasaki, Japan.

The pilot urine plutonium bioassay testing program engaged 100 atomic veterans and 58 persons representative of the United States general population using the fission track analysis technique, developed by Brookhaven National Laboratory. The methodology, collected data, scientific challenges, and test results are presented herein. The Defense Threat Reduction Agency and its predecessor agencies conducted this program in the late 1990s to the mid-2000s to determine the feasibility of bioassay testing to supplement dose reconstruction to estimate atomic veterans' doses under the agency's Nuclear Test Personnel Review Program. In summary, Pu urine bioassay using fission track analysis showed inconsistencies in sample measurement repeatability. To use the analysis as a tool to enhance current processes to perform accurate dose reconstructions requires extensive understanding and resolution of the measurement inconsistencies well beyond the scope of this pilot study. Therefore, fission track analysis (FTA) was found not feasible for implementation on a wide scale basis for atomic veterans.

Geological evolution of the southwestern part of the Veporic Unit (Western Carpathians): based on fission track and morphotectonic data

AbstractZircon and apatite fission track (FT) and morphotectonic analyses were applied in order to infer quantitative constraints on the Alpine morphotectonic evolution of the western part of the Southern Veporic Unit which is related to: (1) Eo-Alpine Cretaceous nappe stacking and metamorphism of the crystalline basement in the greenschist facies. (2) Exhumation phase due to underthrusting of the northerly located Tatric-Fatric basement (~ 90–80 Ma), followed by a passive en-block exhumation with cooling through ~ 320–200 °C during the Palaeocene (ZFT ages of ~ 61–55 Ma). (3) Slow Eocene cooling through ~ 245–90 °C, which most likely reflected erosion of the overlying cover nappes and the Gosau Group sediments. Cooling reached up to 60 °C till the Oligocene (AFT ages of ~ 37–22 Ma) in association with erosion of cover nappes. The efficient Eocene erosion led to the formation of the first Cenozoic planation surface with supergene kaolinization in many places. (4) The early Miocene erosion coincided with surface lowering and resulted in the second planation surface favourable for kaolinization. (5) In the middle Miocene, the study area was covered by the Poľana, Javorie, and Vepor stratovolcanoes. (6) The late Miocene stage was related to the erosion and formation of the third Cenozoic planation surface and the final shaping of the mountains was linked to a new accelerated uplift from the Pliocene.

Palaeogene growth of the northeastern Tibetan Plateau: Detrital fission track and sedimentary analysis of the Lanzhou basin, NW China

The Palaeogene evolution of the northeastern Tibetan Plateau remains elusive, because of the paucity of well-dated long-lasting and continuous sedimentary records. In this work, we present a detailed detrital apatite fission track thermochronology and stratigraphic study in the Lanzhou basin, located on the eastern-most margin of northeastern Tibet, where Eocene to middle Miocene sediments are well preserved. Combining the analysis of sedimentology records and synsedimentary tectonic structures, four regional tectonic phases are deciphered (52–47Ma, 40–32Ma, 23–20Ma and <16Ma, respectively). These episodes were spaced out by three periods of tectonic stability or quiescence linked to regional relief-reduction. Our apatite fission-track data suggest that far-field stress due to the India-Asia collision had been propagated to this region since early Eocene, and the modern northeastern portion of the Tibetan Plateau has been buildup since then. Thereafter, the growth of the northeastern Tibetan Plateau was dominated by a tectonic activity/inactivity cycle until late Miocene. Thus, the regional growth history appears to have been episodic in nature during the Cenozoic.

Cenozoic tectonic and thermal history of the Nenana basin, central interior Alaska: new constraints from seismic reflection data, fracture history, and apatite fission-track analyses

The Nenana basin of interior Alaska forms a segment of the diffuse plate boundary between the Bering and North American plates and is located within a complex zone of crustal-scale strike-slip deformation that accommodates compressional stresses in response to oblique plate convergence to the south. The basin is currently the focus of new oil and gas exploration. Integration of seismic reflection and well data, fracture data, and apatite fission-track analyses with regional data improves our understanding of the tectonic development of this continental strike-slip basin. The Nenana basin formed during the Late Paleocene as a 13 km wide half-graben, affected by regional intraplate magmatism and localized crustal thinning across the Minto Fault in south-central Alaska. The basin was uplifted and exhumed along this faulted margin in the Early Eocene through to Late Oligocene in response to oblique subduction along the southern Alaska margin. This event resulted in the removal of up to 1.5 km of Late Paleocene strata from the basin. Renewed rifting and subsidence during the Early Miocene widened the basin to the west resulting in deposition of Miocene non-marine clastic rocks in reactivated and newly formed extensional half-grabens. In the Middle to Late Miocene, left lateral strike-slip faulting was superimposed on this half-graben system, with rapid subsidence beginning in the Pliocene and continuing to the present day. At present, the Nenana basin is in a zone of transtensional deformation that accommodates compressional stresses in response to oblique plate convergence and allows tectonic subsidence by oblique extension along major basin-bounding strike-slip faults.

Post-Variscan thermal history of the Moravo-Silesian lower Carboniferous Culm Basin (NE Czech Republic - SW Poland)

Apatite fission track analysis (AFT) and zircon (U-Th)/He thermochronology (ZHe) have been carried out for a lower Carboniferous greywacke succession of the Moravo-Silesian Culm Basin in the Nízký Jeseník Mountains. The range of apparent zircon helium ages is 303–233Ma (late Carboniferous to Early Triassic) in the eastern part of the basin, whilst they are significantly younger in the western part, ranging from 194 to 163Ma (Early-Middle Jurassic). Apatite fission track central ages range from 152 (Latest Jurassic) to 44Ma (Eocene), with the majority being grouped between 114 (Aptian) and 57Ma (Paleocene). All samples experienced substantial post-depositional thermal reset; both the AFT ages and the ZHe are considerably younger than the depositional ages. The mean track length varies in the range between 12.5 and 15.4μm. The unimodal track length distribution, the relatively short mean track length (in most samples), and their rather low standard deviation values (1.2 to 2.1μm) indicate that their thermal history was determined by Variscan and post-Variscan heating event(s) followed by a prolonged residence in the apatite partial annealing zone in the Mesozoic and finally by cooling in the Paleogene. Geological evidence combined with thermal modeling based on AFT and ZHe data indicate that the lower Carboniferous strata had already reached maximum palaeotemperatures in the late Carboniferous, however, they were presumably later re-heated during the Permian-Triassic. Post-Variscan extensional tectonics events were responsible for high heat flow that together with Carboniferous burial could account for the reset of both thermochronometers. A major phase of cooling occurred in the Late Cretaceous. Finally, exhumation was probably faster in the Paleogene, causing the present-day exposure of the studied rocks.

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.