Fission-track Ages Research Articles

Evolution of the Adria-Europe plate boundary in the northern Dinarides: From continent-continent collision to back-arc extension

The Sava Zone of the northern Dinarides is part of the Cenozoic Adria‐Europe plate boundary. Here Late Cretaceous subduction of remnants of Meliata‐Vardaroceanic lithosphere led to the formation of a suture, across which upper plate European‐derived units of Tisza‐Dacia were juxtaposed with Adria‐derived units of the Dinarides. Late Cretaceous siliciclastic sediments,deposited on the Adriatic plate, were incorporated into an accretionary wedge that evolved during the initial stages of continent‐continent collision. Structurally deeper parts of the exposed accretionary wedge underwent amphibolite‐grade metamorphism. Grt‐Pl‐ Ms‐Bt thermobarometry and multiphase equilibria indicate temperatures between 550°C and 630°C and pressures between 5 and 7 kbar for this event. Peak metamorphic conditions were reached at around 65 Ma. Relatively slow cooling from peak metamorphic conditions throughout most of the Paleogene was possibly induced by hanging wall erosion in conjunction with southwest directed propagation of thrusting in the Dinarides. Accelerated cooling took place in Miocene times, when the Sava Zone underwent substantial extension that led to the exhumation of the metamorphosed units along a low‐angle detachment. Footwall exhumation started under greenschist facies conditions and was associated with top‐to‐the‐north tectonic transport, indicating exhumation from below European plate units. Extension postdates the emplacement of a 27 Ma old granitoid that underwent solid‐state deformation under greenschist facies conditions. The 40Ar/39Ar sericite and zircon and apatite fission track ages from the footwall allow bracketing this extensional unroofing between 25 and 14Ma. This extension is hence linked to Miocene rift‐related subsidence in the Pannonian basin, which represents a back‐arc basin formed due to subduction rollback in the Carpathians.

Spatiotemporal variation in exhumation of the Crystallines in the NW-Himalaya, India: Constraints from fission track dating analysis

During Himalayan orogeny, coeval thrusting along the Main Central/Munsiari Thrust (MCT/MT) and extension along the South Tibetan-Detachment System (STDS) are widely responsible for rapid exhumation of the Higher Himalayan Crystalline (HHC) zone. Apatite and zircon fission-track data along the Kaliganga and Darma valleys in the Kumaon Himalaya serve to document the shallow bedrock exhumation history of the HHC. Taking into account sample location within the HHC with respect to the MCT/MT, the apatite fission track (AFT) data-sets along the Darma (1.0 ± 0.1 to 2.8 ± 0.3 Ma) and Kaliganga (1.4 ± 0.2 to 2.4 ± 0.3 Ma) which are sharing same structural setting and rock types and being separated by 40 km, show very similar patterns of exhumation histories since Plio-Quaternary in the Kumaon Himalaya. Data sets along Darma and Kaliganga are very similar to data set of adjacent traverse (50 km away) along the Goriganga valley studied by Patel and Carter (2009). Whole data sets within the HHC in Kumaon Himalaya provide clear evidence for Plio-Quaternary tectonic activity along the Vaikrita Thrust (VT). Precipitation in this region exerts a strong influence on erosional surface processes. Fluvial erosional unloading along the Himalaya is focused on the high mountainous region of the HHC, where the orographic barrier forces out the maximum percentage of annual rainfall. FT cooling ages reveal coincidence between rapid erosion and exhumation that is focused in a ~ 25–30 km wide sector of the HHC, rather than covering the entire orogen. Similarity of AFT age pattern and exhumation rates along all three major traverses (Goriganga, Darma and Kaliganga) indicates that the region has been experiencing constant rate of crustal uplift and erosion since long time. Comparison of fission track ages from the Kumaon Himalaya with other segments of the NW-Himalaya shows spatiotemporal variation in exhumation. It is described due to the development of local structures such as dome/window in Jammu and Kashmir and Himachal Himalayas and Quaternary active thrusting along the VT and MCT/MT in the Garhwal-Kumaon Himalaya.

Fission-track dating of Quaternary volcanic glass by stepwise etching

The fission-track method has been applied to date volcanic glass from Quaternary tephra. The size of the spontaneous tracks in the volcanic glass is reduced at ambient temperatures, so the calculated age is often less than the expected eruption age. The underestimation of fission-track ages has been corrected using several methods, some of which have disadvantages in terms of time-consuming experiments and the use of radioactive materials. We examined a new protocol for the fission-track dating of glass that involves estimation of the track number per unit volume in a stepwise etching experiment and measurement of the 238 U concentration by laser ablation inductively coupled plasma mass spectrometry. The bulk etching rate ( V eg ) is estimated in track geometry analyses using an assumed track etching model. As a result, reliable values for V eg and the track density are obtained, suggesting that this new dating protocol has the potential to provide reliable fission-track ages without nuclear irradiation and complicated age correction procedures.

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.

Testing inferences from palaeocurrents: application of zircon double-dating to Miocene sediments from the Hengchun Peninsula, Taiwan

Detrital zircon provenance studies that combine low-temperature fission-track and hightemperature U-Pb single-grain age dating are powerful in constraining sediment provenance by documenting the tectono-thermal evolution of the sediment source(s). We apply these techniques to Miocene (12-6 Ma) sandstones of the Hengchun Peninsula, southern Taiwan which, based on diametrically opposite palaeocurrents, have a controversial provenance. UPb grain ages range from Miocene (8 Ma) to Archaean (2.5 Ga). Cretaceous thermal cooling is recorded by detrital zircon fission-tracks at Loshui and Lilongshan. Permian fission-track grain ages account for <33% of zircon ages from Loshui, while at Lilongshan Jurassic/Triassic grain ages account for 33-66% of all zircon fission-track ages. Minor (<12%) Miocene age components are detected in both formations. These new data suggest that the primary source of these Miocene sediments was similar. A simple model is proposed invoking sediment reworking in this complex tectonic setting. Author Final Draft or ‘Post-Print’ Version. The final version was published in Terra

Fission‐track thermochronology, vertical kinematics, and tectonic development along the western extension of the North Anatolian Fault zone

We have investigated the low‐temperature history of pre‐Neogene basement areas adjacent to the western extension of the North Anatolian Fault zone (NAFZ) by apatite fission track thermochronology of 33 samples taken from Marmara island, Kapidag peninsula (both in the Sea of Marmara), Samothrake island, and Chalkidike peninsula (both in the North Aegean region). Together with already published apatite fission track data from 30 other sampling localities of the region, these data (63 in total) have been evaluated mainly with regard to the transitional period from the Mesohellenic orogeny (Eocene‐Oligocene) to the plate‐tectonical individualization of the Aegean‐Antolian microplate, to gain a better understanding of the tectonical chronology that has led to modern east Mediterranean plate tectonics. Apatite fission track investigations of the area under discussion reveal two major events of cooling and exhumation of pre‐Neogene rocks. A first frequency maximum of cooling ages between 35 and 25 Ma (late Eocene to early Oligocene) is due to postorogenic regional erosion after the climax of the Mesohellenic orogeny, which was caused by continental collision of the Pelagonian platform and the Rila‐Rhodope zone after the final closure of the Axios‐Vardar Ocean. A second frequency maximum of cooling ages between 17 and 11.5 Ma (early to middle Miocene) depicts more localized uplift and exhumation in consequence of transpressive and/or transtensive movements along early structural discontinuities that gave rise to the later fault systems in the western extension of the NAFZ. The earliest fault structures of this system have developed shortly after the orogenic collapse and crustal extension of the Rila‐Rhodope area but quasi simultaneously with the onset of subduction along the modern Hellenic trench. The regional distribution of fission track ages of the Miocene age group (17 to 11.5 Ma), and corresponding thermochronological calculations based on frequency distributions of confined track lengths suggest that an early fault propagation has occurred from east to west during the Miocene. The onset of subduction along the Hellenic trench and subsequent gravitational retreat of the subducting oceanic slab are considered as a major cause for transtension of the Aegean crust, especially with regard to the development of north Aegean pull‐apart basins. Such early fault structures have facilitated the westward propagation of a continuous NAFZ that took place during the Pliocene from the easternmost Marmara region to the Gallipoli peninsula and to the North Aegean trough.

Spatial and temporal patterns of exhumation across the Venezuelan Andes: Implications for Cenozoic Caribbean geodynamics

[1] The Venezuelan Andes formed by complex geodynamic interaction between the Caribbean Plate, the Panama Arc, the South American Plate and the continental Maracaibo block. We study the spatial and temporal patterns of exhumation across the Venezuelan Andes using 47 new apatite fission track (AFT) ages as well as topographic analyses. This approach permits the identification of at least seven tectonic blocks (Escalante, Cerro Azul, Trujillo, Caparo, Sierra Nevada, Sierra La Culata and El Carmen blocks) with contrasting exhumation and cooling histories. The Sierra Nevada, Sierra La Culata and El Carmen blocks, located in the central part of the Venezuelan Andes and separated by the Bocono fault system, cooled rapidly but diachronously during the late Miocene–Pliocene. Major surface uplift and exhumation occurred in the Sierra Nevada block since before 8 Ma. A second phase of uplift and exhumation affected the El Carmen and Sierra La Culata blocks to the north of the Bocono fault during the late Miocene-Pliocene. The highest topography and steepest relief of the belt coincides with these blocks. The Caparo and Trujillo blocks, located at the northeastern and southwestern ends of the orogen, cooled more slowly from the Oligocene to the late Miocene. These blocks are characterized by significantly lower mean elevations and slightly lower mean slopes than the central blocks. Unraveling the cooling history of the individual blocks is important to better understand the control of preexisting faults and regional Caribbean geodynamics on the evolution of the Venezuelan Andes. Our data indicate a strong control of major preexisting fault zones on exhumation patterns and temporal correlation between phases of rapid exhumation in different blocks with major tectonic events (e.g., collision of the Panama arc; rotation of the Maracaibo block).

Late Mesozoic‐Cenozoic exhumation history of the Lesser Hinggan Mountains, NE China, revealed by fission track thermochronology

AbstractThis study uses zircon and apatite fission‐track (FT) analyses to reveal the exhumation history of the granitoid samples collected from the Lesser Hinggan Mountains, northeast China. A southeast to northwest transect across the Lesser Hinggan Mountains yielded zircon FT ages between 89.8 ± 5.7 and 100.4 ± 8.6 Ma, and apatite FT ages between 50.6 ± 13.8 and 74.3 ± 4.5 Ma with mean track lengths between 11.7 ± 2.0 and 12.8 ± 1.7 µm. FT results and modelling identify three stages in sample cooling history spanning the late Mesozoic and Cenozoic eras. Stage one records rapid cooling from the closure temperature of zircon FT to the high temperature part of the apatite FT partial annealing zone (∼210–110 °C) during ca. 95 to 65 Ma. Stage two records a period of relative slow cooling (∼110–60 °C) taking place between ca. 65 and 20 Ma, suggesting that the granitoids had been exhumed to the depth of ∼1−2 km. Final stage cooling (60–20 °C) occurred since the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. The maximum exhumation is more than 5 km under a steady‐state geothermal gradient of 35 °C/km. Integrated with the tectonic setting, this exhumation is possibly led by the Pacific Plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Tectonothermal history of the Schwarzwald Ore District (Germany): An apatite triple dating approach

Exceptionally large, U-rich apatite from the Schwarzwald ore district (Germany) was triple dated using ID-TIMS U–Pb, fission track (FT) and (U–Th)/He dating methods. The aim of adopting the multi-dating approach on a single sample was to (i) further constrain thermo-tectonic evolution of the Schwarzwald from the formation of crystalline basement through to cooling and exhumation, and (ii) test potential and limitations of in-situ FT–(U–Th)/He double dating. The apatite U–Pb age of 289.5 ± 4.5 Ma records the cooling of the basement through the 550 °C during the Early Permian and brackets the age of the apatite-hosting rhyolite between 289.5 ± 4.5 Ma and 314 ± 6 Ma, which is the age of rhyolite-hosting granite. This is in excellent agreement with radiometric ages of related rhyolites and fits with the magmatic evolution of the Schwarzwald during Variscan orogeny. In the Eocene between ~ 50 and 40 Ma, the basement resided at ≥ 100 °C and then cooled to surface temperatures as revealed by the FT age (37.6 ± 1.3 Ma) and thermal modelling results based on track lengths. During the cooling event, the basement was likely exhumed as a rift shoulder of newly formed Upper Rhine Graben rift. Nine apatite grains yielded (U–Th)/He ages from 30.7 ± 1.5 to 65.0 ± 2.6 Ma (mean age: 45.7 ± 1.3 Ma). Three possible reasons for the lack of reproducibility are: (i) non-homogeneous distribution of parent isotopes evidenced by strong U zoning, (ii) ‘bad neighbourhood causing implantation of extraneous He, and (iii) radiation damage leading to increased He retentivity, implied by the high ( > 250 ppm) U concentration. These problems could not be circumvented by mechanical abrasion of outermost rim of dated crystals which yielded (U–Th)/He ages from 26.0 ± 1.0 to 56.2 ± 2.2 Ma (mean age: 37.2 ± 1.4 Ma). In-situ FT and traditional (U–Th)/He double dating of single apatite crystals revealed that the vast majority of (U–Th)/He ages were younger than, indistinguishable from or overlap within error with corresponding FT ages, proving the robustness of this approach. The results also show that apparent discrepancies between FT and (U–Th)/He ages reported in the literature may arise simply as a result of statistical misconceptions and conventional data treatment. Considering the range of single grain FT ages and the oldest single grain FT age (in addition to the central FT age), may provide an effective basis on which to evaluate the viability of (U–Th)/He data.

Polyphase thrusting and dyke emplacement in the central Southern Alps (Northern Italy)

The Triassic succession of the central Southern Alps (Italy) is stacked into several units bounded by south-verging low-angle thrust faults, which are related to two successive steps of crustal shortening. The thrust surfaces are cut by high-angle extensional and strike-slip faults, which controlled the emplacement of hypabissal magmatic intrusions that post-date thrusts motions. Intrusion ages based on SHRIMP U–Pb zircon dating span between 42 ± 1 and 39 ± 1 Ma, suggesting close time relationships with the earliest Adamello intrusion stages and, more in general, with the widespread calc-alkaline magmatism described in the Southern Alps. Fission-track ages of magmatic apatites are indistinguishable from U–Pb crystallization ages of zircons, suggesting that the intrusion occurred in country rocks already exhumed above the partial annealing zone of apatite (depth < 2–4 km). These data indicate that the central Southern Alps were already structured and largely exhumed in the Middle Eocene. Although we describe minor faults affecting magmatic bodies and local reactivations of older structures, no major internal deformations have occurred in the area after the Bartonian. Neogene deformations were instead concentrated farther south, along the frontal part of the belt.

Constraining the post-emplacement evolution of the Hebridean Igneous Province (HIP) using low-temperature thermochronology: how long has the HIP been cool?

Abstract: The thermal history of the Hebridean Igneous Province has been determined through the application of low-temperature thermochronology to the four central complexes in the province. The zircon (U–Th)/He age (59.4 ± 3.3 Ma, 1σ, n = 18) and the ages from each complex (60.7 ± 2.3 Ma for Skye; 58.0 ± 0.4 Ma for Mull; 55.9 ± 3.2 Ma for Ardnamurchan; 55.6 ± 2.7 Ma for Rum) are indistinguishable from the crystallization ages. Apatite fission-track ages (61.2–57.2 Ma, mean of 59.3 ± 3.4 Ma) from the major plutonic units also overlap crystallization ages, implying that on a regional scale the Hebridean Igneous Province cooled rapidly to near-surface temperatures immediately after emplacement. However, apatite fission-track ages and track lengths and apatite (U–Th)/He ages from some small-volume intrusions in the Skye and Rum central complexes identify localized mid-Eocene (45–47 Ma) cooling. Forward and inverse modelling suggests a discrete heating–cooling event at c . 47 Ma, which may have been caused by structurally controlled localized advection of heat above shallow emplacement. This is the first suggestion of Eocene magmatism in the Hebridean Igneous Province.

Tectonic uplift history of the Yitong Basin since the Oligocene, Northeast China: Evidence from apatite fission track ages and geological relationship

The Yitong Basin is an oil-bearing basin with unique characteristics in Northeast China. On the basis of apatite fission track ages and geological relationship, the tectonic uplift history of the Yitong Basin since the Oligocene was discussed. Based on apatite fission track analysis of five samples from the Luxiang and Chaluhe fault depressions and basin modeling study, it can be concluded that since the Oligocene (36.6 Ma) in the Yitong Basin, the Chaluhe fault depression has undergone two episodes of uplift during 24.9–19.1 Ma and 6.9–4.9 Ma. And the Luxiang fault depression also had undergone two episodes of uplift during 30–27.8 Ma and 22.6–11.1 Ma. Moreover, the average apparent exhumation rates for the Chaluhe fault depression and Luxiang fault depression, could be calculated to be 70.34 and 60.33 m/Ma since 21.8 Ma and 18.9 Ma, respectively. The results of thermochronological analysis can also be supported by the evidence from geological relationships such as geodynamics, volcanic activity, and stratigraphic division and correlation.

Radiation damage and uranium concentration in zircon as assessed by Raman spectroscopy and neutron irradiation

Radiation damage of natural and synthetic zircon grains is evaluated by Raman spectroscopy to understand annealing and stability of fission tracks. Analyses focus on a suite of 338 Paleozoic detrital zircon grains from metamorphosed strata in the Hellenic forearc that were variably annealed by a Miocene thermal event, as well as a suite of 97 synthetic zircon grains. The Raman wavenumber shift of ν 3 [SiO 4 ] reveals that radiation damage and damage distribution in this suite mainly depends on uranium concentration. In zircon with similar uranium concentration, the Raman wavenumber shift allows for the determination of radiation damage in different crystals, which is a function of effective accumulation time. Nine detrital zircons grains with moderate radiation damage were stepwise annealed at 1000 and 1400 °C, which resulted in progressive removal of radiation damage revealed in an increase of ν 3 [SiO 4 ] peak positions. For a partly reset sample that was brought to temperatures of ~350 °C in a geologic setting (Hellenic forearc), we use the Raman measurements and uranium determination to estimate a Zircon Damage Discrimination Factor (Z RDD ), which is our attempt to estimate only radiation damage in single grains by accounting for affects of the uranium atom in the Raman wavenumber. This discrimination allows for a separation of zircon fission track (ZFT) ages of single ages based on grains that have a low track retention (high damage, fully reset grain), thus refining the age determination of cooling in a rock that shows variable resetting.

Integrated provenance and detrital thermochronology studies on the ANDRILL AND-2A drill core: Late Oligocene-Early Miocene exhumation of the Transantarctic Mountains (southern Victoria Land, Antarctica)

Terra Nova, 22, 361–368, 2010 Detrital apatite fission-track data combined with provenance analysis of gravel-fraction detritus are used in this study to investigate the Neogene exhumation of the West Antarctic Rift System. Samples have been collected from the upper 1000 m of the ANDRILL AND-2A drill core, which documents the Miocene history of the Victoria Land Basin. Most samples have apatites with fission-track ages between 23 and 36 Ma and gravel clasts derived from the Erebus Volcanic Province and the Transantarctic Mountains exposed along the southern Victoria Land coast. These results indicate the existence of a so far undetected Oligocene/Early Miocene exhumation phase in the region between the Blue–Koettlitz and Skelton–Mulock glaciers, suggesting the importance of fluctuation of local glaciers in the Royal Society Range and/or expansions into the McMurdo Sound of an ice sheet mainly sourced by larger outlet glaciers located further south.

Inversion of thermochronological age–elevation profiles to extract independent estimates of denudation and relief history — II: Application to the French Western Alps

Thermochronologic data collected along age–elevation profiles are commonly interpreted as recording temporally variant but spatially constant exhumation rates. However, thermochronologic age–elevation relationships are known to be perturbed by topographic effects and potential changes in relief, which are neglected in the inherently 1-D interpretation commonly applied. Such data thus potentially record both the denudation and relief history of the sampled region but extracting this information is challenging. In a companion paper, we develop a methodology for rigorously interpreting thermochronologic age–elevation profiles in terms of exhumation rates and relief development through time, and to independently quantify the resolution of these constraints. Here we test this approach using a thermochronological dataset consisting of apatite and zircon fission-track and (U–Th)/He data, collected at La Meije Peak in the Pelvoux-Ecrins massif (French Western Alps). Our data and models suggest a three-phase exhumation history in the Pelvoux–Ecrins massif, including a pulse of rapid exhumation at ∼ 6–5.5 Ma, preceded and followed by more moderate rates of denudation in the order of 0.3–0.4 km Myr − 1 . This rapid exhumation event appears to occur coevally in other external crystalline massifs in the Alps but is not detected by qualitative inspection of the age–elevation relationships. Both our synthetic results and inversion of the La Meije data strongly suggest that apatite fission-track age–elevation relationships alone cannot resolve both denudation and relief histories independently and that multiple thermochronometers are required. Combining apatite fission-track and (U–Th)/He ages and, particularly, including fission-track-length data greatly improves the resolution of the inferred exhumation histories. Although denudation rates through time and the timing of rate changes are generally well resolved, our data have insufficient resolution to satisfactorily constrain the relief history. Synthetic results reported in the companion paper suggest that the reason for this limitation is that relief increase through valley carving has been insufficient with respect to the regional denudation rates to be unambiguously extracted from the data.

Dating of shallow faults: New constraints from the AlpTransit tunnel site (Switzerland)

Research Article| June 01, 2010 Dating of shallow faults: New constraints from the AlpTransit tunnel site (Switzerland) Horst Zwingmann; Horst Zwingmann 1CSIRO Earth Science and Resource Engineering, PO Box 1130, Bentley, WA 6102, Australia, and School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Neil Mancktelow; Neil Mancktelow 2Department of Earth Sciences, ETH-Zurich, CH-8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Marco Antognini; Marco Antognini 3Museo Cantonale di Storia Naturale, Viale C. Cattaneo 4, CH-6900 Lugano, Switzerland Search for other works by this author on: GSW Google Scholar Raffaele Lucchini Raffaele Lucchini 4Geolog.ch SA, Viale Stazione 16a, CH-6501-Bellinzona, Switzerland Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (6): 487–490. https://doi.org/10.1130/G30785.1 Article history received: 16 Oct 2009 rev-recd: 21 Dec 2009 accepted: 30 Dec 2009 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Horst Zwingmann, Neil Mancktelow, Marco Antognini, Raffaele Lucchini; Dating of shallow faults: New constraints from the AlpTransit tunnel site (Switzerland). Geology 2010;; 38 (6): 487–490. doi: https://doi.org/10.1130/G30785.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Methods for dating shallow faults in the Earth's crust are still evolving and their reliability remains controversial. Based on results from well-preserved fault gouge samples from the AlpTransit deep tunnel site in Switzerland, we propose a simplified and readily applicable approach to investigate and date clay-rich gouge zones. We collected 10 samples covering a north-south section of 15 km. All sampled faults are developed in orthogneiss host rock, have a rather consistent approximately northwest-southeast strike, are mainly normal, and are related to northeast-southwest orogen-parallel extension during Alpine exhumation. Fine-grained clay separates from the fault gouges consist of illite 1M/1Md and 2M1 polytypes, smectite and chlorite, with minor amounts of feldspar in some sample fractions. The K-Ar ages for <2 μm illite fractions range between 7.1 and 9.5 Ma (mean 8.5 ± 1.4 [2σ] Ma) and for <0.1 μm range between 3.9 and 7.2 Ma (mean 6.0 ± 2.1 [2σ] Ma); ages consistently decrease with grain size. These ages are close to published apatite fission track ages of ca. 6 Ma from the immediate vicinity, consistent with illite stability in fault gouges generally occurring at temperatures slightly above the partial annealing zone of apatite. The influence on authigenic illite gouge ages of potential contamination by fine-grained cataclastic protolith was evaluated by dating coarse K-feldspar fractions from host-rock clasts in two samples. The K-feldspar ages are significantly older, ca. 13.5 Ma. However, measured illite K-Ar ages are quite constant and do not correlate with amount of K-feldspar impurity, which suggests that fine-grained cataclastic feldspar grains have isotopically reequilibrated, presumably due to fluid-rock interaction within the fault zone. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Exumação tectônica e reativação de paleolineamentos no Arco de Ponta Grossa: termocronologia por traços de fissão em apatitas

The South American Platform in southeastern Brazil is characterized by numerous dikes, shear zones, faults and fractures originated or even reactivated by several times between Upper Cretaceous and Paleogene/Neogene, and caused block tilting and normal faulting. Apatite fission-track ages (obtained in samples collected from Parana Basin, dikes, Precambrian basement and alkaline rocks in Ponta Grossa Arch) in association with geological evidences, allowed the cooling history reconstruction since approximately 60 Ma, in the central axis of Ponta Grossa Arch. Transtensional tectonic movements occurred between Paleogene and Neogene were the responsible for the reactivation of ancient shear zones and origin of southern portion of the Brazilian Southeastern Continental Rifts, represented in the study area by Curitiba basin, Guaraquecaba, Sete Barras and Cananeia Grabens, and Pariquera-Acu and Alexandra formations. In the Paleogene-Neogene limit, trantensional events related to the rearrangement of South American Platform deformed the sedimentary sections in the rift basins.

Old Crow tephra across eastern Beringia: a single cataclysmic eruption at the close of Marine Isotope Stage 6

Old Crow tephra is the largest and most widespread Quaternary eruption presently known in eastern Beringia. Its major- and trace-element geochemistry, Fe-Ti oxides, and stratigraphic and paleoecological context indicate that it is the result of a single cataclysmic eruption. The proximal region may well have experienced tephra fallout from small eruptions just prior to or after the Old Crow event, but there is no evidence to indicate that the distal area was affected. We recalculate the glass fission-track age at 124 ± 10 ka, which, coupled with stratigraphic and paleoenvironmental reconstructions, indicates that deposition occurred prior to development of the last interglacial boreal forest, which suggests a latest Marine Isotope Stage (MIS) 6 age. The bulk tephra volume erupted is estimated by three different approaches, that are in broad agreement at ∼200 km 3, but this result must be considered as tentative given the poor controls on definition of isopachs over such a large area. The source caldera, although presently unrecognized, is located in the eastern Aleutian arc, possibly at or near the Emmons Lake volcanic center.

Timing of brittle deformation within the Nojima fault zone, Japan

K–Ar ages from clay fault gouges of outcrop and core samples (UG500 well) from the Nojima fault zone, Awaji Island, Japan are consistent internally with established field constraints, and with apatite and zircon fission track ages, demonstrating the suitability of this method for dating of brittle deformation. Six unspiked and thirty conventional K–Ar data of clay minerals separated from gouge zones were analysed. Characterized by protoliths of high-grade metamorphic and magmatic rocks, the samples from the Nojima fault zone offer the opportunity to distinct identify newly grown illite in the fault planes. All dated sample fractions were characterized by XRD, SEM and TEM. K–Ar ages of 29 separates have an age range from 63.4±1.3Ma (Early Palaeocene–Danian) to 42.2±1.0 (Palaeogene–Middle Eocene–Lutetian). Some <0.1 and <0.4μm fractions of samples in close proximity to a pseudotachylyte zone are thermally influenced and indicate loss of radiogenic Ar. The K–Ar data from illite support a model that the Nojima fault zone was initiated ∼55Ma ago based on ZFTA data. The data confirm elevated temperatures and a heterogeneous thermal history within the study area. The <2μm ages of the outcropping samples document the age of gouge formation, whereas the <0.1 and <0.4μm illite fractions suggest the influence of a secondary thermal heating event probably caused by circulation of hot fluids within the fault zone about 31–38Ma ago and even a potential influence of Quaternary faulting. Ar diffusion models support elevated temperatures for outcrop samples collected near the Hirabayashi site. The thermal anomalies documented in the outcrop samples could not be observed in the UG500 core samples supporting further a heterogeneous thermal history model within the Nojima fault zone.

Inversion of thermochronological age-elevation profiles to extract independent estimates of denudation and relief history — I: Theory and conceptual model

We determine to what extent low-temperature thermochronology data, in particular from age-elevation profiles, provide independent and quantitative estimates on denudation rates and relief development. Thermochronological age-elevation profiles have been widely used to infer exhumation histories. However, their interpretation has remained inherently one-dimensional, neglecting potential effects of lateral offsets between samples. Furthermore, the potential effects of transient topography on crustal isotherms and consequently on thermochronological data have not yet been addressed in detail. We investigate this problem with the aim of deriving independent estimates of both denudation rates and relief history from low-temperature thermochronometers, measuring the relative uncertainties on these parameters and finally constraining the timing of potential variations in denudation rate and/or relief development. We adopt a non-linear inversion method combining the three-dimensional thermal-kinematic model Pecube, which predicts thermal histories and thermochronological ages from an input denudation and relief history, with an inversion scheme based on the Neighbourhood Algorithm. We use synthetic data predicted from imposed denudation and relief histories and quantitatively assess the resolution of thermochronological data collected along an age-elevation profile. Our results show that apatite fission-track (AFT) ages alone do not provide sufficient quantitative information to independently constrain denudation and relief histories. Multiple thermochronometers (apatite (U–Th)/He (AHe) ages and/or track-length measurements combined with AFT ages) are generally successful in constraining denudation rates and timing of rate changes, the optimum combination of thermochronometers varying with the input scenario (relief change or varying denudation rates). However, relief changes can only be quantified and precisely constrained from thermochronological age-elevation profiles if the rate of relief growth is at least 2–3 times higher than the background denudation rate. This limited resolution is due to the depth of the closure isotherm (between ∼ 70 and 110 °C) for the AFT and AHe systems, which only partly record topographic change. New thermochronometers (such as 4He/ 3He or OSL) that are sensitive to lower temperatures may be the key for resolving this issue.