FT Ages Research Articles

Evidence from zircon and apatite thermochronology provides evidence for the tectonic-thermal evolution and denudational processes in Dulan, Eastern Kunlun Mountains, China

Since the Middle Jurassic, the East Kunlun orogenic belt in China has undergone significant denudation. This region offers crucial insights into the tectonic processes that have shaped the Tibetan Plateau. Moreover, the region boasts abundant mineral resources. This study uses techniques, such as zircon and apatite fission track (ZFT&AFT) analyses as well as citing several apatite (U-Th)-He (AHe) ages and FT ages data from the study areas, to unravel the tectonic-thermal evolution and denudation processes. The results reveal ZFT ages spanning from 167 ± 5 to 92 ± 4 Ma, and AFT ages spanning from 116 ± 6 to 64 ± 4 Ma. Based on the AFT, ZFT, and AHe data, this paper constructed a tectonic-thermal history model. In the first stage (ca. 170–120 Ma), rapid cooling at 2.00 ℃/Ma and denudation of 2.85 km occurred. The second stage (ca. 120–20 Ma) showed cooling rates of 0.60 ℃/Ma and denudation of 1.70 km. In the third stage (ca. 20–0 Ma), rapid cooling with rates of 1.50 ℃/Ma and denudation of 0.86 km were speculated. The first stage indicates the closure of the Paleo-Tethys Ocean in the Middle Jurassic, with the collision of the Lhasa and Qiangtang blocks with Eurasia along the Bangong-Nujiang suture zone in the Cretaceous. The second stage is mainly related to the northward subduction of the Neo-Tethys Ocean and the far-flung effects of the India-Eurasia collision. The uplift of the Tibetan plateau due to the Himalayan orogeny likely contributed to the third stage. The denuded thickness in the study area since 170 Ma is 5.41 km.

Miocene development of the Main Boundary Thrust and Ramgarh Thrust, and exhumation of Lesser Himalayan rocks of the Kumaun-Garhwal region, NW-Himalaya (India): Insights from Fission Track Thermochronology

• New FT ages suggest that the MBT and Saleri Thrust were initiated during ∼14 Ma. • The Amritpur granite was emplaced over the Siwalik along the MBT during late Miocene . • The Ramgarh Thrust sheet was emplaced over the LHMS zone along the RT during ∼24 Ma. • Old ZFT ages suggest that the metamorphism of the outer LHMS zone occurred < 250 °C. • Reactivation of the MBT and its associated faults occurred since Plio-Plistocene time. Bounded between the northeast dipping Main Boundary Thrust (MBT) at the base and the Saleri Thrust (ST) at the top, the Amritpur granite in the Kumaun-Garhwal region, NW-Himalaya is emplaced over the Siwalik sediments. As it is devoid of microscopic/mesoscopic imprints of the Himalayan tectonics, there is a debate regarding its emplacement along the MBT. While some interpret it as a klippe, others describe it as an intrusive body. The new apatite (AFT) and the zircon (ZFT) fission-track thermochronological data from the Amritpur granite provide similar age ranges and trends from the MBT to the ST. AFT ages range between 11.3 ± 1.6 and 14.7 ± 1.0 Ma with a weighted mean of 13.4 ± 1.5 Ma and ZFT ages range between 12.4 ± 1.2 and 15.4 ± 1.7 Ma with a weighted mean of 13.9 ± 1.2 Ma. Such age ranges and trends could be explained by rapid upliftment and exhumation of the Amritpur granite as horse rooted from the basement along the MBT from a depth of ∼8–10 km during mid-Miocene (∼14 to 13 Ma). We interpret this age as the time of development of the MBT. The new ZFT ages from the Jaunsar Group of the outer Lesser Himalayan meta-sedimentary (LHMS) zone range between 49.4 ± 2.0 and 80.3 ± 4.1 Ma (with a weighted mean of 67.5 ± 4.01 Ma) while the AFT ages range between 1.7 ± 0.2 and 4.2 ± 1.0 Ma (with a weighted mean of 3.17 ± 0.6 Ma). The old ZFT ages indicate that the outer LHMS zone underwent low-grade pre-Himalayan metamorphism (∼250 °C) around ∼67–72 Ma and the ZFT age of outer LHMS was not reset during the Himalayan orogeny. Young AFT ages from the Jaunsar Group of the outer LHMS zone reflect accelerated exhumation, likely due to reactivation of the MBT and its associated faults, since Plio-Pleistocene time. The new ZFT ages from the Ramgarh Thrust (RT) sheet range between 23.2 ± 2.4 and 24.2 ± 2.7 Ma with a weighted mean of 23.7 ± 2.5 Ma. The mean ZFT age of the RT sheet is similar to the age range of the Main Central Thrust (MCT) movements. This indicates that the RT sheet has been thrusted along the RT during 24 Ma followed by the exhumation of the RT sheet.

Serum testosterone concentrations in male patients with end-stage kidney disease treated with haemodialysis.

Testosterone deficiency is frequently found in male patients with chronic kidney disease (CKD) and may participate in the pathogenesis of osteoporosis, sarcopaenia, anaemia, impotence, infertility, and other comorbidities observed in these patients. The aim of the study was the evaluation of the frequency of testosterone deficiency in male patients with CKD on maintenance haemodialysis (HD). In 79 male HD patients, serum total (TT), free (FT) testosterone, C-reactive protein (CRP), and interleukin 6 (IL-6) serum concentrations were assessed before an HD procedure. Patients were divided into three subgroups based on age categories: 19-39 years (18 patients), 40-59 years (34 patients), and ≥ 60 years (27 patients). TT insufficiency and deficiency were diagnosed when the serum TT concentration was below 4.0 ng/mL and 2.9 ng/mL, respectively. FT deficiency was diagnosed in patients with serum FT concentration below 8.9, 6.6, and 4.9 pg/mL in the abovementioned age subgroups, respectively. In the abovementioned age subgroups the serum TT concentration was 5.9 (4.6-7.1), 4.8 (3.9-5.4), and 4.6 (3.9-5.3) ng/mL, respectively. The serum FT concentration was 7.9 (5.2-10.1), 6.1 (5.1-7.2), and 6.0 (5.0-7.1) pg/mL, respectively. In the whole group TT insufficiency was found in 40%, TT deficiency in 15% of patients, and FT deficiency in 50% of patients. Significant negative correlations were found between both serum TT and FT concentrations and age (r = -0.23, p = 0.05 and r = -0.27, p = 0.02, respectively). Additionally, negative correlations were found between both serum TT and FT and IL-6 concentrations (r = -0.43, p < 0.05 and r = -0.29, p < 0.05), respectively. 1. Testosterone deficiency is common in male patients with chronic kidney disease treated with HD. 2. In HD patients the serum testosterone concentration decreases with age. 3. Chronic inflammation may participate in the pathogenesis of testosterone deficiency in haemodialysis patients.

Apatite fission-track dating by LA-Q-ICP-MS imaging

Obtaining accurate and precise apatite fission-track (AFT) ages depends on the availability of high-quality apatite grains from a sample, ideally with high spontaneous fission-track densities (c. >1.105 tracks.cm−2). However, many natural samples, such as bedrock samples from young orogenic belts or low-grade metamorphic samples with low U contents yield low spontaneous fission-track densities. Such apatites must be counted to avoid biasing the resultant FT age. AFT dating employing LA-Q-ICP-MS spot ablation works very well for grains with high spontaneous fission-track densities. This approach allows detection of potential U zoning, while also removing the need for an irradiation step and facilitating simultaneous acquisition of U-Pb and trace element data. The LA-Q-ICP-MS spot ablation thus offers several advantages compared to the External Detector Method (EDM). However, the spot ablation approach requires the counted area to mimic exactly the site and size of the laser spot, which for grains with low spontaneous fission-track densities (<105 tracks.cm−2), implies fewer track counts and impairs the precision of the resultant AFT age. Here we present an alternative approach to LA-Q-ICP-MS spot analysis of low fission-tracks density grains by generating a U distribution (238U/43Ca) map of the entire apatite surface by LA-Q-ICP-MS elemental mapping, which enables characterization of U zonation. The Monocle plugin for the Iolite LA-ICP-MS data reduction software is used to display elemental maps and extract mean 238U/43Ca values of the same area counted for the fission-tracks. A typical grain-mapping session takes <5 h to map 80 grains. The method was employed on the Durango and Fish Canyon Tuff apatite reference materials, six bedrock apatite samples with low fission-track densities (≤1.105 track.cm−2), and one bedrock apatite sample with high fission-track density (>1.106 track.cm−2) to assess the precision and accuracy of our approach. Most apatite samples investigated here were previously dated by the EDM or the LA-Q-ICP-MS ablation spot method. The AFT grain-mapping ages agree with previously published EDM or LA-Q-ICP-MS spot ablation ages at the 2σ level. For each apatite sample, we simultaneously acquired U-Pb age and trace element data (Mn, Sr, La, Ce, Sm, Eu, Gd, Lu); here again the data agree with literature constraints (when available) within uncertainties. The mapping approach is therefore a practical solution to low-temperature thermochronology studies facing apatite grains with low spontaneous fission-track densities, while also facilitating investigation of the spatial relationships between thermo- and geochronometric ages and grain chemistry.

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.

Uplift and denudation history of the Akaishi Range, a thrust block formed by arc‐arc collision in central Japan: Insights from low‐temperature thermochronometry and thermokinematic modeling

AbstractFission track (FT), (U‐Th)/He (He), and U‐Pb data were used to identify the denudation history of the Akaishi Range, central Japan. The northern Akaishi Range is bounded on the east by the Itoigawa‐Shizuoka Tectonic Line Fault Zone (ISTL‐FZ). The thermochronometric ages progressively decrease with the decreasing distance to the ISTL‐FZ. Thermokinematic calculations suggest that the age pattern observed can be explained by 5–7.5 mm/yr reverse slip on the ISTL‐FZ that dips 34–45° west and soles onto detachment at 20–22.5 km depth. By assuming the same geometry and slip rate of the fault, the bedrock uplift rates and denudation rates are estimated at ~4 mm/yr. Thus, the uplift and denudation style of the northern Akaishi Range is well explained as a simple tilted thrust block that has been exhumed along the listric ISTL‐FZ. On the other hand, considering both the difference in apatite FT age and the active fault distribution, the southern Akaishi Range might be different in uplift origin and timing, although the difference in apatite FT ages between them may be attributable to the chlorine content variation in apatite. The inferred total denudation is larger than several kilometers and likely exceeds 10 km since the beginning of the northern Akaishi Range uplift, suggesting that the low‐relief surfaces on the ridges and the relatively constant elevations of the summits reflect postuplift denudation rather than preexisting low‐relief landforms.

Uplift History of Syenite Rocks of the Sushina Hill, Tamar Porapahar Shear Zone (TPSZ), Purulia: Constraints from Fission-track Ages of Two Cogenetic Minerals

Fission-track ages of cogenetic minerals namely apatite, and zircon from syenite rocks of the Sushina hill, Tamar Porapahar Shear Zone (TPSZ), coupled with the corresponding closure temperatures of the minerals have been used to reveal the uplift history of syenite rocks. Offset of their FT ages indicates that the samples uplifted at the rate of 9.97 m/Ma during the period 535 Ma-970 Ma.

Basin provenance and post-depositional thermal history along the continental P/T boundary of the Raniganj basin, eastern India: Constraints from apatite fission track dating

Apatite fission-track analysis has been applied to the Raniganj and Panchet formations of Raniganj basin of Gondwana Supergroup to unravel its thermal and provenance history. Apatite fission track age population from both Raniganj and Panchet formations indicate partial annealing and point to a maximum temperature of around ∼100-110°C during their post depositional evolution. The sandstone of Raniganj Formation has five peak ages at 26.3, 59.3, 109.7, 173.7 and 299.9 Ma, while Panchet Formation has three peak ages at 25.4, 143.5 and 281.3 Ma. This implies that the provenance of the Raniganj Formation of late Permian and Panchet Formation of early Triassic changed obviously. According to thermotectonic evolution of the Gondwana basin, these apatites with different FT ages possibly represent different source components, although partial annealing had occurred to these apatites. Possibly all the apatites had transported from the Precambrian basement which was undergoing deformation due to Gondwana rifting initiated during Carboniferous period. Due to this, the basement was undergoing inhomogeneous thermal history which became source of sediments for Raniganj basin.

Novel calibration for LA-ICP-MS-based fission-track thermochronology

We present a novel age-equation calibration for fission-track age determinations by laser ablation inductively coupled plasma mass spectrometry. This new calibration incorporates the efficiency factor of an internal surface, [ηq]is, which is obtained by measuring the projected fission-track length, allowing the determination of FT ages directly using the recommended spontaneous fission decay constant. Also, the uranium concentrations in apatite samples are determined using a Durango (Dur-2, 7.44 μg/g U) crystal and a Mud Tank (MT-7, 6.88 μg/g U) crystal as uranium reference materials. The use of matrix-matched reference materials allows a reduction in the uncertainty of the uranium measurements to those related to counting statistics, which are ca. 1 % taking into account that no extra source of uncertainty has to be considered. The equations as well as the matrix-matched reference materials are evaluated using well-dated samples from Durango, Fish Canyon Tuff, and Limberg as unknown samples. The results compare well with their respective published ages determined through other dating methods. Additionally, the results agree with traditional fission-track ages using both the zeta approach and the absolute approach, suggesting that the calibration presented in this work can be robustly applied in geological context. Furthermore, considering that fission-track ages can be determined without an age standard sample, the fission-track thermochronology approach presented here is assumed to be a valuable dating tool.

New geochronological constraints on the thermal and exhumation history of the Lesser and Higher Himalayan Crystalline Units in the Kullu–Kinnaur area of Himachal Pradesh (India)

New geochronological, petrological and structural data from the Beas–Sutlej area of Himachal Pradesh (India) are used to reconstruct the tectonothermal and exhumation history of this part of the Himalayan orogen.Sm–Nd garnet ages at 40.5±1.3Ma obtained on a pegmatoid from the inverse metamorphic High Himalayan Crystalline (HHC) in the Malana–Parbati area probably mark local melting during initial decompression. Ongoing exhumation in ductilely deformed leuco-gneiss is constrained by Sm–Nd garnet ages at 29±1Ma and white mica Rb–Sr ages around 24–20Ma, while Bt Rb–Sr ages indicate a drop of regional metamorphic temperatures below 300°C between 15 and 12Ma.The deep Sutlej gorge exposes medium-grade paragneisses and Proterozoic orthogneisses of the Lesser Himalayan Crystalline (LHC), overthrust by the HHC along the Main Central Thrust (MCT). Mica cooling ages in the HHC are in the range of 14–11Ma. Above the extruded wedge of the HHC, the Leo Pargil leucogranite and associated dykes intrude the Haimanta Unit (HU) below the weakly metamorphic Palaeo-Mesozoic sediments of the Tethyan Himalayas (TH). The Leo Pargil leucogranite yielded a mean Sm–Nd garnet age of 19±1Ma and Rb–Sr muscovite and biotite cooling ages between 16.4 and 11.6Ma.Marked young extrusion of LHC units resulted in differentiated exhumation/cooling of more frontal parts of the orogen. Very young ductile deformation of LHC gneisses near Wangtu is constrained by late-kinematic pegmatite intrusions crosscutting the main mylonitic foliation. Sm–Nd garnet and Rb–Sr muscovite ages of these pegmatites range between 7.9±0.9 and 5.5±0.1Ma. Published apatite FT ages down to 0.6Ma also document accelerated diachronous sub-recent exhumation of different parts of the orogen.Together with geochronological data from the literature, the new results demonstrate that the HHC and the HU were deformed by shortening and crustal thickening during the Eohimalayan phase (Late Eocene–Oligocene), followed by a strong thermal overprint and intrusions of granitoids during the Neohimalayan Phase (Early to Middle Miocene). The LHC experienced amphibolite facies metamorphic conditions in the Late Miocene prior to extrusion between the HHC and the very low-grade Lesser Himalayan sediments. In conjunction with climate changes, young tectonic activity in this central part of the Himalayan orogen may have strongly influenced fluvial incision and erosion, and therefore, contributed to the accelerated uplift, as indicated by extensive accumulation of Late Miocene to Early Pleistocene fluviatile–lacustrine sediments in the Zanda basin, the Transhimalayan headwaters of the Sutlej, in Western Tibet.

Denudation history of the Kiso Range, central Japan, and its tectonic implications: Constraints from low‐temperature thermochronology

AbstractFission‐track (FT) and (U–Th–Sm)/He (He) analyses are used to constrain the denudation pattern and history of the Kiso Range, a Japanese fault‐block mountain range which has been uplifted sinceca0.8 Ma. We obtained nine zircon FT ages ranging 59.3–42.1 Ma, 18 apatite FT ages ranging 81.9–2.3 Ma, and 13 apatite He ages ranging 36.7–2.2 Ma. The apatite FT and He ages are divided into an older group comparable to the zircon FT age range and a younger group of &lt;18 Ma. The younger ages are interpreted as a reflection of uplift of the Kiso Range because they were obtained only to the east of the Seinaiji‐touge Fault, and the event age estimated from apatite FT data is consistent with the timing of the onset of the Kiso Range uplift. On the basis of the distribution of the younger ages, we propose westward tilting uplift of the Kiso Range between the boundary fault of the Inadani Fault Zone and Seinaiji‐touge Fault, which implies a model of bedrock uplift that is intermediate between two existing models: a pop‐up model in which the Kiso Range is squeezed upward between the two faults and a tilted uplift model which assumes that the Kiso Range is uplifted and tilted to the west by the Inadani Fault Zone. The original land surface before the onset of uplift/denudation of the Kiso Range is estimated to have been uplifted to an elevation of 2700–4900 m. We estimated denudation rates at 1.3–4.0 mm/y and maximum bedrock uplift rates at 3.4–6.1 mm/y sinceca0.8 Ma. The Seinaiji‐touge fault is interpreted as a back thrust of the west‐dipping Inadani Fault Zone. The older group of apatite FT and He ages is interpreted to reflect long‐term peneplanation with a probable denudation rate of &lt;0.1 mm/y.

Forward kinematic modelling of a regional transect in the Northern Emirates using geological and apatite fission track age constraints on paleo-burial history

This paper aims to simulate the kinematic evolution of a regional transect crossing the Northern Emirates in the northernmost part of the Semail Ophiolite and the Dibba zone, just south of the Musandam Platform exposures. The studied section comprises, from top to bottom and from inner to outer zones, (1) the erosional remnants of the Semail Ophiolite, mainly made up of serpentinized ultramafics in the west and gabbros in the east, (2) high-grade metamorphic rocks which are currently exposed in the core of a nappe anticline near Masafi, (3) far-travelled Hawasina basinal units and Sumeini paleo-slope units of the Dibba Zone, (4) parautochthonous platform carbonates, which are currently well exposed in the Musandam area, and (5) a flexural basin filled with uppermost Cretaceous to Neogene sediments. Two main compressional episodes are generally identified, resulting first in the obduction of the Semail Ophiolite and then in the stacking of underlying platform carbonate units of the former Arabian passive margin, thus accounting for the present architecture of this transect: (1) first, deformation at the plate boundary initiated in the Late Cretaceous, resulting in the obduction of the Semail Ophiolite and the progressive accretion of the Hawasina and Sumeini tectonic wedge on top of the Arabian foreland, leading to a progressive bending of its lithosphere and development of a wide flexural basin; (2) compression resumed during the Neogene, leading to the tectonic stacking of the parautochthonous platform duplexes of Musandam and Margham trends, the development of out-of-sequence thrusts and triangle zones, refolding of the sole thrust of the former Late Cretaceous accretionary wedge and coeval normal (?) high-angle faulting along the contact between the Musandam and Dibba zones. However, seismic profiles and paleo-thermometers also help in identifying another erosional event at the boundary between the Paleogene Pabdeh and the Neogene Fars series. Evidenced by the local erosional truncation of the Pabdeh series in the vicinity of the frontal triangle zone (i.e. the inner part of the former Late Cretaceous foredeep), this Paleogene uplift/unroofing episode is tentatively interpreted here as an evidence for a continuum of compressional deformation lasting from the Late Cretaceous to the Middle Miocene although one may alternatively speculate that it was related to the detachment of the subducted slab. Although carbonate facies are usually not suitable for apatite fission track (AFT) studies, we were able to extract detrital apatites from quartz-bearing Triassic dolomites in the Musandam area. However, the yield and the quality were both poor and too few fission track lengths could be measured, making it difficult to interpret the meaning of the FT ages. The FT dates obtained in this study are therefore compared with those existing in the literature. Fortunately enough, for each sample, at least ten apatite crystals could be used for fission track dating, except for site 6 with only five datable apatite grains. The obtained apatite fission track dates between 28 and 13 Ma, much younger than the Triassic age of the series, are taken to represent reset fission track ages, implying erosion of an up-to-3-km-thick pile of Jurassic–Cretaceous carbonates and Hawasina allochthon during the Neogene. Apatite fission track dates from the ~95 M-old plagiogranites of the Semail complex (Searle and Cox, Geol Mag 139(3):241–255, 2002) obtained in this study and compared with those recently published provide evidences for more than one cooling event. An early unroofing of the ophiolite during the Late Cretaceous is revealed in fission track dates of 72–76 Ma at the top of the ophiolite in the east, which are coeval and also consistent with the occurrence of paleo-soils, rudists and paleo-reefs on top of serpentinized ultramafics in the west. High-pressure rocks at As Sifah in the southeast near Muscat revealed apatite fission track data ranging from ~46 to 63 Ma (Gray et al. 2006). The leucocratic part of the ophiolite (sample UAE 180) yielded comparable young apatite (40.6 ± 3.9 Ma) and zircon (46.6 ± 4.3 Ma) FT dates. A Cenozoic (~20–21 Ma) exhumation has been determined for the Bani Hamid metamorphic sole in northern Oman, applying low temperature geochronology and combining apatite FT and apatite (U–Th)/He analyses (Gray et al. 2006). In this study, young apatite fission track dates of 20 Ma have also been found but at the base of the ophiolite near Masafi, in the core of the nappe anticline, thus indicating a Neogene age for the refolding of the allochthon and stacking of underlying parautochthonous platform carbonate units. During the subsequent 2D forward Thrustpack kinematic modelling of the regional transect, these AFT data-set has been used, together with available subsurface information, to reconstruct the past architecture of the structural sections through time, accounting for incremental deformation along the various decollement levels, synorogenic sedimentation and erosion, as well as for successive bending and unbending episodes of the Arabian lithosphere.

Extension and exhumation of the HP/LT rocks in the Hellenic forearc ridge

Localized intensive ductile and ductile-brittle extension of along-the-arc orientation is prominent in Phyllite-Quartzite unit (PQU) rocks in Kythera and the adjacent southeastern Peloponnese but declines in prominence to the north and to the south along the Cretan-Peloponnese ridge. Using zircon and apatite fission track dating we find that this structural characteristic is correlated with the youngest zircon fission track cooling ages of 14 to 9 Ma, and with the highest pressure-temperature condition recorded in the metamorphic rocks below the structural detachment. The cooling ages of the high pressure Phyllite-Quartzite unit (PQU) rocks along the Hellenic forearc ridge show that exhumation migrated from both Crete and from the Peloponnese to the area with the youngest ages, in Kythera and the southeastern Peloponnese. Starting in the Early Miocene, and continuing to the present, trench-rollback and slab retreat expanded the Hellenic arc, and this bending of the arc from an initial more rectilinear geometry increased areas of oblique convergence and we suggest this localized the subsequent arc-parallel extension and local enhanced exhumation of HP-rocks. The Zircon FIT exhumation ages from the arc-parallel stretching episode restrict the ductile part of this episode in exposed rocks to between about 14 and 9 Ma. Apatite FT ages suggest brittle along-arc extension continued to about 7 Ma. This episode is proposed to be the result of a temporary higher rate of rollback and slab retreat. Younger normal faults, including those which define the margins of the present Hellenic Arc, show return to arc-normal extension.

フィッション・トラック熱年代に基づいた六甲地域の冷却・削剥史

Quantitatively estimating denudation is generally difficult because it essentially involves the removal and loss of materials in situ. The denudation rate of mountainous areas in Japan has commonly been studied from the volume of sediment in a basin or catchment. Nonetheless, the availability of these methods is constrained spatially by upstream area and temporally by depositional age. In the last few decades, thermochronometric methods that evaluate thermal history using radiometric-dating methods have been used to evaluate the denudation and tectonic history of orogenic belts around the world. The advantages of thermochronometric methods are that we can calculate the denudation rate at each sampling point and that combining multiple methods and/or target minerals enables us to calculate denudation rates in multiple periods. However, thermochronometric methods have been applied to areas with extraordinarily high denudation rates in island arc areas such as Japan. Thus, the effectiveness of thermochronometric methods for estimating denudation rates in island arc areas has not been demonstrated. We applied apatite and zircon fission-track thermochronometry to granitic rock samples collected from outcrops and a borehole to estimate the tectonic history of the Rokko area, southwest Japan. Previous studies suggested that the Rokko Mountains have been uplifted by active faulting along their northern and southern margins during the Rokko movements, a Quaternary tectonic movement in the Kinki district. However, the tectonic history of the Rokko area prior to 1 Ma has not been revealed due to a lack of prevalent geologic markers. We estimated average denudation rates in various periods based on apatite and zircon FT ages and previously reported radiometric ages. We obtained denudation rates at about 0.04-0.10 mm/yr after about 30 Ma, 0.05-0.7 mm/yr during 50-30 Ma, 0.7-4.0 mm/yr during 70-50 Ma. The denudation rate after the deposition of the Kobe Group (36.9-30.4 Ma) is estimated to be in the 0.01-0.1 mm/yr order, while bedrock uplift rate after about 1 Ma is estimated to be about 0.5 mm/yr. Thus, the tectonic activity of the Rokko Mountains area prior to 1 Ma has been relatively low.

Fission track data from the Mesohellenic Trough and the Pelagonian zone in NW Greece: Cenozoic tectonics and exhumation of source areas

The Mesohellenic Trough (MHT) trends parallel to the structural fabric of the Hellenides and covers the suture between the Apulian microplate and the Pelagonian block. It comprises an up to ∼4 km thick middle Eocene to upper Miocene sedimentary sequence. We have studied the thermal history of the Pelagonian basement and the provenance of the detrital material in the clastic sediments of the MHT by fission track dating. Apatite and zircon fission track (AFT, ZFT) analysis is applied to samples from the Pelagonian microcontinent along the eastern border of the MHT, and AFT analysis to the sedimentary rocks in the southern MHT. Eocene AFT age populations in the sedimentary strata indicate a proximal position of the Pelagonian microcontinent, which shows the same or even younger AFT ages, as the source area of the detrital material in the MHT. Late Cretaceous to Palaeocene age populations point to a more distant or structurally higher (now eroded) source area. The Eocene orogenic event caused only weak thermal overprinting in rocks of the Pelagonian microcontinent. In its eastern part, the AFT ages show only partial resetting, if any, whereas in its western part the ages were clearly reset during the Eocene event. AFT age‐elevation relations, correlation of zircon and apatite FT ages from the same samples, and thermal modelling based on AFT ages and track length distributions were all used to reconstruct the low‐temperature cooling history of the Pelagonian basement adjacent to the MHT. The results document fast cooling and exhumation in the Eocene that were possibly related to thrusting and associated erosion, followed by slow cooling and exhumation during Oligocene and Miocene time. This scenario is confirmed by the AFT data from the detrital material in the MHT sedimentary strata. The slow cooling period coincides with a stagnation period or crustal extension and possible reheating, which is probably also responsible for the partial rejuvenation of the ages of the detrital apatites in the oldest (Eocene) formation in the sediment sequence of the MHT.

A fission-track and (U–Th)/He thermochronometric study of the northern margin of the South China Sea: An example of a complex passive margin

Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6 ± 4.7 Ma and 87.3 ± 4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9 ± 3.6 and 37.3 ± 2.3 Ma with mean track lengths between 13.26 ± 0.16 µm and 13.95 ± 0.19 µm whilst AHe ages are marginally younger 47.5 ± 1.9–15.3 ± 0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~ 70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin.

福岡県西部，那珂川上流，五ヶ山地域に分布する糸島花崗閃緑岩と早良花崗岩のFT年代

福岡県西部，那珂川上流，五ヶ山地域に分布する糸島花崗閃緑岩と早良花崗岩のFT年代

Fission-track analysis of the Atotsugawa Fault (Hida Metamorphic Belt, central Japan): fault-related thermal anomaly and activation history

Abstract Fission-track (FT) thermochronology was applied to the Atotsugawa Fault in the Hida Metamorphic Belt, central Japan, to detect any ancient thermal anomaly associated with fault displacements. Apatite and zircon grains from gouges ( c . 2 cm wide) and fractured rocks ( c . 10 cm) at six fracture zones within a 15 m-wide fault zone were dated. Most of the zircon (120–150 Ma) and apatite (44–60 Ma) ages agree well with emplacement ages for the granites that intrude the Hida Belt. The discordance in zircon and apatite FT ages is interpreted to reflect cooling due to regional uplift and associated erosion. A thermal anomaly was identified at one of the gouge zones that showed an exceptionally young apatite age ( c . 32 Ma) with an unimodal FT length distribution. It presumably indicates secondary heating induced by frictional slip during an earthquake, possibly giving a younger limit of the initiation of the activity in the Hida Belt.

Unexpected Jurassic to Neogene vertical movements in ‘stable’ parts of NW Africa revealed by low temperature geochronology

AbstractIn Morocco, it is generally considered that post‐Hercynian vertical movements were limited to the Atlas system, the passive continental margin and the Rif. Apatite FT and He ages from the Moroccan Meseta (Rehamna and Zaer Massif) document instead two episodes of subsidence and exhumation in Jurassic‐Early Cretaceous and during the Late Cretaceous to Neogene. The Meseta subsided to &gt;3 km depth during the Late Triassic to Middle Jurassic and was exhumed to the surface before the Late Cretaceous, during the rift and post‐rift stages of Central Atlantic opening. Erosion of the exhuming rocks is responsible for a thick package of terrigenous sands found in the Moroccan offshore and elsewhere along the NW Africa margin. About 1 km of subsidence affected the Meseta during the Late Cretaceous to Eocene. During the Neogene, these areas were brought back to the surface in association with bimodal folding with wavelengths of 100–150 km and &gt;500 km.

フィッション・トラック年代によるむつ燧岳の活動年代の再検討

During the latest phase of eruptive activity by the Mutsu-Hiuchidake volcano, Northeast Japan, large pyroclastic flows were estimated by previous geological and tephrochronological studies to have occurred at about 0.3 Ma. In this re-examination of the latest volcanic history, we present new fission-track ages for two zircons from the pyroclastic flow deposits from the volcano and two from the associated basement rocks. FT ages determined for the pyroclastic flow deposits are 0.81 ± 0.14 Ma and 0.77 ± 0.10 Ma compared to the 2.6 ± 0.2 Ma and 3.0 ± 0.3 Ma ages for the basement rocks. These results confirm that the lavas at the top of the volcanic edifice are younger than the pyroclastic flow deposits. In addition, the volcano is estimated to have been active from 1.2 to 0.5 Ma, based on other K-Ar ages, TL ages and stratigraphic relationships.