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The Denali Fault System and Alaska Range of Alaska: Evidence for Underplated Mesozoic Flysch from Magnetotelluric Surveys

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The Denali Fault System and Alaska Range of Alaska: Evidence for Underplated Mesozoic Flysch from Magnetotelluric Surveys The Denali fault system and Alaska Range of Alaska: Evidence for underplated Mesozoic flysch from magnetotelluric surveys ^i^'^L f Ji^lV^ } U.S. Geological Survey, Box 25046, M.S. 964, Denver Federal Center, Denver, Colorado 80225 VICTOR F. LABSON I WARREN J. NOKLEBERG ) BELA CSEJTEY, JR. } U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 MICHAEL A. FISHER J ABSTRACT others, 1973; Smith and others, 1974; Turner and others, 1974; Eisbacher, 1976; Nokleberg and others, 1985; Plafker and others, 1989a). Regional magnetotelluric surveys recently completed across the This region has been studied geophysically as part of two programs. central and eastern Alaska Range of Alaska provide evidence for large Magnetotelluric (MT) surveys of the Alaska Range and Mesozoic sedi- volumes of conductive rocks beneath the core of the range. These mentary rocks east of Mount McKinley were conducted as part of a U.S. conductive rocks may represent a formerly extensive, but now col- Department of Energy (DOE) program to locate deeply emplaced sedi- lapsed, Mesozoic flysch basin formed on the leading edge of the Tal- mentary rocks that could serve as sources for methane and other hydro- keetna superterrane (amalgamated Wrangellia, Peninsular, and carbons. The central and east-central Alaska Range and terranes to the Alexander terranes). The docking of the Talkeetna superterrane south of the fault system were investigated as part of U.S. Geological caused large-scale oblique thrusting, folding, and metamorphism in Survey (USGS) deep continental investigations, called TACT (Trans- the flysch basin, and formation of a megasuture along which the Alaska Crustal Transect). These latter studies, still in progress, have been Cenozoic strike-slip Denali fault system developed. The deep magne- integrated with extensive seismic refraction, reflection, gravity, magnetic, totelluric soundings and seismic reflection data suggest the possibility and geologic surveys of the transect (Fisher and others, 1988). Extensive that the highly conductive rocks were tectonically emplaced beneath geologic studies of the central and eastern Alaska Range and surrounding the thin crystalline sheet constituting the southern Yukon-Tanana ter- regions have been carried out as part of the USGS Alaska Mineral Re- rane over a broad region of the Alaska Range. The conductive rocks sources Assessment Program (AMRAP). are locally correlated with surface outcrops of Mesozoic black shales that are part of Upper Jurassic and Cretaceous flysch but may be GEOLOGIC SETTING OF THE CENTRAL AND EASTERN composed of Paleozoic carbonaceous shales as well. In either case, ALASKA RANGE AND DENALI FAULT SYSTEM their extremely low resistivities make them a valuable marker horizon for tectonic studies. The conductive rocks are interpreted to extend to Major Tectonic Units depths of greater than 20 km and were mapped north and northeast of the Denali fault for more than 50 km. The magnetotelluric surveys The Denali fault occurs along the core of the central and eastern represent the first large-scale surveys done in Alaska, but the struc- Alaska Range between mostly crystalline, continental-affinity terranes to tures mapped are similar to those observed in large, compressed flysch the north and oceanic-affinity terranes to the south (Fig. 1). The Denali basins in the eastern Alps and Carpathian Mountains of Europe. The fault is a major suture zone from its westernmost mappable extent to its results of these surveys bear on several key tectonic questions, includ- southeastern extremity in the lower panhandle of Alaska. It has been ing development of the ancestral Denali fault, and collapse and possi- postulated to have been a locus of major amounts of strike slip (Gabrielse, ble underplating of an extensive Mesozoic flysch system and 1985; Nokleberg and others, 1985; Mortensen and Jilson, 1985; Plafker associated igneous arc. and others, 1989a). The geophysical interpretations in this paper do not place direct constraints on the proposed amounts of strike slip, although INTRODUCTION we assume oblique convergence between the docking Talkeetna superter- rane and proto-Alaska. The Denali fault has been portrayed primarily as a The Alaska Range and Denali fault system of southern Alaska are vertical feature, but evidence provided in this paper establishes the possibil- two of the most dramatic tectonic features of North America, arcing across ity that movement on the Denali fault may be located in thrust planes Alaska in near-small-circle fashion (Stout and Chase, 1980) for a distance associated with the suture zone. of more than 1,200 km (Fig. 1). The Alaska Range contains the conti- To the north of the Denali fault is mainly the Devonian-Mississippian nent's highest peak, Mount McKinley, with an elevation of greater than or older Yukon-Tanana terrane, which consists of polymetamorphosed, 6.0 km. The Denali fault system was initially defined by Sainsbury and deformed metasedimentary and intermediate-composition metavolcanic Twenhofel (1954), St. Amand (1954, 1957), Twenhofel and Sainsbury rocks and lesser Devonian and Mississippian metagranitic rocks. Present (1958), and Grantz (1966). The fault system has been postulated to repre- interpretations suggest formation of this terrane in a continental- sent part of the Mesozoic continental margin and to have been the locus of margin/arc setting (Nokleberg and others, 1985, 1986; Aleinikoff and 400-1,000 km of right-lateral slip since the early Tertiary (Forbes and others, 1987; Foster and others, 1987; Jones and others, 1987). The Geological Society of America Bulletin, v. 102, p. 160-173, 11 figs., February 1990. 160 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/2/160/3380734/i0016-7606-102-2-160.pdf by guest on 02 October 2021 DEN ALI FAULT SYSTEM AND ALASKA RANGE, ALASKA 161 104° ALASKA CANADA MESOZOIC FLYSCH mk WRANG., PEN., ALEX. TERRANES Figure 1. Simplified geology and YUKON-TANANA tectonostratigraphic terranes of Alaska, TERRANE modified from Howell and others FA YKlr 3NIXON FORK,(Pz) (1985). Abbreviations: ALX, Alex- DILLINGER TERRANES ander; CHG, Chugach; DIL, Dillinger; STIKINE TERRANE KND, Kandik River; MAN, Manley; NXF, Nixon Fork; PEN, Peninsular; OTHER PMW, Pingston, McKinley, and Windy terranes combined; WRN, Wrangellia; YAK, Yakutat; YKT, Yukon-Tanana; KAH, Kahiltna flysch terrane; STK, Stikine; TKU, Taku; MG, Maclaren; KS, Kluane Schist; RRB, Ruby Range batholith; TNF, Tintina fault; GNB, Gravina-Nutzotin flysch belt; TT, Tal- keetna thrust; YUK, Koyukuk; DNF, Denali fault. Denali and Tintina fault systems are indicated by the dashed lines. -- MT PROFILE REFLECTION PROFILE Yukon-Tanana terrane is bounded on the north (Fig. 1) by the Tintina adjacent and parallel to the Denali fault (Jones and others, 1987). A fault (Foster and others, 1987), a regional right-lateral slip zone somewhat western flysch belt, the Kahiltna terrane (Fig. 1), is defined as a structurally analogous to the Denali fault. In the west-central Alaska Range region is a disrupted assemblage of Upper Jurassic and mainly Lower Cretaceous collage of terranes (Fig. 1), including parts of the Nixon Fork and Dillinger shale-graywacke flysch with metamorphic grade ranging from zeolite to terranes and miniterranes such as the Mystic, Pingston, and Windy ter- amphibolite facies, containing minor amounts of volcanic rocks. ranes (Jones and others, 1987). The Kahiltna terrane (KAH) is a major Csejtey and others (1982) have described the relationship of the feature south of the Alaska Range and consists of highly deformed Upper Cenozoic Denali fault system to the Cretaceous accretionary development Jurassic and Lower Cretaceous shale flysch, with lesser volcanic and vol- of southern Alaska. They interpreted that the deformed flysch of the caniclastic rocks. Kahiltna terrane south of the Denali fault was deposited in the narrowing The Wrangellia terrane is an extensive terrane found south of the and subsequently collapsed oceanic basin between converging blocks of Denali fault between the Kahiltna and Chugach (CHG) terranes but also the Talkeetna superterrane (amalgamated Wrangellia, Peninsular, Alex- at various places along the continental margin as far south as Vancouver ander terranes) and the pre-Jurassic core of Alaska. Both the superterrane Island. Wrangellia consists mainly of an upper Paleozoic island-arc se- and adjoining flysch complex may have been transported a considerable quence of volcanic and sedimentary rocks and an Upper Triassic sequence distance in a right-lateral sense after initial deposition of the flysch. Incor- of rift basalts. Farther south is the Peninsular terrane (Fig. 1), which porated into the flysch complex through large-scale tectonic transport are a consists mainly of Jurassic sedimentary, volcanic, and granitic rocks, inter- number of miniterranes, such as the Chulitna terrane, that dip to the preted to have formed in an island-arc setting (Fig. 1) (Jones and others, northwest (Jones and others, 1982). 1987; Plafker and others, 1989b). The Peninsular and Wrangellia terranes In addition to the Kahiltna terrane south of the Denali fault, less are adjoined on the south, in the Alaska Peninsula, by the Alexander extensive Jurassic and Cretaceous flysch also occurs in the upper part of terrane, composed of upper Precambrian to Triassic metabasalts, carbon- the McKinley terrane to the north. Gilbert and Bundtzen (1983) suggested ates, and greenschist- to amphibolite-facies schist and gneiss (Silberling
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