Mark T. Brandon

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.

Topographic controls on erosion rates in tectonically active mountain ranges

The functional relationship between erosion rate and topography is central to understanding both controls on global sediment flux and the potential for feedback between tectonics, climate, and erosion in shaping topography. Analysis of a high-resolution (10-m-grid) DEM transect across the convergent orogen of the Olympic Mountains reveals a non-linear relation between long-term erosion rates and mean slope, similar to a model for hillslope evolution by landsliding in steep terrain. The DEM data also reveal a relation between mean slope and mean local relief. Coarser-scale (1-km-grid) global analysis of the relation between erosion rate and mean local relief reveals different trends for areas with low erosion rates and tectonically active mountain ranges, with the composite relation being well-described by non-linear models. Together these analyses support the emerging view that erosion rates adjust to high rates of tectonically driven rock uplift primarily through changes in the frequency of landsliding rather than hillslope steepness, and imply that changes in local relief play a minor role in controlling landscape-scale erosion rates in tectonically active mountain ranges. - 2002 Elsevier Science B.V. All rights reserved.

On steady states in mountain belts

The dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state. This concept is often invoked, but the nature of the steady state is commonly not specified. We identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical-model results and natural examples. These types are (1) flux steady state, (2) topographic steady state, (3) thermal steady state, and (4) exhumational steady state: they refer to the erosional flux, the topography, the subsurface temperature field, and the spatial pattern of cooling ages, respectively. Models suggest that the topography will reach a steady mean form at the scale of an orogenic belt, but perfect topographic steady state is unlikely to be achieved at shorter length scales. Thermal steady state is a precondition for exhumational steady state and in the case of temperature-dependent deformation, topographic steady state. Exhumational steady state is characterized by reset age zones spatially nested according to closure temperature, as illustrated in natural systems from New Zealand, the Cascadia accretionary margin, and Taiwan.

Probability density plot for fission-track grain-age samples

Abstract 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 zi to represent the FT grain ages τi, 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.

Mantle flow at a slab edge: Seismic anisotropy in the Kamchatka region

The junction of the Aleutian Island and the Kamchatka peninsula defines a sharp turn in the boundary of the Pacific and North American plates, terminating the subduction zones of the northwest Pacific. The regional pattern of shear-wave birefringence near the junction indicates that trench-parallel strain follows the seismogenic Benioff zone, but rotates to trench-normal beyond the slab edge. Asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge, and may influence the shallowing dip of the Benioff zone at the Aleutian junction.

Steady-state exhumation of the European Alps

Fission-track grain-age distributions for detrital zircon are used in this study to resolve the late Cenozoic exhumation history of the European Alps. Grain-age distributions were determined for six sandstone samples and one modern river sediment sample, providing a record from 15 Ma to present. All samples can be traced to sources in the Western and Central Alps. The grain-age distributions are dominated by two components, P1 (8–25 Ma) and P2 (16–35 Ma), both of which show steady lag times (cooling age minus depositional age), with an average of 7.9 m.y. for P1 and 16.7 m.y. for P2. These results indicate steady-state exhumation in the source region at rates of ∼0.4–0.7 km/m.y. since at least 15 Ma.

A zero-damage model for fission-track annealing in zircon

Abstract A zircon fission track-annealing model is calculated on the basis of annealing experiments from the literature with induced tracks in α-decay event damage-free zircon samples. Empirically derived parallel and fanning equations for this “zero-damage” model yield an excellent fit to the data, with the fanning model providing slightly better statistical parameters. A comparison between annealing models with fanning iso-annealing lines but different α-decay event damage densities reveals that annealing temperatures and closure temperatures for the estimated partial annealing zone are highest for the zero-damage model. Compilations of existing geologic constraints on the zircon partial-annealing zone on one hand and the zircon closure temperature on the other show that these constraints do not or only partly overlap with curves of proposed models for the zircon partial-annealing zone and closure temperature. This finding is consistent with the fact that the annealing behavior of zircon from long-duration temperature evolutions is increasingly influenced by the accumulated α-decay event damage. Zircon samples of young age or low U content show a behavior closest to the predictions of the zero-damage model, and are in the predicted range of published models with low α-decay event damage density. For thermal events of more than 10 myr duration, however, constraints from field studies show marked differences from proposed partial-annealing zone boundaries of the zero- or low-damage models. The applicability of the zero-damage model is threefold. (1) It predicts correct closure temperatures in the case of very rapid cooling across the partial annealing zone where basically no α-decay event damage is accumulated. (2) It predicts an uppermost boundary for complete annealing of a mixture of zircon components of different age, as found in sedimentary samples, and in this case may be used as a thermometer. (3) It represents an important reference for the establishment of a more comprehensive model of zircon fission-track annealing that also includes the influence of α-decay event damage. For such a model, two different equations are discussed. However, additional detailed experimental and field data are needed for a more robust annealing model that includes the influence of α-decay event-damage annealing.

Glaciation as a destructive and constructive control on mountain building

Theoretical analysis predicts that enhanced erosion related to late Cenozoic global cooling can act as a first-order influence on the internal dynamics of mountain building, leading to a reduction in orogen width and height. The strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion, producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude, where long-term glacier mass gain equals mass loss. However, acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive. Here we present an extensive new low-temperature thermochronologic data set from the Patagonian Andes, a high-latitude active orogen with a well-documented late Cenozoic tectonic, climatic and glacial history. Data from 38° S to 49° S record a marked acceleration in erosion 7 to 5 Myr ago coeval with the onset of major Patagonian glaciation and retreat of deformation from the easternmost thrust front. The highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen, as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity. In contrast, towards higher latitudes (49° S to 56° S) a transition to older bedrock cooling ages signifies much reduced late Cenozoic erosion despite dominantly glacial conditions here since the latest Miocene. The increased height of the orogenic divide at these latitudes (well above the equilibrium line altitude) leads us to conclude that the southernmost Patagonian Andes represent the first recognized example of regional glacial protection of an active orogen from erosion, leading to constructive growth in orogen height and width.

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 (>50 ppm), and peak-ages statistics can be improved by counting numerous apatite grains (>100).

Regionally extensive mid-Cretaceous west-vergent thrust system in the northwestern Cordillera: Implications for continent-margin tectonism

The Intermontane-Insular superterrane boundary zone represents a fundamental crustal boundary separating the two largest allochthonous crustal fragments in the North American Cordillera. Structural, stratigraphic, and geochronologic relations along this boundary indicate that substantial west-vergent compression and concomitant crustal thickening occurred there in mid-Cretaceous time. This orogenic zone extends for more than 1200 km along strike length, between southern southeast Alaska and northern Washington. In southern southeast Alaska and northwest British Columbia, rocks of the Insular superterrane were imbricated along a series of west- to southwest-vergent thrust faults. In northern Washington and southwestern British Columbia, a wide zone encompassing the margins of the two superterranes, as well as numerous intervening smaller fragments, was shortened principally along west-vergent thrusts. Known geologic relations do not discriminate among existing tectonic models that explain the origin of the mid-Cretaceous thrust system.