Structural Relationships in the Eastern Syntaxis of the St. Elias Orogen, Alaska

Neogene Tectonics and Climate-tectonic Interactions in the Southern Alaskan Orogen themed issue Structural relationships in the eastern syntaxis of the St. Elias orogen, Alaska James B. Chapman1,*,†, Terry L. Pavlis1,†, Ronald L. Bruhn2,†, Lindsay L. Worthington3,†, Sean P.S. Gulick4,†, and Aaron L. Berger5,† 1University of Texas at El Paso, Department of Geological Sciences, 500 West University Boulevard, El Paso, Texas 79968, USA 2University of Utah, Department of Geology and Geophysics, 115 S. 1460 E, Salt Lake City, Utah 84112, USA 3Texas A&M University, Department of Geology and Geophysics, MS 3115, College Station, Texas, 77843, USA 4University of Texas Institute for Geophysics, J.J. Pickle Research Campus, Building 196, 10100 Burnet Road (R2200), Austin, Texas 78758, USA 5ConocoPhillips Company, Houston, Texas 77252, USA ABSTRACT the eastern syntaxis. Exposures of basement styles (Fig. 1). The Samovar Hills offer singular and fault patterns within the syntaxis have insight into the evolution of the YFTB and pro- The eastern syntaxis in the St. Elias oro- implications for tectonic reconstructions of vide a template for interpreting geologic obser- gen (Alaska, USA) is one of the most com- the Yakutat microplate and the geodynamics vations throughout the Yakutat microplate. Our plex and least understood regions within of the orogen. study of the eastern syntaxis helps to resolve the the southern Alaska coastal mountain belt. geodynamics of the St. Elias orogen in a criti- The syntaxis contains many features unique INTRODUCTION cal, and poorly understood, location within the to the orogen that are essential to under- Yakutat microplate. standing the structural architecture and tec- Throughout most of the Neogene, the Yaku- tonic history of the collision between North tat microplate traveled northward relative to TECTONIC SETTING America and the allochthonous Yakutat North America, impinging upon the southern microplate. The eastern syntaxis contains margin of Alaska and initiating both proximal In the eastern Yakutat microplate, onshore the transition from transpressional struc- and far-fi eld orogenesis (Plafker, 1987; Pavlis basement exposures consist of fl ysch and accre- tures associated with the Queen Charlotte– et al., 2004; Leonard et al., 2007). The Yaku- tionary mélange of the Yakutat Group, part of Fairweather fault system in the east to the tat microplate is located between the transition the late Mesozoic Chugach accretionary com- Yakataga fold-and-thrust belt (YFTB) to from dextral strike-slip motion along the Queen plex (Fig. 1) (Plafker et al., 1977). The entire the west. Throughout the eastern syntaxis, Charlotte–Fairweather fault system to oblique Mesozoic accretionary complex from southern a prominent uncon formity at the base of the convergence in the core of the St. Elias orogen Alaska to Vancouver Island was intruded by synorogenic Yakataga Formation records an and ultimately to subduction at the Aleutian distinctive near-trench plutons and locally meta- erosional event related to the development of Trench. One of the most dynamic areas within morphosed to high-grade low-pressure–high- the YFTB. Strain accumulations in the east- the St. Elias orogen is the eastern syntaxis, an temperature metamorphic assemblages during ern YFTB predate the deposition of the Yaka- area of high topography and structural diversity an Early Eocene oceanic spreading ridge sub- taga Formation, extending estimates for the where northwest-striking, basement-involved duction event (Pavlis and Sisson, 1995; Bradley early development of the St. Elias orogen. trans pressional systems to the southeast are et al., 2003; Sisson et al., 2003). A portion of Structural and stratigraphic relationships in juxta posed against the east- to northeast-strik- this accretionary complex was excised from the the eastern syntaxis suggest that forethrusts ing, thin-skinned Yakataga fold-and-thrust belt Cordilleran margin to form part of the Yakutat associated with the transpressional system (YFTB) to the west (Fig. 1). The intersection microplate, and transported northward. As a shut down and were overprinted by fold- of these two structural systems is poorly under- result, in the present collision, variably meta- and-thrust structures in the Early to latest stood, but critical to unraveling the tectonic his- morphosed assemblages of the Chugach accre- Miocene. Basement in the eastern syntaxis tory of the Yakutat microplate. tionary complex form both the autochthonous consists of the Yakutat Group, part of the We present an updated description of the east- backstop and part of the allochthonous base- Chugach accretionary complex, which is car- ern syntaxis, characterize the structural architec- ment (Plafker, 1987). ried by numerous low-angle thrust faults in ture of the YFTB, and utilize stratigraphic and The autochthonous rocks are equivalent to structural relationships to help constrain the tim- the Late Cretaceous to Paleocene Valdez and *Present address: SandRidge Energy, 123 Robert S. ing and evolution of deformation in the eastern Orca Groups, which were variably metamor- Kerr Avenue, Oklahoma City, Oklahoma 73102, USA. syntaxis, including the initiation of the YTFB. phosed from greenschist to upper amphibo- †Emails: Chapman: jchapman@sandridgeenergy In addition to an overview of the eastern YFTB, lite facies (Richter et al., 2005). The Valdez .com; Pavlis: [email protected]; Bruhn: ron.bruhn @utah.edu; Worthington: [email protected]; we present a focused study of the Samovar Group is part of the Chugach terrane, which Gulick: [email protected]; Berger: Aaron.L Hills, a key exposure located in the core of the was accreted to the southern Alaska margin in [email protected]. eastern syntaxis at the intersection of structural the Late Cretaceous (Amato and Pavlis, 2010). Geosphere; February 2012; v. 8; no. 1; p. 105–126; doi:10.1130/GES00677.1; 17 fi gures; 1 table. For permission to copy, contact [email protected] 105 © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/1/105/3341010/105.pdf by guest on 27 September 2021 Chapman et al. 142° W 140° W 138° W Chugach Terrane Denali Fault Bagley/Contact Fault PWT Mt. Logan Alaska Area of Eastern CSEF Syntaxis Bering ? Glacier ? Mt. St. Elias Yakataga ovar Hil Canada am ls 1 S Esker Creek Fault 60° N È Malaspina Fault Insular terrane Icy BayÈ2 Malaspina Glacier 50 km Fig. 3 Bor der R Yakutat Bay ang a Valley Yakutat Foothills es utat Se F Pamplona fold ak au and thrust 200belt m Y Yakutat lt Alsek River “ Yakutat D a Chugach n Microplate g FairweatherTerrane Fault e Pacific r o u 59° N s Yakutat R i v e r Z o Lituya Bay Fault n 1 e È well ” suture fault international border bathymetric contour Pacific Plate “Dangerous River Zone” Yakutat microplate Prince William terrane Transition Fault 58° N Yakutat sedimentary rocks Insular terrane 20 mm/yr Yakutat Group Chugach terrane Figure 1. Overview map of the eastern Yakutat microplate and surrounding area. Outline of Alaska is inset in white. The interpretation of the Dangerous River zone is from Plafker et al. (1994). Plate motion velocity vectors are from Elliott et al. (2010). Wells: 1—Chaix Hills #1A, 2—Riou Bay #1. PWT is Prince William terrane, CSEF is Chugach St. Elias fault. The Orca Group is part of the Prince William The Yakutat Group is regionally metamor- cores (Jones and Clark, 1973; Rau et al., 1983) terrane, which was accreted in the Early Eocene phosed to zeolite to prehnite-pumpellyite suggest that the Yakutat Group is Campanian to (Plafker et al., 1994). The Contact fault forms facies and locally metamorphosed to green- Maastrichtian in age (65.5–83.5 Ma; Gradstein the suture between the Prince William ter- schist facies (Dusel-Bacon, 1994; this study). et al., 2005). Haeussler et al. (2005) estimated rane and Chugach terrane (Plafker, 1987). The Building upon mapping by Plakfer and Miller a maximum deposition age of 72–74 Ma from Chugach St. Elias fault (CSEF) forms the suture (1957), Richter et al. (2005) subdivided the detrital zircon data for the Yakutat Group, an between the Prince William terrane and the Yakutat Group into fl ysch and mélange assem- age they interpret as within ~5 Ma of the true Yakutat microplate (Fig. 1). blages. Paleontological data from outcrop and depositional age. 106 Geosphere, February 2012 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/1/105/3341010/105.pdf by guest on 27 September 2021 Eastern syntaxis, St. Elias orogen In contrast to the eastern Yakutat microplate, 1987), herein referred to as the Oily Lake mem- on the continental shelf (Eyles and Lagoe, geophysical studies indicate that basement in the ber of the Kulthieth Formation (Fig. 2). Forami- 1990), and are the youngest within the orogen central and western Yakutat microplate consists nifera analysis for the Oily Lake member in (Middle Miocene to present). The 0–6-km-thick of a 15–30-km-thick oceanic plateau that is sub- the Samovar Hills indicates an Early to Middle Yakataga Formation contains numerous internal ducting beneath and accreting to North America Eocene age (Ulatisian Stage, ca. 42.5–49.5 Ma; angular unconformities associated with fold and (Ferris et al., 2003; Pavlis et al., 2004; Eberhart- McDougall, 2007) (Plafker et al., 1994). fault growth (Miller, 1957; Worthington et al., Phillips et al., 2006; Gulick et al., 2007; Christe- The Kulthieth Formation is typically 2–3 km 2008, 2010; Broadwell, 2001). son et al., 2010). The relationships and transi- thick and consists of fl uvial to shallow-marine tion between basement lithologies across the deltaic deposits including coarse arkosic sand- Yakataga Fold-and-Thrust Belt Yakutat microplate are unclear, although Plafker stone, shale, and coal beds (Plafker, 1987; (1987) and Plafker et al. (1994) suggested that Landis , 2007). West of the study area, the In the western and central Yakutat micro- the transition occurs along the Dangerous River Tokun and Stillwater Formations are the marine plate, the sedimentary cover is decoupled from zone (DRZ) (Fig.

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