Apatite Partial Annealing Zone Research Articles

The influence of lithospheric inheritance and Cenozoic magmatism on Phanerozoic rock cooling in the São Francisco Craton and adjacent orogens

The South American Platform encompasses the São Francisco Craton (SFC) and the Araçuaí-Ribeira orogenic system (AROS) located in the eastern Brazil. These geologic domains hold records of differential exhumation, sedimentation, volcanic activity, and post-rift reactivation of faults and shear zones. Therefore, these terranes provide outstanding opportunities for studying the dynamics of lithospheric processes, particularly in their interaction zones, characterized by the transition from cratonic to orogenic lithosphere. To investigate the processes involved in the contrasting behavior of the upper crust, we provide 23 new apatite fission track (AFT) ages and thermal histories from the northern segment of the AROS and from the eastern area of the SFC. The AFT ages range from 154.4 ± 20.1 and 37.1 ± 3.0 Ma and the mean track lengths range from 10.9 and 13.2 μm. We compiled thermal information from previous thermochronological analysis from 357 sites and constructed inverse distance weighted maps that depict the spatial distribution of AFT ages and the temperature evolution from 360 to 30 Ma. The results indicate that the SFC experienced its final exhumation within the apatite partial annealing zone (APAZ) at temperatures ranging from 110 °C to 60 °C during the late Paleozoic. In contrast, the AROS went through its last cooling within the APAZ during the Mesozoic or even in the Cenozoic. This suggests that reactivation and reheating, possibly associated with the South Atlantic rift, influenced the orogenic basement of the passive margin. We highlight that the reheating associated to the Abrolhos magmatic province in the northern segment of the AROS caused the youngest AFT ages of the study area. Our work demonstrates the importance of considering contrasting rheology when analyzing regional exhumation patterns, the structural fabric of orogens through reactivation of crustal discontinuities generating heating and exhumation, and how volcanism can disrupt the thermal record, inducing crustal reheating.

Late Cretaceous exhumation of the Little Belt Mountains and regional development of the Helena salient, west-central Montana, USA

The timing of deformation within and adjacent to the Helena salient of west-central Montana is poorly constrained relative to other segments of the Sevier fold-and-thrust belt. This study presents low-temperature thermochronology data from the Little Belt Mountains, a basement-cored Laramide uplift that is juxtaposed with the leading edge of the salient. We analyzed eight samples of Paleoproterozoic basement for apatite fission-track (AFT) and zircon (U-Th)/He (ZHe) thermochronology. Four samples yielded AFT ages ranging from ca. 80 Ma to 73 Ma and associated long, unimodal confined track lengths, indicating rapid cooling and exhumation of Little Belt Mountains basement during the Late Cretaceous. The other four samples are characterized by younger AFT ages (ca. 55 Ma), which suggest a combination of prolonged residence in the apatite partial annealing zone and postexhumation magmatic reheating. In total, 20 new ZHe dates range from ca. 236 Ma to 28 Ma and show a correlation between date and effective uranium. Forward model results for ZHe data are consistent with upper-crustal residence during the Proterozoic followed by Phanerozoic burial and rapid Late Cretaceous cooling. Cross sections across the Little Belt Mountains display the geometry of the Volcano Valley fault zone, an array of down-to-the-south Proterozoic normal faults that profoundly influenced the development of the Cordilleran thrust belt. Our new constraints from the Little Belt Mountains when integrated with published kinematic constraints from the Helena salient reveal significant out-of-sequence deformation in this portion of the thrust belt between ca. 80 Ma and 55 Ma. A kinematic model is proposed that involves Late Cretaceous (ca. 80 Ma) exploitation of rheologically incompetent units at the base of the Belt Supergroup within the Helena Embayment, facilitating early exhumation in the Little Belt Mountains. Our new data and synthesis are consistent with previous interpretations in which an inherited stratigraphic and structural architecture of Proterozoic ancestry was the predominant control on the development of the Helena salient during Cretaceous−early Eocene time.

Origin and timing of spilitic alterations in volcanic rocks from Głuszyca Górna in the Intra-Sudetic Basin, Poland

The formation of spilitic assemblages (i.e. chlorite and albite) has been ubiquitously involved during the evolution of continental early-Permian volcanics from the Intra-Sudetic Basin (ISB). Based on the investigation of laccolith-type and variably-altered trachyandesite exposure in the vicinity of Głuszyca Górna (Lower Silesia, Poland), we have demonstrated that apatite fission-track dating (AFT), coupled with chlorite geothermometry, can be successfully applied to denote the timing of low-temperature alterations within volcanic rocks. The primary magmatic assemblages of the trachyandesites (i.e. augite and andesine-labradorite) have been affected by chloritization and alblitization respectively, followed by the formation of secondary titanite, celadonite, and calcite. The chlorite species have crystallized in the range of 106–170 °C, that exceeds Apatite Partial Annealing Zone (70–110 °C). The secondary, nearly pure albite (Ab ~ 99 mol.%) with weak to dark-brown cathodoluminescence replaces primary plagioclase (~ An37–50Ab47–58Or2–4) along the cleavage and/or twinning planes during Al3+–conservative reaction. The accessory apatite is marked by swallow-tail terminations indicative of rapid cooling formation conditions. It shows homogenous chemical composition, high F− content, and pink to yellow (REE3+ and Mn2+-activated, respectively) cathodoluminescence. Based on the AFT dating, the development of spilitic alterations within the early-Permian (ca 290 Ma) laccolith from Głuszyca could not only span the range of 182–161 Ma (Middle Jurassic), but also occurred prior to large-scale geological events in the ISB, such as burial under late-Mesozoic sediments, as well as tectonic inversion and exhumation. Whole-rock geochemistry of trachyandesites altered to various extent, indicates that original trace elements concentrations, except for i.e. Sr, Cs, and Ba, could be preserved during low-temperature alteration (spilitization). Meanwhile, geochemical fingerprint of the volcanics (i.e. humped-shaped mantle normalized trace element diagrams and positive Zr–Hf anomaly) points to the crustal contamination during magma evolution, combined with the mantle metasomatism in the source via subduction-derived components (i.e. fluids), as shown by i.e. low Nb/Th and Nb/LREE ratios.

Thrusting sequence in the Western Himalayan foreland basin during the late phase of continental collision defined by low-temperature thermochronology

Foreland basins associated with collisional mountain belts undergo postdepositional deformation in response to compressive stress exerted from the evolving orogen. The low thermal regime of such processes requires sensitive methods to unravel the structural evolution of such regions. In this study, we conducted apatite fission track (AFT) and (U-Th)/He thermochronology on the sedimentary succession of the Subathu basin in the Himalayan foreland of the Himachal Pradesh region in India, an area preserving the sedimentary record of the India-Asia collision since the late Paleocene-early Eocene. The results show that the Subathu basin exposes a fossilised apatite partial annealing zone (APAZ) depicted by incompletely reset AFT ages and shortened track lengths in the Paleocene-middle Miocene pre-Siwalik units and unreset ages in the Neogene Siwalik Group. We associate the partial annealing of the pre-Siwalik rocks with thickening due to internal imbrication and nappe stacking triggered by shearing along the Main Boundary Thrust and its splays since the middle Miocene. Thermal modelling of the Siwalik rocks projects a rapid heating up to approximately 90–100 °C reached between 4 and 1 Ma. Following the thermal peak, the Siwalik rocks cooled rapidly, as indicated by 2.6–1.2 Ma AHe ages. We associate this recent cooling event with SW-directed thrusting along the Main Frontal Thrust, which placed the foreland basin over the Gangetic Plains. Our results are similar to those obtained for the foreland basin in Nepal, indicating that the movement along the youngest major discontinuity of the Himalaya is synchronous along the orogen.

Burial and Exhumation History of the Lujing Uranium Ore Field, Zhuguangshan Complex, South China: Evidence from Low-Temperature Thermochronology

The Zhuguangshan complex hosts the main uranium production area in South China. We report (U-Th)/He and fission track thermochronological data from Triassic–Jurassic mineralized and non-mineralized granites and overlying Cambrian and Cretaceous sandstone units from the Lujing uranium ore field (LUOF) to constrain the upper crustal tectono-thermal evolution of the central Zhuguangshan complex. Two Cambrian sandstones yield reproducible zircon (U-Th)/He (ZHe) ages of 133–106 Ma and low effective uranium (eU) content (270–776 ppm). One Upper Cretaceous sandstone and seven Mesozoic granites are characterized by significant variability in ZHe ages (154–83 Ma and 167–36 Ma, respectively), which show a negative relationship with eU content (244–1098 ppm and 402–4615 ppm), suggesting that the observed age dispersion can be attributed to the effect of radiation damage accumulation on 4He diffusion. Correspondence between ZHe ages from sandstones and granites indicates that surrounding sedimentary rocks and igneous intrusions supplied sediment to the Cretaceous–Paleogene Fengzhou Basin lying adjacent to the LUOF. The concordance of apatite fission track (AFT) central ages (61–54 Ma) and unimodal distributions of confined track lengths of five samples from different rock units suggest that both sandstone and granite samples experienced a similar cooling history throughout the entire apatite partial annealing zone (~110–60 °C). Apatite (U-Th-Sm)/He (AHe) ages from six non-mineralized samples range from 67 to 19 Ma, with no apparent correlation to eU content (2–78 ppm). Thermal history modeling of data suggests that the LUOF experienced relatively rapid Early Cretaceous cooling. In most samples, this was followed by the latest Early Cretaceous–Late Cretaceous reheating and subsequent latest Late Cretaceous–Recent cooling to surface temperatures. This history is considered as a response to the transmission of far-field stresses, involving alternating periods of regional compression and extension, related to paleo-Pacific plate subduction and subsequent rollback followed by Late Paleogene–Recent India–Asia collision and associated uplift and eastward extrusion of the Tibetan Plateau. Thermal history models are consistent with the Fengzhou Basin having been significantly more extensive in the Late Cretaceous–Early Paleogene, covering much of the LUOF. Uranium ore bodies which may have formed prior to the Late Cretaceous may have been eroded by as much as ~1.2 to 4.8 km during the latest Late Cretaceous–Recent denudation.

Cenozoic exhumation history at the core of the Andes at 31.5°S revealed by apatite fission track thermochronology

The Andes at ~31°-32°S lie above the Chilean-Pampean flat slab zone (~27–33°S), where several morphostructural units developed resulting in a large orogenic width. The core of the Andes is composed of the La Ramada fold-and-thrust belt in Principal Cordillera and the basement blocks of Frontal Cordillera. While rock uplift of these blocks has been broadly constrained to the middle Miocene based on structural and provenance studies, thermochronologic approaches with the potential to directly constrain the timing and amount of exhumation have not been exploited until recently. Apatite fission track data from a ~1 km vertical profile collected within the Carboniferous Pico Los Sapos Batholith in the High Andes at 31.5°S places some constraints on the thermal evolution of the region since the Paleocene. The age-elevation profile combined with inverse thermal modeling and previous AHe thermochronology, indicates an episodic cooling/exhumation history. Rocks cooled rapidly in the early Cenozoic (ca. 65-55 Ma), followed by a period of relative thermal and tectonic stability when residence in an apatite partial annealing zone (PAZ) from at least ~52 Ma to ca.15 Ma, followed by final rapid cooling beginning ca. 15 Ma. We interpret early Cenozoic and middle Miocene rapid cooling events as to be related to erosional exhumation during Andean contractional phases, associated with thrust activity along the Mondaquita Fault. The age-elevation profile is partially duplicated, with upper samples being offset ~500 m due to back-thrusting since the late Miocene. The preservation of part of an exhumed PAZ indicates 3–5 km of exhumation since the onset of rapid cooling/exhumation at ~15 Ma. Although evidence for an early Cenozoic compressional phase in the High Andes at this latitude is scarce, the occurrence of a regional K-T (~65 Ma) unconformity supports our results. The Eocene Inca phase, registered north of 30°S, on the other hand, is not shown in our thermochronological data, suggesting that this tectonic phase did not affect the core of the Andes south of this latitude. Independent geological evidence both from the hinterland (structural, geochemical and thermochronological analyses) and foreland (provenance studies) corroborate our findings of a middle Miocene deformational event in the core of the Andes at 31.5°S.

Cretaceous exhumation history of the southwestern South China Block: Constraints from fission‐track thermochronology

To detect the exhumation history of the southwestern South China Block (SCB), we performed fission track (FT) analysis on zircon and apatite grains in granitoid and sedimentary samples. From east to west, the study area includes the Yunkai Massif, Lingshan granitic belt, Shiwandashan Basin, and eastern margin of the Youjiang Basin. The granite sample of the Yunkai Massif has two peaks of 75 and 92 Ma for zircon FT dating. The Lingshan granitic belt samples yield zircon FT central ages from 135 to 132 Ma. The Permian bauxite samples of the Youjiang Basin yield zircon FT central ages from 109 to 93 Ma and display age peaks at ~97–93 Ma. The Yunkai Massif granite sample has two peaks at 74 and 104 Ma for apatite FT dating, while the Lingshan granitic belt sample has two peaks at 81 and 111 Ma. The cooling paths of the Yunkai Massif sample started with an Early Cretaceous (~120–100 Ma) cooling event that brought the granitoids into the apatite partial annealing zone (PAZ). Through comparative studies, we suggest that the southwestern SCB underwent three periods of crustal uplift during ~135–132, ~111–92, and ~ 81–74 Ma, respectively, which is consistent with the regional stratigraphic unconformity and crustal extension. Only the Yunkai Massif and Lingshan granitic belt were uplifted during ~135–132 Ma. During ~111–92 Ma, the southwestern SCB experienced crustal uplift, and the basal conglomerate began to deposit in the Shiwandashan Basin. Late Cretaceous uplifted occurred during ~81–74 Ma, and the Yunkai Massif was uplifted throughout the Cretaceous. From 97 to 92 Ma, the Permian bauxite deposits in the Youjiang Basin were uplifted, which provided the basis of the later transformation to Quaternary bauxite.

Cretaceous Exhumation of Proterozoic Carbonatite on the Northern Margin of the North China Craton Constrained by Apatite Fission-Track and (U-Th)/He Geochronology

AbstractSome carbonatites related to Proterozoic rift activities developed on the northern margin of the North China Craton and have a close genetic relationship with certain types of mineralization. We obtained new fission-track and (U-Th)/He geochronological data from fragments of pegmatitic apatite that coexists with Proterozoic igneous carbonatite along the border of three provinces (Hebei, Shanxi, and Inner Mongolia). Twenty fragments yielded a fission-track central age of 124.1 ± 6.5 Ma (1σ). Thermal-history inversion and track-length distribution suggest rapid Early Cretaceous cooling through the apatite partial-annealing zone. Sixteen single-fragment apatite (U-Th)/He ages range between 82.2 ± 1.8 and 109.6 ± 2.3 Ma, with the age variation much larger than the analytical uncertainty. Several reasons could account for this dispersion. While alpha-particle redistribution resulted from U and Th inhomogeneties, chemical variations and radiation damage are the most probable causes. We calculated the me...

Post-Variscan thermal history of the Moravo-Silesian lower Carboniferous Culm Basin (NE Czech Republic - SW Poland)

Apatite fission track analysis (AFT) and zircon (U-Th)/He thermochronology (ZHe) have been carried out for a lower Carboniferous greywacke succession of the Moravo-Silesian Culm Basin in the Nízký Jeseník Mountains. The range of apparent zircon helium ages is 303–233Ma (late Carboniferous to Early Triassic) in the eastern part of the basin, whilst they are significantly younger in the western part, ranging from 194 to 163Ma (Early-Middle Jurassic). Apatite fission track central ages range from 152 (Latest Jurassic) to 44Ma (Eocene), with the majority being grouped between 114 (Aptian) and 57Ma (Paleocene). All samples experienced substantial post-depositional thermal reset; both the AFT ages and the ZHe are considerably younger than the depositional ages. The mean track length varies in the range between 12.5 and 15.4μm. The unimodal track length distribution, the relatively short mean track length (in most samples), and their rather low standard deviation values (1.2 to 2.1μm) indicate that their thermal history was determined by Variscan and post-Variscan heating event(s) followed by a prolonged residence in the apatite partial annealing zone in the Mesozoic and finally by cooling in the Paleogene. Geological evidence combined with thermal modeling based on AFT and ZHe data indicate that the lower Carboniferous strata had already reached maximum palaeotemperatures in the late Carboniferous, however, they were presumably later re-heated during the Permian-Triassic. Post-Variscan extensional tectonics events were responsible for high heat flow that together with Carboniferous burial could account for the reset of both thermochronometers. A major phase of cooling occurred in the Late Cretaceous. Finally, exhumation was probably faster in the Paleogene, causing the present-day exposure of the studied rocks.

Thermal history of the westernmost Eastern Alps (Penninic Rhenodanubian Flysch nappes, Helvetic nappes, and Subalpine Molasse thrust sheets)

The frontal part of the westernmost Eastern Alps comprises from top to bottom of the Austroalpine and Penninic nappes, Ultrahelvetic slices, and two Helvetic thrust sheets, thrust upon the northern Alpine Molasse Basin. The thermal evolution of the Penninic Rhenodanubian Flysch nappes, the Helvetic nappes, and the allochthonous part of the Alpine Molasse Basin is constrained by vitrinite reflectance measurements and apatite fission track dating and implemented in a tectonic evolution scheme. Within the Helvetic nappes, vitrinite reflectance increases regionally from north to south and stratigraphically from the Campanian–Maastrichtian Wang Formation to the Toarcian Mols Member. Apatite fission track ages from Penninic and Subalpine Molasse units are consistently younger than the deposition age. They indicate therefore a post-depositional thermal overprint exceeding approximately 120 °C, the upper temperature limit of the apatite partial annealing zone. 1D thermal modelling suggests that the Penninic nappes attained deepest burial between the latest Cretaceous and Early Palaeocene with the Penninic basal thrust being located at approximately 8 km in the north compared to approximately 12 km in the south. Deepest burial of the upper Helvetic nappe occurred between the latest Eocene and Early Miocene. Its base was buried down to approximately 10.5 km in the north compared to 11.5 km in the south. Exhumation of the entire nappe stack started in the Early to Middle Miocene. For both Penninic and Helvetic models, a heatflow minimum during the Cenozoic deformation (max. 27–32 mW/m2), followed by an increase from the Middle Miocene onwards (up to 60 mW/m2), was assumed.

Thermochronometric evidence for Miocene tectonic reactivation of the Sevan–Akera suture zone (Lesser Caucasus): a far-field tectonic effect of the Arabia–Eurasia collision?

AbstractLow-temperature thermochronological data for the Eurasian foreland north of the Bitlis–Zagros suture zone suggest that the tectonic stresses related to the Arabian collision during mid-Miocene time were transmitted efficiently over large distances, focusing preferentially at rheological discontinuities. Since the late Middle Miocene a new tectonic regime has been active as the westwards translation of Anatolia is accommodating most of the Arabia–Eurasia convergence, thus precluding the efficient transfer of stress northwards. Apatite fission-track data from the central Lesser Caucasus show that a portion of this orogen underwent a discrete phase of cooling/exhumation at 18–12 Ma (late Early–early Middle Miocene) as a result of the structural reactivation of a segment of the Late Cretaceous–Palaeogene Sevan–Akera suture zone. This inference contradicts the notion that the post-collisional history of the study area was dominated by strike-slip tectonics with relatively minor dip-slip components. Reactivation and exhumation was focused along those segments of the suture zone at high angles to the inferred collisional stress field; the remaining areas were not exhumed enough to expose a new apatite partial annealing zone and thus retained the thermochronological record of a phase of Late Cretaceous cooling/exhumation associated with ophiolite obduction and the following continental collision along the suture zone.

Thermal history of the Sabero Coalfield (Southern Cantabrian Zone, NW Spain) as revealed by apatite fission track analyses from tonstein horizons: implications for timing of coalification

Apatite fission track (AFT) central ages from Carboniferous (Stephanian) tonsteins of the Sabero Coalfield, NW Spain, range from 140.8 ± 7.5 to 65.8 ± 8.1 Ma (Cretaceous), with mean c-axis projected track length values ranging from 12.5 to 13.4 μm. Mean random vitrinite reflectance (R r) of these samples ranges from 0.91 to 1.20 %, which can be translated into maximum palaeotemperatures of ca. 130 to 180 °C. All analysed samples experienced substantial post-depositional annealing. The considerably younger AFT ages compared to the depositional ages of the samples and R r data indicate the certainty of the occurrence of at least one heating event after the deposition of strata. The unimodal track length distributions, the relatively short mean track length, and the rather low standard deviation (SD) (1.0–1.6 μm) indicate a relatively simple thermal history that could be related to the post-Late Variscan heating event followed by prolonged residence in the apatite partial annealing zone (APAZ). Geological data combined with thermal models of AFT data indicate that Stephanian strata reached the maximum palaeotemperatures in the Permian period, which was therefore the major time of the coalification processes. The Permian magmatic activity was responsible for a high heat flow, which, with the added effect of sedimentary burial, could account for the resetting of the AFT system. It appears that the fault-related hydrothermal activity could have redistributed heat in areas of significant subsidence. Cooling occurred in the Triassic–Cretaceous times after a high heat flow Permian regime. A post-Permian maturation of the Stephanian organic matter is not very likely, since there is no evidence of a high Mesozoic burial that was sufficient to cause a significant increase in the palaeotemperatures. Finally, exhumation and associated erosion rates may possibly have been faster in the Tertiary, causing the present exposure of the studied rocks.

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 (<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 (>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 (<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.

Upper Cretaceous exhumation of the western Rhodope Metamorphic Province (Chalkidiki Peninsula, northern Greece)

The Vertiskos Unit of northern Greece is an elongated basement belt with a complex poly-metamorphic history. It extends from Greece (Chalkidiki peninsula), to the south, up to Serbia, in the north, and arguably represents the westernmost part of the Rhodope Metamorphic Province (northern Greece to southern Bulgaria). The Vertiskos Unit experienced a medium pressure lower amphibolite-facies metamorphic overprint during the Alpine Orogeny. The available medium-temperature geochronology implies that it remained at temperature of approximately 300°C (or slightly higher) during Lower Cretaceous. In order to constrain its post-Lower Cretaceous thermal history, until near-surface exposure, we applied apatite fission track analysis. The central ages obtained range from 68.5 ± 3.8 to 46.6 ± 3.6 Ma (uppermost Cretaceous to Middle Eocene) and mean track lengths between 13 and 13.5 μm. We applied two inverse thermal modeling approaches using either each sample independently (high degree of freedomin the thermal history, better data fit) or all samples together interpreting them as a vertical profile (simpler thermal history, worse data fit). Irrespective of the modeling approach, we conclude that the bulk thermal history of the Vertiskos Unit crosses the high-temperature limit of the apatite partial annealing zone by the uppermost Cretaceous and reaches near-surface conditions as early as lower/middle Eocene. These results contrast with the thermal history of the other domains of the Rhodope Metamorphic Province further east (namely the Southern Rhodope Core Complex and the Northern Rhodope Complex) and establish the Vertiskos basement complex as the oldest exhumed coherent basement fragment of the Rhodope Metamorphic Province and Greece.

Apatite fission-track dating and low-temperature history of the Bavarian Forest (southern Bohemian Massif)

Apatite fission-track (AFT) dating applied to uplifted Variscan basement blocks of the Bavarian Forest is employed to unravel the low-temperature history of this segment of the Bohemian Massif. Twenty samples were dated and confined track lengths of four samples were measured. Most samples define Cretaceous APT ages between 110 and 82 Ma (Albian to Campanian) and three samples give older ~148–140 Ma (Jurassic–Cretaceous boundary) ages. No discernible regional age variations exist between the areas north-east and south-west of the Pfahl shear zone, but >500 m post-Jurassic and post-Cretaceous vertical offsets along this and other faults can be inferred from elevation profile analyses. The AFT ages clearly postdate the Variscan exhumation history of the Bavarian Forest. Thermal modeling reveals that the ages are best explained by a slight reheating of the basement rocks to temperatures within the apatite partial annealing zone during the middle and late Jurassic and/or by late Cretaceous marine transgression causing burial heating, which affected marginal low-lying areas of the Bohemian Massif and the Bavarian Forest. Late Jurassic period was followed by enhanced cooling through the 120–60 °C temperature interval during the subsequent exhumation phase for which denudation rates of ~100 m myr−1 were calculated. On a regional scale, Jurassic–Cretaceous AFT ages are ubiquitous in marginal structural blocks of the Bohemian Massif and seem to reflect the exhumation of these zones more distinctly compared to central parts.

Post-Triassic thermal history of the Tazhong Uplift Zone in the Tarim Basin, Northwest China: Evidence from apatite fission-track thermochronology

The Tarim Basin is a representative example of the basins developed in the northwest China that are characterized by multiple stages of heating and cooling. In order to better understand its complex thermal history, apatite fission track (AFT) thermochronology was applied to borehole samples from the Tazhong Uplift Zone (TUZ). Twelve sedimentary samples of Silurian to Triassic depositional ages were analyzed from depths coinciding with the apatite partial annealing zone (∼60–120 °C). The AFT ages, ranging from 132 ± 7 Ma (from a Triassic sample) to 25 ± 2 Ma (from a Carboniferous sample), are clearly younger than their depositional ages and demonstrate a total resetting of the AFT thermometer after deposition. The AFT ages vary among different tectonic belts and decrease from the No. Ten Faulted Zone (133–105 Ma) in the northwest, the Central Horst Zone in the middle (108–37 Ma), to the East Buried Hill Zone in the south (51–25 Ma). Given the low magnitude of post-Triassic burial heating evidenced by low vitrinite reflectance values (Ro < 0.7%), the total resetting of the AFT system is speculated to result from the hot fluid flow along the faults. Thermal effects along the faults are well documented by younger AFT ages and unimodal single grain age distributions in the vicinity of the faults. Permian–early Triassic basaltic volcanism may be responsible for the early Triassic total annealing of those samples lacking connectivity with the fault. The above arguments are supported by thermal modeling results.

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 2. The Biella Volcanic Suite

The high-pressure metamorphic rocks of the Sesia–Lanzo zone are partly covered by a volcano-sedimentary unit, the Biella Volcanic Suite. Calc-alkaline and shoshonitic lavas extruded sub-aerially on the Oligocene surface. Uranium–Lead zircon dating yields 32.44–32.89 Ma for the eruption of the calc-alkaline lavas and therefore fixes a minimum age for the paleosurface. The Biella Volcanic Suite consists mainly of epiclastic rocks deposited in a high-energy fluvial environment and minor lava flows. The rocks of the suite display widespread post-eruption transformations. Illite and chlorite thermometry as well as fission track dating suggest a thermal overprint related to burial of the Biella Volcanic Suite. An upper crustal rigid block tilting in the area causes this burial. Hydrothermal tourmaline and ankerite veins related to the intrusion of the nearby Valle del Cervo Pluton crosscut the already tilted Biella Volcanic Suite. The intrusion age of Valle del Cervo Pluton at 30.39 ± 0.50 Ma sets therefore the lower time limit for tectonic processes responsible for the tilting and burial. After the burial, the Biella Volcanic Suite remained for around 20 million years between the zircon and the apatite partial annealing zone. The apatite fission track ages spread between 16 and 20 Ma gives the time frame for the second exhumation of these units. The Biella Volcanic Suite and the adjacent rocks of the Sesia–Lanzo zone were the second time exhumed to the surface in Messinian times, after a long residence time within the apatite partial annealing zone.

Vertical tectonics at a continental crust‐oceanic plateau plate boundary zone: Fission track thermochronology of the Sierra Nevada de Santa Marta, Colombia

The topographically prominent Sierra Nevada de Santa Marta forms part of a faulted block of continental crust located along the northern boundary of the South American Plate, hosts the highest elevation in the world (∼5.75 km) whose local base is at sea level, and juxtaposes oceanic plateau rocks of the Caribbean Plate. Quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary, and the driving forces of rock uplift along the active margin. The Sierra Nevada Province of the southernmost Sierra Nevada de Santa Marta exhumed at elevated rates (≥0.2 Km/My) during 65–58 Ma in response to the collision of the Caribbean Plateau with northwestern South America. A second pulse of exhumation (≥0.32 Km/My) during 50–40 Ma was driven by underthrusting of the Caribbean Plate beneath northern South America. Subsequent exhumation at 40–25 Ma (≥0.15 Km/My) is recorded proximal to the Santa Marta‐Bucaramanga Fault. More northerly regions of the Sierra Nevada Province exhumed rapidly during 26–29 Ma (∼0.7 Km/My). Further northward, the Santa Marta Province exhumed at elevated rates during 30–25 Ma and 25–16 Ma. The highest exhumation rates within the Sierra Nevada de Santa Marta progressed toward the northwest via the propagation of NW verging thrusts. Exhumation is not recorded after ∼16 Ma, which is unexpected given the high elevation and high erosive power of the climate, implying that rock and surface uplift that gave rise to the current topography was very recent (i.e., ≤1 Ma?), and there has been insufficient time to expose the fossil apatite partial annealing zone.

Thermotectonic evolution of the Ukrainian Donbas Foldbelt revisited: new constraints from zircon and apatite fission track data

ABSTRACTThe Donbas Foldbelt (DF) is the compressionally deformed segment of a large Late Palaeozoic rift cross‐cutting the southern part of the East European Craton and is traditionally described as a classic example of an inverted intracratonic rift basin. Proposed formational models are often controversial and numerous issues are still a matter of speculation, primarily due to the lack of absolute time constraints and insufficient knowledge of the thermal evolution. We investigate the low‐temperature thermal history of the DF by means of zircon fission track and apatite fission track (AFT) thermochronology applied to Upper Carboniferous sediments. In all samples, the AFT chronometer was reset shortly after deposition in the Early Permian (∼275 Ma). Samples contained kinetically variable apatites that are sensitive to different temperatures and using statistic‐based component analysis incorporating annealing characteristics of individual grains assessed by Dpar , we identified several distinct age populations, ranging from the Late Permian (∼265 Ma) to the Late Cretaceous (∼70 Ma). We could thus constrain the thermal history of the DF during a ∼200 Myr long period following the thermal maximum. We found that earliest cooling of Permian and Permo‐Triassic age is recorded on the basin margins whereas the central parts were residing in or slowly cooling through the apatite partial annealing zone during Jurassic and most of Cretaceous times, and then finally cooled to near‐surface conditions latest around the Cretaceous/Palaeogene boundary. Our data show that Permian erosion was less significant and Mesozoic erosion more significant than generally assumed. Inversion and pop‐up of the DF occurred in the Cretaceous and not in the Permian as previously thought. This is indicated by overall Cretaceous AFT ages in the central parts of the basin.

Low-temperature exhumation history of Variscan-age rocks in the western Cantabrian Mountains (NW Spain) recorded by apatite fission-track data

This paper presents the first regional study of apatite fission-track (AFT) thermochronology to be undertaken in the western termination of the Cantabrian Mountains (NW Spain). The mountains reach elevations of over 2600 m along the northern coast of Spain and are comprised of a Variscan crustal section uplifted due to Cenozoic shortening along the northern Iberian Plate. The study constrains the pattern and history of exhumation within the Paleozoic bedrock over the past c. 240 Ma. Twenty-one apatite fission-track samples range in age from 246.7 (± 26.9) Ma to 78.1 (± 3.7) Ma, with mean track lengths between 10.4 (± 1.8) µm and 12.4 (± 1.4) µm. Time–temperature path modelling of the data indicates that different rates of continuous cooling took place during the three main tectonic events that affected the area. A rapid cooling event that ended by the Late Jurassic corresponds to topographic decay during unroofing of the Variscan orogen and the break-up of Pangea, and is responsible for the largest amount of exhumation. Westernmost samples cooled coinciding with rifting in the North Atlantic and Bay of Biscay during the Late Jurassic to Early Cretaceous. By about 100–80 Ma most samples had reached, or passed through, the upper boundary of the apatite partial annealing zone, which indicate that regional denudation has not exceeded c. 1.7 km since then, for geothermal gradients ≥ 27 °C/km and a surface temperature of 15 °C. Only three samples next to fault escarpments in the west cooled below 70 °C since 80 Ma, reaching below 65 °C before initiation of incipient subduction along the northern Iberian Margin by 46 Ma. An average cooling rate of ≤ 1 °C/Ma reflects latest denudation as the new mountainous relief developed since then due to shortening and incipient subduction associated with convergence along the northern Iberian Plate. The Cantabrian Mountains are one of the few natural examples of a coastal orogen in a juvenile stage of evolution.