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Analysis of late Mesozoic and Cenozoic sedimentary basins, metamorphic rocks, and major faults in the eastern and central Alaska Range documents the progressive development of a suture zone that formed as a result of collision of an island-arc assemblage (the Wrangellia composite terrane) with the former North American continental margin. New basin-analysis, structural, and geochronologic data indicate the following stages in the development of the suture zone: (1) Deposition of 3-5 km of Upper Jurassic-Upper Cretaceous marine strata (the Kahiltna assemblage) recorded the initial collision of the island-arc assemblage with the continental margin. The Kahiltna assemblage exposed in the northern Talkeetna Mountains represents a Kimmeridgian-Valanginian backarc basin that was filled by northwestward-flowing submarine-fan systems that were transporting sediment derived from Mesozoic strata of the island-arc assemblage. The Kahiltna assemblage exposed in the southern Alaska Range represents a Valanginian-Cenomanian remnant ocean basin filled

Mesozoic and Cenozoic sedimentary strata exposed throughout southern Alaska contain a rich archive of information on the growth of collisional continental margins through the processes of terrane accretion, magmatism, accretionary prism development, and subduction of oceanic spreading ridges. Two major collisional events define the tectonic growth of southern Alaska: Mesozoic collision of the Wrangellia composite terrane and Cenozoic collision of the Yakutat terrane. The sedimentary record of these two collisional events can be summarized as follows. (1) Middle Jurassic volcaniclastic and sedimentary strata represent shallow-marine deposition in narrow subbasins adjacent to the volcanic edifice of the south-facing, intraoceanic Talkeetna arc. (2) Late Jurassic syndepositional regional shortening resulted in thick sections of conglomerate in proximal parts of both retroarc and forearc basins. In distal retroarc depocenters, fine-grained turbidite sedimentation commenced in a series of basins that presently extend for Ͼ2000 km along strike. This time interval also marked cessation of magmatism and rapid exhumation of the Talkeetna oceanic arc. We interpret these observations to reflect initial oblique collision, younging to the northwest, of the Wrangellia composite terrane with the continental margin of western North America. (3) During Early Cretaceous time, Jurassic retroarc basin strata were incorporated into an expanding north-verging thrust belt, and sediment was bypassed into more distal collisional retroarc basins located within the suture zone. Compositional data from these collisional basins show that the Wrangellia composite terrane was exhumed to deep stratigraphic levels. Detrital zircon ages from strata in these basins record some sediment derivation from source areas with North American continental margin affinity. Our data clearly show that the Wrangellia composite terrane and the former continental margin were in close proximity by this time. Accretion of this oceanic terrane and associated basinal deposits marked one of the largest additions of juvenile crust to western North America. The collision of the Wrangellia composite terrane also resulted in a change in subduction parameters that eventually prompted development of a new south-facing arc system, the Chisana arc. Construction of this arc was contemporaneous with renewed forearc basin subsidence and sedimentation. (4) Late Early Cretaceous to early Late Cretaceous time was characterized by regional deformation of retroarc collisional basin strata by south-verging thrust faults that are part of a regional thrust belt that

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

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