Zircon Fission-track Ages Research Articles

Heat influx and exhumation of the Shimanto accretionary complex: Miocene fission track ages from the Kii Peninsula, southwest Japan

Abstract To determine how local geological events contributed to the evolution of accretionary complexes and eventual exposure of rocks with different structural levels, geochronological mapping was carried out using fission track (FT) analysis at the Kii Peninsula, southwest Japan. At this site, the original zonal structure of Cretaceous accretionary complexes parallel to the subduction zone is disturbed by the northward projection of the Shimanto accretionary complex. Twenty‐six zircon FT ages were obtained from an area of ∼12 km in an east–west direction and ∼15 km in a north–south direction, and classified into three groups: (i) ages ∼15 Ma (range ∼10–20 Ma), which are distributed along the northwest–southeast valley; (ii) ages of ∼50 Ma in the northwest of the study area; and (iii) ages older than those in Groups 1 and 2. Based on results from eight zircon FT length distributions, the Miocene ages appear to be the result of spatial variations in heat influx and cooling after the regional exhumation of the area, as recorded by FT ages of ∼50 Ma.

Contemporaneous plutonism and strike‐slip faulting: A case study from the Tonale fault zone north of the Adamello pluton (Italian Alps)

A field, microstructural, and geochronological study of contemporaneous plutonism and strike‐slip faulting along the eastern Tonale fault zone provides new insights into the interrelationships between magmatic emplacement, contact metamorphism and shearing, and it places new time constraints for the Periadriatic Fault System. Although pluton emplacement and shearing were not caused by each other, they mutually interacted during contact metamorphism. The character of the composite Tonale fault zone varies considerably with its proximity to the northern margin of the Adamello pluton, from a paired greenschist facies mylonite belt (Northern mylonite zone) and cataclastic fault zone in the west, to a fault zone consisting of Northern mylonite zone, Cataclastic fault zone and a second mylonite belt (Southern mylonite zone) formed by contemporaneous shearing and contact metamorphism in the east. Ongoing strike‐slip motion led to telescoping and advective cooling of the deforming contact aureole. New U/Pb zircon ages for the northernmost Adamello intrusions (the Avio at 34.6‐1.0 Ma and the Presanella at 32.0‐2.3 Ma) date the onset of contact metamorphism. Rb‐Sr and K‐Ar cooling ages and new zircon fission track ages provide evidence that postintrusive cooling to below 300°C was achieved rapidly, i.e., approximately 30 Myr ago being consistent with a shallow crustal emplacement at ambient temperatures of approximately 250°C. Cataclastic dextral strike‐slip faulting continued until about 20 Ma, when the Tonale fault zone became offset by a number of minor sinistral shears, and by the Giudicarie fault. Hence dextral strike‐slip motion along the Periadriatic Fault System east of the Bergell pluton lasted from approximately 35–20 Myr ago.

Timing of Upper Carboniferous-Permian horst-basin formation and magmatism in the NW Thuringian Forest, central Germany: a review

Abstract During Late Carboniferous-Early Permian times dextral transtensional movements along the NW-trending Franconian Fault System and parallel faults caused complex block faulting in the Thuringian Forest region, Germany, accompanied by intense magmatism. This is well constrained by geochronological data ( 207 Pb/ 206 Pb single zircon, SHRIMP, 40 Ar/ 39 Ar mica, zircon fission-track ages), field relations, and the sedimentary record from the Ruhla Crystalline Complex (RCC) and surroundings. The combined dataset indicates that the Ruhla Crystalline Complex was faulted into three nearly N-S-trending segments, which underwent different exhumation histories during Late Carboniferous-Permian times. The central segment of the RCC was exhumed by several kilometres as a horst block, while the eastern and western segments subsided simultaneously, forming the basement to the Oberhof and Eisenach molasse basins, respectively. Late Carboniferous-Permian uplift of the central segment is constrained by 40 Ar/ 39 Ar cooling ages of 311 ± 3 (muscovite) and 294–288 ± 3 Ma (biotite), a weighted zircon fission-track age of 272 ± 7 Ma and overlying Zechstein sediments. In contrast, the eastern segment shows much older 40 Ar/ 39 Albiotite cooling ages between 336 ± 4 and 323 ± 3 Ma, was exposed at c. 300 Ma, and subsequently covered by molasse sediments and volcanic rocks between 300 and c. 275 Ma. A similar Late Carboniferous evolution is inferred for the western segment, as it is also overlain by Lower Permian volcanic rocks and has a 297 ± 29 Ma single zircon fission-track age. Simultaneous horst and basin formation is additionally constrained by granite pebbles in conglomerates of the Oberhof and Eisenach basins. These pebbles can partly be derived from granites in the central segment of the RCC. Age data and the orientation of granitoid bodies and dykes in the Ruhla Crystalline Complex and its surroundings provide evidence for the opening of NE-trending structures between 300 and 294 Ma, and formation or reactivation of W- to NW-trending structures between 290 and 275 Ma. Magmatic activity in the Thuringian Forest region may have been caused by widespread mantle upwelling in central Europe during the Late Carboniferous-Early Permian.

Detrital-zircon fission-track ages for the “Hoh Formation”: Implications for late Cenozoic evolution of the Cascadia subduction wedge

We report new fission-track (FT) ages for detrital zircons for 34 sandstone samples and 2 volcanic ash beds from the “Hoh Formation,” exposed along the western side of the Olympic Mountains of western Washington State. The “Hoh Formation” is now formally known as the coastal unit of the Olympic Structural Complex, or Coastal OSC for short. About 35 zircons were dated per sample. The fission-track grain-age (FTGA) distributions are all strongly discordant; grain ages range from 10 to older than 100 Ma. Low vitrinite-reflectance values, short etch times for the zircons, and a broad range of grain ages indicate that the zircon FT ages are unreset and thus preserve information about cooling events in the source region for these sedimentary rocks. Five areas were sampled repeatedly and yield similar FTGA distributions, demonstrating that sampling errors are not a problem. We show that almost all of the samples contain a well-defined young component that was probably derived from a contemporaneous active volcanic source, presumably the adjacent Cascadia arc. Binomial peak-fitting was used to estimate the FT minimum age, which is the age of the youngest concordant fraction of zircon FT grain ages in a FTGA distribution. In most cases, minimum ages are similar to fossil ages where available. This result supports our contention that zircon FT minimum ages from volcaniclastic sandstones commonly can be used as a proxy for depositional age. Our zircon FT minimum ages indicate that the Coastal OSC is made up mainly of lower Miocene (ca. 24 to 16 Ma) sedimentary rocks. We use these age data, together with other geologic constraints, to reconstruct a tectonic history. Sedimentary rocks of the Coastal OSC were derived from a mixed-source region that included an active volcanic arc and also older units, including Cretaceous metamorphic rocks, probably located in the Omineca crystalline belt in the Canadian Rockies. The upper part of the Clallam Formation, located on the northern side of the Olympic Peninsula, appears to be a remnant of the sedimentary system that fed the Coastal OSC. The sediments that formed the Coastal OSC were initially deposited seaward of the Cascadia trench, at water depths of >2000 m. This debris was deposited seaward of the Cascadia trench, at water depths of >2000 m, and subsequently accreted beneath the frontal 50 to 100 km of the wedge. Owing to continued accretion at the front of the wedge, and erosion of the forearc high in back of the wedge, these lower Miocene sediments were moved rearward within the Cascadia subduction wedge. A simple relationship based on the cross-sectional area of the wedge and a steady accretion flux indicates that it would have taken ∼22 m.y. for the Coastal OSC to reach its present position 140 km landward of the toe of the wedge. This estimate is in good agreement with the unit9s early Miocene age.

Downstream Changes of Alpine Zircon Fission-Track Ages in the Rhone and Rhine Rivers

Zircons from 13 sediment samples from the Rhone and Rhine drainages were dated by the fission-track method to study downstream changes in detrital fissiontrack grain-age distributions in large river systems draining the European Alps. The orogen-parallel Rhone River shows a zircon fission-track grain-age distribution similar to that in its Alpine source areas. This signal is well preserved because there are Alpine sources along its full length and input from non-Alpine sources is small. In contrast, only the headwaters of the north-flowing Rhine River are located in the Alps. As a consequence, Alpine-derived zircons, which are distinguished by young fission-track ages, become progressively diluted downstream by older ages from zircon sources external to the Alps. Nevertheless, the Alpine component is persistent and can easily be detected in sediments more than 1000 km downstream at the Rhine delta. These results demonstrate that fission-track dating of detrital zircons can provide useful information about orogenic processes, even where sediments have been transported hundreds of kilometers from the orogenic source, crossing ephemeral lakes and subsiding basins. Deposits along the lower reaches of the river appear to have a short-residence time (< 1 Myr), and thus many of these deposits serve to smooth out variations in the supply of sediment from fast-eroding heterogeneous sources in the Alpine headwaters of these drainages. For the Rhone and the Rhine rivers, we show that fast erosion in the Alps accounts for most of the sediment load. This finding supports a widespread observation that the sediment in most large continental drainages is usually derived from a small part of the drainage, where uplift, relief, and erosion rates are greatest.

Fission track age of granite batholith from Southern Tibet: implications for the plateau uplift *

Abstract Fission track (FT) ages of apatite and zircon from four granite batholiths from Lhasa and Shannan areas are measured. The FT ages of apatite range from 3.2 to 8.3 Ma, corresponding to the uplift rates of 0.12 to 0.20 mm·a−1 during this period. The uplift height is 580 m, showing that there is not large-scale rapid uplifting in southern Tibet from 3.2 to 8.3 Ma. The zircon FT ages of Lhasa batholith are 25.9 ± 1.7 and 32.7 ± 2.8 Ma, yielding an uplift rate of 0.08mm·a−1 between 26 and 33 Ma. Combining this work with other studies, it is suggested that the average uplift rate in southern Tibet is low from the time of collision between India and Asian continents to ∼3 Ma. The uplift of Tibetan Plateau seems to have finished in multi-stage processes with varied rates.

Fast extension but little exhumation: the Vari detachment in the Cyclades, Greece

Markedly different cooling histories for the hanging- and footwall of the Vari detachment on Syros and Tinos islands, Greece, are revealed by zircon and apatite fission-track data. The Vari/Akrotiri unit in the hangingwall cooled slowly at rates of 5–15 °C Myr−1 since Late Cretaceous times. Samples from the Cycladic blueschist unit in the footwall of the detachment on Tinos Island have a mean zircon fission-track age of 10.0±1.0 Ma, which together with a published mean apatite fission-track age of 9.4±0.5 Ma indicates rapid cooling at rates of at least ∼60 °C Myr−1. We derive a minimum slip rate of ∼6.5 km Myr−1 and a displacement of &lt;∼20 km and propose that the development of the detachment in the thermally softened magmatic arc aided fast displacement. Intra-arc extension accomplished the final ∼6–9 km of exhumation of the Cycladic blueschists from ∼60 km depth. The fast-slipping intra-arc detachments did not cause much exhumation, but were important for regional-scale extension and the formation of the Aegean Sea.

Exhumation of the central Wasatch Mountains, Utah: 1. Patterns and timing of exhumation deduced from low‐temperature thermochronology data

The Wasatch Mountains are often cited as an example of normal fault growth and footwall flexure. They represent a tilted footwall at the edge of the Basin and Range extensional province, a major rift basin. Thus understanding the detailed spatial and elevation changes in coupled thermochronometer data, and how these changes can be interpreted, may aid in the analysis of thermochronometer data from other extensional regions around the world. We present a dense data set from the Cottonwood Intrusive Belt (CIB) of the Wasatch that includes apatite fission track (AFT), zircon fission track (ZFT), and apatite (U‐Th)/He ages. ZFT, AFT, and apatite (U‐Th)/He ages are 10, 5, and 3 Ma, respectively, adjacent to the Wasatch fault. AFT and (U‐Th)/He ages increase slightly with distance east of the fault until about 15–20 km, where a more abrupt increase in these ages occurs at or near the Silver Fork‐Superior fault zone. ZFT and AFT ages are concordant with 31–38 Ma pluton emplacement ages on the eastern side of range. Modeling of the data leads to the following interpretation: (1) Early cooling and ∼3–4 km of exhumation for the middle and eastern parts of the range occurred in the late Oligocene‐middle Miocene. (2) Beginning at 10–12 Ma, the locus of exhumation shifted westward toward the present range front, where the rocks cooled from &gt;200°C in the last 10–12 Myr. Our data and interpretations are consistent with a model in which the locus of faulting and exhumation shifted opposite the direction of tilt, similar to that predicted by rolling‐hinge extensional models. However, this westward shift and rapid Miocene to recent exhumation may be a local effect superimposed on lower fault displacement and exhumation rates elsewhere along the Wasatch.

Erosion rates and orogenic-wedge kinematics in Taiwan inferred from fission-track thermochronometry

New apatite and zircon fission-track ages and previously published thermochronometric data are used to evaluate erosion rates and particle paths within the active Taiwan arc-continent collision. We present 20 new apatite fission-track ages and 6 new zircon fission-track ages. Apatite and zircon ages are all reset in the northern and eastern parts of Taiwan, although the region of reset apatite ages is larger. We interpret this pattern as resulting from crustal accretion at the western margin of the orogenic wedge combined with southward propagation of the collision zone. A onedimensional thermal model including erosion provides prediction of the fission-track ages. The distribution of reset ages is best explained with an erosion rate of 4‐6 mm/yr. Given a propagation velocity of 60 mm/yr, this erosion rate implies that nearly 25 km of material has been eroded from northern Taiwan. The lack of reset 40 Ar/ 39 Ar ages from muscovite and biotite suggests that rockparticle paths have a large horizontal component, a result consistent with an eroding orogenic-wedge model.

Geodynamic evolution of southern Costa Rica related to low-angle subduction of the Cocos Ridge: constraints from thermochronology

The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting. Fission track thermochronology using zircon and apatite, as well as 40Ar– 39Ar and Rb–Sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. A weighted mean zircon fission track age (13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement. In some cases 40Ar– 39Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages, which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma.

Thermobarometric data from a fossil zircon partial annealing zone in high pressure–low temperature rocks of eastern and central Crete, Greece

A fossil partial annealing zone of fission tracks in zircon is described from high pressure–low temperature (HP–LT) rocks of the Phyllite–Quartzite Unit (PQ) on the island of Crete, Greece. Correlation of regional trends in fission track age populations with independent thermobarometric and microstructural data, and with new experimental annealing results, allows a calibration of this low temperature thermochronological method to a degree hitherto not available from other field examples. The zircon fission track (FT) ages of samples from the PQ across Crete range from original detrital signature through reduced to completely reset. The annealing is the result of a single heating period related to the HP–LT metamorphism with near-peak temperatures lasting for only a few million years some time between 24±1 and 20±1 Ma. In eastern Crete, where rocks have experienced temperatures of 300±50 °C and pressures of 0.8±0.3 GPa, zircon FT ages range from 414±24 to 145±10 Ma. Ages above 300 Ma occur mostly near the east coast of the island in rocks which have not been heated to above ca. 280 °C and probably represent a pre-Variscan source. Track lengths are already indicative of a substantial annealing at this temperature. Most of the zircon FT ages from eastern Crete scatter within error around the stratigraphic age. Samples with apparent zircon FT ages significantly younger than the depositional age are only observed in areas where temperatures exceeded ca. 320 °C. Towards the west, a sudden decrease to very young ages ranging from 17±2 to 18±1 Ma reflects a complete resetting at ca. 350 °C. Short tracks, however, are still observed. Throughout the central and western part of the island, ages are consistently below 22 Ma. Thermobarometric data for this area indicate maximum temperatures of 400±50 °C and pressures of 1±0.3 GPa. Only samples from western Crete, which have been exposed to 400±50 °C, show exclusively long tracks. Consequently, the high temperature limit of the zircon partial annealing zone (ZPAZ) appears to be between 350 and 400 °C. A significant influence of elevated confining pressure on the stability of fission tracks in zircon is ruled out by the results of annealing experiments at 0.5 GPa and at different temperatures, which fit the curves previously obtained by other authors at ambient pressure.

Thermo-tectonic Implications of Zircon and Apatite FT Data of the Marlborough Region, South Island, New Zealand

Zircon and apatite fission track (FT) reveal some of the thermo-tee- tonic features of the Marlborough Region, South Island, New Zealand. The very young FT ages (＜10 Ma) of zircon and apatite in the vicinity of the Alpine Fault bend and Seaward Kaikoura Range coincide with the recent rapid uplift/erosion. Four samples with reset zircon ages in the Alpine Fault bend reveal that the host rocks in this area cooled below the closure termperature of zircon (~240℃) in the late Miocene. Unlike these four zircon FT ages, most zircon FT ages are consistent with depositional ages. Annealed apatite and unannealed zircon FT ages show that the Marlborough did not experience exposure to the closure temperature of zone (PAZ) of apatite (~60-100℃). The host rocks in the north rather than those in the south passed through the lower part of apatite PAZ. In addition, most of the zircon samples with low P( X2) values (＜5 %) show that the samples have been slightly annealed, implying that the host rocks might have experienced the upper part of the partial annealing zone of zircon (~175℃) during the Mesozoic cooling.

Low-temperature cooling history of the Shuswap metamorphic core complex, British Columbia: constraints from apatite and zircon fission-track ages

Nine zircon and 18 apatite fission-track ages are used to determine the low-temperature cooling history of part of the Shuswap metamorphic core complex of the Canadian Cordillera. The zircon ages range from 54 to 38 Ma and the apatite ages from 49 to 28 Ma. These ages reveal a similarity in cooling histories across the Shuswap units until temperatures of ~250°C were reached at about 45 Ma. From this time onwards, the regional cooling pattern within the core complex was controlled by the relative movements on two normal faults, the Victor Creek fault and the Columbia River fault. Cooling since 45 Ma was variable, depending on the structural level of the sample. On this basis four thermotectonic units are defined. These units are controlled by normal faults that crosscut the lithological units of the core complex and reflect the latest stage of its evolution.

Mio–Pliocene adakite generation related to flat subduction in southern Ecuador: the Quimsacocha volcanic center

New geochemical and geochronological data on the Miocene–Pliocene Quimsacocha volcanic center (QVC) have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern Ecuador and northern Peru. The QVC, located in the presently inactive southern part of the Ecuadorian arc, was built up during three distinctive volcanic phases. The first phase generated a basal edifice with mainly andesitic lava flows, while the second phase is characterized by the emplacement of cryptodomes, domes and related outflow breccias comprised of andesites and some dacites. The last phase released rhyolitic ignimbrites associated with the formation of a large caldera, which was later partly filled by dacitic–rhyolitic domes. Geochemical data for the QVC indicate higher Al 2O 3, TiO 2, Na 2O, Zr and Sr contents and lower Fe 2O 3*, MgO, Y, MREE and HREE abundances, compared to other eruptive rocks of the Plio–Quaternary volcanic front of Ecuador. Such geochemical features, as well as the frequent presence of an associated epithermal gold deposit, are characteristic of the involvement of slab melts, also known as adakites [1,2], in the generation of these magmas. After a calc-alkaline arc magmatism phase, slab horizontalization – in response to the subduction of a buoyant oceanic plateau – results in increased involvement of a slab melting component in the magmas produced. However, pristine adakites were generated and emplaced during a relatively short period, as indicated by zircon fission-track ages. Then volcanic activity stopped and a volcanic gap formed. The identification of these adakites, their location and age support a model of slab melting associated with flat slab subduction [M.A. Gutscher et al., Geology 28 (2000) 535–538].

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.

Provenance Ages of Cretaceous Strata in the Songliao Basin, Northeast China Inferred from Fission Track Data

The Songliao Basin in northeast China is an intra-continental fossil rift basin mainly developed from the latest Jurassic and throughout the Cretaceous. The Daqing oil field in the basin is one of the largest hydrocarbon exploitation areas in China. A fission track (FT) study has been carried out on sandstone well samples in the Songliao Basin to constrain the depositional ages of Cretaceous strata and to ascertain their source region. FT dating also has been carried out on welded tuffs in the Dahinganling Mountains to determine their eruption ages. The detrital zircon FT age suggests that the depositional age of the middle Quantou Formation is younger than 109 Ma. Zircon FT ages for Cretaceous strata (from the Nenjiang Formation to the Quantou Formation) suggest that the source region has a fairly homogeneous cooling history at ∼120-100 Ma. Eruption ages of welded tuffs in the Dahinganling Mountains show an age range from 110 to 92 Ma. The regional geology and FT data suggest that volcanic rocks in the Dahinganling Mountains were eroded soon after their deposition and were reworked toward the Songliao basin.

Mesozoic‐Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes

Fission track (FT) analysis is applied to assess the Mesozoic and Cenozoic thermal and denudational history of the Patagonian Andes between 44° and 51°S and the geologic and geomorphic response of late Cenozoic subduction of the active Chile rise mid‐oceanic spreading center on the overriding plate. Seventy‐two FT ages from 43 samples are presented. Zircon FT ages indicate fast post intrusion cooling of Cretaceous parts of the Patagonian batholith and previously unreported Miocene magmatic rocks south of 48°S. Metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early Carboniferous and Late Permian. Apatite FT data reveal initiation of accelerated cooling and denudation at ca. 30 Ma at the western margin of southern continental South America followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca. 12–8 Ma at the position of the present‐day main topographic divide. This migration is proposed to be related to either coeval eastward migration of the retroarc deformation, the effects of subduction erosion in the overriding plate at the Peru‐Chile trench or less likely, shallowing of the angle of subduction. East of the divide, &lt;3 km of denudation has occurred since the Late Cretaceous. Enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca. 28–26 Ma that triggered orographically enhanced precipitation on the west side of the Patagonian Andes allowing increased erosion by fluvial incision and mass transport processes. The actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation.

Cretaceous–Tertiary Rhenodanubian flysch wedge (Eastern Alps): clues to sediment supply and basin configuration from zircon fission‐track data

The provenance of Cenomanian to Eocene flysch deposits accreted along the northern margin of the Eastern Alps has been investigated by means of zircon fission‐track (FT) geochronology and zircon morphology. The Rhenodanubian flysch and Ybbsitz klippen zone comprise several nappes representing the Main flysch and Laab basins. The Laab basin received sediments of stable European provenance, indicated by pre‐Variscan, Variscan, and Permian–Triassic zircon FT ages, and was thus located in the immediate south of the European margin. The Main flysch basin was supplied mainly from the evolving Eastern Alps and was therefore situated south of the Laab basin. Zircon populations with Permian to Jurassic cooling ages in the Main flysch basin are related to increased heat fluxes during the break‐up of Pangaea and are probably derived from the northwestern part of the Eastern Alps. The dominant Cretaceous zircon FT cooling ages reflect Eoalpine metamorphism in the Austroalpine realm.

Petrographical, geochemical and geochronological constraints on igneous clasts and sediments hosted in the Oligo-Miocene Bakony Molasse, Hungary: evidence for a Paleo-Drava River system

The provenance of igneous clasts and arenitic sediment enclosed within the Bakony Molasse was studied using geochemical and geochronological methods. The majority of igneous clasts were eroded from the Oligocene Periadriatic magmatic belt. A part of the andesite material has Eocene formation age. Rhyolitic pebbles originated from Permian sequences of the Greywacke zone or the Gurktal Alps. Apatite fission track (FT) ages from the sandstone matrix (age clusters at ~75 and ~30 Ma) are typical for the Austroalpine nappe pile and for the cooling ages of Periadriatic magmatic belt. Variscan detrital zircon FT ages indicate source areas that had not suffered Alpine metamorphism, such as the Bakony Mountains, Drauzug and the Southern Alps. Another group of detrital zircon grains of Late Triassic–Jurassic FT age (mean: ~183 Ma) marks source zones with Mesozoic thermal overprint such as the Gurktal Alps and some Austroalpine regions. Zircon grains with Oligocene FT age (mean: ~34.7 Ma) were derived from the Periadriatic intrusives and their contact zones. On the basis of the new data, we propose that the ancestor of the recent Drava River had already existed in Oligo-Miocene time and distributed eroded material of the southern Eastern Alps to the east.

Combination of Single-Grain Fission-Track Chronology and Morphological Analysis of Detrital Zircon Crystals in Provenance Studies: Sources of the Macigno Formation (Apennines, Italy)

Fission track (FT) analyses on unannealed detrital min- erals provide a powerful tool both for refining provenance models de- rived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies con- sistently identify the uplifting western Central Alps as the main source region for the Macigno Formation. Most zircon grains fall into a young age cluster ( ; 40-30 Ma), de- rived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distin- guished at 30 and 40 Ma. In the younger subgroup, the zircon mor- phology supports the presence of two main populations: (1) from ig- neous rocks (S-type euhedral zircons), which appear to be partly de- rived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission- track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and mig- matitic rocks affected by an Eocene high-temperature phase. A Late Cretaceous age cluster ( ; 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively. Finally, an ill-defined Jurassic age cluster, with a mean in Late Ju- rassic times, is related to rift-shoulder heating of the Austroalpine/ South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neo- gene erosion is suggested.