Fission-track Grain Research Articles

Testing inferences from palaeocurrents: application of zircon double-dating to Miocene sediments from the Hengchun Peninsula, Taiwan

Detrital zircon provenance studies that combine low-temperature fission-track and hightemperature U-Pb single-grain age dating are powerful in constraining sediment provenance by documenting the tectono-thermal evolution of the sediment source(s). We apply these techniques to Miocene (12-6 Ma) sandstones of the Hengchun Peninsula, southern Taiwan which, based on diametrically opposite palaeocurrents, have a controversial provenance. UPb grain ages range from Miocene (8 Ma) to Archaean (2.5 Ga). Cretaceous thermal cooling is recorded by detrital zircon fission-tracks at Loshui and Lilongshan. Permian fission-track grain ages account for <33% of zircon ages from Loshui, while at Lilongshan Jurassic/Triassic grain ages account for 33-66% of all zircon fission-track ages. Minor (<12%) Miocene age components are detected in both formations. These new data suggest that the primary source of these Miocene sediments was similar. A simple model is proposed invoking sediment reworking in this complex tectonic setting. Author Final Draft or ‘Post-Print’ Version. The final version was published in Terra

Pliocene onset of rapid exhumation in Taiwan during arc–continent collision: new insights from detrital thermochronometry

ABSTRACTThe Coastal Range in eastern Taiwan contains the remnants of the Pliocene–Pleistocene retro‐foredeep basin of the ongoing Penglai orogeny. These sedimentary successions record the earliest exhumation of the Central Range, Taiwan. We dated detrital Plio‐Pleistocene sediments in the Coastal Range using multiple thermochronometers [fission‐track, zircon (U–Th)/He and U/Pb dating] to document changes in exhumation rate through time. Fission‐track grain ages in 2–4‐Myr‐old sediments were not reset by the Penglai orogeny and reflect the early stage removal of the sedimentary cover. This early stage, when exhumation rates were low, could encompass both the accretionary wedge phase of the orogen and the early arc–continent collision. Sediments younger than 2‐Myr‐old yield Pliocene zircon fission‐track grain ages and suggest that exhumation, transport and deposition occurred within 0.4–1.5 Myr. The recorded onset of rapid exhumation in the Pliocene is contemporaneous with other major tectonic changes in the region, including an increase in subsidence rate in both the pro‐ and retro‐foredeep basins and a change in the wedge kinematics from internal shortening to underplating.

Pebble population dating as an additional tool for provenance studies - examples from the Eastern Alps

Abstract Detrital fission-track (FT) dating can be successfully used in provenance studies of siliciclastic sediments to define the characteristic cooling ages of the source regions during erosion and sedimentation. In order to obtain more specific information about potential source regions we have developed the pebble population dating (PPD) method in which pebbles of specific lithotype are merged and dated. Dating of both zircon and apatite crystals from such pebble populations yields age distributions, which reflect the cooling ages of the given lithotype in the source area at the time of sedimentation. By this technique it is possible to define ‘FT litho-terrains’ in the source regions and thus outline palaeogeological maps. Two examples are presented from the Eastern Alps. (i) Comparison of FT ages from a sandstone sample and a gneiss PPD sample from an Oligocene conglomerate of the Molasse Basin shows that the youngest age cluster is present only in the sand fraction and derived from the Oligocene volcanic activity along the Periadriatic zone. The lack of the youngest ages in the gneiss pebble assemblage excludes the Oligocene exhumation of the crystalline basement from mid-crustal level. (ii) Pebble assemblages of red Bunter sandstone, gneiss and quartzite were collected from an Upper Miocene conglomerate of the Molasse Foreland Basin and merged as PPD samples. Apatite and zircon FT grain age distributions of these PPD samples, representing the largest ancient East Alpine catchment, allow generating a new combination of palaeogeological and palaeo-FT-age maps of the Eastern Alps for the Late Miocene.

Late Oligocene–Early Miocene initiation of shortening in the Southwestern Chinese Tian Shan: Implications for Neogene shortening rate variations

The Cenozoic Tian Shan is one of the preeminent examples of an intracontinental orogen. However, there remains a significant controversy over when deformation related to the India-Asia collision commenced and therefore how shortening within the mountains has been partitioned over time. One approach has been to look at the modern shortening rate as measured by geodetic studies, combined with estimates of the total shortening across the range and extrapolate backwards. This approach suggests that the onset of range construction was ca. 10 Ma [K.Y. Abdrakhmatov, S.A. Aldazhanov, B.H. Hager, M.W. Hamburger, T.A. Herring, K.B. Kalabaev, K.B. Kalabayev, V.I. Makarov, P. Molnar, S.V. Panasyuk, M.T. Prilepin, R.E. Reilinger, I.S. Sadybakasov, B.J. Souter, Y.A. Trapeznikov, V.Y. Tsurkov, A.V. Zubovich, Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates, Nature 384 (6608) (1996) 450–453]. An alternate method is to determine the age of the onset of exhumation using thermochronology. We present 19 new apatite fission-track (AFT) results from the southwestern Chinese portion of the belt; this region represents the first area exhumed during the late Tertiary along a transect at ca. 76°E. Exhumation commenced at the Oligocene–Miocene boundary (∼ 24 Ma) along the Maidan and Muziduke thrusts, which bound the southern side of the Kokshaal range. Subsequently, deformation propagated ca. 20 km south to the Kashi basin-bounding thrust (KBT), which was exhumed by no earlier than 18.9 ± 3.3 Ma. Three detrital AFT samples from Plio-Pleistocene strata deposited ca. 20 km farther south contain fission track grain age peaks that young monotonically upwards from 20.9 + 7.0/− 5.3 Ma to 15.9 + 5.4/− 4.0 Ma with a fairly constant lag time of 16 to 18 Ma. These ages, combined with structural data, suggest that both the hanging wall and the footwall of the KBT experienced a renewed episode of exhumation during the latest Cenozoic. The discrepancy between the Late Oligocene–Miocene initiation of significant exhumation shown herein and the 10 Ma initiation estimate from geodesy suggests that the Tian Shan has undergone a complex Late Cenozoic shortening history. Assuming that the present shortening rate could account for the total amount of Cenozoic shortening in 10 Ma and realizing that shortening initiated at least 15 Myr earlier, we conclude that the shortening rate must have varied over time, possibly in pulsed-southward migrating events, and that the present rate may not reflect the average rate since initiation of range uplift.

Provenance and tectonic settings of accretionary wedge sediments on northeastern Karaginski Island (Kamchatka, Russian Far East)

The provenance and tectonic setting of the sediments and rare cobbles from the accretionary wedge on Karaginski Island were examined. This study provides a picture of the nature of the NE Asian paleomargin in the middle Eocene to early Miocene. The trace element geochemistry of shale from the flysch and melange indicates (a) their common provenance with the quartzofeldspathic and arkosic sandstones, and (b) derivation of these sediment types from active continental and island arcs with partially dissected basement rocks. Previous studies of the sandstones (Shapiro et al., 2000) showed that they dominate in debris whose composition is very similar to those of the felsic extrusive and intrusive rocks in cobbles and boulders embedded in the sediments. Fission-track (FT) ages from detrital zircon from sandstones, and felsic cobbles show their derivation from sources with several cooling histories in the Late Cretaceous and Early Tertiary. The youngest detrital component probably represents continuous younging population from active volcanism in the source that is nearly the same age of deposition. The felsic cobbles represent the essentially random selection of the older material component. They include three distinct types: (a) subduction-related low-K tonalites and trondhjemites and their extrusive analogues; (b) crustally-derived med-K two-mica leucogranites, and (c) crustally-derived high-K biotite leucogranites. The felsic rock cobbles of all types have a narrow range of zircon FT grain ages that indicate cooling in the source in the Late Cretaceous to Early Paleogene. Tonalites and trondhjemites (low-K) cooled at 53.8 ( 3.5/+3.8) Ma and 78.5 ( 6.7/+7.6) Ma. The rhyolites (med-K) cooled at 70.1 ( 3.7/+3.9) Ma. The biotite leucogranites (high-K) cooled between 59.1 Ma ( 3.3/+3.9) and 58.6 ( 3.2/+3.3) Ma (two clasts). The Late Cretaceous cooling ages (70-78 Ma) probably reflect original magmatic activity whereas the Early Eocene cooling ages may indicate timing of exhumation. The chemical and age data obtained on the felsic cobbles suggest derivation of the accretionary wedge sediments from a source formed either by: (a) subduction in the Late Cretaceous and collision/exhumation in the Early Paleogene; or by (b) continuous subduction. The closest petrochemical and age intrusive and extrusive analogues of the felsic cobbles occur only locally in the Olyutorsky Peninsula subterrane. Nonetheless, this structure is too small and too remote from Karaginski Island to supply enough material for the middle Eocene-early Miocene accretionary wedge sediments. We speculate that the structure of the Olyutorsky Peninsula subterrane might be extended in the shelf where several highs exist.

Detrital-zircon fission-track ages for the “Hoh Formation”: Implications for late Cenozoic evolution of the Cascadia subduction wedge

We report new fission-track (FT) ages for detrital zircons for 34 sandstone samples and 2 volcanic ash beds from the “Hoh Formation,” exposed along the western side of the Olympic Mountains of western Washington State. The “Hoh Formation” is now formally known as the coastal unit of the Olympic Structural Complex, or Coastal OSC for short. About 35 zircons were dated per sample. The fission-track grain-age (FTGA) distributions are all strongly discordant; grain ages range from 10 to older than 100 Ma. Low vitrinite-reflectance values, short etch times for the zircons, and a broad range of grain ages indicate that the zircon FT ages are unreset and thus preserve information about cooling events in the source region for these sedimentary rocks. Five areas were sampled repeatedly and yield similar FTGA distributions, demonstrating that sampling errors are not a problem. We show that almost all of the samples contain a well-defined young component that was probably derived from a contemporaneous active volcanic source, presumably the adjacent Cascadia arc. Binomial peak-fitting was used to estimate the FT minimum age, which is the age of the youngest concordant fraction of zircon FT grain ages in a FTGA distribution. In most cases, minimum ages are similar to fossil ages where available. This result supports our contention that zircon FT minimum ages from volcaniclastic sandstones commonly can be used as a proxy for depositional age. Our zircon FT minimum ages indicate that the Coastal OSC is made up mainly of lower Miocene (ca. 24 to 16 Ma) sedimentary rocks. We use these age data, together with other geologic constraints, to reconstruct a tectonic history. Sedimentary rocks of the Coastal OSC were derived from a mixed-source region that included an active volcanic arc and also older units, including Cretaceous metamorphic rocks, probably located in the Omineca crystalline belt in the Canadian Rockies. The upper part of the Clallam Formation, located on the northern side of the Olympic Peninsula, appears to be a remnant of the sedimentary system that fed the Coastal OSC. The sediments that formed the Coastal OSC were initially deposited seaward of the Cascadia trench, at water depths of >2000 m. This debris was deposited seaward of the Cascadia trench, at water depths of >2000 m, and subsequently accreted beneath the frontal 50 to 100 km of the wedge. Owing to continued accretion at the front of the wedge, and erosion of the forearc high in back of the wedge, these lower Miocene sediments were moved rearward within the Cascadia subduction wedge. A simple relationship based on the cross-sectional area of the wedge and a steady accretion flux indicates that it would have taken ∼22 m.y. for the Coastal OSC to reach its present position 140 km landward of the toe of the wedge. This estimate is in good agreement with the unit9s early Miocene age.

Towards a more complete record of magmatism and exhumation in continental arcs, using detrital fission-track thermochrometry

The NE Asian margin was the locus of an Andean-style subduction zone during most of the Mesozoic and Cenozoic. The Ukelayat flysch is a 10 to 15 km thick section of marine turbidites that were deposited in a 500 km long forearc basin along the outboard edge of this convergent margin. It was structurally overridden during the early Tertiary by an obducted island arc, the Olytorsky terrane. We present new fission-track (FT) grain ages from unreset detrital zircons. These data provide precise information about depositional age of the Ukelayat and the temporal evolution of the Asian margin continental arc, which provided much of the sediment for the Ukalayat. Because all sandstones contain a large fraction of first-cycle volcanic zircons (euhedral, colorless), we infer that the youngest component of the FT grain age distribution, designated P1, closely approximates the time of deposition. Twenty-seven samples have yielded P1 ages ranging from ~44 to 88 Ma, which indicates continuous magmatic activity in the arc from the Late Cretaceous to Middle Eocene. A second component (P2) records progressive exhumation of the basement to the Okhotsk-Chukotka arc. The difference between P1 and P2 ages indicates steady exhumation at ~200–400 m/Myr from ~90 to 44 Ma. This finding implies the removal of 9 to 18 km of rock in the most deeply exhumed parts of the source region. A change in exhumation rates at ~70 Ma may coincide with an eastward shift in the locus of volcanism from the Okhotsk-Chukotka arc to the younger Western Kamchatka-Koryak arc.

Exhumation history of orogenic highlands determined by detrital fission-track thermochronology

Abstract A relatively new field in provenance analysis is detrital fission-track thermochronology which utilizes grain ages from sediment shed off an orogen to elucidate its exhumational history. Four examples highlight the approach and usefulness of the technique. (1) Fission-track grain age (FTGA) distribution of apatite from modern sediment of the Bergell region of the Italian Alps corresponds to ages obtained from bedrock studies. Two distinct peak-age populations at 14.8 Ma and 19.8 Ma give calculated erosion rates identical to in situ bedrock. (2) Zircon FTGA distribution from the modern Indus River in Pakistan is used to estimate the mean erosion rate for the Indus River drainage basin to be about 560 m Ma −1 , but locally it is in excess of 1000 m Ma −1 . (3) FTGA distribution of detrital apatite and zircon from the Tofino basin records exhumation of the Coast Mountains in the Canadian Cordillera. Comparison of detrital zircon and apatite FT ages gives exhumation rates of c. 200 m Ma −1 during the interval between c. 34 and 54 Ma, but higher rates ( c. 1500 m Ma −1 ) at c. 56 Ma. (4) FTGA analysis of apatite grain ages from a young basin flanking Fiordland in New Zealand indicates that removal of cover strata was followed by profound exhumation at c. 30 Ma, which corresponds to plate reorganization at this time. Exhumation rates at the onset of exhumation were c. 2000–5000 m Ma −1 . These studies outline the technique of detrital FTGA applied to exhumation studies and highlight practical considerations: (1) well-dated, stratigraphically coordinated suites of samples that span the exhumation event provide the best long-term record; (2) strata from the basin perimeter are the most likely to retain unreset detrital ages; (3) the removal of ‘cover rocks’ precedes exhumation of deeply buried rocks, which retain a thermal signal of the exhumation event; (4) steady-state exhumation produces peak ages that progressively young with time and have a constant lag time; (5) same-sample comparison of zircon and apatite peak ages is best in sequences with high-uranium apatite grains (&gt;50 ppm), and peak-ages statistics can be improved by counting numerous apatite grains (&gt;100).

Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State

The apatite fission-track method is used to determine the exhumation history of the Olympic subduction complex, an uplifted part of the modern Cascadia accretionary wedge. Fission-track ages are reported for 35 sandstones from the Olympic subduction complex, and 7 sandstones and 1 diabase from the Coast Range terrane, which structurally overlies the Olympic subduction complex. Most sandstone samples give discordant results, which means that the variance in grains ages is much g reater than would be expected for radioactive decay alone. Discordance in an unreset sample is caused by a mix of detrital ages, and in a reset sample is caused by a mix of annealing properties among the detrital apatites and perhaps by U loss from some apatites. Discordant grainage distributions can be successfully interpreted by using the minimum age, which is the pooled age of the youngest group of concordant fission-track grain ages in a dated sample. The inference is that this fraction of apatites has the lowest thermal stability, and will be the first to reset on heating and the last to close on cooling. Comparison of the minimum age with depositional age provides a simple distinction between reset samples (minimum age younger than deposition) and unreset samples (minimum age older than deposition). The success of the minimum-age approach is demonstrated by its ability to resolve a well-defined age-elevation trend for reset samples from the Olympic subduction complex. Microprobe data suggest that the apatites that make up the minimum-age fraction are mostly fluorapatite, which has the lowest thermal stability for fission tracks among the common apatites. Reset minimum ages are all younger than 1 5Ma, and show a concentric age pattern; the youngest ages are centered on the central massif of the Olympic Mountains and progressively older ages in the surrounding lowlands. Unreset localities are generally found in coastal areas, indicating relatively little exhumation there. Using a stratigraphically coordinated suite of apatite fission-track ages, we estimate that prior to the start of exhumation, the base of the fluorapatite partial annealing zone was located at ~10 0°C and ~4.7 km depth. The temperature gradient at that time was 19.6 ± 4. 4°C/km, similar to the modern gradient in adjacent parts of the Cascadia forearc high. Apatite and previously published zircon fission-track data are used to determine the exhumation history of the central massif. Sedimentary rocks exposed there were initially accreted during late Oligocene and early Miocene time at depths of 12.1‐14.5 km and temperatures of ~242‐28 9°C. Exhumation began at ca. 18 Ma .A rock currently at the local mean elevation of the central massif (1204 m) would have moved through the α-damaged zircon closure temperature at about 13.7 Ma and ~10.0 km depth, and through the fluorapatite closure temperature at about 6.7 Ma and ~4. 4km depth. On the basis of age-elevation trends and paired cooling ages, we find that the exhumation rate in the central massif has remained fairly constant, ~0.75 km/m.y., since at least 14 Ma. Apatite fission-track data a re used to construct a contour map of long-term exhumation rates for the Olympic Peninsula. The average rate for the entire peninsula is ~0.28 km/m.y., which is comparable with modern erosion rates (0.18 to 0.32 km/m.y.) estimated from sediment yield data for two major rivers of the Olympic Mountains. We show that exhumation of this part of the Cascadia forearc high has been dominated by erosion and not by extensional faulting. Topography and erosion appear to have been sustained by continued accretion and thickening within the underlying Cascadia accretionary wedge. The rivers that drain the modern Olympic Mountains indicate that most of the eroded sediment is transported into the Pacific Ocean, where it is recycled back into the accretionary wedge, either by tectonic accretion or by sedimentary accumulation in shelf and slope basins. The influx of accreted sediments is shown to be similar to the outflux of eroded sediment, indicating that the Olympic segment of the Cascadia margin is currently close to a topographic steady state. The record provided by our fissiontrack data, of a steady exhumation rate for the central massif area since 14 Ma, suggests that this topographic steady state developed within several million years after initial emergence of the forearc high.

Probability density plot for fission-track grain-age samples

This paper outlines the statistical basis, construction and interpretation of probability-density (PD) plots as applied to the analysis of mixed distributions of fission-track (FT) grain ages. Such distributions can be viewed as being composed of a mixture of elemental component distributions, with each component characterized by a unique average FT age. The original PD plot of Hurford et al. is shown to have a clear statistical basis, as defined by the Gaussian-kernel method of density estimation. This background is used to develop and justify an improved version of the PD plot. Three modifications are recommended: (1) as originally suggested by Galbraith, the PD plot should be constructed using the transform variable z, which is approximately proportional to the logarithm of FT age. By using z i to represent the FT grain ages τ i , each component distribution within a mixed FTGA sample becomes approximately Gaussian-distributed. This approximation is shown to work well for FT dating of zircon grains, which generally have relatively high uranium content and high track densities, and less so for FT dating of apatite grains; (2) estimation of the PD plot is optimized by setting the width of the Gaussian-kernel equal to α( SE( z ̂ i) , where α is a scaling factor with an optimal value of ∼ 0.6 and SE( z ̂ i) is the standard error of the FT estimate for z for the ith grain. This arrangement ensures the best compromise between resolution and smoothness for the final PD plot; (3) probability density is estimated as a function of z but it is best presented as a function of FT age τ. This objective is accomplished by transforming the z coordinates of the PD plot to τ and plotting τ on a logarithmically-scaled axis, which ensures that the original scaling of the PD plot is preserved. With these modifications, the component distributions in a PD plot will appear as symmetric Gaussian-shaped peaks and the area beneath each peak will be proportional to the relative size of the component. Several examples are given that illustrate the general concepts behind the PD plot and the advantages of the recommended modifications.