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Upper-Crustal Cooling of the Wrangellia Composite Terrane in the Northern St. Elias Mountains, Western Canada

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Upper-Crustal Cooling of the Wrangellia Composite Terrane in the Northern St. Elias Mountains, Western Canada RESEARCH Upper-crustal cooling of the Wrangellia composite terrane in the northern St. Elias Mountains, western Canada Sarah Falkowski1 and Eva Enkelmann2 1UNIVERSITY OF TÜBINGEN, DEPARTMENT OF GEOSCIENCES, 72074 TÜBINGEN, GERMANY 2UNIVERSITY OF CINCINNATI, DEPARTMENT OF GEOLOGY, CINCINNATI, OHIO 45221-0037, USA ABSTRACT This study presents the long-term exhumation history of the Wrangellia composite terrane of the remote and ice-covered northern St. Elias Mountains in southwest Yukon, northwest British Columbia, and adjacent Alaska. Detrital zircon and apatite fission-track age distributions are presented from 21 glacial catchments. The detrital sampling approach allows for a large spatial coverage (~30,000 km2) and access to material eroded beneath the ice. An additional five bedrock samples were dated by zircon fission-track analysis for a comparison with detrital results. Our new thermochronology data record the Late Jurassic–mid-Cretaceous accretion of the Wrangellia composite terrane to the former North American margin and magmatism, which reset the older thermal record. The good preservation of the Jurassic–Cretaceous record suggests that Cenozoic erosion must have been limited overall. Nonetheless, Eocene spreading-ridge subduction and Oligocene–Neogene cooling in response to the ongoing Yakutat flat-slab subduction are evident in the study area despite its inboard position from the active plate boundary. The results further indicate an area of rapid exhumation at the northern end of the Fairweather fault ca. 10–5 Ma; this area is bounded by discrete, unmapped structures. The area of rapid exhumation shifted southwest toward the plate boundary and the center of the St. Elias syntaxis after 5 Ma. Integrating the new data with published detrital thermochronology from the southern St. Elias Mountains reveals an evolving concentration of deformation and exhumation, possibly within a large-scale, transpressional structure providing impor- tant constraints for geodynamic models of syntaxes. LITHOSPHERE; v. 8; no. 4; p. 359–378; GSA Data Repository Item 2016150 | Published online 20 May 2016 doi:10.1130/L508.1 INTRODUCTION 1 TiF 10 2 DF Yakutat slab YCT The Wrangellia composite terrane is the classic example of an accreted 3 WT G and displaced terrane within the terrane mosaic of the western North 5 PT 7 Fig. 2 American margin (Jones et al., 1972; Coney et al., 1980). The composite KT 4 BRF A 6 terrane has been studied to understand continental growth by terrane sub- CMC 8 AT duction and accretion processes and its spatial and temporal transitions 6 9 between compressional, transpressional, and extensional deformation 60ºN (e.g., Coney et al., 1980; Rusmore and Woodsworth, 1991; Andronicos YM FF et al., 1999, 2003; Gehrels et al., 2009; Israel et al., 2013). Due to the CPWT Alaska n Fig. 4 TF WT long history of accretion, terranes experience continued deformation dur- ko NWT Yu ing their accretion, possible postaccretional displacement, and continued 60ºN PAC B.C. QCF subduction and accretion at their outboard margins. Arc AMT 140ºW This study investigates the Wrangellia composite terrane of the north- Aleutian 140ºW ern St. Elias Mountains in southwest Yukon, northwest British Columbia, Figure 1. Terrane map of southern Alaska and western Canada after Silber- and adjacent Alaska (Figs. 1 and 2). This area has been influenced by two ling et al. (1994). The thick, dashed line indicates the surface projection major accretion events—the Late Jurassic–mid Cretaceous Wrangellia of the Yakutat slab after Eberhart-Phillips et al. (2006). Plate vectors after composite terrane accretion to the North American margin and the Late Plattner et al. (2007) and Elliott et al. (2010). TiF—Tintina fault; DF—Denali fault; BRF—Border Ranges fault; FF—Fairweather fault; QCF—Queen Eocene–Present oblique accretion and flat-slab subduction of the Yaku- Charlotte fault; TF—Transition fault; AMT—Aleutian megathrust; YCT— tat microplate (Fig. 1) (e.g., Nokleberg et al., 1994; Plafker et al., 1994). Yukon composite terrane; KT—Kahiltna terrane; WT—Wrangellia terrane; Evidence of the Yakutat flat-slab subduction is found in areas above the PT—Peninsular terrane; AT—Alexander terrane; CPWT—Chugach–Prince subducted slab located in south-central Alaska and the Alaska Range (Fig. William terrane; YM—Yakutat microplate; PAC—Pacific plate; G—Gravina- 1) (e.g., Enkelmann et al., 2008; Benowitz et al., 2011, 2014; Finzel et al., Nutzotin belt; CMC—Chugach metamorphic complex; A—Anchorage; 2011; Brennan and Ridgway, 2015). Our study area is located east of the NWT—Northwest Territories; B.C.—British Columbia (inset map). Numbers indicate locations of 1—eastern Alaska Range; 2—central Alaska Range; 3—Talkeetna Mountains; 4—Matanuska Basin; 5—western Alaska Range; *Emails: Falkowski: [email protected]; Enkelmann: eva.enkelmann@ 6—eastern and western Chugach Mountains; 7—Wrangell Mountains; 8— uc.edu St. Elias Mountains; 9—St. Elias syntaxis; 10—Mackenzie Mountains. LITHOSPHERE© 2016 Geological | Volume Society 8 of| AmericaNumber 4| |For www.gsapubs.org permission to copy, contact [email protected] 359 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/8/4/359/3040051/359.pdf by guest on 30 September 2021 FALKOWSKI AND ENKELMANN on 30 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/8/4/359/3040051/359.pdf 360 0 25 50 A B km Alaska Yu ko n KLD110 Aishih Wr angell volcanicKlutlan belt Gl. Ruby Range ik La Kluane Ranges Kluane KLD20 KLD78 ke DRF Lak KLD23 Chitina Rive ? Cc e KLD27 F KLD Downstream r KLD106 KLD Alsek River 61°N 65 67 Donjek Gl. 61°N . Border Ranges Faul KLD105 Kluane Gl KLB5 St. Elias Mountains Kaskawulsh Gl. Haines Junction t ? KLD66 5 KLD 39 Bagley Ice Bagley Fault KLB44 KLD8 lleVa Seward Hubbard Gl y . ? KLB41 Gl Dusty Gl . CSEF ? KLD40 KLB91 . CHF KLD . Lowell Gl KLB42 9 Fisher Gl. KLD Denali Fault Yukon EC KLD13 17 60°N Tw F eets British Columbia KLD Ta Malaspina muir Gl 60°N Malaspina Fault Alsek Rive 18 tshenshini Glacier . Rive KLD2 Batt r l Detrital sample location with KLD2 6 www.gsapubs.org e Gl Yakutat Bay Fairweather Range . corresponding catchment 5 Ya r kutat Faul Fairweather Fault KLD Detrital sample location along the 33 Russell Fjord Alsek River (from N to S: KLD20, Nunatak Fjord KLD23, KLD27) t KLD Bedrock sample location 29 0 25 50 km 140°W Mount Logan (5959 m) 140°W | Volume 8 Volume Figure 2. (A) Satellite image (Google Earth) and (B) digital elevation model (ASTER GDEM, 30 m; NASA, USA, and METI, Japan) of the St. Elias Mountains and Fairweather, Kluane, and Ruby ranges in western Canada and southeastern Alaska. Names of main glaciers draining the northern St. Elias Mountains are given (Gl.—Glacier). Note that the large Alsek River catchment comprises most of the glacial catchments of the eastern study area (KLD67, 85, 39, 40, 9, 13, 17, 18, 26, and 25). DRF—Duke River fault; CcF—Connector fault; CSEF— | Chugach–St. Elias fault; CHF—Chaix Hills fault; ECF—Esker Creek fault. Number 4 | LITHOSPHERE Upper-crustal cooling in the northern St. Elias Mountains | RESEARCH Yakutat slab and north and east of the indenting Yakutat plate corner where transcurrent faults since the Late Cretaceous, even though plate motion deformation transitions from transpressional to compressional (Fig. 2B). directions changed through time (e.g., Doubrovine and Tarduno, 2008). This area is known as the St. Elias syntaxis and gained recent attention due Today, the geodynamics of southern Alaska are dominated by the col- to its complex structural setting and locally concentrated, high deforma- lision of the Yakutat microplate, which is a 15–30-km-thick, westward tion rates (e.g., O’Sullivan et al., 1997; Pavlis et al., 2004; Enkelmann et tapering oceanic plateau capped by a remnant Campanian–Maastrichtian al., 2009, 2015a, 2015b; Koons et al., 2010). The Wrangellia composite accretionary prism (Yakutat Group) in the east and Cenozoic sediments terrane has generally been ascribed the role of the backstop to Cenozoic in the west (Fig. 3) (Plafker, 1987; Worthington et al., 2012). The Fair- accretion with limited amounts of exhumation (e.g., Pavlis and Roeske, weather–Queen Charlotte transform plate boundary (Fig. 1) formed in the 2007; Enkelmann et al., 2008, 2010; Spotila and Berger, 2010). However, mid-Eocene and resulted in the northwestward translation of the Yakutat geophysical data and large-scale geodynamic models suggest that deforma- microplate (e.g., Plafker et al., 1994; Haeussler et al., 2003). A prominent, tion at the current plate boundary is partly transferred inland to the north- westward bend in the plate boundary and associated topography marks the ern margin of the Wrangellia composite terrane and farther to the eastern St. Elias syntaxis, which is the transition zone between transform motion deformational front of the North American Cordillera (e.g., Mackenzie and subduction (Fig. 3). During the past decade, various data have been Mountains in the Northwest Territories; Fig. 1) (e.g., Lahr and Plafker, published that illuminate the structural situation and geologic evolution 1980; Mazzotti and Hyndman, 2002; Koons et al., 2010; Doser, 2014). of the St. Elias orogen with a strong emphasis on the southern flanks We use detrital zircon and apatite fission-track (ZFT and AFT, respec- located in Alaska. Particularly geophysical data, glacial geomorphology, tively) analyses on modern sand deposits of glacial and fluvial catch- and detrital thermochronology
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