Intense localized rock uplift and erosion in the St Elias orogen of Alaska

Abstract

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

Evaluation of detrital thermochronology for quantification of glacial catchment denudation and sediment mixing

“…Most studies use thermochronometry on bedrock samples but the use of detrital samples has been shown to be a powerful tool and became increasingly popular (e.g., Enkelmann et al, 2009Enkelmann et al, , 2011Enkelmann et al, , 2015Stock et al, 2006;Rahl et al, 2007;Vermeesch, 2007Vermeesch, , 2013Whipp et al, 2009;Avdeev et al, 2011;Thomson et al, 2013). Although there is a general loss of the spatial information of where the cooling age is sourced within the catchment, there are several advantages that come with dating detrital material: (1) the age distribution of a detrital sample can provide an integrated picture of the cooling age pattern for the entire catchment, (2) using detrital thermochronometry on sedimentary strata of known depositional age allows quantification of changes in either the source rock exhumation rates, or catchment area through time (e.g., Bernet and Garver, 2005;Bernet et al, 2009), and (3) detrital samples can provide age information from regions that may be otherwise inaccessible for bedrock sampling due to thick vegetation cover, lacking infrastructure, political reasons, landscapes that are too steep, or ice coverage.…”

“…For example, in the St. Elias Mountains in southeast Alaska, massive glaciers and ice fields obstruct the study of rock exhumation by means of bedrock thermochronology. Detrital zircon and apatite FT dating of sediment derived from more than 50 glacial catchments was able to reveal the spatial and temporal pattern of rock exhumation across the entire mountain range (Enkelmann et al, 2008(Enkelmann et al, , 2009Falkowski et al, 2014).…”

Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

“…These strata are interpreted to have been deposited along the northern margin of flat-slab subduction ( Fig. 16.1 Berger et al, 2008;Enkelmann et al, , 2009Arkle et al, 2009;Sendziak et al, 2009;Benowitz et al, 2011). We interpret exhumation as a product of insertion of buoyant, thick oceanic crust of the Yakutat microplate beneath southern Alaska (Finzel et al, 2011).…”

Seismic and non-seismic influences on coastal change in Alaska--Fieldtrip guide and conference abstracts, 5th International Conference of IGCP 588

“…The St. Elias Range provides the opportunity for direct field investigations of glacial erosion processes in an active orogen and testing glacial erosion models (e.g. Spotila et al, 2004;Enkelmann et al, 2009Enkelmann et al, , 2010Spotila and Berger, 2010;Headley et al, 2013).…”

“…Headley et al, 2013 Enkelmann et al (2008Enkelmann et al ( , 2009Enkelmann et al ( , 2010. zHe and aHe: zircon and apatite U-Th/He ages of bedrock samples (data from O'Sullivan and Currie, 1996;Enkelmann et al, 2010 Grabowski et al, 2013 Grabowski et al, 2013) Portage lies along the axis of maximum coseismic subsidence in 1964.…”