Track Length Distributions Research Articles

Meso‐Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite fission track thermochronology

Apatite fission track (AFT) thermochronology of Precambrian and Paleozoic basement samples from southern Norway reveals a post‐Paleozoic exhumation history, related to offshore Mesozoic and Cenozoic extensional basin development. The data indicate two major phases of rapid exhumation. A first Mesozoic phase started in the Triassic (∼220 Ma) in the east and south of the study area and migrated to the west where Jurassic (∼160 Ma) ages of exhumation predominate. A second event is indicated by thermal history modeling of AFT ages and track length distributions. It is inferred to be Neogene in age, initiated at about 30 Ma, and it produced a domal pattern of AFT isochrons which follow present‐day topographic elevation. Youngest AFT ages (∼100 Ma) are encountered at sealevel in the inner fjords near the areas of highest topography; ages increase radially outward to the mountain peaks and the coastlines. Forward modeling of age‐elevation patterns suggests that Mesozoic geothermal gradients were 10–15°C/km higher than the present value of 20°C/km. During the Triassic and Jurassic, a total of 1.3–3.5 km of overburden was removed from the study area, assuming a 30°C/km geothermal gradient for that period. We attribute this to rift margin erosion as a result of erosional base level lowering and flank uplift, as evidenced by thick continental clastic sequences deposited in Triassic‐Jurassic half grabens in the North Sea basins. We propose that 1.5–2.5 km of Neogene exhumation were a result of late stage domal uplift. This is supported by basinward dipping pre‐Neogene strata in the basins surrounding southern Norway and the infill of a 1‐ to 2‐km‐thick Neogene sediment wedge containing various internal unconformities. Domal uplift probably started in the Late Oligocene, may have been amplified in the Pliocene, and was overprinted by Plio‐Pleistocene glacial erosion. Maximum Neogene tectonic uplift is estimated at approximately 1–1.5 km, radially decreasing outward to a value <500 m near the shoreline. Neogene domal uplift is coincident with Oligocene and Pliocene plate reorganizations in the North Atlantic; similar Neogene domes are found around the Norwegian‐Greenland Sea (i.e., Svalbard and the Barents Sea, northern Norway, east Greenland), suggesting a regional tectonic cause. The onset of Neogene uplift postdates major volcanism and continental breakup by ∼25 m.y. and predates Plio‐Pleistocene glaciations. Its origin is possibly a combination of induced mantle convection, resulting in thermal erosion of the lithosphere, and the operation of intraplate stresses.

Evidence for unetchable gaps in apatite fission tracks

The apparent shape of etched confined fission tracks in apatite depends on their crystallographic orientation, on the direction of observation and on the distribution of longitudinal etching velocities along the track. Overetched blade-like tracks, which are approximately perpendicular to the c-axis, sometimes exhibit an extremely discontinuous etching behaviour resulting from the existence of so-called “unetchable gaps”. The full length of such tracks might not be revealed under normal etching conditions. In the context of thermal history analysis, the occurrence of unetchable gaps is very important for the understanding of the relationship between measured track-length distributions and areal track densities. In contrast to previous statements concerning Durango apatite, it can be shown that the phenomenon of fragmentation is not restricted to short tracks with a length of < 11 μm but may also affect rather long tracks. During this study a significant number of natural tracks with a clearly discontinuous etching behaviour and with a total length above 14 μm has been found in samples from different locations and with different cooling histories. In the investigated samples the percentage of obviously fragmented natural tracks varies between 3% and 8%. Fragmented tracks have also been found in the fresh induced populations of two samples. Therefore, the process of fading appears to be a combination of shortening and fragmentation even at the early stages of annealing. Proportionality between the reductions of the areal track density and the etchable track length, as has been assumed in some models dealing with projected track-length distributions, does not describe the true behaviour of strongly annealed apatites.

Fennoscandian Borderzone: thermal and tectonic history of a tuffaceous sandstone and granite from fission track analysis, Bornholm, Denmark

Fission track ages for zircon and apatite together with track length distributions for apatite have been obtained from a tuffaceous sandstone of Wenlock age from Bornholm, Denmark. The zircon yields an age close to the depositional age of c. 420 Ma, which is similar to the age of bentonites from Gotland, Sweden. The apatite yields a much younger age, which is interpreted as a cooling/uplift age on the basis of the track length distribution. It is concluded that, due to burial since deposition, the sediment has been heated into the 130–190°C interval prior to 261 Ma. The complex apatite fission track age and length distribution obtained for a granite suggest reheating or prolonged heating in the 70–130°C interval probably associated with individual block movements in the area. Bornholm is located at the rim of the Fennoscandian Borderzone and thus fission track analysis of the thermal and tectonic history is important in understanding the evolution of this structure which is part of a major structural discontinuity stretching from Oslo to the Black Sea.

One‐dimensional models of groundwater flow, sediment thermal history and petroleum generation within continental rift basins

AbstractWe investigate the effects of convective heat transfer on the thermal history of sediments and petroleum formation within continental rift basins using one‐dimensional mathematical modelling. The transport equations used in this study to describe vertical groundwater flow and conductive/convective heat transfer are solved by the finite element method. Sediment thermal history is quantitatively represented using first‐order rate kinetic expressions for kerogen degradation and an empirical fanning Arrhenius model for apatite fission track annealing. Petroleum generation is also represented in the model by a suite of first‐order rate kinetic expressions. The analysis provides insights into how pore fluid circulation patterns are preserved in the rock record as anomalies in palaeogeothermometric data within continental rifts. Parameters varied in the numerical experiments include the ratio of conductive to convective heat transfer (thermal Peclet number; Pe) and the composition of the disseminated organic matter in the sediment (type II and III kerogen).Quantitative results indicate that vertical groundwater flow rates on the order of a mm/yr cause a change in computed vitrinite reflectance of the rocks and a shift in the depth to oil generation by as much as 3000 m. Differences in thermal gradients between recharge and discharge areas (Pe= 0.6) also change the width of the zone of oil generation by a factor of two. Even more dramatic, however, are the large changes in predicted apatite fission track length distributions and model ages between recharge and discharge areas. For example, a sediment package buried to a depth of 2400 m over 200 Myr within the groundwater recharge column had a fission track length distribution with a computed mean and standard deviation of 12.83 μm and 0.77 μm, respectively. The fission track model age for this sediment package was 209 Ma. The same sediment package in the discharge area has a distribution with a mean track length of 5.68 μm, a standard deviation of 3.37 μm, and a fission track model age of 2.6 Ma.Transient groundwater flow simulations, in which fluid circulation ceases after a period of time within the rift basin, are also presented to illustrate how disturbances in palaeogeothermometric parameters are preserved on geological time‐scales. Vitrinite reflectance profiles require about 10 Myr to return to conductive conditions within groundwater recharge areas while the convective disturbances are preserved indefinitely along the discharge column, as long as further subsidence does not occur. Ancient groundwater flow systems are preserved as anomalies in computed apatite fission track model ages and distributions much longer after groundwater flow stops, relative to organic‐based geothermometers. Significant differences exist in model ages between recharge (145 Ma) and discharge (90 Ma) areas 200 Myr after flow has ceased. However, calculated fission track histogram distributions are virtually identical in recharge and discharge areas after about 50 Myr.Our study suggests that ancient groundwater flow systems can be detected by comparing thermochronometric data between suspected recharge and discharge areas within continental rifts. Vitrinite reflectance profiles, observed offsets in the depth to the onset of petroleum generation, and apatite fission track annealing studies are all well suited for detecting groundwater flow systems which have been relatively long lived (107 years). Apatite fission track age data are probably best suited for identifying ancient groundwater flow systems within rifts long (&gt;200 Myr) after flow ceases.

Applications of nuclear tracks in geoscience

Geoscience is one of the application fields of nuclear tracks. In the Laboratory of Nuclear MicroAnalysis, applications in geochronology, micromapping and radon dosimetry are studied. For these purposes, theoretical models are developed and applied to mineral and polymeric detectors. First, uranium fission tracks (FT) are analysed in order to trace the thermal history of a rock. The apatite/fission track system is particularly well suited to such studies due to its low closing temperature. Then, several different models are developed to understand, and to use, the partial annealing phenomena. With this aim in mind, we propose a model allowing simulation of the projected track length distributions. According to this model, track distributions are transferred onto a time scale graph (the “age function”). This highlights the type of thermal history of the mineral and corrects the apparent FT age. Using the traditional law for the production of tracks, and experimental results for annealing kinetics, an analytical model based on a reaction situation associated with one or several activation energies has been developed. The aim of this approach is to obtain a single solution for both direct and inverse problems linking thermal history and track length data. The fission track etching process in apatite is also studied in order to model the evolution of track characteristics. A computer program has been developed, taking into account track segmentation during annealing and crystallographic parameters of apatite. Secondly, the track etching process is studied in polymeric detectors. A geometrical approach based on two distinct etching velocities is used. For light ions such as alpha particles with non-relativistic energies, etching times are short, so the bulk velocity can be considered constant. The track velocity, however, is variable. The critical angle of registration can then be determined as a function of incident energy and etching conditions. Such a model can be applied in alpha microcartography, for example, in order to determine the uranium and thorium content of geological materials. Finally, in the context of this work, a mathematical model to determine the solid state nuclear track detector (SSNTD) detection efficiency for an alpha particle emitter has been developed. This work is specially intended to measure radon using a cylindrical cell. This theoretical work makes it possible to choose the size of the cell for which the SSNTD response does not depend on the radon daughters ( 214Po and 214Po) behaviour. This size guarantees the reproducibility of the results. Such a device can be used to study radon emanation over wide areas in radioprotection and earth science applications (thermal prospecting, volcanology, etc.…).

Fission track analysis of the crystalline basement rocks of the calabrian arc, southern italy: evidence of oligo-miocene late-orogenic extension and erosion

Fission track analysis is used to provide low temperature and time constraints on the late-orogenic cooling and exhumational history of the crystalline basement rocks of the Calabrian Arc, southern Italy. 65 samples yielded 57 apatite fission track ages, 54 zircon fission track ages and 25 apatite-confined fission track length distributions. Interpretation of this data reveals a phase of increased cooling rates related to exhumation between approximately 35 Ma (mid-Oligocene) and 15 Ma (Middle Miocene). Evidence from the sedimentary record indicates significant Oligo-Miocene erosion related to this period of exhumation. New chronological constraints are also applied to localised late-orogenic extensional tectonism that has recently been identified within the basement rocks of the Calabrian Arc. This information is used to produce a new model of the Oligo-Miocene tectonic evolution of the Calabrian Arc. It proposes that the crystalline basement rocks were part of a critical orogenic wedge between the mid-Oligocene and the Middle Miocene.

Apatite fission track thermochronometry from central Alberta: Implications for the thermal history of the Western Canada Sedimentary Basin

Apatite separated from 47 Cretaceous and Tertiary drill core and surface samples from the foreland basin strata of the Western Canada Sedimentary Basin (WCSB) have been analyzed for their apatite fission track (AFT) age and confined track length distribution. Thermal histories of the fission track data were then estimated using a constrained random search inversion technique based on the Durango apatite annealing model. Most importantly, this technique provides an estimate of the peak postdepositional temperature experienced by each sample with error bounds determined by the precision of the fission track data. Most apatite samples retain at least some tracks that formed prior to the time of maximum Cenozoic burial, assumed to be coincident with maximum temperature. Seven apatite samples were fully annealed during burial, during or following the Laramide Orogeny, and provide a minimum age of 42 Ma for heating. Lower Cretaceous samples from near the deformation front were fully annealed and attained temperatures of at least 119°–138°C during maximum burial. In contrast, stratigraphically equivalent samples at the northeastern end of the transect near the Cold Lake heavy oil and tar sand deposits did not exceed 80°C. Paleogeothermal gradients calculated using bounded estimates of the peak temperatures experienced by the samples, together with maximum burial estimates from coal moisture studies, range from ∼20°C/km near the deformation front to as high as 60°C/km near the cratonic edge of the basin. This variation has the same trend as present geothermal gradients, which range from ∼30°C/km to 45°C/km, but indicates a greater contrast at the end of the Laramide. The results are consistent with the concept of heat transport by basinal scale fluid flow.

Syn-rift thermal structure and post-rift evolution of the Oslo Rift (southeast Norway): New constraints from fission track thermochronology

The Permo-Carboniferous (240–305 Ma) Oslo Graben in southeast Norway is a classical magmatic continental rift. We present fission track (FT) data on apatites, zircons and sphenes from the rift and surrounding areas in order to clarify its syn- and post-rift thermal evolution. Zircons and sphenes within the rift record FT ages of between ∼ 270 and 180 Ma (i.e., syn-post-rifting). In contrast, the Precambrian basement surrounding the graben yields zircon FT/sphene FT ages of ∼ 500–650 Ma, indicating that heating associated with rifting was focused inside the rift as a result of advective heat flow. Syn-rift to early post-rift temperatures reached > 240°C at the present erosion level in the graben. Syn-rift advective heating ( ∼ 260–270 Ma) was probably a result of large-scale batholith intrusion; post-rift ( ∼ 180–220 Ma) heating seems to have occurred by hydrothermal circulation of high-temperature (100–300°C) fluids. The FT data, organic geochemical indicators and fluid inclusions suggest that hydrothermal circulation followed a highly complex pattern controlled by the tectonic and volcanic structure of the rift. Apatite FT ages and confined track length distributions reveal the post-rift exhumation history of the area. Apatite FT ages are continuous within and outside the graben and decrease from Triassic (200–240 Ma) in the southeast to Jurassic ( ∼ 160 Ma) in the northwest, indicating that post-rift exhumation took place on a much larger scale than Permian rifting. Modelling of apatite FT ages and track lengths suggests cooling/denudation events in the Triassic, Jurassic and Neogene, accounting for a total post-Permian erosion of 3–4 km. Triassic-Jurassic denudation coincides with the migration of rifting from the Oslo-Skagerrak area to the North Sea/Danish Basin, as revealed by offshore seismic data, and caused destruction of the hydrothermal regime. Neogene uplift and erosion of the area is also supported by seismic evidence. The inferred timing and amount of regional post-rift erosion suggests that post-rift subsidence and sedimentation within the Oslo Graben was minimal.

The denudation history of the onshore continental margin of SE Brazil inferred from apatite fission track data

A suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast Brazil are presented. Some of the results reflect resetting related to the eruption of the Paraná continental flood basalt (∼130 Ma), and four others are directly related to Late Cretaceous magmatism (∼80 Ma). The majority of the results were obtained from Precambrian basement rocks and sediments from the Paraná Basin. The fission track ages broadly increase inland from 60–90 Ma on the coastal plain to &gt;300 Ma in the continental hinterland. The track length distributions show a characteristic trend, being negatively skewed at the coast and showing increasing bimodality inland, except for the oldest samples which are more unimodal. These trends do not reflect rift‐related heating and resetting but are the result of protracted denudation since the opening of the South Atlantic. The results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland. The topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology. For the region further south, model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long‐wavelength topography, unless the flexural rigidity is more than 1025 N m. In this case the initial elevation of the rift margin would need to be &gt;2.5 km in order to accommodate the amount of denudation inferred from the fission track data. However, it is possible that the effective flexural rigidity varies across the margin, becoming greater in the continental interior. Alternatively, the broad scale wavelength topography may be the result of magmatic underplating related to the Paraná continental flood basalt event. If so, estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5.

Major Late Cretaceous cooling events in the eastern Peninsular Ranges, California, and their implications for Cordilleran tectonics

Fission-track ages from the Peninsular Ranges batholith in the San Jacinto Mountains near Palm Springs, California, indicate two major late Cretaceous cooling events. Concordant sphene and zircon fission-track ages of plutonic rocks (tonalite and low-K granodiorite) range from ca. 85 Ma to 98 Ma, with a mean of 92 Ma. The concordance of these ages over 2.5 km of exposed vertical section, coupled with previous U-Pb zircon dating (94-93 Ma), implies major cooling from crystallization temperatures to below ∼235 °C between 94 Ma and ca. 92 Ma. Cooling is attributed to initial intrusion of the tonalite and granodiorite into an evolving brittle-ductile normal fault zone (detachment) followed by ∼10-14.5 km of exhumation during continued crustal extension along the Santa Rosa mylonite belt. Field relationships, microstructural fabrics, and amphibole thermobarometry from the mylonite belt support the notion that extensional faulting was preceded by west-directed thrusting. This sequence of tectonic thickening followed by extension is similar to other areas in the southwestern Cordillera and may represent the creation and failure of an unstable regional crustal welt in Late Cretaceous time. A second event is evident from concordant fission-track ages and indistinguishable tracklength distributions from apatite at all elevations. These data indicate very rapid cooling of the tonalitic plutons through the apatite partial annealing zone (∼130-70 °C) between ca. 74 Ma and 80 Ma (mean age of 76 Ma). The occurrence of Late Cretaceous-age regional unconformities throughout Baja and southern California suggests that this cooling event was the result of surface uplift and erosion. By using a reasonable range of geothermal gradients (20-40 °C/km), the cooling data imply a minimum of 4-5 km of exhumation in conjunction with the ca. 76 Ma event. Surface uplift coincided with several important changes in the North American and Farallon (or Kula) plate motions. These include an increase in convergence rate, an increase in the absolute velocity of the North American plate, and a change in the direction of convergence between the plates. The changes in plate motions are thought to have initiated low-angle or flat-slab subduction that, in turn, was responsible for the Laramide deformation of the Cordilleran foreland. Surface uplift, erosion, and associated cooling during exhumation of the eastern Peninsular Ranges are considered to be an initial expression of Laramide tectonism. Phenomena documented here are consistent with uplift brought on by tectonic erosion of the North American lithospheric mantle due to low-angle subduction and by subsequent underplating of lower crustal rocks.

Middle tertiary hydrothermal activity and uplift of the northern flinders ranges, South Australia: Insights from apatite fission‐track thermochronology

Apatite fission‐track thermochronology reveals that hydrothermal activity was pervasive during mid‐Tertiary time within the Paralana Fault zone and adjacent country rocks of the northern Flinders Ranges, South Australia. Apatite fission‐track ages from the Mt Painter block, west of the Paralana Fault zone, range between ∼ 175 and 260 Ma. The mean lengths of confined fission‐tracks for these samples are ∼ 12 μm with s.d. of the track length distributions of ∼ 2 μm. These data suggest that rocks exposed in the Mt Painter block cooled slowly though the apatite partial annealing zone (∼ 110–50°C) starting before 200 Ma and continuing into the Tertiary. In contrast, apatite samples from the Paralana Fault zone yield apparent ages as young as 25–30 Ma and mean track lengths mostly < 12 μm, with s.d. ∼ 3 μm. These young apparent ages and relatively short track lengths require mid‐Tertiary palaeotemperatures of 90–105°C for the fault zone, assuming a heating time in the order of 1 Ma. This heating is interpreted to be the result of hydrothermal activity within the fault zone when it was buried beneath Mesozoic and Tertiary sedimentary strata. At least one stage of uplift of the northern Flinders Ranges during Miocene time elevated these rocks above the basin sediments, resulting in cooling due to a significant reduction in hot springs activity.

An Evolutionary Approach to Thermal History Modelling with Fission Track Data

Recent advances in our understanding of fission track annealing have led to empirical predictive models such that, given various assumptions, it is possible to predict the expected fission track parameters (age, track length distribution) for a given thermal history (Laslett et al. 1985; Green et al. 1989; Carlson 1991). In practice, however, it is the fission track parameters that are measured, and the thermal history is unknown. In sedimentary basins, the thermal history is intimately linked to the burial history and a reasonable reconstruction of both is often possible when a suite of downhole samples is used. However, in situations where only surface samples are analysed, the thermal history and cooling/exhumation rates must often be constrained from the fission track data alone. In principle, analytical inversion methods could be used to determine the thermal history directly from the data, but the mathematical formulations are nonqinear. This non-lineadty can lead to computational problems with analytical methods (e.g. instabilities in matrix inversions and partial 9 derivative calculations, an initial guess very close to the true thermal history). The resulting method may be unstable and a rigorous resolution analysis of the solution is difficult. Alternatively, more robust optimisation methods which avoid many of these pitfalls can be used.

Apatite fission-track evidence for Laramide and post-Laramide uplift and anomalous thermal regime at the Beartooth overthrust, Montana-Wyoming

The crystalline rocks of the Beartooth overthrust in southwest Montana and northwest Wyoming were uplifted and thrust over the northwest margin of the Bighorn basin during the Laramide orogeny. The timing and geometry of that event is well documented in the synorogenic Paleocene and early Eocene sedimentary rocks that were deposited ahead of the rising Beartooth basement block and partially over-ridden as the block advanced. The absence of post-Laramide sedimentary rocks on the block and in the adjacent Bighorn Basin has, however, precluded the reconstruction of the post-orogenic tectonothermal history of this region. We report here the results of fission-track analysis of apatite from the three components of the Beartooth over-thrust with the aim of reconstructing the Laramide and post-Laramide tectono-thermal history of the southeast margin of the Beartooth Mountains. Apatite fission-track ages and corresponding horizontal confined track length distributions (HCTLDs) from Precambrian basement rocks constituting the upper plate of the Beartooth overthrust indicate that from 7 to 12 km of uplift of the Red Lodge corner of the Beartooth block has occurred since early Paleocene time. This amount of uplift occurred in two stages, with an intervening mid-Tertiary period of tectonic quiescence. The latter was a period of either (1) Oligocene and/or Miocene deposition or (2) tectonic quiescence. Uplift of 4 to 8 km occurred during the first phase of cooling, which lasted from early Paleocene time (∼61 Ma) to early Eocene time (∼52 Ma). During the second phase, which began in late Miocene-early Pliocene time and continues to the present day, about 4 km of uplift occurred. Our fission-track data suggest that the thermal regime in rocks above the Beartooth overthrust was relatively stable during Tertiary time. The maximum geothermal gradient permitted by model thermal histories generated from our observed fission-track data during post-Laramide time is 17 °C/km. This value is the same as that of the present-day geothermal gradient measured in the Amoco Beartooth well and suggests that a low geothermal gradient prevailed throughout Tertiary time. Apatite fission-track ages and HCTLDs from Jurassic and Cretaceous sedimentary rocks beneath the Beartooth overthrust indicate that these rocks have remained 10-20 °C cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since ∼61 Ma, an observation that is not consistent with the stratigraphic position of these rocks. We interpret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate cooled them and insulated them from conductive heat transfer from hotter overlying rocks; this ground-water circulation may have been responsible for flushing hydrocarbons out of the rock column explored by the Beartooth well. Shear-zone rocks experienced a higher temperature during Cenozoic time than did rocks of the upper and lower plates; this condition was maintained by flow through the shear zone of ground water heated to higher temperatures at deeper levels along the thrust.

On the use of 60°C “cooling ages” obtained using projected fission-track lengths in apatite

The estimation of fission-track ages using counts of projected track lengths, in particular, 60°C cooling ages, is based on determining the proportion of tracks greater than a particular length. We show that errors on 60°C cooling ages are significantly greater than have been previously estimated; for example, a minimum error on such ages as they are conventionally determined, is ±22% at the 95% confidence level. In addition, the equation used to determine projected track length ages, involving a constant relationship between track shortening, conventional age reduction, and the proportion of projected track lengths greater than 10 μm, is shown not to apply generally. It is recommended that the practise of estimating 60°C cooling ages using projected track length distributions be substantially re-evaluated.

Possible secondary apatite fission track age standard from altered volcanic ash beds in the middle Jurassic Carmel Formation, Southwestern Utah

Secondary age standards are valuable in intra- and interlaboratory calibration. At present very few such standards are available for fission track dating that is older than Tertiary. Several altered volcanic ash beds occur in the Middle Jurassic Carmel Formation in southwestern Utah. The formation was deposited in a shallow marine/sabhka environment. Near Gunlock, Utah, eight ash beds have been identified. Sanidines from one of the ash beds (GUN-F) give a single-crystal laser-probe 40Ar/ 39Ar age of 166.3±0.8 Ma (2 σ). Apatite and zircon fission track ages range from 152–185 Ma with typically 15–20 Ma errors (2σ). Track densities in zircons are high and most grains are not countable. Apatites are fairly common in most of the ash beds and have reasonable track densities ranging between 1.2–1.5 × 10 6 tracks/cm 2. Track length distributions in apatites are unimodal, have standard deviations <1 μm, and mean track lengths of about 14–14.5 μm. High Cl apatites (F:Cl:OH ratio of 39:33:28) are particularly abundant and large in ash GUN-F, and are fairly easy to concentrate, but the concentrates contain some siderite, most of which can be removed by sieving. GUN-F shows evidence of some reworking and detriaal contamination based on older single grain 40Ar/ 39Ar analyses and some rounding of grains, but the apatite population appears to be largely uncontaminated. At present BJK has approximately 12 of apatite separate from GUN-F.

The use of the Projected Track Length Distributions of fission tracks in apatite for thermal hisory analysis

The use of the Projected Track Length Distributions of fission tracks in apatite for thermal hisory analysis

Results of interlaboratory comparison of fission-track ages for the 1992 fission track workshop : Miller, Donald S., Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, Crowley, Kevin D., Geology Department, Miami University, Oxford, OH 45056, DOKKA, Roy K., Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803 Galbraith, R.F., Department of Statistical Science,

New apatite (56), sphene (5) and zircon (22) fission-track ages from granitoid rocks are combined with K-Ar ages (both new and published) to reveal a thermo-tectonic history of the West Coast, South Island, New Zealand. Although many of the intrusives are thought to have been emplaced in Palaeozoic time, later thermal events are revealed as a result of resetting of the various isotopic clocks. Modal ages of 105, 95 and 75 Ma on biotite (K-Ar), zircon and apatite (fission track), respectively, represent a Mesozoic cooling/uplift of the granites. A few zircon and many apatite ages contain further overprinting, representing later uplift during the Miocene. On the basis of these events, the West Coast can be subdivided into three tectonic regions.Several of the present granitoid mountains were the sites of NE-SW elongate sedimentary basins in the Early Tertiary. A direct correlation can be made between coal rank (percent vitrinite reflectance) of Eocene coals immediately above basement granites and the underlying granite apatite “age”. Granites, with associated overlying coals having a percent vitrinite reflectance of > 0.7, have been buried sufficiently deeply for the apatites to be reset prior to basin uplift in the Miocene. Those coals with a reflectance of generally < 0.55 overlie granites that have retained a Late Mesozoic age. Intermediate or mixed ages (55 Ma) are found where the vitrinite reflectance of coal lies between these two values, although there is some overlap. These are the product of partial annealing, a conclusion supported by confined track-length distributions. Ages on samples on either side of major faults suggest that these faults may have been active both during basin development and Miocene uplift; hence, their sense of movement has reversed with time.

Palaeogeothermal interpretation of surface track-length distribution in apatite

Palaeogeothermal interpretation of surface track-length distribution in apatite

Post-Palaeozoic cooling and uplift of the Brabant Massif as revealed by apatite fission track analysis

AbstractA fission track study has been carried out on apatite from the igneous rock belt running along the southern border of the Brabant Massif. The study includes age determinations and a length analysis of both surface tracks and confined tracks. Apatite fission track ages vary between 146 Ma and 209 Ma. Confined track length distributions and the projected length age spectra indicate that the rocks cooled relatively rapidly from above 100 °C to ambient temperatures. The fission track ages therefore date a cooling phase of the Brabant Massif which is interpreted as reflecting an important uplift during the major part of the Jurassic, related to the Cimmerian tectonism which affected the North Sea basin and adjacent areas. Two apatite samples from the southerly Dinant Basin yield fission track ages around 200 Ma, similar to the oldest ages observed in the Brabant Massif, and with comparable track length characteristics. This indicates that the uplift was not limited to the Brabant region but also affected the Hercynian basement to the south.

Burial and exhumation of the Long Range Inlier and its surroundings, western Newfoundland: results of an apatite fission-track study

The Long Range Inlier, a steep-sided plateau underlain mainly by Grenvillian gneisses, is the most prominent topographic feature of western Newfoundland. Apatite fission-track analysis of 31 samples from the Long Range Inlier and its surroundings yielded measured apparent ages of 343–152 Ma. Age versus elevation plots, track-length distributions, and model thermal histories indicate that the region experienced slow cooling in the late Paleozoic, with apparent exhumation rates of 7–9 m∙Ma−1 and cooling rates of 0.08–0.28 °C∙Ma−1. Model thermal histories suggest that the present upper surface of the Long Range plateau cooled below ~120 °C in Ordovician times. The thermal histories are compatible with, but do not require, some exhumation of the Long Range Inlier along Acadian thrust faults. Results from Early Carboniferous sedimentary rocks of the Deer Lake Basin are similar to Long Range Inlier data from similar elevations, implying that at some time between ~350 and 300 Ma, the entire region was buried to depths sufficient to induce total annealing (T &gt; 120 °C) in these samples. Closure ages determined from model thermal histories indicate that regional cooling to temperatures below ~120 °C began before 300 Ma. The Carboniferous sedimentary cover was largely removed by Jurassic time, perhaps in response to lowering of regional base level by rifting associated with the opening of the Atlantic Ocean.