Constraining the area of rapid and deep‐seated exhumation at the St. Elias syntaxis, Southeast Alaska, with detrital zircon fission‐track analysis

Abstract

The Chugach-St. Elias orogen, Southeast Alaska/Southwest Yukon, formed in response to the ongoing Yakutat microplate subduction-collision with the North American Plate. Due to heavy glaciation, the region is a prime location to study active convergent orogenesis and climate-tectonic interactions. This study focuses on the long-term distribution of deformation in the St. Elias syntaxis area, where dextral motion along the plate-bounding Fairweather Fault transitions into convergence. We present 2718 new zircon fission-track single-grain ages from 26 glaciofluvial outwash samples. The grain ages range from 293 Ma to 0.2 Ma, and each sample contains two to five age populations with peaks between 267 ± 64 Ma and 1.2 ± 0.7 Ma (1σ). The rocks of the Yakutat microplate are dominated by latest Cretaceous and Eocene fission-track ages, while the rocks of the North American Plate in the syntaxis region reveal two exhumation phases at~5.1 Ma and 2.7 Ma. The spatial pattern of ≤5 Ma cooling ages shows that the area of rapid and deep exhumation at the St. Elias syntaxis is more extensive than previously known and confined to the south by the Fairweather Fault, to the west by the Seward Glacier catchment, and to the east possibly by the inferred Connector Fault. The area seems to be~4800 km 2 large and may extend farther to the north. The new zircon fission-track data further suggest a transpressional plate boundary since~30 Ma and the onset of plate collision 15-12 Ma.

Initiation of the Wrangell Arc: A Record of Tectonic Changes in an Arc-Transform Junction Revealed by New Geochemistry and Geochronology of the ~29–18 Ma Sonya Creek Volcanic Field, Alaska

“…Modern seismic imaging below south-central Alaska show the presence of thickened (11-22 km thick) oceanic crust as far north as the central Alaska Range (Ferris et al, 2003;EberhartPhillips et al, 2006;Gulick et al, 2007;Christeson et al, 2010;Worthington et al, 2012) and the presence of a subducting slab below the modern WA (Eberhart-Phillips et al, 2006;Bauer et al, 2014). These data indicate that subduction of the leading edge of the Yakutat microplate began was active at ~35-30 Ma, subduction of a slab steep enough to cause arc magmatism has been active since ~30 Ma in the WA, and continental collision of the thicker parts of the Yakutat microplate began by ~15-12 Ma, (Christeson et al, 2010;Worthington et al, 2012;Falkowski et al, 2014Falkowski et al, , 2016.…”

“…The Yakutat microplate increases in thickness from 15 km in the west to 30 km in the east, and subduction and underthrusting has resulted in increased topography and exhumation in the Chugach and St. Elias Ranges on the overriding Alaskan continental crust (Enkelmann et al, 2010;Finzel et al, 2011;Worthington et al, 2012;Pavlis et al, 2014;Falkowski et al, 2014;Enkelmann et al, 2017). The Yakutat microplate is bounded to the northwest by the Aleutian megathrust, to the north by the Chugach-St. Elias thrust fault, the east by the Fairweather rightlateral transform fault, and to the southwest by the Transition fault ( Fig.…”

“…It is in this complex setting that the WA is located. Recent studies have provided major advancements in the understanding of recent (e.g., Neogene) deformation in the zone between Yakutat convergence and the former continental margin in south-central Alaska (e.g., Gulick et al, 2007;Enkelmann et al, 2010;Worthington et al, 2012;Pavlis et al, 2014;Falkowski et al, 2014;Finzel et al, 2015;Finzel et al, 2016;Enkelmann et al, 2017), but the connection between subduction of the northeastern corner of the Yakutat microplate and volcanism in the WA remains largely understudied, including the earliest phases of volcanism.…”

“…Trop, pers. comm Pliocene-Pleistocene time (Enkelmann et al, 2008;Berger et al, 2008aBerger et al, , 2008bGrabowski et al, 2013;Enkelmann et al, 2010;Falkowski et al, 2014Falkowski et al, , 2016Falkwoski and Enkelmann, 2016;Enkelmann et al, 2017). However, thermochronologic data from the Chugach range to the west of the syntaxis shows that the Chugach and Prince William terranes were not being uplifted and exhumed from ~45-35 Ma (Enkelmann et al, 2010;Falkowski et al, 2016).…”

“…Recent thermochronology research in the Chugach-St. Elias Range to the south of the SCVF reveals a record of uplift and exhumation as a result of the progressive subduction, collision, and underthrusting of the Yakutat microplate beneath North America (Berger et al, 2008a(Berger et al, , 2008bMcAleer et al, 2009;Enkelmann et al, 2008Enkelmann et al, , 2010Arkle et al, 2013;Grabowski et al, 2013;Falkowski et al, 2014Falkowski et al, , 2016Falkowski and Enkelmann, 2016;Enkelmann et al, 2017;Dunn et al;. After the subduction of a spreading ridge in Eocene time, the eastern Chugach-western St. Elias range experienced relative quiescence until the onset of Yakutat subduction no earlier than 35 Ma (Enkelmann et al, 2010;Falkowski et al, 2014Falkowski et al, , 2016. Uplift and exhumation of the Chugach-St. Elias range accelerated in mid-Miocene, and the St. Elias syntaxis experienced rapid exhumation since ~5 Ma (Enkelmann et al, 2010;Grabowski et al, 2013;Falkowski et al, 2016).…”