Fission Track Length Distributions Research Articles

Deconvolution of fission-track length distributions and its application to dating and separating pre- and post-depositional components

Abstract. To enable the separation of pre- and post-depositional components of the length distribution of (partially annealed) horizontal confined fission tracks, the length distribution is corrected by deconvolution. Probabilistic least-squares inversion corrects natural track length histograms for observational biases, considering the variance in data, modelization, and prior information. The corrected histogram is validated by its variance–covariance matrix. It is considered that horizontal track data can exist with or without measurements of angles to the c axis. In the latter case, 3D histograms are introduced as an alternative to histograms of c-axis-projected track lengths. Thermal history modelling of samples is not necessary for the calculation of track age distributions of corrected tracks. In an example, the age equations are applied to apatites with pre-depositional (inherited) tracks in order to extract the post-depositional track length histogram. Fission tracks generated before deposition in detrital apatite crystals are mixed with post-depositional tracks. This complicates the calculation of the post-sedimentary thermal history, as the grains have experienced different thermal histories prior to deposition. Thereafter, the grains share a common thermal history. Thus, the extracted post-depositional histogram without inherited tracks may be used for thermal history calculation.

Exhumation and landscape evolution in eastern South China since the Cretaceous: New insights from fission-track thermochronology

Widespread extensional half-/grabens with red bed sedimentation, dome structures, and magma intrusions/eruptions with polymetallic mineralization within South China during the Cretaceous-Cenozoic provide a natural laboratory to understand intracontinental deformation and plate tectonics. In order to decipher the tectonic and landscape evolution of eastern South China, this study presents new low-temperature thermochronology from a broad region covering major tectonic units including the Xuefeng, Jiuling, Nanling, and Wuyi mountains. Zircon and apatite fission track dates, length distribution, and thermal history modeling reveal a prominent and rapid cooling phase during ~125–80 Ma across the paleo-highland in eastern South China. A synthesis of existing thermochronologic data and geological observations suggests that NE/NNE-striking syn-sedimentary faults system with half-/graben basins, rapid exhumation of Cretaceous magma and pre-Cretaceous basement, and final extension of dome structures were all initiated in a back-arc setting during this period (~80-125 Ma). Eastern South China during the Paleogene was characterized by a tectonic quiescence during which insignificant exhumation, peneplanation, and local sedimentation occurred, forming regional low-amplitude, long-wavelength topography. Accelerated regional cooling commenced in the late Oligocene or early Miocene in response to the far-field effect of the India-Asia continental convergence. Additionally, the termination of red-colored gypsum-salt-bearing sediment in the Neogene suggests a paleo-environmental change within South China, from an arid to a monsoonal climate; such change was likely the result of the topographic uplift of the Tibetan Plateau and its margins.

Post-Variscan tectonic and landscape evolution of the Elbe Fault Zone and the Lusatian Block based on apatite fission-track data and geomorphologic constraints

The Elbe Fault Zone is a continental scale NW striking structure that contains faults of different kinematics. Some faults were active at different times since at least the Variscan orogenesis in the Late Carboniferous. The Lusatian Fault (LF) was one of the major structures of the Elbe Fault Zone in the Late Cretaceous, when NW striking faults of Central Europe experienced basin inversion and thrusting during a short lived pulse of contraction. The LF displays a curved appearance on earth surface, what is not compatible with its activity at only one well defined event. Possibly, different faults where active along strike of the LF at different times.Apatite fission-track thermochronology can distinguish differential exhumation or burial of neighbouring fault blocks, if the vertical difference comprised at least 1–2 km. This study presents apatite fission-track data from the footwall and the hangingwall block of the LF and some other faults in its vicinity. Our new data show a clear difference between the hangingwall block of the LF (the Lusatian Block) with uniform 70–85 Ma apparent ages and relatively long track lengths and the footwall-block of the LF with 66–173 Ma apparent ages and shorter and more variable track lengths. For one section of the LF, we can show, that the Cretaceous fault is not visible geomorphologically, but was substituted by a neighbouring Cenozoic reverse fault with a distinct fault scarp. We used thermal history modelling to determine the timing and magnitude of exhumation and burial events from apparent apatite fission track ages and track length distributions. We infer, that the present morphological expression of the LF consist of smaller faults of different kinematics and age and present an evolution model for this section of the LF.This study establishes the existence of similar striking reverse faults in direct vicinity and partly intersecting each other in the Elbe Fault Zone. This observation emphasizes the significance of the Elbe Fault Zone as a continental scale zone of crustal weakness. It has not only been reactivated several times since the Variscan orogenesis, but represents a cluster of similar striking faults of different age and kinematics.

Thermochronological insights into the morphotectonic evolution of the Eastern Highlands, Zimbabwe: Implications for thermal history modelling of multi-thermochronometer data

The Eastern Highlands form a north-south trending high relief region along the eastern border of Zimbabwe, which is also spatially coincident with the eastern Zimbabwe cratonic margin. Uncertainty surrounds when this mountain range formed and how the region has evolved, particularly as there are no Phanerozoic cover rocks preserved. Here, we use low-temperature thermochronology (apatite fission track, apatite (U–Th-Sm)/He and zircon (U–Th)/He dating) and inverse thermal history modelling to reveal the morphotectonic evolution of this region since the mid-Paleozoic. The data suggest that structural reactivation occurred in the Late Devonian and Jurassic-Cretaceous that caused tectonic denudation and crustal cooling in the northern Eastern Highlands. The first period is thought to be associated with post-orogenic extension following Pan-African activity. The ensuing denudation continued into the Triassic and substantiates previous suggestions that these mountains were a source region of Lower Karoo sediments. The Jurassic-Cretaceous episodes are most likely associated with stress transmission from the progressive breakup of Gondwana. These events include the mid-to Late Cretaceous reactivation of the Mutare Shear Zone, which caused upward displacement (∼800 m) of the southern Eastern Highlands. The Eastern Highlands then entered a relatively quiescent phase with all samples residing close to the surface (<60 °C) since at least the beginning of the Cenozoic, which limits subsequent denudation of these mountains to less than ∼3 km. An investigation of the different multi-thermochronometer modelling combinations reinforces concerns with the current zircon radiation damage accumulation and annealing model (ZRDAAM). However, the findings also suggest that presenting the observed versus predicted apatite fission track length distribution should be an essential reporting requirement for thermal history modelling.

Thermal History since the Paleozoic in the Eastern Qaidam Basin, Northwest China

The Qaidam Basin is the one of the three major petroliferous basins in northeastern Tibetan Plateau, which has experienced multiphase superimposition and transformation. The study of thermal history not only plays an important role on revealing the tectonic origin of the Qaidam Basin and the forming mechanism and uplift history of the Tibetan Plateau, but also can provide scientific evidence for the assessment of oil and gas resources. This work used balanced cross‐section technique and apatite fission track ages with modeling of fission track length distribution to infer that the eastern Qaidam Basin has experienced significant tectonic movement in the Early Jurassic movement (∼200 Ma), which caused the carboniferous uplift and denudation, the geological movement in the Late Cretaceous, characterized by early stretching and late northeast‐southwest extrusion; the Himalayan movement in multi‐stage development in eastern Qaidam Basin, which can be divided into the early Himalayan movement (41.1–33.6 Ma) and the late Himalayan movement (9.6–7.1 Ma, 2.9–1.8 Ma), and large‐scale orogeny caused pre‐existing faults reactivated in late Himalayan movement. On the basis of burial history reconstruction, the thermal history of eastern Qaidam Basin was restored. The result shows that the thermal history in eastern Qaidam Basin shows slow cooling characteristics; the paleo‐geothermal gradient of eastern Qaidam Basin was 38–41.5°C/km, with an average value of 39.0°C/km in the Late Paleozoic, 29–35.2°C/km, with an average value of 33.0°C/km in the Early Paleogene; the geothermal gradient of the Qaidam Basin increased in the Late Paleogene, which was similar to the present geothermal gradient in the Late Neogene. The characteristics of the tectono‐thermal evolution since Paleozoic in the eastern Qaidam Basin are mainly controlled by magmatic thermal events in the study area.

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.

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.

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.

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.

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.

Mesozoic fault reactivation along the St. Lawrence rift system, eastern Canada: Thermochronologic evidence from apatite fission-track dating

The St. Lawrence rift system is an active fault zone where reactivation of Late Proterozoic, Iapetus-related faults is believed to occur. It forms both a NE-SW–trending half graben and NW-SE–trending grabens that link and include the Ottawa-Bonnechere and Saguenay River grabens, both interpreted as aulacogens. The rift faults locally preserve kinematic evidence for normal-sense displacement. The lack of isotopic chronological constraints and the absence of strata younger than the Ordovician make it difficult to determine the timing of fault increments. Field relations in the Quebec City, Charlevoix, and Saguenay River regions suggest faulting event(s) younger than the Devonian-age Charlevoix crater. We determined apatite fission-track (AFT) ages for Grenvillian basement rocks from the hanging wall and footwall of typical rift faults at different locations of the St. Lawrence and Saguenay River fault systems along the St. Lawrence rift system. The footwall rocks of major faults yielded Late Triassic–Early Jurassic AFT ages ranging from 235 ± 28 Ma to 184 ± 22 Ma. The hanging-wall samples yielded older ages, between 443 ± 44 Ma and 279 ± 28 Ma along the NW-trending Saguenay River fault system that are consistent with Triassic–Early Jurassic normal faulting at ca. 250–200 Ma based on HeFTy inverse models of fission-track length distributions. Younger Late Jurassic AFT ages, ranging from 152 ± 17 Ma to 149 ± 14 Ma in the hanging wall of the NE-SW–trending St. Lawrence fault system, suggest the occurrence of localized faulting increments during the Late Jurassic. The latter AFT age discontinuities are interpreted as the result of normal faulting at ca. 250–200 Ma, followed by tectonic inversion. Evidence for the reactivation and inversion of preexisting normal faults during or after Jurassic time is also found in the Mesozoic rift-related basins of the Atlantic coast. The NW-SE–trending compressive stress field currently documented along the St. Lawrence River is consistent with tectonic inversion along the St. Lawrence rift system. This study provides geochronological support for Atlantic rift-related extensional deformation followed by compressional deformation in the interior of the Canadian Shield, more than 500 km west of the axis of the Mesozoic Atlantic rift basins.

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.

Determination of Onset of Uplifting for the Mid‐Upper Yangtze Area Since Indosinian Event

AbstractThe onset of uplifting of the Mid‐upper Yangtze area is constrained by the combination of analysis of apatite fission track (AFT) age and modeling of fission track length distribution. The results show that the general trend of onset of uplifting is characterized by decreasing westward in geological time for the different tectonic units in the Mid‐upper Yangtze area. Two episodes of uplifting in Jianghan basin occurred within 157~97 Ma and 10~0 Ma; Xiang'exi‐Wuling area experienced an uplifting in 137~0 Ma; E'xi Yudong area, Chuandong folds, Qianzhong Hights and Dian‐Qian‐Gui areas in 97~0 Ma; Northern Sichuan basin and Chuanzhong areas in 56~0 Ma; and Chuanxi depression and Dianxi region in 23~0 Ma. The major faults of Qiyueshan, Huanyinshan and Longquanshan controlled the boundaries of the onsets of uplifting. The onsets of uplifting for the areas east of Qiyueshan fault, in between Qiyueshan and Huayinshan faults, in between Huayinshan and Longquanshan faults, west of Longquanshan fault are late Jurassic‐early Cretaceous (J3‐K1), late Cretaceous (K2), Paleogene (E) and Neogene (N), respectively.

Denudation history of the Bocaina Plateau, Serra do Mar, southeastern Brazil: Relationships to Gondwana breakup and passive margin development

The Bocaina Plateau, which is situated on the eastern flank of the continental rift of southeastern Brazil, is the highest part of the Serra do Mar. Topographic relief in this area is suggested to be closely related to its complex tectono-magmatic evolution since the breakup of Western Gondwana and opening of the South Atlantic Ocean. Apatite fission track ages and track length distributions from 27 basement outcrops were determined to assess these hypotheses and reconstruct the denudation history of the Bocaina Plateau. The ages range between 303 ± 32 and 46 ± 5 Ma, and are significantly younger than the stratigraphic ages. Mean track lengths vary from 13.44 ± 1.51 to 11.1 ± 1.48 μm, with standard deviations between 1.16 and 1.83 μm. Contrasting ages within a single plateau and similar ages at different altitudes indicate a complex regional tectonothermal evolution. The thermal histories inferred from these data imply three periods of accelerated cooling related to the Early Cretaceous continental breakup, Early Cretaceous alkaline magmatism, and the Paleogene evolution of the continental rift of southeastern Brazil. The oldest fission track ages (> 200 Ma) were obtained in the Serra do Mar region, suggesting that these areas were a long-lived source of sediments for the Paraná, Bauru, and Santos basins.

Oldest record of brittle deformation along the Median Tectonic Line: fission-track age for pseudotachylyte in the Taki area, Mie Prefecture

Fission-track (FT) dating of pseudotachylyte (PST) associated with the Median Tectonic Line (MTL) in the Taki area, Mie Prefecture, SW Japan, provides new constraints on the timing of movement upon this fault. A PST vein with a thickness of about 5 cm yields a zircon FT age of 60.0 ± 3.5 Ma (1σ). In contrast, a weighted average of zircon FT ages obtained for protolith samples (cataclastic mylonitized Hatai Tonalite) collected 10 cm and 15 m from the PST vein boundary is 69.8 ± 1.2 Ma, which is significantly older than the age of the PST. Decomposition of feldspars in the PST suggests that the temperature exceeded 1100°C, which is sufficient to completely erase previous fission tracks in zircon within several seconds. The distribution of fission-track lengths in zircon from the PST vein also supports the interpretation that the zircon FT age was completely reset during frictional fusion of the PST vein. Considering an apatite FT age of 38.0 ± 1.5 Ma for the host rock, the age of the PST indicates that the frictional fusion was occurred during cooling of the Ryoke granite at temperatures between 250°C (closure temperature of the zircon FT system) and 100°C (closure temperature of the apatite FT system). This PST age is comparable with the oldest K-Ar age obtained for fine fractions of MTL fault gouge derived from both the Sanbagawa pelitic schist and the Izumi Group in Shikoku, indicating that the initiation of brittle fault movement associated with formation of the PST and/or fault gouges along the MTL had occurred by 60 Ma.

Effect of alpha-damage annealing on apatite (U–Th)/He thermochronology

Recent data on He diffusion challenge the temperature sensitivity of apatite (U–Th)/He thermochronology: the damage induced by recoil of U and Th decay series during emission of α particles (α-recoil damage) has been proposed to modify He-diffusion properties through time. However, we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly-cooled or reheated basement rocks. To test this hypothesis, we developed a quantitative model including an explicit treatment of α-recoil damage, annealing, and their effect on He-diffusion kinetics, and calibrate it against literature data. Our model is based on two hypotheses: (1) helium is in equilibrium between an apatite crystal and its defects and (2) alpha-recoil damage annealing can be described analogously to fission-track annealing. This model has been embedded into a Monte Carlo simulation of helium production/ejection/diffusion and applied to data from the French Massif Central; a complex slowly-cooled terrain with burial reheating, where the thermal history has been constrained by previous fission-track (FT) data including FT length distributions. (U–Th)/He ages are close to the FT ages from the same samples and are generally reproducible among replicates, but some samples present He-age dispersion that is not correlated with crystal size. Our model reproduces the Massif Central data very well except for three samples where He ages are older than corresponding FT ages. We show that annealing of irradiation damage has an important impact on retentivity of helium and that the He content, [He] is only a rough approximation of the damage level. In particular our results show that independence of He ages on crystal sizes, in case of reheated samples, is a clear indication of the higher He retentivity induced by α-recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of (U–Th)/He ages in such a case. More generally a correlation with the crystal size can bring information on the thermal path only if the age of defects, well represented by the fission-track age, is available, due to the dependence of the partial retention zone on damages. Conversely, in case of rapid cooling or for samples having low U and Th contents, damage effects can be ignored without significant effects on He ages.

Crustal Thermal Processes and the Interpretation of Thermochronometer Data

Many thermochronology studies focus on extracting the thermal history of a sample using fission-track length distributions and track annealing models (e.g., Ketcham 2005, and references therein). Recent developments in measuring the 3He/4He concentration profile across apatite grains (e.g., Shuster and Farley 2005) offer a new approach for quantifying sample cooling histories over a broader range of temperatures with the (U-Th)/He system. The motivation behind calculating thermal histories from thermochronometer data has traditionally been to date events such as the onset of exhumation or fault motion, erosion, or for quantifying hydrocarbon maturation in sedimentary basins. The calculation of thermal histories from thermochronometer data is a routine aspect of most thermochronology studies and produces valuable geologic information. A relevant question for thermochronology studies is what can be learned from forward modeling crustal thermal fields from principles of heat conduction and advection beyond what is already learned from the thermal history extracted from the data? Forward modeling of crustal thermal fields requires a physically based model for heat transfer in the geologic setting of interest. Specific geologic processes such as magmatism, fault motion, fluid flow, as well as the kinematic, topographic, and erosional evolution of an orogen can significantly influence the thermal history of thermochronometer samples. These processes can be simulated with thermal models and then compared to thermochronometer derived cooling histories and ages. Thus, predicted thermal histories and thermochronometer ages generated under a known set of conditions can be compared to observed thermal histories and ages to quantify the geologic processes associated with sample cooling. To answer the question posed earlier, forward modeling crustal thermal fields allows robust, or at least constrained, interpretations of what geologic events could have occurred to produce an observed suite of thermochronometer ages. Hence, comparison of thermochronometer data with model predicted thermal histories …