Zircon Fission Track Data Research Articles

東北日本男鹿半島，真山流紋岩類の放射年代

The Shinzan Rhyolites are the uppermost part of the Monzen Formation and discordantly cover the Kamo Lavas of the same formation. The volcanic succession from the Kamo Lavas and correlative Kuguriiwa Lavas to the Shinzan Rhyolites has been accepted to range in isotopic age from 34 Ma to 24 Ma with a large time gap of 2 or 3 million years between the Shinzan Rhyolites and the underlying two units. This paper reports new isotopic ages of the Shinzan Rhyolites, ca. 34 Ma by Ar-Ar and K-Ar dating of biotite and ca. 37 Ma by fission-track dating of zircon. A new fission-track date of the welded felsic tuff from the Kamo Lavas is ca. 36 Ma. These new age data are mutually consistent and much more reliable than the ever-reported isotopic age, and obviously indicate that 1) the Monzen Formation is to be Late Eocene to Early Oligocene in age and 2) the time gap between the Shinzan Rhyolites and the Kamo Lavas should be much smaller than or negligible relative to the time span of the entire volcanic succession. The main part of the Shinzan Rhyolites would be slightly younger than the shallow marine Shiose-no-misaki Sandstone and Conglomerate (34-36 Ma) and perhaps demonstrates the Late Eocene to Early Oligocene rifting precedent to the rapid opening of the Japan Sea.

A thermomechanical wedge model of Taiwan constrained by fission-track thermochronometry

We present 31 new apatite fission-track (AFT) ages for the island of Taiwan that, when combined with existing AFT and zircon fission-track (ZFT) data, provide regional spatial coverage of the island with respect to low-temperature thermochronometry. The overall pattern of ZFT and AFT ages in Taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the Central Range and eastern Taiwan. This pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone. In this model, new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west. The southward propagating collision is consistent with reset ages in the north, where erosional exhumation has been ongoing for longer, and unreset ages in the south, where the younger collision implies less time for erosional exhumation. Despite the variation in the age of the collision along the strike of the island, the widths of the AFT and ZFT reset age zones remain nearly constant between 23° 00′N to ∼ 24° 00′N and 23° 20′N to ∼ 24° 00′N, respectively, suggesting that the orogen is in an exhumational steady state over these regions with respect to the AFT and ZFT thermochronometers. We use the fission-track data in conjunction with observations of crustal structure, crustal fabric, and heat flow measurements to constrain a time-dependent, two-dimensional, thermomechanical model of orogen evolution. By accounting for the heat transfer, tectonic and erosion processes needed to predict AFT and ZFT ages, we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen. With our model we conclude that: (1) roughly half of the material accretion in Taiwan occurs through underplating over an approximately 40 km wide region, (2) current average erosion rates are ∼ 3.3 mm/yr in the eastern Central Range and ∼ 2.3 mm/yr over the whole island, (3) the collision has been propagating southward at a rate between 20 and 51 km/Ma over the past 2–3 Ma, and (4) central Taiwan is in a topographic, thermal and exhumational steady state.

The Carboniferous to Jurassic evolution of the pre-Alpine basement of Crete: constraints from U-Pb and U-(Th)-Pb dating of orthogneiss, fission-track dating of zircon, structural and petrological data

Abstract The pre-Alpine evolution of the external Hellenides is poorly constrained because of the Alpine impact which largely erased the older orogenic imprints. Only a few outcrops with pre-Alpine basement exist, one of which is located in eastern Crete. The pre-Alpine basement, part of the Phyllite-Quartzite Unit, is composed of four sub-complexes, which are different in protolith age, type and age of metamorphism, and postmetamorphic cooling history. The lowermost, Kalavros crystalline complex (KCC) underwent Permian amphibolite-facies metamorphism related to top-to-the-NE shearing. The KCC exhibits a four-stage garnet zonation and a late, high-temperature event associated with the growth of K-feldspar. The KCC is overlain by the Myrsini crystalline complex (MCC), which underwent Carboniferous amphibolite facies metamorphism associated with top-to-the-north shearing. Late cooling of the MCC is documented by Jurassic fission track ages of zircon. The Chamezí crystalline complex underwent upper greenschist-facies metamorphism related to top-to-the-north shearing. In addition, the Vaí crystalline complex, in an uncertain structural position, is characterized by Triassic emplacement of granite, followed by amphibolite-facies top-to-the-NW shearing and cooling, as is indicated by Jurassic fission-track ages of zircon. A preliminary tectonic model is presented, which invokes south-directed subduction, collision and accretion of the crystalline complexes to the northern margin of Gondwana.

Fission-track age of the volcanic ash layer in the Bisan-seto Group around the Bannosu region of the eastern parts of the Seto Inland Sea, Japan

Fission-track dating of zircon and volcanic ash analysis were carried out for Bannosu volcanic ash in the Bisan-seto Group around the Bannosu region in the northeast Shikoku. The first reliable radiometric age, 0.63±0.14 Ma (1σ), was obtained for the volcanic ash layer. On the other hand, the refractive indexes of the volcanic glasses show the 1.5004-1.5071 (mode: 1.504-1.506). On the basis of these values and other petrographical data, this volcanic ash layer is correlated to the Kb-Ks volcanic ash. These results are important to discuss the sedimentary environment of the Seto Inland Sea in Pleiocene to Holocene.

Age and significance of core complex formation in a very curved orogen: Evidence from fission track studies in the South Carpathians (Romania)

Twenty-four new zircon and apatite fission track ages from the Getic and Danubian nappes in the South Carpathians are discussed in the light of a compilation of published fission track data. A total of 101 fission track ages indicates that the Getic nappes are generally characterized by Cretaceous zircon and apatite fission track ages, indicating cooling to near-surface temperatures of these units immediately following Late Cretaceous orogeny. The age distribution of the Danubian nappes, presently outcropping in the Danubian window below the Getic nappes, depends on the position with respect to the Cerna-Jiu fault. Eocene and Oligocene zircon and apatite central ages from the part of the Danubian core complex situated southeast of this fault monitor mid-Tertiary tectonic exhumation in the footwall of the Getic detachment, while zircon fission track data from northwest of this fault indicate that slow cooling started during the Latest Cretaceous. The change from extension (Getic detachment) to strike-slip dominated tectonics along the curved Cerna-Jiu fault allowed for further exhumation on the concave side of this strike-slip fault, while exhumation ceased on the convex side. The available fission track data consistently indicate that the change to fast cooling associated with tectonic denudation by core complex formation did not occur before Late Eocene times, i.e. long after the cessation of Late Cretaceous thrusting. Core complex formation in the Danubian window is related to a larger-scale scenario that is characterized by the NNW-directed translation, followed by a 90° clockwise rotation of the Tisza-Dacia “block” due to roll-back of the Carpathian embayment. This led to a complex pattern of strain partitioning within the Tisza-Dacia “block” adjacent to the western tip of the rigid Moesian platform. Our results suggest that the invasion of these southernmost parts of Tisza-Dacia started before the Late Eocene, i.e. significantly before the onset of Miocene-age rollback and associated extension in the Pannonian basin.

FORMATION OF A PALEOTHERMAL ANOMALY AND DISSEMINATED GOLD DEPOSITS ASSOCIATEDWITH THE BINGHAM CANYON PORPHYRY Cu-Au-Mo SYSTEM, UTAH--A REPLY

The thermal history of the Oquirrh Mountains, Utah, indicates that hydrothermal fluids associated with emplacement of the 37 Ma Bingham Canyon porphyry Cu-Au-Mo deposit extended at least 10 km north of the Bingham pit. An associated paleothermal anomaly enclosed the Barneys Canyon and Melco disseminated gold deposits and several smaller gold deposits between them. Previous studies have shown the Barneys Canyon deposit is near the outer limit of an irregular distal Au-As geochemical halo, about 3 km beyond an intermediate Pb-Zn halo, and 7 km beyond a proximal pyrite halo centered on the Bingham porphyry copper deposit. The Melco deposit also lies near the outer limit of the Au-As halo. Analysis of several geothermometers from samples collected up to 22 km north of the Bingham Canyon porphyry Cu-Au-Mo deposit indicate that most sedimentary rocks of the Oquirrh Mountains, including those at the gold deposits, have not been regionally heated beyond the oil window (less than about 150°C). For geologically reasonable heating durations, the maximum sustained temperature at Melco, 6 km north of the Bingham pit, and at Barneys Canyon, 7.5 km north of the pit, was between 100°C and 140°C, as indicated by combinations of conodont color alteration indices of 1.5 to 2, mean random solid bitumen reflectance of about 1.0 percent, lack of annealing of zircon fission tracks, and partial to complete annealing of apatite fission tracks. The pattern of reset apatite fission-track ages indicates that the gold deposits are located approximately on the 120°C isotherm of the 37 Ma paleothermal anomaly assuming a heating duration of about 106 years. The conodont data further constrain the duration of heating to between 5 × 104 and 106 years at approximately 120°C. The δ 18O of quartzite host rocks generally increases from about 12.6 per mil at the porphyry to about 15.8 per mil approximately 11 km from the Bingham deposit. This change reflects interaction of interstitial clays in the quartzite with circulating meteoric water related to the Bingham Canyon porphyry system. The δ 18O and δ 13C values of limestone vary with respect to degree of recrystallization and proximity to open fractures. Recrystallized limestone at the Melco and Barneys Canyon gold deposits has the highest δ 18O values (about 30‰), whereas limestone adjacent to the porphyry copper deposit has the lowest values (about 10‰). The high δ 18O values for the recrystallized limestone at Barneys Canyon and Melco strongly suggest that mineralization was related to low temperature fluids with exceptionally high δ 18OH2O values such as could be derived from water in a crater lake of an active volcano. The age of formation of the gold deposits has been interpreted to range from Jurassic to Eocene. The mineralized rocks at the Barneys Canyon and Melco deposits are likely the same age as the geochemically similar deposits that are present in north-striking, late faults that cut the Bingham Canyon porphyry. The patterns of apatite and zircon fission-track data, conodont color alteration indices, solid bitumen reflectivity, stable isotope data, and mineral zoning are consistent with the gold deposits being genetically related to formation of the 37 Ma Bingham porphyry deposit. We interpret the disseminated gold mineralization to be related to collapse of the Bingham Canyon hydrothermal system in which isotopically heavy, oxidizing, acidic waters, possibly from an internally draining acidic crater lake, mixed with and were entrained into reduced gold-bearing meteoric water fluids in the collapsing main-stage hydrothermal system. Most of this fluid mixing and cooling was probably located close to the hydrologic interface between the sedimentary basement rocks and overlying volcanic rocks.

Internal dynamics of a paleoaccretionary wedge: insights from combined isotope tectonochronology and sandbox modelling of the South-Central Chilean forearc

Forearc accretionary wedges are cyclic systems in which material is frontally and/or basally accreted. Material cycling involves underthrusting, subduction, underplating, exhumation, erosion, transfer to the trench and underthrusting again. In this study we present a novel, tectonochronologic approach to constrain long-term exhumation rates of basally accreted wedge complexes, based on isotopic dating of structural features, on petrological data and sandbox analogue simulations. Congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics. For the present-day surface material of the paleoaccretionary wedge of South-Central Chile (Valdivia area, 40°S), published U–Pb ages of detrital zircon place a maximum age of ∼278 Ma for subduction. Prograde metamorphism at transitional greenschist to blueschist facies conditions (420 °C, 8–9 kbar) was immediately followed by progressive penetrative deformation associated with basal accretion, dated at ∼250–245 Ma using Rb/Sr internal mineral isochrons. The accretion process involved duplex tectonics and antiformal stacking, with formation of near-horizontal mylonitic shear zones at around 241 Ma. Continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels. Both semi-ductile, small-scale extensional shear zones and post-kinematic vein mineralizations yield Rb/Sr ages of ∼235 Ma. Tension gashes, representing the latest isotopically dateable stage of structural evolution, were formed at ∼210 Ma, at conditions of ∼230 °C at 1.5–3 kbar, as constrained by fluid inclusion data. Zircon fission track data indicate final cooling to below ∼200 °C at 186±24 Ma. The results suggest continuous basal accretion for at least 50 Ma, with long-term average exhumation rates of 0.6±0.2 mm/a, most probably outbalanced by similar long-term average erosion rates. Changing plate boundary conditions at about 210–200 Ma terminated the accretion process, as evident from a dramatic decrease of exhumation rates at that time. Since then, the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the Andean active margin. The tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high-temperature increments of P– T trajectories.

Evolution of the Himalayan foreland basin, NW India

AbstractThis paper provides new information on the evolution of the Himalayan foreland basin in the under‐reported region of the Kangra and Subathu sub‐basins, NW India. Comparisons are made with the better documented co‐eval sediments of Nepal and Pakistan to build up a broader picture of basin development. In the Subathu sub‐basin, shallow marine sediments of the Palaeocene–lower Lutetian Subathu Formation are unconformably overlain by the continental alluvial Dagshai and Kasauli Formations and Siwalik Group. The start of continental deposition is now dated at younger than 31 Ma from detrital zircon fission track data, thereby defining the duration of this major unconformity, which runs basin‐wide along strike. Final exhumation of these basin sediments, as thrusting propagated into the basin, occurred by 5 Ma constrained from detrital apatite fission track data. In the Kangra sub‐basin, the Subathu Formation is not exposed and the pre‐Siwalik sediments consist of the Dharamsala Group, interpreted as the deposits of transverse‐draining rivers. In this area, there is no evidence of westerly axial drainage as documented for coeval facies in Nepal. Similar to data reported along strike, facies analysis indicates that the sediments in NW India represent the filled/overfilled stages of the classic foreland basin evolutionary model, and the underfilled stage is not represented anywhere along the length of the basin studied to date.

ジルコンＦＴ年代測定における試料評価と測定値の解釈

How to deal with and to evaluate fission track data of zircons from the point of view of users were presented. Analytical diagrams including a grain-age histogram, a correlation between spontaneous and induced track counts, a correlation between spontaneous and induced track densities, an age spectra, a radial plot and a uranium-concentration histogram, were explained to read grain data. Grain-age distributions were simulated for a sample with an identical age but various conditions of track counts per grain. Data from about 2, 500 samples previously determined were presented and categorized for assessment of unknown samples. These are important as a guide to evaluate fission track data with relatively large uncertainties. Fission track dating of zircon is performed as follows: 1) assessment of a sample, 2) dating and analysis of grain data, and 3) interpretation of ages. For sample assessment, it is important that grouping of zircons in the sample based on their appearances such as color, morphological features and fission track densities on external surfaces. For interpretation of fission track ages, thermal history analysis with track length distributions is useful. Crosscheck of fission track ages between external and internal surfaces of zircon is also important. For examples, three samples from deep boreholes of the Niigata sedimentary basin were investigated and the meaning of zircon ages was discussed.

Two Stages of Deformation and Fluid Migration in the West-Central Brooks Range Fold and Thrust Belt, Northern Alaska

The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (60 and 45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (250–300C) event that was buffered by limestone and a younger, lower temperature (150C) event that had distinctly lower 13C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160–120 Ma, whereas the lower temperature event probably occurred at about 60–45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by far-traveled allochthons with relatively low structural relief, because it involved a thin (1–4-km [0.6–2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5–10 km (3–6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100–90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.

Zircon and apatite fission-track thermochronology of Late Carboniferous volcanic rocks of the NE German Basin

Zircon and apatite fission-track analyses from Late Carboniferous felsic volcanic rocks of the NE German Basin (Halle area and Friedland drilling) reveal at least two major post-emplacement thermal events. After initial cooling at ca. 300 Ma, the volcanic pile underwent a major thermal event at ca. 200 Ma that reached in most places ca. 250–280 °C and led to the new growth of clay minerals. This event is recorded in the zircon fission-track data and can be related to Jurassic–Triassic rifting in Europe. Another thermal event is recorded in the apatite samples at ca. 100 Ma. A close correlation is observed between apatite fission-track age, texture, and sample alteration. Coarse-grained samples are stronger altered and have younger fission-track ages. Hydrothermal alteration coupled with advective heating probably caused these Upper Cretaceous fission-track ages. The latter event can be related to block faulting and inversion of the European plate during the early stages of the Alpine orogeny.

Thermal history of the Cretaceous Sindong Group, Gyeongsang Basin, Korea based on fission track analysis

ABSTRACT Apatite and zircon fission track (FT) analyses were carried out to reconstruct the thermal history of the Lower Cretaceous Sindong Group, which is the lowermost stratal unit of the Gyeongsang Basin, Korea. Zircon FT central ages show a wide range from 83 ± 5 to 157 ± 18 Ma, and single‐grain age spectra have multiple age populations, whereas all apatites have very consistent FT ages of c. 60 Ma, suggesting a totally reset cooling age. Co‐existence of both older and younger ages compared with the depositional age and relatively short mean track length indicate that the Sindong zircons were partially annealed. The Sindong Group was heated into the zircon partial annealing zone (ZPAZ) around 80 Ma, and cooled below the apatite closure temperature at c. 60 Ma. Based on the zircon FT results combined with vitrinite reflectance data, the maximum palaeotemperature to which the Sindong Group had been subjected can be inferred to be about 260 °C. Zircon FT data from a granite body that is in contact with the Sindong Group and sandstones close to the granite body indicate that thermal influence caused by Upper Cretaceous intrusive rocks was limited in close vicinity of the intrusion and that the major heat source of the Sindong Group was burial. The thickness of uplifted and eroded section is estimated to be about 7 km.

Deformation and the timing of gas generation and migration in the eastern Brooks Range foothills, Arctic National Wildlife Refuge, Alaska

Along the southeast border of the 1002 Assessment Area in the Arctic National Wildlife Refuge, Alaska, an explicit link between gas generation and deformation in the Brooks Range fold and thrust belt is provided through petrographic, fluid inclusion, and stable isotope analyses of fracture cements integrated with zircon fission-track data. Predominantly quartz-cemented fractures, collected from thrusted Triassic and Jurassic rocks, contain crack-seal textures, healed microcracks, and curved crystals and fluid inclusion populations, which suggest that cement growth occurred before, during, and after deformation. Fluid inclusion homogenization temperatures (175–250C) and temperature trends in fracture samples suggest that cements grew at 7–10 km depth during the transition from burial to uplift and during early uplift. CH4-rich (dry gas) inclusions in the Shublik Formation and Kingak Shale are consistent with inclusion entrapment at high thermal maturity for these source rocks. Pressure modeling of these CH4-rich inclusions suggests that pore fluids were overpressured during fracture cementation.Zircon fission-track data in the area record postdeposition denudation associated with early Brooks Range deformation at 64 3 Ma. With a closure temperature of 225–240C, the zircon fission-track data overlap homogenization temperatures of coeval aqueous inclusions and inclusions containing dry gas in Kingak and Shublik fracture cements. This critical time-temperature relationship suggests that fracture cementation occurred during early Brooks Range deformation. Dry gas inclusions suggest that Shublik and Kingak source rocks had exceeded peak oil and gas generation temperatures at the time structural traps formed during early Brooks Range deformation. The timing of hydrocarbon generation with respect to deformation therefore represents an important exploration risk for gas exploration in this part of the Brooks Range fold and thrust belt. The persistence of gas high at thermal maturity levels suggests, however, that significant volumes of gas may have been generated.

Cooling and exhumation of the Rieserferner Pluton (Eastern Alps, Italy/Austria)

The Defereggen–Antholz–Vals (DAV) Line marks the southern border of alpine greenschist facies metamorphism within the Austroalpine basement to the south of the western Tauern window. During the Oligocene sinistral-strike slip, coeval with vertical displacement, was accompanied by the syntectonic emplacement of the Rieserferner Pluton along the north side of the fault. To the north of the DAV Line Rb/Sr as well as K/Ar-biotite cooling ages continuously increase from 16 Ma at the Rensen Pluton in the west to 30 Ma at the eastern end of the Rieserferner Pluton. Similar trends are apparent from zircon and apatite fission-track data, which increase eastward from 19 to 29 Ma, and 13 to 16 Ma, respectively. Low cooling rates allow a direct transformation into exhumation rates assuming an average geothermal gradient of 28 °C km–1. The resulting exhumation rates of less than 1 mm year–1 do not affect the depth of isotherms in a geologically significant manner. Hence, variable cooling rates within the basement north of the DAV Line are a result of differential exhumation of individual blocks. High exhumation rates bracketed between the intrusion age of the Rieserferner Pluton at 31±3 Ma and the apatite partial annealing zone (PAZ) range between 0.4 and 0.6 mm year–1, whereas final exposure takes place at reduced exhumation rates of 0.2 mm year–1. East-side-down tilting of the block north of the DAV Line, as indicated by variations in exhumation rates, strongly affects the western part of the area studied between the Rensen and the Central Rieserferner Pluton whereas, further east, block rotation becomes less important. A total difference in exhumation of about 2.2 km corresponding to 5° tilting between the Western and Eastern Rieserferner, as indicated by K/Ar-biotite and zircon fission-track data, occurred before going through the PAZ for apatite fission-tracks. Furthermore, the significance of the cooling ages, especially for biotite, is tested by calculating a finite element approximation. Different numerical models indicate fast cooling of the intrusive melts prior to approaching the closure temperature of the K/Ar system in biotite.

Low-temperature thermochronological record of exhumation of the Bitterroot metamorphic core complex, northern Cordilleran Orogen

The Bitterroot metamorphic core complex is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the North American Cordilleran Orogen, in the northern Idaho batholith region. Extension was accommodated mainly on the Bitterroot mylonite zone, a 500–1500-m-thick shear zone that deforms granitic intrusive rocks as young as 53–52 Ma, as well as older high-grade metamorphic rocks and plutons. Exhumation of the core complex, in Eocene time, is marked in the shear zone by the transition from amphibolite-facies mylonitization, to greenschist-facies mylonitization, chloritic brecciation, to brittle faulting that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 53 –30 Ma based on U–Pb, Ar–Ar, and fission-track data. Apatite and zircon fission-track data record the lower-temperature part of the exhumation history and help define when the shear zone became inactive, as well as the transition from rapid, core complex-style extension to slower basin-and-range-style extension. They indicate that the western part of the complex was exhumed to within 1–2 km of the surface by ∼48–45 Ma, while the eastern part of the complex was still at amphibolite-facies conditions and that the eastern part of the complex was not exhumed below ∼60 °C until after 30 Ma. Younger apatite fission-track ages (≤26 Ma) on the eastern range front of the Bitterroot Mountains suggest that the present topographic expression of the mylonite front was due to Miocene high-angle faulting and widening of the Bitterroot Valley.

Phanerozoic tectonothermal history of the Arabian–Nubian shield in the Eastern Desert of Egypt: evidence from fission track and paleostress data

To constrain the post-Pan-African evolution of the Arabian–Nubian Shield, macro-scale tectonic studies, paleostress and fission track data were performed in the Eastern Desert of Egypt. The results provide insights into the processes driving late stage vertical motion and the timing of exhumation of a large shield area. Results of apatite, zircon and sphene fission track analyses from the Neoproterozoic basement indicate two major episodes of exhumation. Sphene and zircon fission track data range from 339 to 410 Ma and from 315 to 366 Ma, respectively. The data are interpreted to represent an intraplate thermotectonic episode during the Late Devonian–Early Carboniferous. At that time, the intraplate stresses responsible for deformation, uplift and erosion, were induced by the collision of Gondwana with Laurussia which started in Late Devonian times. Apatite fission track data indicate that the second cooling phase started in Oligocene and was related to extension, flank uplift and erosion along the actual margin of the Red Sea. Structural data collected from Neoproterozoic basement, Late Cretaceous and Tertiary sedimentary cover suggest two stages of rift formation. (1) Cretaceous strike-slip tectonics with sub-horizontal σ1 (ENE/WSW) and σ3 (NNW/SSE), and sub-vertical σ2 resulted in formation of small pull-apart basins. Basin axes are parallel to the trend of Pan-African structural elements which acted as stress guides. (2) During Oligocene to Miocene the stress field changed towards horizontal NE–SW extension (σ3), and sub-vertical σ1. Relations between structures, depositional ages of sediments and apatite fission track data indicate that the initiation of rift flank uplift, erosion and plate deformation occurred nearly simultaneously.

Study of the tectonic uplift history of the Sillai Patti granitic gneiss, Pakistan: constraints from zircon fission-track dating

A group of alkaline igneous complexes called the Peshawar Plain Alkaline Igneous Province (PAIP) is exposed in an arcuate fashion north of the Peshawar Plain in northwestern Pakistan. The PAIP, which stretches from Tarbela in the east to Loe-Shilman near the Pakistan–Afghanistan border in the west, consists mainly of granites, syenites, gabbros, ijolites and carbonatites. Field relationships suggest that the Malakand granite and Sillai Patti carbonatite are younger than the Malakand and Sillai Patti granitic gneisses. However, a zircon fission-track age from the Sillai Patti granitic gneiss is less than the absolute ages of the Malakand granite (zircon U–Pb age=270 Ma ) and Sillai Patti carbonatite (calcite U–Pb age=30 Ma, biotite K–Ar age=31±2 Ma, zircon fission-track age=32.3±1.4 Ma ). Therefore, the zircon fission-track age of 24.4±2.9 Ma from the Sillai Patti granitic gneiss represents a time of post metamorphic denudation history of the area, when these rocks passed through the 210°C isotherm. This isotherm corresponds to a depth of about 6.7 km within the earth's crust from the present-day surface.

Alpine metamorphic evolution and cooling history of the Veporic basement in northern Hungary: new petrological and geochronological constraints

Petrological and geochronological investigations were carried out on metamorphic rocks of the Veporic unit (Inner Western Carpathians) in northern Hungary. K/Ar and Ar/Ar data on micas and amphibole show only Alpine ages (mostly in the range of 87–95 Ma) in this basement unit. Thermobarometric calculations yield lower amphibolite facies peak conditions (ca. 550±30 °C and 9±1 kbar) for the Eoalpine metamorphic event. Complex evolution of gneissic rocks is reflected by the presence of discontinuously zoned garnets, the cores of which may represent relics of a pre-Alpine (presumably Variscan) thermal event. Zircon fission track (FT) data in the narrow range of 75–77.5 Ma indicate that this portion of the Veporic unit was emplaced to shallow crustal levels already during the Senonian time. The relative minor difference between zircon FT and K/Ar or Ar/Ar ages suggests very rapid cooling during the Late Cretaceous, most probably related to the extensional unroofing of the Veporic core complex. The obtained cooling ages do not support previous models of Tertiary uplift and exhumation of the Veporic unit along the Hurbanovo-Diosjenő Line.

Possible Jurassic age for part of Rakaia Terrane: Implications for tectonic development of the Torlesse accretionary prism

Greywacke sandstone and argillite beds comprising Rakaia Terrane (Torlesse Complex) in mid Canterbury, South Island, New Zealand, are widely regarded as Late Triassic (Norian) in age based on the occurrence of Torlessia trace fossils, Monotis, and other taxa. This paleontological age assignment is tested using published 40Ar/39Ar mica and U‐Pb zircon ages for these rocks and published and new zircon fission track (FT) ages. The youngest U‐Pb zircon ages in the Rakaia Terrane rocks in mid Canterbury are Norian, whereas 10–20% of the 40Ar/ 39Ar muscovite ages are younger than Norian. Numerical modelling of these mica ages shows that they cannot have originated from partial thermal overprinting in the Torlesse prism if the thermal maximum was short‐lived and early in the prism history (210–190 Ma), as commonly inferred for these rocks. The young component of mica ages could, however, be explained by extended residence (200–100 Ma) at 265–290°C in the prism. Early Jurassic (c. 189 Ma) zircon FT ages for sandstone beds from Arthur's Pass, the Rakaia valley, and the Hermitage (Mt Cook) are interpreted not to have experienced maximum temperatures above 210°C, and therefore cannot have been reduced as a result of partial annealing in the Torlesse prism. This is based on identification of a fossil Cretaceous, zircon FT, partial annealing zone in low‐grade schists to the west, and the characteristics of the age data. The Early Jurassic zircon FT ages and the young component of 40Ar/39Ar mica ages are regarded therefore as detrital ages reflecting cooling in the source area, and constrain the maximum depositional age of parts of the Rakaia Terrane in mid Canterbury. The zircon FT data also show the initiation (c. 100 Ma) of marked and widespread Late Cretaceous cooling of Rakaia Terrane throughout Canterbury, which is attributed to uplift and erosion of inboard parts of the Torlesse prism due to continuing subduction accretion at its toe. The critical wedge concept is proposed as a new framework for investigating the development of the Torlesse Complex. The Rakaia Terrane may have formed the core of an accretionary wedge imbricated against the New Zealand margin during the Middle or Late Jurassic. Late Jurassic nonmarine sediments (e.g., Clent Hills Formation) accumulated upon the inner parts of the prism as it enlarged, emerged, and continued to be imbricated. Exhumation of Otago Schist from c. 135 Ma may mark the development of a balance (steady state) between sediments entering the prism at the toe and material exiting at the inboard margin. The enlargement of the area of exhumation to all of Canterbury from c. 100 Ma may reflect a dynamic response to widening of the prism through the accretion of Cretaceous sediments. The model of a dynamic critical wedge may help to explain the various expressions of the Rangitata Orogeny.

Unroofing history of the Sillai Patti granite gneiss, Pakistan: constraints from zircon fission-track dating

A group of alkaline igneous rocks is exposed in the north of the Peshawar Plain, extending from Tarbela in the east up to Loe–Shilman near the Pakistan–Afghanistan border in the west. The alkaline rocks consist mainly of granites, syenites, gabbros, ijolites and carbonatites. Granitic gneisses of Paleozoic age are exposed at the Malakand and further westward at Sillai Patti. However, the fission-track dating studies on zircon, based on the present work, indicate that the age of the Sillai Patti granite gneiss is less than the absolute age of granite gneisses. Therefore, the zircon fission-track age of 24.28±2.97 Ma of the Sillai Patti granite gneiss, represents a time of post-metamorphic denudation history of the area, when these rocks passed through the 210°C isotherm, corresponding to a depth of about 6.7 km inside the earth's crust from their present position if a paleogeothermal gradient of 30°C/km is assumed to have prevailed. Our average fission-track zircon age of 24.28±2.97 Ma is very similar to the average fission-track zircon age of 25.4±0.7 Ma of Mansehra granites. Average cooling rates of the Mansehra and Sillai Patti granite gneisses have been computed to be (8.00±0.22)°C/Ma and (8.00±0.98)°C/Ma, while the average denudation rates of the Mansehra and Sillai Patti granite gneisses have been computed to be (0.262±0.007) and (0.274±0.034) mm/ yr , respectively, on the basis of zircon fission-track ages for the period between 25 Ma and the present time. This indicates that the two complexes have experienced similar average cooling/uplift-induced denudation histories during the last 25 Ma or so.