Fission Track Length Research Articles

Fission-track evidence of tectonic evolution in the northwestern Qaidam Basin, China

Fission-track dating was conducted on zircons and apatites from 11 cores of the upper Xiaganchaigou Formation and lower Shangganchaigou Formation (northwestern Qaidam Basin). The obtained apatite fission-track age is 3.1–61.9 Ma, and the zircon fission-track age is 49.2–123.5 Ma. Although the average apatite age is consistent with ages predicted from the stratigraphy, nine of the 11 apatite fission-track ages have \(\hbox {P}(\upchi ^{2}) < 5\%\), indicating that the grains experienced heterogeneous annealing after sedimentation. The average zircon age is greater than that indicated by stratigraphy, and all eight zircon fission ages have \(\hbox {P}(\upchi ^{2})>5\%\), exhibiting consistent characteristics and indicating that zircons retain provenance age information after burial. From the zircon and apatite ages, the fission-track length distribution, and the geological setting, the northwestern Qaidam Basin has experienced two tectonothermal events since the Late Mesozoic, at \(39.1 \pm 9.3\) to \(133.7\,\pm \,6.6\,\hbox {Ma}\) and \(1.2 \pm 0.6\) to \(32.0\,\pm \,3.0\,\hbox {Ma}\). The earlier (39.1–133.7 Ma) tectonothermal event resulted from the initial collision of the Indian and Eurasian plates. As a consequence of the collision, the Altyn Tagh fault, which forms the northwestern boundary of the Qaidam Basin, began to develop. Subsequently, uplift of the Altyn Tagh mountains began and the northwestern depression of the Qaidam Basin started to form. The later (1.2–32.0 Ma) tectonothermal event resulted from further collision of the Indian and Eurasian plates along the Yarlung Tsangpo suture zone. Strata in the Qaidam Basin were further deformed by transpression in this period and this period played a crucial role in petroleum accumulation.

Timing of exhumation and deformation across the Taimyr fold–thrust belt: insights from apatite fission track dating and balanced cross-sections

Abstract The exhumation and shortening history associated with the Taimyr fold–thrust belt is determined using apatite fission track and balanced cross-section analysis. Eighteen samples from across northern, central and southern Taimyr are used for apatite fission track analysis. These include granite, meta-arenite and sandstone samples with stratigraphic ages ranging from the late Proterozoic to Early Cretaceous. Fission track lengths and central ages are used to model the thermal history of the region and indicate three episodes of cooling in the Early Permian, earliest Triassic and Late Triassic. The thermochronological data are integrated with two balanced regional cross-sections. The regional structural style of deformation reflects a thick-skinned thrust system with 15% shortening (minimum estimate). This is consistent with thickening during early Permian Uralian orogenesis, followed by later heating, uplift and cooling associated with Siberian Trap magmatism and/or Mesozoic transpression.

Tracking the multi-stage exhumation history of the western Chinese Tianshan by apatite fission track (AFT) dating: Implication for the preservation of epithermal deposits in the ancient orogenic belt

The western Chinese Tianshan, located in the southern domain of the Central Asian Orogenic Belt (CAOB), was originally established by multiple accretion-collision processes in the Paleozoic, and was modified by complicated intracontinental tectonic evolution in the Mesozoic to Cenozoic. Understanding the timing and mechanism of the latter geological processes is critical to unravel the preservation conditions of the epithermal deposits in the western Chinese Tianshan. This work presents new apatite fission track (AFT) data for three mountain ranges (Keguqinshan, Wusunshan and Nalatishan) of the western Chinese Tianshan to track their exhumation history. Our AFT data gave a wide range of ages from 76.8±5.5Ma to 182.3±9.9Ma, and the mean confined fission track lengths are between 9.8±0.5μm and 12.3±0.2μm. The new data, in combination with the thermal history modeling results, allow us to summary three main episodes of exhumation in the western Chinese Tianshan, including the Early Permian (300–280Ma), Late Triassic–Late Jurassic (230–140Ma), and Late Oligocene–Early Miocene (30–20Ma). The first episode may have been caused by terrane accretion-collision in the late Paleozoic, leading to uplift of the Keguqinshan that supplied abundant sediments to form a critical cover of ca. 4km thick on these epithermal deposits. The second episode was likely related to closure of the Mongol-Okhotsk Ocean in the Mesozoic, and an extremely slow exhumation rate (ca. 0.0083mm/y) as well as a moderate erosion of ca. 0.8km thick are estimated. The last episode is regarded to be the result of collision between the Indian Plate and the Eurasia Plate in the Cenozoic, and this significant tectonic event triggered rapid exhumation (up to ca. 0.1mm/y) of the whole region in the western Chinese Tianshan, which removed more than ca. 3.2km pre-Cenozoic sedimentary cover and sent the epithermal deposits to the subsurface level.

CENOZOIC TECTONIC EVOLUTION OF THE EASTERN ALPS – A RECONSTRUCTION BASED ON 40AR/39AR WHITE MICA, ZIRCON AND APATITE FISSION TRACK, AND APATITE (U/Th)-He THERMOCHRONOLOGY

The Cenozoic tectonic evolution of the Eastern Alps is defined by nappe assembly within the Penninic and Subpenninic units and their subsequent exhumation. The units above, however, are affected by extension and related faulting. By applying distinct thermochronological methods with closure temperatures ranging from ~450° to ~40°C we reveal the thermochronological evolution of the eastern part of the Eastern Alps. 40Ar/39Ar dating on white mica, zircon and apatite fission track, and apatite U/Th-He thermochronology were carried out within distinct tectonic units (Penninic vs. Austroalpine) and on host rocks and fault- related rocks (cataclasites and fault gouges) along major fault zones. We use particularly the ability of fission tracks to record the thermal history as a measure of heat transfer in fault zones, causing measurable changes of fission track ages and track lengths. Additionally, these studies will provide a general cooling and exhumation history of fault zones and adjacentcrustal blocks.

Exhumation history of the Jiaodong and its adjacent areas since the Late Cretaceous: Constraints from low temperature thermochronology

The thermal history of the Jiaodong region and adjacent provinces (Shandong and northern Jiangsu) have been extensively studied, particularly by apatite fission track (AFT) dating. However, the AFT ages from surface outcrops range broadly and do not show an apparent relationship between age and elevation. This work provides a multiple low temperature thermochronological dataset including zircon and apatite (U-Th)/He ages (ZHe and AHe), and AFT ages from a 1000-m-deep borehole at the Jiaojia goldfield in the northwest of Jiaodong Peninsula. ZHe, AFT and AHe ages range from ~100–70, ~85–50 and ~65–50 Ma, respectively. These data conform to the principles of age vs. closure temperature and age vs. elevation and thus can be employed to estimate the exhumation history. Based on the density histogram of fission track length calculation, thermal history modeling, and previously published AFT ages from the Chinese Continental Science Drill program, this work concludes that compared to the AFT ages from surface outcrops, the low temperature thermochronological ages from the boreholes show a better relationship between age, elevation and closure temperature, and the age becomes younger with increasing depth. In addition, the exhumation history in the Jiaodong and adjacent areas can be divided into two distinct stages: a short, rapid tectonic exhumation (~100–95 Ma) and a long, slow exhumation since 95 Ma. The rate and amount of tectonic exhumation since 95 Ma are inferred as ~30 m Ma - 1 and ~3 km, respectively.

Thermochronologic constrains on the processes of formation and exhumation of the Xinli orogenic gold deposit, Jiaodong Peninsula, eastern China

The Xinli orogenic gold deposit, with gold resources of 40t, located in the northwestern part of the giant Jiaodong gold province, eastern China, is controlled by the Sanshandao Fault and is one of a few deposits hosted by the Early Cretaceous ~129–128Ma Guojialing granitoid. Soon after intrusion, the granitoid underwent ductile shearing at >400–500°C marked by recrystallized quartz ribbons and bending of plagioclase lamellae. With rapid cooling from zircon crystallization temperature of ~750–800°C at ~129–128Ma (U–Pb) to closure temperature of 300±50°C for biotite (40Ar/39Ar method) at 124±1Ma, the ductile deformation lasted <4 million years. Mineralization was associated with subsequent brittle reactivation of normal movement on the fault, indicated by a SE plunging, downdip lineation on the fault. A hydrothermal sericite 40Ar/39Ar age of ~121Ma, inferred to date the alteration associated with gold mineralization, has been previously obtained for the adjoining Cangshang gold deposit also controlled by the Sanshandao Fault. At the Xinli deposit, the 121.5±1.3Ma and 120.5±1.2Ma 40Ar/39Ar plateau ages of weakly-fractured igneous K-feldspar record closely the time of normal faulting and cooling within the range of mineralization temperatures (350–250°C).Two zircon fission-track (ZFT) ages of 91±4Ma and 90±3Ma constrain the time of cooling through ~240±50°C. Unimodal distribution of apatite fission-track (AFT) lengths with a slightly negative skew and mean fission-track lengths of 12.3±0.2μm indicate relative slow continuous cooling through 125–60°C at 60±6Ma. The slight acceleration of cooling around ~90Ma constrained by the 40Ar/39Ar, ZFT and AFT data, and thermal modelling may have resulted from the late normal reactivation of the Sanshandao Fault. In summary, extension and normal faulting not only created the channelways for the mineralizing fluids, but also gave rise to the subdued topography and post-mineralization exhumation that preserved the deposit.

複数の熱年代学的手法に基づいた江若花崗岩敦賀岩体の冷却・削剥史

In this study, multi-system thermochronology, i.e., fission-track (FT), K-Ar and U-Pb methods are used to identify the cooling and denudation history of the Tsuruga body of Kojaku granite, southwest Japan. Apatite FT age of 51.8 ± 6.5 Ma, zircon FT age of 70.4 ± 2.0 Ma, biotite K-Ar ages of 66.7–62.0 Ma, and zircon U-Pb age of 68.5 ± 0.7 Ma were obtained for granitic samples, whereas plagioclase K-Ar ages of 19.1–18.8 Ma and whole-rock K-Ar age of 19.0 ± 2.9 Ma were inferred for the basaltic dyke intruding into the granite. The zircon FT lengths are not significantly shorter than their initial lengths, implying rapid cooling at the zircon FT partial annealing zone (PAZ). On the other hand, the apatite FT length distribution shows a typical pattern for granitic pluton without reheating, indicating a slow cooling history at the apatite FT PAZ. Based on the results of these thermochronometric analyses, inverse thermal calculations using the FT data, and simple thermal conduction modeling of the granitic body, the cooling and denudation histories of the Tsuruga body are reconstructed: (1) the Tsuruga body intruded at ca. 68 Ma, late Cretaceous, at a depth of several kilometers, (2) rapidly cooled to below the zircon FT PAZ by heat conduction within a few million years or less, and (3) slowly cooled due to peneplanation during the past 50–60 million years. On the other hand, the whole-rock Rb-Sr age previously reported for the Kojaku body is younger than when the cooling curve of the Tsuruga body obtained by this study intersects with the closure temperature of the whole-rock Rb-Sr system. This may imply a time lag between the formation ages of these bodies, but more thermochronometric studies are required to draw a definitive conclusion. The K-Ar ages of the basaltic dyke are interpreted as its formation age, indicating that dyke intrusion was associated with the Green Tuff movement.

Fission track and U–Pb zircon ages of psammitic rocks from the Harushinai unit, Kamuikotan metamorphic rocks, central Hokkaido, Japan: constraints on metamorphic histories

AbstractAccurate pressure–temperature–time (P–T–t) paths of rocks from sedimentation through maximum burial to exhumation are needed to determine the processes and mechanisms that form high‐pressure and low‐temperature type metamorphic rocks. Here, we present a new method combining laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb with fission track (FT) dates for detrital zircons from two psammitic rock samples collected from the Harushinai unit of the Kamuikotan metamorphic rocks. The concordant zircon U–Pb ages for these samples vary markedly, from 1980 to 95 Ma, with the youngest age clusters in both samples yielding Albian‐Cenomanian weighted mean ages of 100.8 ± 1.1 and 99.3 ± 1.0 Ma (2σ uncertainties). The zircon U–Pb ages were not reset by high‐P/T type metamorphism, because there is no indication of overgrowth within the zircons with igneous oscillatory zoning. Therefore, these weighted mean ages are indicative of the maximum age of deposition of protolithic material. By comparison, the zircon FT data yield a pooled age of ca. 90 Ma, which is almost the same as the weighted mean age of the youngest U–Pb age cluster. This indicates that the zircon FT ages were reset at ca. 90 Ma while still at their source, but have not been reset since. This conclusion is supported by recorded temperature conditions of less than about 300 °C (the closure temperature of zircon FTs), as estimated from microstructures in the deformed detrital quartz grains in psammitic rocks, and no shortening of fission track lengths in the zircon. Combining these new data with previously reported white mica K–Ar ages indicates that the Harushinai unit was deposited after ca. 100 Ma, and underwent burial to its maximum depth before being subjected to a localized thermal overprint during exhumation at ca. 58 Ma.

Long-term exhumation history of the Inner Mongolian Plateau constrained by apatite fission track analysis

The Inner Mongolian Plateau, along the southeastern flank of the wider Mongolian Plateau, is a vast undulating surface ranging in elevation between 900 and 1500m above sea level. The peculiar topography of this area is assumed to be closely related to its complex tectono-thermal evolution since Late Paleozoic. The lithospheric structure of the Plateau includes three continental blocks: the Mandula and the Bart Obo blocks form the southern margin of the Central Asian Orogenic Belt in that area, and to the south, the Plateau includes the northern margin of the North China Craton. Apatite fission track (AFT) ages and track length distributions from 13 basement outcrops situated in the main tectonic blocks forming the Inner Mongolian Plateau were determined in order to reconstruct its denudation history. The thermal histories inferred from these data imply multi-phased, differential exhumation/burying processes from the Late Paleozoic to the Early Cretaceous. This complex thermal history is largely related to the Early/Middle Triassic closure of the Paleo-Asian Ocean, the Jurassic closure of the Mongol–Okhotsk Ocean, and the Early Cretaceous orogenic collapse of the Mongol–Okhotsk belt. Finally, since Late Cretaceous, no further major tectonic movement occurred and the Inner Mongolian Plateau has been largely peneplained.

A reporting protocol for thermochronologic modeling illustrated with data from the Grand Canyon

Apatite (U–Th)/He and fission-track dates, as well as 4He/3He and fission-track length data, provide rich thermal history information. However, numerous choices and assumptions are required on the long road from raw data and observations to potentially complex geologic interpretations. This paper outlines a conceptual framework for this path, with the aim of promoting a broader understanding of how thermochronologic conclusions are derived. The tiered structure consists of thermal history model inputs at Level 1, thermal history model outputs at Level 2, and geologic interpretations at Level 3. Because inverse thermal history modeling is at the heart of converting thermochronologic data to interpretation, for others to evaluate and reproduce conclusions derived from thermochronologic results it is necessary to publish all data required for modeling, report all model inputs, and clearly and completely depict model outputs. Here we suggest a generalized template for a model input table with which to arrange, report and explain the choice of inputs to thermal history models. Model inputs include the thermochronologic data, additional geologic information, and system- and model-specific parameters. As an example we show how the origin of discrepant thermochronologic interpretations in the Grand Canyon can be better understood by using this disciplined approach.

Inter-laboratory comparison of fission track confined length and etch figure measurements in apatite

Apatite fission-track length and etch figure data are powerful tools for obtaining thermal history information, but both require human analysts making manual measurements and reproducibility is not assured. We report the results of an inter-laboratory study designed to clarify areas of congruence and divergence for these measurements and provide a basis for evaluating best practices to enhance intercompatibility of data sets. Four samples of megacrystic apatite from Durango, Mexico, with induced tracks, one unnannealed and three thermally annealed by varying amounts, were distributed internationally. In all, 55 analysts in 30 laboratory groups participated in the experiment. Relative mean track lengths among the samples were consistent across all analysts, but measurements for each sample showed scatter among labs and analysts considerably in excess of statistical expectation. Normalizing measurements of annealed samples using the unannealed sample improved consistency, as did normalizing for track angle using c-axis projection. Etch figure data also showed variability beyond statistical expectation, and consistency was improved by normalizing. Based on these data we recommend rigorous analyst training for length and etch figure measurement that includes measurement of standards, and that each analyst’s data on unknowns be normalized by that analyst’s own measurements on standards when using thermal history inverse modeling as part of the interpretation process.

Tectonothermal history of the NE Jiangshan–Shaoxing suture zone: Evidence from 40Ar/39Ar and fission-track thermochronology in the Chencai region

The Jiangshan–Shaoxing suture zone, a component of the Neoprotozoic orogenic belt between the Yangtze and Cathaysia blocks in South China, has experienced a complex history of tectonic reactivation. Previous investigations of this significant tectonic boundary have focused on earlier stages of its tectonic history pre-dating the Early Paleozoic Wuyi–Yunkai orogeny, with more recent episodes rarely addressed. We here improve understanding of the later reactivation history of this suture zone through new 40Ar/39Ar and fission track thermochronological constraint derived for the ductilely strained Chencai Complex and the Lipu–Huangshan quartz diorite, together with sedimentological investigations of the proximal Wuzao Formation.This combined constraint demonstrates the occurrence of as many as five discrete episodes of tectonic reactivation and deformation of the Jiangshan–Shaoxing suture zone. The 431±4Ma 40Ar/39Ar biotite cooling age of the Lipu–Huangshan quartz diorite records retrograde cooling of the Chencai region following high-grade metamorphism at c. 460–450Ma during the Early Paleozoic Wuyi–Yunkai orogeny. Consistent younger 40Ar/39Ar ages of the gneissic Chencai Complex suggest that this retrograde cooling episode may extend to c. 390–400Ma, with later ductile strain and/or thermal overprinting of biotite at c. 386Ma suggesting that the terminal stage of the Early Paleozoic Wuyi–Yunkai orogeny may have continued significantly later than previously assumed. Although not reflected directly in the thermotectonic records compiled for the Chencai Complex, lithoclasts of these distinctive ductilely deformed gneisses represent a significant component of the conglomeratic gravels appearing in the late Triassic Wuzao Formation, demonstrating substantial uplift and erosion of the suture zone at this time, coincident with the Indosinian orogeny. Zircon fission track thermochronology of the samples from both Lipu–Huangshan quartz diorite and Chencai Complex yield much younger Cretaceous cooling ages of 144±8 to 103±6Ma, with apatite fission track ages ranging from 66±4 to 41±3Ma and mean apatite fission track lengths between 12.1 and 13.1μm. Thermal history modeling of these low-temperature controls demonstrates significant cooling during the Cenozoic. These later activities reflect brittle deformation in the upper crust and erosional exhumation of these rocks coincident with the Yanshanian extension event and Himalayan tectonism.These results together imply that the Jiangshan–Shaoxing suture zone represents a significant and long-lived lithospheric weakness that was preferentially reactivated during at least two episodes of tectonic activity in regional far-field domains. Examining the complex reactivation history of the fossil suture zone thus provides a useful window into the timing of episodes of broader tectonic activity across the wider region.

Low‐temperature thermochronologic record of Eocene migmatite dome emplacement and late Cenozoic landscape development, Shuswap core complex, British Columbia

AbstractExhumed mid‐to‐lower crustal rocks offer an opportunity to determine the mechanisms, conditions, timing, and consequences of the ascent of hot rocks from deep to shallow crustal levels. We used results of low‐T thermochronology (zircon and apatite (U‐Th)/He, apatite fission track) to document the very shallow emplacement (&lt;2 km) of high‐grade metamorphic rocks and to determine the timing and rates of Cenozoic cooling, exhumation, and subsequent incision of the Thor‐Odin migmatite dome of the Shuswap metamorphic core complex, British Columbia (Canada). Samples collected at high elevation in the dome (&gt;1800 m) have preserved Eocene fission‐track ages and evidence of rapid cooling (≥60°C/Myr). This Eocene cooling event corresponds to rapid exhumation by upward flow of partially molten crust and final exhumation by detachment faulting. Samples collected below 1800 m in elevation display a wide range of apatite fission track ages (43–15 Ma) and track length distributions that reflect prolonged residence in the apatite partial annealing zone. These age‐elevation relations imply that the dome rocks reached the near surface (&lt;2 km) during initial upward flow and tectonic exhumation in the Eocene and that little erosion of the Eocene surface has occurred since that time. Thermal modeling of the lowest elevation samples (≤ ~600 m) and intrasample apatite (U‐Th)/He age variations reveal enhanced erosion and relief production at the onset of continental glaciations at ~3 Ma. Our work illustrates the dynamic links between deep and shallow crustal processes and the evolution of topography in a deeply incised hot orogen.

Mesozoic and Cenozoic uplift and exhumation of the Bogda Mountain, NW China: Evidence from apatite fission track analysis

Apatite fission track (AFT) analysis on samples collected from a Paleozoic series is used to constrain the cooling history of the Bogda Mountain, northwest China. AFT ages range from 136.2 to 85.6 Ma and are younger than rock depositional ages and the mean confined track lengths (11.0–13.2 μm) mostly showing unimodal distribution are shorten, indicating significant track-annealing. Thermal histories modeling based on the distribution of fission-track lengths combined with the regional geological data show that two rapid cooling phases occurred in the latest Jurassic–early Cretaceous and the Oligocene–Miocene. Those new data together with previous published data show that the AFT ages become younger from the southwest to northeast in the western Bogda Mountain and its adjacent areas. The fission-track ages of the southwest area are relatively older (>100 Ma), recording the earlier rapid uplift phase during the late Jurassic–Cretaceous, while the ages in the north piedmont of the Bogda Mountain (namely the northeast part) are younger (<60 Ma), mainly reflecting the later rapid uplift phase in the Oligocene–Miocene. The trend of younger AFT ages towards the northeast might be explained by post-Cretaceous large-scale crustal tilting towards the southwest. In the thrust fault-dominated northern limbs of the Bogda Mountain, AFT ages reveal a discontinuous pattern with age-jumps across the major fault zones, showing a possible strata tilting across each thrust faults due to the thrust ramps during the Cenozoic. The two rapid uplift stages might be related to the accretion and collision in the southern margin of the Asian continent during the late Jurassic and late Cenozoic, respectively.

Timing of coalification of the upper carboniferous sediments in the upper silesia coal basin on the basis of by apatite fission track and helium dating

Streszczenie Przeprowadzono datowania za pomocą metody trakowej i helowej dla apatytów z utworów karbońskich w Górnośląskim Zagłębiu Węglowym w celu określenia ram czasowych procesów uwęglcnia. Pomierzone centralne wieki trakowe apatytów mieszczą się w przedziale od 259±11 (późny perm) do 103±6 milionów lat (wczesna kreda), a średnia długość traków waha się od 11,7±0,2 do 13,7±0,1 (im. Wszystkie wieki trakowe są młodsze od wieku stratygraficznego analizowanych próbek, wskazując znaczne zaawansowanie procesów diagcnctycznych. Próbki z zachodniej i środ- kowej części GZW mają wieki trakowe od późnego permu do środkowego/późnego triasu (259±11 do 214±10 min lat). Jcdnomodalnc rozkłady długości traków i ich średnie wartości wskazują na poje- dyncze, względnie szybkie zdarzenie postwaryscyjskicgo wychładzania do temperatury poniżej 60°C, co jest zgodne zc znaczną erozją postinwersyjną utworów gómokarbońskich po fazie asturyjskicj. W pozostałej części mczozoiku następowało wolniejsze wychładzanie. Próbki zc wschodniej i NE części GZW mają wieki trakowe od późnego triasu do wczesnej kredy (210±10 do 103±6 milionów lat). Charakteryzuje je względnie krótsza średnia długość traków i wyższe odchylenia standardowe, a także w przypadku części próbek bimodalny i/lub mieszany charakter rozkładów długości. Jest to razem wskazówką bardziej złożonej historii termicznej, z długim okresem przebywania w PAZ i możliwym drugim zdarzeniem termicznym. Wieki helowe apatytów w całym basenie są wczcs- nokredowc(144,l±l 1 do 108,1±8 milionów lat), wskazując raczej na wolne postwaryscyjskic wychła- dzanie lub możliwe mczozoicznc podgrzanie karbonu do temperatury nic większej niż 60-70°C, które spowodowało częściową dyfuzję helu i odmłodzenie wieków helowych, ale równocześnie nic spo- wodowało znaczącego zabliźniania traków na większości obszaru GZW. Jedynie w NE części GZW podgrzanie mczozoicznc mogło być nieco wyższe, do temperatury 70-85°C, powodując odmłodzenie wieków trakowych, zwłaszcza przy długim okresie przebywania w PAZ. Mczozoiczny impuls ter- miczny był przypuszczalnie spowodowany cyrkulacją gorących roztworów związaną z procesami ekstensji. Powyższe zakresy temperatur i czas ich występowania świadczą, żc uwęglenie materii organicznej w GZW nastąpiło zasadniczo z końcem okresu karbońskiego.

Exhumation history and faulting activity of the southern segment of the Longmen Shan, eastern Tibet

The Longmen Shan (LMS), which constitutes the eastern border of the Tibetan Plateau, is about 400km in length and characterized by a steep topographic transition from the Sichuan Basin to the plateau. The 2008 Mw7.9 Wenchuan earthquake and 2013 Mw6.6 Lushan earthquake were associated with the central to northern segments and southern segment of the LMS fault belt, respectively. In this paper, zircon and apatite fission track (ZFT and AFT, respectively) dating in combination with previously published low temperature thermochronology studies are used to constrain both the exhumation history and fault activity along the LMS, with a special focus on the southern segment. In the southern segment of the LMS, the ZFT ages in the hanging wall of the Wulong-Yanjing fault 10–14Ma, increasing to ca. 30Ma to the northwest of the faults and to 100–200Ma in the plateau region. The AFT ages are 3–5Ma at the mountain front and increase to 8–26Ma in the plateau. We show that these age distributions are controlled by fault geometry. Two stages of rapid exhumation were identified using apatite fission track length modeling and the age distributions in the southern segment of the LMS. The first stage is from ca. 30Ma and the second stage is from 3–5Ma to present. In contrast with the middle segment of the LMS, the Cenozoic exhumation rate is higher in the southern segment of the LMS, which may be due to the influence of the collision between the India and Eurasia plates and/or different faulting mechanisms in the different segments.

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

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

Geological framework and fission track dating of pseudotachylyte of the Atotsugawa Fault, Magawa area, central Japan

This paper describes newly discovered pseudotachylyte along the Atotsugawa Fault at the Magawa outcrop, where this fault divides Quaternary deposits in the SW from Triassic Hida granitic rocks to the NE. Within several meters of the fault surface, pseudotachylyte veins are found with a thickness of less than 10 cm, but are displaced by fault brecciation. Zircon fission track dating of pseudotachylyte samples yields ages of 48.6–50.2 Ma (sample AT-A), 55.1 Ma (AT-A'-1) and 60.9 Ma (AT-D-1); the latter is similar to the fission track ages of 56.1–60.1 Ma for granitic protoliths. The results of fission track length analyses in zircon suggest that pseudotachylytes (AT-A and AT-D-1) and protolith granite are mostly annealed. Consequently, the pseudotachylyte (AT-A) reached the highest temperature during 48.6–50.2 Ma, thereby resetting the fission track system totally in zircon during faulting. Another pseudotachylyte (AT-A'-1) and its wall rock granite contain shortened tracks within zircon grains suggesting partial annealing. The age distribution pattern of the former also contains decomposed age after the normality test (Shapiro–Wilk test) in which the major age yields 52.5 Ma. Accordingly, these pseudotachylytes yield a peak age of about 50 Ma, whereas the peak ages of one pseudotachylyte (AT-D-1) and the protolith Hida granitic rocks are about 60 Ma, representing the thermal effects not caused by frictional heating but by intrusions of Late Cretaceous to Paleogene granitoids that are probably concealed below the exposed Triassic Hida granitic rocks. Such thermal effects did not affect the K–Ar muscovite age (149 Ma) for the protolith granite because of the higher closure temperature of this system. Using the new geochronological data, we can elucidate the cooling history of the Hida granitic rocks, and constrain the timing of the main pulse of pseudotachylyte generation along the Atotsugawa Fault at about 50 Ma.

Further investigation of the initial fission-track length and geometry factor in apatite fission-track thermochronology

The external detector method (EDM) is a widely used technique in fission track thermochronology (FTT) in which two different minerals are concomitantly employed: spontaneous tracks are observed in apatite and induced ones in the muscovite external detector. They show intrinsic differences in detection and etching properties that should be taken into account. In this work, new geometry factor values, g, in apatite, were obtained by directly measuring the ρ ed /ρ is ratios and independently determined [GQR] ed/is values through the measurement of projected lengths. Five mounts, two of which were large area prismatic sections and three samples composed of random-orientation pieces have been used to determine the g-values. A side effect of applying EDM is that the value of the initial confined induced fission track, L 0 , is not measured in routine analyses. The L 0 -value is an important parameter to quantify with good confidence the degree of annealing of the spontaneous fission tracks in unknown-age samples, and is essential for accurate thermal history modeling. The impact of using arbitrary L 0 -values on the inference of sample thermal history is investigated and discussed. The measurement of the L 0 -value for each sample to be dated using an extra irradiated apatite mount is proposed. This extra mount can be also used for determining the g value as an extension of the ρ ed /ρ is ratio method. Eight apatite samples from crystalline basement, with grains at random orientation, were used to determine the g-values. The results found are statistically in agreement with the values found for apatite samples (from Durango, Mexico) measured in prismatic section and also measured at random orientation. There was no observable variation in efficiency regarding crystal orientation, showing that it is relatively safe using non-prismatic grains, especially in samples with paucity of grains, as it is the case of most basin samples. Implications for the ζ-calibration and for the calibration of the direct (spectrometer-based) fission-track dating are also discussed.

The Exhumation Process of Mufushan Granite in Jiangnan Uplift Since Cenozoic: Evidence from Low‐Temperature Thermochronology

AbstractJiangnan uplift located in the collision zone between Yangtze and Cathaysia block is the key to interpret the tectonic evolution of South China block. The Cenozoic thermal history and the uplift processes of Mufushan pluton are constrained by the combination of analysis of apatite fission track and (U‐Th‐Sm)/He ages with modeling of fission track length distribution. Based on the analysis of thermochronological dating and regional tectonic setting, the authors probed into the relationship between the thermochronological evolution and the tectonic development. The exhumation of plutons arrived at about 4800 m since 80 Ma, and the uplift processes were not uniform in space, and different plutons had somewhat different uplift processes. Thermal history modeling of the AFT data shows that the Mufushan pluton experienced three episodes of cooling event, two rapid exhumation events happened in Upper Cretaceous‐Paleocene (about 80∼50 Ma) and Upper Miocene (since 10 Ma). There is a period of relative thermal stability between 50 and 10 Ma. The first stage of rapid uplifting during 80∼50 Ma in this region is coeval with extension episode of Mid‐Upper Yangtze during Himalayan which resulted in the formation of fault basin and exhumation of shoulder block; the Upper Miocene rapid cooling since 10 Ma is attributed to the westward movement and subduction of Pacific plate. The uplift and exhumation of Mufushan pluton since Late Yanshanian led to a good deposition response in basins and the cooling events of three phases just coincided with the regional tectonic settings of study area.