Apatite Fission Track Results Research Articles

Post-Variscan thermal history of the Intra-Sudetic Basin (Sudetes, Bohemian Massif) based on apatite fission track analysis

The Intra-Sudetic Basin, a ~ 12 km deep Variscan intramontane basin, has the best preserved post-orogenic sedimentary record available at the NE margin of the Bohemian Massif. Apatite fission track (AFT) analyses have been performed on 16 sedimentary and volcanic samples of Carboniferous to Cretaceous age from the Intra-Sudetic Basin to improve understanding of the post-Variscan thermal evolution. AFT central ages range from 50.1 ± 8.8 to 89.1 ± 7.1 Ma (Early Eocene to Coniacian), with 13 of them being Late Cretaceous. The mean track length values range from 12.5 ± 0.4 to 13.8 ± 0.5 (except for one sample 14.4 ± 0.2) µm. This relatively short mean track length together with the unimodal track length distributions and rather low standard deviation (0.8 to 1.7 µm) in most samples indicate a long stay in the partial annealing zone during slow cooling. However, in the northern part of the Intra-Sudetic Basin, samples show a wider track length distribution (standard deviation of 1.8 to 2.1 µm) that could indicate a more complex thermal evolution possibly related to Mesozoic reheating. Vitrinite reflectance data combined with thermal models based on the AFT results indicate that the Carboniferous strata reached maximum palaeotemperatures in the latest Carboniferous to Early Permian time, corresponding to a major coalification event. The second phase of temperature rise occurred due to Late Mesozoic sedimentary burial, but it had no influence on maturation of the Carboniferous organic matter. Final cooling phase in the Late Cretaceous–Paleogene was related to tectonic inversion of the Intra-Sudetic Basin, which occurred after deposition of a significant thickness of Cenomanian–Turonian sediments. Thermal modelling demonstrates that ~ 4 km thick cover of Upper Cretaceous sediments is required to obtain a good match between our AFT data and modelled time–temperature paths. This outcome supports a significant amount of Late Cretaceous–Paleogene inversion within the Variscan belt of Central Europe.

Application of apatite fission-track analysis and zircon U-Pb geochronology to study the hydrothermal ore deposits in the Lesser Hinggan Range: Exhumation history and implications for mineral exploration

This study uses apatite fission track (AFT) and zircon U-Pb data to reveal the cooling history of bedrocks from the Dong'an, Cuihongshan and Pingdingshan deposits in the Lesser Hinggan Range, which is one of the most important metallogenic districts in NE China. AFT results and modeling of samples identified three cooling phases since late Mesozoic times. Stage one recorded rapid cooling to the low temperature part of the fission track partial annealing zone at circa 100 ± 10 °C during ca. 90–70 Ma with an average exhumation rate of ~29–58 m/Ma. Stage two was a relatively slow cooling period occurring between ~70 and 25 Ma with an average exhumation rate of ~12 m/Ma, suggesting that the Lesser Hinggan Range had been denuded to an area of low relief. Stage three was an increased cooling period took place from the Miocene to Quaternary (<25 Ma) with an average exhumation rate of ~47 m/Ma, which transported the sampled rocks to the present surface at an increased exhumation rate. Zircon U-Pb dating of magmatic rocks from the three deposits yielded Concordia ages of 188.8 ± 0.5 Ma (alkali-feldspar granite) and 115.6 ± 0.4 Ma (rhyolitic porphyry) at the Dong'an deposit, yielded Concordia ages of 195 ± 1 Ma (alkali-feldspar granite) and 200.3 ± 0.5 Ma (monzonitic granite) at the Cuihongshan deposit, and yielded Concordia ages of 259.4 ± 0.82 Ma (Biotite granit) and 225.9 ± 1.1 Ma (dioritic dyke) at the Pingdingshan deposit, which are notably older than their corresponding AFT ages. Our results, combined with other data in and/or around NE Asia (Great Hinggan, Changbai Mts, Yanji, Sikhote Alin Mts, South Korea and SW Japan), suggest micro-continental blocks in NE Asia underwent different uplifting/denudation rates since the Late Cretaceous, which indicates that the Xing'an, Songliao and Nadanhada-Sikhote-Alin accretionary complex blocks may have good potential for mineral exploration to great depths.

Thermochronological insights into reactivation of a continental shear zone in response to Equatorial Atlantic rifting (northern Ghana)

West Africa was subjected to deformation and exhumation in response to Gondwana break-up. The timing and extent of these events are recorded in the thermal history of the margin. This study reports new apatite fission track (AFT) data from Palaeoproterozoic basement along the primary NE-SW structural trend of the Bole-Nangodi shear zone in northwestern Ghana. The results display bimodality in AFT age (populations of ~210-180 Ma and ~115-105 Ma) and length distributions (populations of 12.2 ± 1.6 and 13.1 ± 1.4 µm), supported by differences in apatite chemistry (U concentrations). The bimodal AFT results and associated QTQt thermal history models provide evidence for multiple cooling phases. Late Triassic – Early Jurassic cooling is interpreted to be related with thermal relaxation after the emplacement of the Central Atlantic Magmatic Province (CAMP). Early to middle Cretaceous cooling is thought to be associated with exhumation during the Cretaceous onset of rifting between West Africa and Brazil. Late Cretaceous – Cenozoic cooling can be related with exhumation of the Ivory Coast – Ghana margin and NNW-SSE shortening through western Africa. Furthermore, our data record differential exhumation of the crust with respect to the Bole-Nangodi shear zone, preserving older (CAMP) cooling ages to the south and younger (rifting) cooling ages to the north of the shear zone, respectively. This suggests that the Palaeoproterozoic BN shear zone was reactivated during the Cretaceous as a result of deformation in the Equatorial Atlantic region of Africa.

Meso-Cenozoic tectonic evolution of the Talas-Fergana region of the Kyrgyz Tien Shan revealed by low-temperature basement and detrital thermochronology

This study provides new low-temperature thermochronometric data, mainly apatite fission track data on the basement rocks in and adjacent to the Talas-Fergana Fault, in the Kyrgyz Tien Shan in the first place. In the second place, we also present new detrital apatite fission track data on the Meso-Cenozoic sediments from fault related basins and surrounding intramontane basins. Our results confirm multi-staged Meso-Cenozoic tectonic activity, possibly induced by the accretion of the so-called Cimmerian blocks to the Eurasian margin. New evidence for this multi-staged thermo-tectonic activity is found in the data of both basement and Meso-Cenozoic sediment samples in or close to the Talas-Fergana Fault. Zircon (U–Th)/He and apatite fission track data constrain rapid Late Triassic–Early Jurassic and Late Jurassic–Early Cretaceous basement cooling in the Kyrgyz Tien Shan around 200 Ma and 130–100 Ma respectively. Detrital apatite fission track results indicate a different burial history on both sides of the Talas-Fergana Fault. The apatite fission track system of the Jurassic sediments in the Middle Tien Shan unit east of the Talas-Fergana Fault is not reset, while the Jurassic sediments in the Fergana Basin and Yarkand-Fergana Basin, west of the fault zone, are partially and in some cases even totally reset. The totally reset samples exhibit Oligocene and Miocene ages and evidence the Cenozoic reactivation of the western Kyrgyz Tien Shan as a consequence of the India-Eurasia convergence.

Cenozoic evolution of the Western Qinling Mt. Range based on thermochronologic and sedimentary records from the Wudu Basin, NE Tibetan Plateau

The Cenozoic exhumation and evolution of the NE Tibetan Plateau remain a topic of debate, although evidence of exhumation and tectonic evolution is well archived in the sediments of related synorogenic basins and in the bedrock of the provenances of these sediments. In the NE Tibetan Plateau, the intermontane Wudu Basin is situated on the southeastern edge of Western Qinling Mt. Range (hereafter, referred to as “Western Qinling”) and contains a well-preserved Neogene succession. We present a comprehensive investigation of the thermochronology and sedimentology of the Wudu Basin and use the findings to decipher the Cenozoic sequence of the exhumation and deformation of Western Qinling. Apatite fission-track results indicate that three rapid cooling events occurred: one each in the late Paleocene to early Eocene (58–45Ma), the late Eocene (38–36Ma), and the early Miocene (23–18Ma). Based on an integration of these data with provenance analysis, the first two events are interpreted as episodic exhumation of Western Qinling linked to subduction-related far-field effects on the Tibetan Plateau, and the third event represents local volcanic activity linked to the formation of the Neogene Wudu Basin and the third deformation phase of Western Qinling. The folded Neogene strata indicate that this region, likely including most of the NE Tibetan Plateau, was deformed by tectonic activity in the late Miocene. The multiple Cenozoic deformation events affecting Western Qinling imply that the NE Tibetan Plateau has experienced a punctuated evolution and exhumation history.

Geometry and Kinematic Evolution of the Hotan-Tiklik Segment of the Western Kunlun Thrust Belt: Constrained by Structural Analyses and Apatite Fission Track Thermochronology

The western Kunlun thrust belt defines the boundary between the stable Tarim Basin in the north and the intensely deformed Cenozoic Tibetan Plateau in the south. Because of its important tectonic position, understanding its tectonic evolution should have important implications for propagation of deformation from Tibet to its neighboring cratonal regions during India-Eurasia convergence. We here present new structural analyses based on field investigations and seismic reflection profiles across the Hotan-Tiklik segment of the western Kunlun thrust belt. The results indicate that the structural section crosses two major thrust zones: the Tiklik zone in the hinterland to the south and the Hotan zone in the foreland to the north. Within these, the Hotan thrust zone is thin skinned, with its deformation characterized by fault-bend folding and fault slipping along detachment layers, whereas the Tiklik thrust zone involves basement, with its deformation driven by the currently steeply dipping Tiklik fault. Results from apatite fission track thermochronology in combination with growth strata and balanced cross section indicate that the Hotan-Tiklik segment underwent two-stage deformation: (1) development of the Tiklik thrust during the late Oligocene–early Miocene and again since the mid- to late Miocene and (2) activity of the Hotan thrust since the mid- to late Miocene as a result of basinward propagation of thrusting. The balanced cross section, combined with the apatite fission track results, suggests that the Hotan-Tiklik segment contributes a total shortening magnitude of more than ca. 34 ± 6 km. Within this, ca. 4 ± 2 and ca. 23 ± 1 km of the shortenings were absorbed by the Hotan anticline and the Hotan detachment fault, respectively, both of which were related to detachment layers. This suggests that detachment layers played an efficient role in propagating deformation from the western Tibetan Plateau into the Tarim Basin.

Thermochronology quantifying exhumation history of the Wudang Complex in the South Qinling Orogenic Belt, central China

AbstractThe Wudang Complex located in the central part of South Qinling, has been inferred to be a segment of the Yangtze Craton involved in the orogen. In this study, the cooling/exhumation history of the Wudang Complex is revealed through combined published geochronology data and new apatite fission-track results. Three rapid exhumation episodes related to relevant geodynamic events have been identified. Previous40Ar–39Ar and (U–Th)/He data indicate that the most significant exhumation, induced by the collision between the North and South China Blocks, occurred fromc.237 to 220 Ma after long-term subsidence and sedimentation of the passive continental margin. The second exhumation event, related to the long-distance effect of the Pacific subduction, occurred during the period fromc.126 to 90 Ma. Following the late Cretaceous – Eocene peneplanation stage, the final late Cenozoic exhumation sincec.15 Ma may be attributed to the combined effect of the eastward growth of the Tibetan Plateau uplift and the Asian monsoon.

Tectonothermal history of an exhumed thrust‐sheet‐top basin: An example from the south Pyrenean thrust belt

AbstractThis paper presents a new balanced structural cross section of the Jaca thrust‐sheet‐top basin of the southern Pyrenees combined with paleothermometry and apatite fission track (AFT) thermochronology data. The cross section, based on field data and interpretation of industrial seismic reflection profiles, allows refinement of previous interpretations of the south directed thrust system, involving the identification of new thrust faults, and of the kinematic relationships between basement and cover thrusts from the middle Eocene to the early Miocene. AFT analysis shows a southward decrease in the level of fission track resetting, from totally reset Paleozoic rocks and lower Eocene turbidites (indicative of heating to Tmax &gt; ~120°C), to partially reset middle Eocene turbidites and no/very weak resetting in the upper Eocene‐lower Oligocene molasse (Tmax &lt; ~60°C). AFT results indicate a late Oligocene‐early Miocene cooling event throughout the Axial Zone and Jaca Basin. Paleomaximum temperatures determined by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous material reach up to ~240°C at the base of the turbidite succession. Inverse modeling of AFT and vitrinite reflectance data with the QTQt software for key samples show compatibility between vitrinite‐derived Tmax and the AFT reset level for most of the samples. However, they also suggest that the highest temperatures determined in the lowermost turbidites correspond to a thermal anomaly rather than burial heating, possibly due to fluid circulation during thrust activity. From these results, we propose a new sequential restoration of the south Pyrenean thrust system propagation and related basin evolution.

Experimental and geologic evaluation of monazite (U–Th)/He thermochronometry: Catnip Sill, Catalina Core Complex, Tucson, AZ

Monazite is a petrologically important and analytically promising target for (U–Th)/He thermochronology. Previous studies have reported highly variable He diffusion results from monazite from a single sample and demonstrated that composition can significantly affect He diffusion parameters. In this study, we performed incremental heating of single monazite grains to experimentally determine the 4He diffusion properties of reference monazite ‘554’ that occurs within a peraluminous two-mica granite from the Catnip Sill within the Catalina Core complex, Arizona. Assuming that the grain size defines the diffusion geometry, the six experiments yielded Ea values of 212 to 238±5 kJmol−1 (1σ) and Do values of 15.7 to 103 cm2s−1 with one value of 784 cm2s−1. Monazite (U–Th)/He data from five grains yielded closure temperatures of 291 to 262 °C (± c. 15 °C) and ages of 23.8–20.3(±∼1.2;2σ) Ma; the weighted mean age is 21.8±0.73 (MSWD=1.83, n=5) and the weighted mean closure temperature is 282±6°C (MSWD=0.96, n=5;1σ). We tested the accuracy of these results by comparing our monazite thermochronology data with monazite Th/Pb depth profiling results, the 40Ar/39Ar thermal history for the Catnip Sill constrained using coexisting muscovite, biotite, and K-feldspar, and published regional zircon and apatite fission track results. The monazite Th/Pb data indicate emplacement of the Catnip Sill at ∼45 Ma. The 40Ar/39Ar muscovite and biotite data indicate cooling from 460 to 350 °C from 27 to 26 Ma. K-feldspar MDD modeling suggests cooling from 360 to 240 °C from 26 to 24 Ma. Zircon fission track data indicate cooling through 250 °C between 29 and 20 Ma. Additional cooling through 110 °C is recorded by apatite fission track ages of 19–16 Ma. Because the monazite thermochronology results are reproducible and consistent with the thermal history constrained by the other chronometers, our results 1) confirm the accuracy of the 4He diffusion kinetics from monazite ‘554,’ and 2) demonstrate that monazite can be an effective (U–Th)/He thermochronometer, provided that He diffusion properties are either measured or reasonably assumed for each sample.

Partitioning of the Cretaceous Pan-Yangtze Basin in the central South China Block by exhumation of the Xuefeng Mountains during a transition from extensional to compressional tectonics?

The formation of a series of intermountain basins is likely to indicate a geodynamic transition, especially in the case of such basins within the central South China Block (CSCB). Determining whether or not these numerous intermountain basins represent a division of the Cretaceous Pan-Yangtze Basin by exhumation of Xuefeng Mountains, is key to understanding the late Mesozoic to early Cenozoic tectonics of the South China Block (SCB). Here we present apatite fission track (AFT) data and time–temperature modeling in order to reconstruct the evolution history of the Pan-Yangtze Basin. Fourteen rock samples were taken from a NE–SW-trending mountain–basin system within the CSCB, including, from west to east, the Wuling Mountains (Wuling Shan), the south and north Mayang basins, the Xuefeng Mountains (Xuefeng Shan) and the Hengyang Basin. Cretaceous lacustrine sequences are well preserved in the south and north Mayang and Hengyang basins, and sporadically crop out in the Xuefeng Mountains, whereas Paleogene piedmont proluvial–lacustrine sequences are only found in the south Mayang and Hengyang basins. AFT results indicate that the Wuling and Xuefeng mountains underwent rapid denudation post-84Ma, whereas the south and north Mayang basins were more slowly uplifted from 67 and 84Ma, respectively. Following a quiescent period from 32 to 19Ma, both the mountains and basins have been rapidly denuded since 19Ma. Both the AFT data and sedimentary facies changes suggest that the Cretaceous deposits that cover the south–north Mayang and Hengyang basins through to the Xuefeng Mountains define the Cretaceous Pan-Yangtze Basin. Integrating our results with tectonic background for the SCB, we propose that rollback subduction of the paleo-Pacific Plate produced the Pan-Yangtze Basin, which was divided into the south–north Mayang and Hengyang basins by the abrupt uplift and exhumation of the Xuefeng Mountains from 84Ma to present, apart from a period of tectonic inactivity from 32 to 19Ma. This late Late Cretaceous to Paleogene denudation resulted from movement on the Ziluo strike–slip fault, which formed due to intra-continental compression most likely associated with the Eurasia–Indian plate subduction and collision. Sinistral transpression along the Ailao Shan–Red River Fault at 34–17Ma probably transformed this compression to the extrusion of the Indochina Block, and produced the quiescent window period from 32 to 19Ma for the mountain–basin system in the CSCB. Therefore, the initiation of exhumation of the Xuefeng Mountains at 84Ma indicates a switch in tectonic regime from Cretaceous extension to late Late Cretaceous and Cenozoic compression.

Apatite fission-track evidence for the Cretaceous development of kilometer-scale relief and steady-state Tertiary topography in New England

Apatite fission track (AFT) ages in samples collected along the 5000 foot relief (1500m) exposed at Mt. Washington in the Presidential Range of New Hampshire are used to constrain the Cretaceous cooling history of this area in the northern Appalachians. Nine AFT ages for samples of the Littleton and Rangeley formations collected along this profile range in age from ~150Ma at the highest elevations of ~1900m to ~100Ma at the base (~500m). Thermal histories based on these results and on model time–temperature histories based on the distribution of fission track lengths in the higher elevations samples show three stages of cooling, from 1.5–2.0°C/m.y. (170–130Ma) to 0.2°C (130–65Ma), to ~0.6°C/m.y. (65Ma to the present). In contrast, the thermal history calculated for the lower elevations sample shows cooling delayed until ~120Ma at ~2°C/m.y. (120–100Ma), followed by monotonic cooling of ~0.6°C/m.y. from 100Ma to the present. The convergence of these histories from high, intermediate, and low elevations suggests a common cooling history independent of elevation differences of >1km. Structural/tectonic explanations for this thermal convergence are implausible, and we conclude that the most likely explanation for the common cooling history across >1km of relief is that the relief was established by the end of the Cretaceous and has persisted with steady-state topography through the Tertiary to the present. The AFT results are consistent with an earlier relief method study employing 40Ar/39Ar muscovite cooling ages. Geothermal gradients calculated from the results of both studies yield ~40°C/km suggesting that this gradient persisted throughout Permian and Mesozoic times.

Interpreting data dispersion and “inverted” dates in apatite (U–Th)/He and fission-track datasets: An example from the US midcontinent

New apatite (U–Th)/He (AHe) and apatite fission-track (AFT) data were acquired for cratonic basement samples from an 80 m span of drillcore in northeastern Kansas. The short depth interval over which the samples were collected indicates that they should have undergone thermal histories that would be indistinguishable using low temperature thermochronometry techniques. Individual AHe dates from four samples range from 99 to 464 Ma. Three samples yield dates <300 Ma that display a correlation with apatite eU (9–34 ppm) and a weaker correlation with grain size. eU concentration maps of apatites from these samples reveal low to moderate zonation in eU. Results for a fourth sample are characterized by dates >300 Ma, higher eU (39–113 ppm), and substantial data dispersion uncorrelated with eU and grain size. These apatites have strong and variable eU zonation. AFT dates for five samples range from 242 to 291 Ma. The sample with the highest eU apatites and oldest AHe dates yields the youngest AFT results. These results are “inverted”, with AHe dates distinctly older than the corresponding AFT date. We explore both the causes of data dispersion and the overall compatibility of this cratonic dataset. We find that geologically reasonable thermal histories can (1) explain the distribution of the moderate eU AHe data when accounting for the influence of radiation damage, grain size, and eU zonation on apatite He diffusivity, (2) reproduce the observed dispersion in the high eU AHe data when using a viable range of eU zonation and grain size, and (3) explain the AFT data for the same samples. The AHe and AFT data are mutually consistent, and viable thermal histories successfully predict the observed pattern of older AHe than AFT dates for the high eU apatites. Together these results suggest that appropriately accounting for the known controls on apatite He diffusivity can explain the observed dispersion and “inverted” AHe and AFT results in some thermochronometry datasets. A range of AHe dates should be especially common in cratonic data, because small differences in apatite He diffusivity are amplified by the thermal histories that typify cratonic settings. We use these results to develop some guidelines for interpreting dispersed AHe datasets. First, date–eU and date–grain size correlations should be evaluated, and if these patterns occur they can be used to better resolve the thermal history. Second, for samples that yield inexplicably large dispersion of AHe dates uncorrelated with eU and crystal size, the appropriate strategy is either to reject these samples from the suite used for thermal history interpretation or to acquire additional data to help decipher the significance of the age distribution.

Eocene to Pliocene exhumation history of the Tianshui-Huicheng region determined by Apatite fission track thermochronology: Implications for evolution of the northeastern Tibetan Plateau margin

This study presents new apatite fission-track (AFT) data from the Tianshui-Huicheng region, which is situated in the northeastern margin of the Tibetan Plateau. AFT results (including 19 detrital and 13 bedrock samples) show that deposition in the Tianshui-Huicheng basin range from early Eocene to late Miocene–Pliocene and most of the apatites derived from the Western Qinling orogenic belt, reflecting one rapid exhumation episode (at 34–29 Ma) and two phases of volcanism (at 23–18 Ma and ca. 13 Ma). The exhumation history of the Tianshui-Huicheng region is likely related to episodic tectonic activity and directly linked to the kinematics of active deformation of the Tibetan Plateau. Active processes deciphered from the detritus indicate that deformation reached the northeastern Tibetan Plateau margin as early as ca. 49 Ma, which is manifested by the initiation of the Tianshui-Huicheng basin. The 23–18 Ma event, linked to the lithosphere deformation of the Western Qinling and the Neogene basin formation along the Tianshui-Huicheng region. By modeling the bedrock samples, an 8–4 Ma event was revealed which resulted from the intense deformation of the Tianshui-Huicheng region.

Rapid extensive erosion of the North Alpine foreland basin at 5-4 Ma

An extensive low-temperature thermochronology study of the Swiss part of the North Alpine Foreland Basin has been conducted with the aim of deciphering the late Neogene basin development. Apatite fission-track (AFT) ages from wells located in the distal and weakly deformed Plateau Molasse reveal rapid, km-scale erosion with an onset in early Pliocene times. The distribution of erosion implies that there was a strong gradient in late Miocene deposition rates along the strike of the basin, with an increase towards the northeast. Additionally, renewed tectonic activity and km-scale out-of-sequence thrusting during Plio-Pleistocene times is indicated by AFT data from wells within the thrusted, proximal Subalpine Molasse. Several different mechanisms driving late Neogene basin erosion and accelerated erosional discharge from the European Alps have been considered in the literature. Based on our AFT results, we reevaluate previously published hypotheses, and suggest that a change in climate and/or drainage reorganisation coincided and possibly interacted with preexisting tectonic and geodynamic forces in the Alpine region.

Evidence for surface uplift of the Atlas Mountains and the surrounding peripheral plateaux: Combining apatite fission-track results and geomorphic indicators in the Western Moroccan Meseta (coastal Variscan Paleozoic basement)

This work represents an initial attempt to link the evolution of the topography in relation to the general tectonic framework of western Morocco. For this purpose, in a section of the Western Moroccan Meseta different tools are combined in order to attain the general objective. Apatite fission-track (AFT) data of granitic rocks of the Rabat–Khenifra area give ages around 200 Ma with track length distributions which are compatible with the thermal models already established for the area. An inverse correlation between AFT ages and elevation is observed which is compatible with previous models indicating northward tilting of the whole Western Moroccan Meseta which is younger than 20–25 Ma. In order to test this possibility a detailed analysis of the topography at different scales in the Western Moroccan Meseta has been performed. Results indicate that two open folds with different amplitudes are recognized and that the one with wider wavelength could correspond to a lithospheric fold as previously stated by other authors on the basis of independent geological arguments. The northward tilting proposed based on the AFT data agrees with the results obtained in the analysis of the topography which reinforces the presence of a very open fold with a wavelength of 200–300 km in the north-western limb of the Western Moroccan Meseta.

A new perspective on the significance of the Ranotsara shear zone in Madagascar

The Ranotsara shear zone in Madagascar has been considered in previous studies to be a >350-km-long, intracrustal strike-slip shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent NW–SE trending brittle faulting along most of its length. We therefore prefer the more neutral term “Ranotsara Zone”. Lithologies, tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of E–W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550–520 Ma). The Ranotsara Zone shows significant NW–SE striking brittle faulting that reactivates part of the NW–SE striking ductile structures in the flexure zone, but also extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern Madagascar. It can therefore not be used as a piercing point in Gondwana reconstructions.

Fission-track thermochronology of the Oman Mountains continental windows, and current problems of tectonic interpretation

AbstractThe Oman Mountain belt offers a unique case study of an Alpine orogen where blueschist- and eclogite-facies metamorphism developed in continental rocks beneath an ophiolite in the absence of continental collision. We present hereafter the first systematic, fission-track (FT) study of the subophiolitic Jebel Akhdar and Saih Hatat windows in the central-southern Oman Mountains. Nineteen samples out of 28 yielded significant zircon and/or apatite FT results. All of the 9 samples from NE (internal) Saih Hatat, where metamorphism occurred at ca. 300°C &amp;lt; T &amp;lt; 550°C (low-grade blueschist to low-grade eclogite facies), yielded FT zircon ages at 66-70 Ma. Four samples from SW Saih Hatat (front of the Fe-Mg carpholite zone, ca. 280°C &amp;lt; T &amp;lt; 300°C) yielded 69-93 Ma dates, and 4 samples from Jebel Akhdar (ca. 250°C &amp;lt; T &amp;lt; 280°C) yielded 80-96 Ma dates, which reflects partial resetting of the zircon FT chronometre at T ≤ ca. 280°C. As eclogite metamorphism of the deepest Saih Hatat units is dated at ca. 80 Ma [Warren et al., 2003], we deduce an early cooling rate of 30°C/Ma for these units. Late thermal evolution is constrained by apatite FT length optimization analysis and includes, i) cooling under 60°C until 60-58 Ma (rate close to 10°C/Ma); and ii) heating over 80°C at ca. 20-8 Ma.The Oman Mountains tectonic building is currently accounted for by contrasting models involving one or two subductions. Taking into account the structural and geophysical data on the Saih Hatat metamorphic pile, we interpret the reported thermal evolution in the framework of the classical, one subduction model: i) Permian rifting and Mesozoic spreading; ii) late Cretaceous intra-oceanic subduction followed by continental margin subduction/ophiolite obduction; iii) late Late Cretaceous-Paleocene synorogenic exhumation of the subducted units through extrusion within the subduction channel (relative extension), then isostatic rebound and extensional inversion of the obduction thrust; and iv) Neogene compression resulting in rejuvenation of the basal thrust beneath the continental windows, which are considered to be parautochthonous.

Slow Senonian and fast Palaeocene–Early Eocene uplift of the granitoids in the Central Eastern Pontides, Turkey: apatite fission-track results

Dating and forward modelling of the fission-track data of apatite samples from the Dereli–Şebinkarahisar region, south of Giresun in the Eastern Turkish Pontides, provides quantitative data on the regional tectonics resulting from the closure of neo-Thetys and the collision of Eurasia and Gondwana. The age vs. elevation profiles identified Senonian (80.7±3.2 to 62.4±2.5 Ma) slow uplift and denudation, interpreted as the result of the diapiric ascent of subduction-related plutons above the neo-Tethyan subduction zone beneath the Eurasian continent. This was followed by rapid differential uplift during the Palaeocene–Early Eocene (57.4±2.4 to 47.8±2.4 Ma), which juxtaposed granitoid units of different ages, compositions, and emplacement levels in the crust, and is thought to be related to the collision between the Pontide (Eurasian) and Anatolide (Gondwana) basements. The modelling results must be interpreted with caution, but appear to indicate a period of Mio-Pliocene (ca. 5 Ma) reheating related to volcanism associated with the westward escape of the Anatolian plate and uplift from the Pliocene (ca. 3.5 Ma) up to the present.

Evidence for post-early Eocene tectonic activity in southeastern Ireland

The role played by Cenozoic deformation in denudation and landscape development in Ireland has historically been difficult to assess because of the lack of widespread pre-glacial Cenozoic deposits onshore. Here we combine analysis of apatite fission-track data and geomorphic observations to place constraints on the timing, kinematics and magnitude of onshore deformation in southeastern Ireland. Relationships between apatite fission-track central age and elevation for samples from the Wicklow and Blackstairs Mountains and Tullow Lowland suggest that these rocks record an exhumed apatite partial annealing zone, which after cooling was dismembered by differential vertical displacements of up to several hundred metres. We use inverted models of sample thermal history to show that samples across the region experienced very similar thermal histories up to and including a cooling event in late Paleocene or early Eocene time. This effectively rules out strongly spatially heterogeneous denudation, and implies that differential rock uplift occurred in post-early Eocene time. The central age–elevation relationships define at least three spatial domains with internally consistent apatite fission-track data, separated by known faults or topographic escarpments. Geomorphic analysis of these structures shows that patterns of catchment incision and sinuosity, as well as the presence of antecedent drainage, are best explained by differential vertical displacements at or near the domain boundaries. The kinematics and magnitudes of these displacements are consistent with those implied by the apatite fission-track results, and are compatible with other examples of known Cenozoic deformation from Ireland and the adjacent continental margin.

Low‐temperature thermochronology of the Mt Painter Province, South Australia

Apatite fission track results are reported for 26 outcrop samples from the Mt Painter Inlier, Mt Babbage Inlier and adjacent Neoproterozoic rocks of the northwestern Curnamona Craton of South Australia. Forward modelling of the data indicates that the province experienced variable regional cooling from temperatures >110°C during the Late Palaeozoic (Late Carboniferous to Early Permian). The timing of this cooling is similar to that previously reported from elsewhere in the Adelaide Fold Belt and the Curnamona Craton, suggesting that the entire region underwent extensive Late Palaeozoic cooling most likely related to the waning stages of the Alice Springs or Kanimblan Orogenies. Results from the Paralana Fault Zone indicate that the eastern margin of the Mt Painter Inlier experienced a second episode of cooling (∼40–60°C) during the Paleocene to Eocene. The entire region also experienced significant cooling (less than ∼40°C) during the Late Cretaceous to Palaeogene in response to unroofing and/or a decrease in geothermal gradient. Regional cooling/erosion during this time is supported by: geomorphological and geophysical evidence indicating Tertiary exhumation of at least 1 km; Eocene sedimentation initiated in basins adjacent to the Flinders and Mt Lofty Ranges sections of the Adelaide Fold Belt; and Late Cretaceous ‐ Early Tertiary cooling previously reported from apatite fission track studies in the Willyama Inliers and the southern Adelaide Fold Belt. Late Cretaceous to Palaeogene cooling is probably related to a change in stress field propagated throughout the Australian Plate, and driven by the initiation of sea‐floor spreading in the Tasman Sea in the Late Cretaceous and the Eocene global plate reorganisation.