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 greater 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 15 Ma, 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 ~100°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 ~242-289°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 4.4 km 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...

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 ϳ0.4-0.7 km/m.y. since at least 15 Ma.

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The Tien Shan are the quintessential intracontinental range, situated more than 1000 km north of the suture between India and Asia. Their initiation and growth in the Cenozoic, however, remain poorly understood. In this study we present stratigraphic, detrital fission-track, and magnetostratigraphic results that provide a basis for reconstructing the Cenozoic tectonic evolution of the Kyrgyz Range and adjacent Chu basin in the northwestern Tien Shan. Detrital fission-track thermochronology indicates that the northwestern Tien Shan was tectonically quiescent for much of the Cenozoic. Prior to uplift and exhumation in the late Miocene, the Kyrgyz Range was buried by sediments shed from highlands to the south and/or east. Paired bedrock fission-track and [U-Th]/He ages from a sampling transect of 2.4 km relief demonstrate that rapid exhumation commenced at ca. 11 Ma. Initial thrusting in the hinterland was followed by evaporite accumulation (ϳ0.4 km/m.y.), which coincided with erosion of the pre-11 Ma strata that mantled the Kyrgyz Range. Between 10 and 3 Ma, bedrockexhumation rates decreased to Ͻ0.3 km/ m.y., while sedimentation rates decelerated initially to ϳ0.25 km/m.y. before accelerating to ϳ0.4 km/m.y. at 4-5 Ma. Detrital fission-track results indicate that by 4.5 Ma,

We investigate the material fluxes in space and time as a result of exhumation and erosion processes at the ongoing Yakutat-North American collision in southeast Alaska. Many thermochronologic studies using a variety of sampling strategies are challenged by the widespread ice cover that limit field observations and accessibility. This paper reviews new and published low-temperature thermochronological data from southeast Alaska to give a comprehensive interpretation of the exhumation patterns through time and how they are influenced by surface processes and climate change. We find that the southeastern margin of Alaska was exhumed and eroded long before the late Miocene-Pliocene Yakutat collision, but since the beginning of the subduction of the Yakutat lithosphere in the Oligocene/early Miocene. Today there is a distinct pattern of exhumation in southeast Alaska with a localized very rapid and deep-seated exhumation at the Yakutat plate corner (St. Elias syntaxis), where strike slip motion changes to convergence. Exhumation is also rapid, but less deep along the dextral Fairweather fault, and in the evolving fold and thrust belt. We present a re-interpretation of the exhumation pattern in the fold and thrust belt and suggest that mass transport by exhumation is parallel to the observed active thrust faults and oblique to the suture zone and orogenic strike. The locus of most rapid exhumation migrated from northwest to southeast with Recent exhumation occurring near the St. Elias syntaxis. Exhumation of the Chugach terrane rocks is still active, however to a lesser degree than on the south side of the orogen where precipitation rates are much higher. The Wrangellia terrane to the north has experienced little exhumation and has essentially formed the backstop for terrane accretion in southeast Alaska since the Early Cretaceous. Apatite U-Th/He ages give the first evidence that rocks of the Wrangell Range have only been recently uplifted and eroded as a consequence of the continuing Yakutat collision. In general the thermochronology in southeast Alaska reveals that climate variations across the region as well as changes through time have a limited influence on the pattern of erosion and that the location of deep exhumation is primarily influenced by tectonic processes.

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A B S T R A C TPaired apatite fission track and U-Th/He dates provide the first Late Cenozoic cooling ages for the northern Tien Shan. These data clearly argue for pulsed deformation since the Late Miocene, with early (10-11 Ma) and late (0-3 Ma) intervals of rapid exhumation separated by an extended interval of much slower rates. By integrating these bedrock cooling rates with shortening estimates derived from a balanced section, detrital cooling ages, and geomorphological estimates of conditions before deformation, we reconstruct a four-stage history of range growth and exhumation. Following ∼100 m.yr. of tectonic quiescence, abruptly accelerated rock uplift, exhumation, and cooling in the Kyrgyz Range commenced at ∼11 Ma with rates exceeding ∼1 km/m.yr. During the subsequent 7 m.yr., deformation and cooling rates decreased three-to sixfold before accelerating by comparable amounts during the past 3 m.yr. Since mid-Miocene times, the surface elevation of the Kyrgyz Range has increased ∼2 km, consistent with the reconstructed magnitude of crustal shortening (∼11 km) and thickening (∼12 km) across the range. The highly pulsed deformation rates indicate that the locus of deformation probably shifted repeatedly within the Tien Shan from the Miocene to present. Even at their most rapid, Cenozoic shortening rates in the Kyrgyz Range were equivalent to only 10%-20% of the modern geodetic convergence rate across the entire Tien Shan. This requires several ranges within the Tien Shan to have deformed simultaneously since the Middle Miocene, a situation analogous to the distributed shortening seen today.

The timing and role of exhumation in the St Elias orogen, the world's highest coastal mountain range, has been unclear. Sampling is limited to high mountain ridges that tower over widespread ice fields that sit in deeply eroded parts of the orogen. Existing bedrock studies 1-3 in the region are therefore prone to bias. Here we analyse detrital material of active sediment systems in the St Elias orogen to obtain age information from the inaccessible ice-covered valley bottoms. We present 1,674 detrital zircon fission-track ages from modern rivers that drain the glaciers. We find a population of very young ages of less than 3 Myr from the Seward-Malaspina glacier systems that is sharply localized in the area of the orogen's highest relief, highest seismicity and at the transition from transform to subduction tectonics. Our data provide evidence for intense localized exhumation that is driven by coupling between erosion and active tectonic rock uplift.The St Elias mountain belt originates from the collision of the Yakutat terrane with North America, at the corner formed by the dextral Fairweather transform and the Aleutian subduction zone (Fig. 1). Initiation of the Fairweather fault and northward transport of the Yakutat terrane started ∼30 Myr ago, but collision began at 10-5 Myr as the thickened crust of the Yakutat terrane accreted to the Aleutian trench 4,5 , stripping sedimentary cover from basement to construct a foreland fold and thrust belt 4,6 (Fig. 2). Along the orogenic belt, the youngest (5-0.5 Myr) low-temperature cooling ages of bedrock (60-110• C closure temperatures (T c ) for apatite (U-Th)/He (ref. 7) and fission track 8 ) are strongly correlated with those areas with the highest precipitation along the southern, seaward flanks of the orogen. Bedrock cooling ages are oldest in the drier northern side 1,2 (Fig. 2). Bedrock zircon fission-track (ZFT) ages (T c ∼ 250• C; ref. 9) give >10 Myr ages 3,10 ( Fig. 2), or are non-reset in the fold-thrust belt, because lateral transport of material into the orogenic wedge results in exhumation restricted to the upper 5 km (ref. 10). Similar to other active orogenic belts with high erosion rates, the St Elias range seems to have developed localized feedback between erosion and crustal strain [11][12][13] . Thus, it is puzzling that no evidence has emerged for locally enhanced exhumation and erosion in the form of localized young, higher-temperature cooling ages. However, the St Elias orogen is unique among active orogens in that more than 50% of its area is covered by glaciers hundreds of metres in thickness (Fig. 2). Besides reducing bedrock exposures in general, this glaciation also prohibits direct sampling of the low-elevation intensely glaciated valley bottoms where the most recently exhumed rocks and youngest cooling ages would be expected.To overcome this sampling obstacle, we analysed detrital zircons from rivers draining the main glacial systems to evaluate the cooling