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

^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 superterrane 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

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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 that the Pingston and McKinley terranes (Fig. 2) represent a former trail- and Jones, 1984). ing continental margin on the Yukon-Tanana block. An eastern flysch belt, the Gravina-Nutzotin belt, occurs stratigraphically on the Wrangellia Mesozoic Flysch terrane in the eastern Alaska Range and southeastern Alaska and is defined as an Upper Jurassic to middle Cretaceous sequence of argillite, Broad complexes of Mesozoic flysch with locally abundant carbona- graywacke, conglomerate, and associated minor volcanic rocks with low ceous shale occur in central, southern, and southeastern Alaska (Fig. 1). metamorphic grade. Two large discontinuous units of mainly Upper Jurassic to Lower Cre- Both the Kahiltna terrane and the Gravina-Nutzotin belt are com- taceous flysch and associated andesitic volcanic rocks occur in a broad posed of flysch containing abundant intermediate-composition volcanic swath along the southern flank of the central and eastern Alaska Range, detritus (Berg and others, 1972; Richter, 1976; Nokleberg and others,

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and folding due to accretionary compression (Berg and Richter, 1972; Csejtey and others, 1982; Jones and others, 1982; Nokleberg and others, 1985). The intense thrust faulting and folding have been modified by concurrent and (or) subsequent strike-slip movement on mainly the Denali fault system (Nokleberg and others, 1985). To the south of the study area, Upper Cretaceous flysch occurs in the Valdez Group of the Chugach terrane (Fig. 1) (Nilson and Zuffa, 1978).

MAGNETOTELLURIC SURVEYS

Deep soundings of the Earth's crust and mantle using the magnetotelluric (MT) and geomagnetic variation (GMV) methods (Rokityansky, 1982) have provided important information on structures in many active tectonic regions of the world. Major conductivity anomalies have been observed in tectonic sutures world-wide. For instance, a large subduction- related anomaly that follows the arc of the Carpathian Mountains in Europe has been investigated in detail with MT and GMV soundings (Jankowski and others, 1984). This very extensive anomaly was interpreted to be caused by subduction-related processes. The conductive rocks have thicknesses of more than 10 km and extend to depths of more than 20 km. Recent deep reflection surveys in Czechoslovakia (Tomek, 1986) confirm the attitude of the conductive rocks and their subduction-related EXPLANATION D Dillinger terrane 7777 setting. Stanley (1989) interpreted that the cause of the deep high conduc- S//S/ / / / Yukon-Tanana terrane My Mystic terrane tivities may be metamorphosed black shales. Other mechanisms such as partial melting or thermal brines are not so easy to accept in the low-heat- Mc McKinley terrane Wrangellia terrane flow regime of the Carpathian region. Stanley and others (1987) have used Pn Pingston terrane Cret. and Jur. Flysch magnetotelluric, gravity, and magnetic data to outline an interpreted Eo- Qt Cenozoic sedimentary terrane cene suture zone in the southern Washington Cascades Range that may and volcanic rocks MT Soundings contain a subduction-related sedimentary system that has been downthrust Figure 2. Tectonostratigraphic terranes (from Jones and others, to depths of more than 15 km. 1982) in the region of Mount McKinley, with locations of MT sound- No MT surveys in Alaska are described in the literature. The compli- ings (triangles) on profiles AA' and BB'. The Hines Creek and McKin- cated nature of the ionospheric electric and magnetic fields in the northern ley strands of the Denali fault system and Talkeetna thrust of Csejtey regions was thought to prevent effective use of the MT method (Quon and and others (1982) are also shown. others, 1979); however, more recent studies of the current sheets associated with electromagnetic field systems at high latitudes reveal that these sheets are very large in dimension (Oguta and Kanji, 1984, 1985) and 1985,1989; Csejtey and others, 1986; Jones and others, 1987). The source represent adequate (approximate plane waves) sources for crustal MT of the detritus is interpreted as an extensive, coeval igneous arc flanked by studies. Repeated MT soundings at different episodes of geomagnetic activ- volcanic-derived sedimentary rocks. This igneous arc was first recognized ity and the correlation of the measured resistivities of similar rocks at by Berg and others (1972), who called it the "Gravina-Nutzotin arc"; more widely scattered localities during 3 yr of MT surveying in central and recently, it has been called the "Chisana arc" by Plafker and others southern Alaska have removed most of our doubts about the effectiveness (1989a). We will refer to the igneous arc that likely existed along the full and utility of the method there. Large-amplitude, natural fields in Alaska length of the flysch system as the "Gravina arc." In the Kahiltna terrane, normally allowed a great deal of efficiency in obtaining MT data, although no volcanic rocks are known, but extensive andesitic detritus occurs in the various natural conditions such as unstable muskeg and high winds caused flysch (Csejtey and others, 1988). As described below, the rest of the problems in some instances. Gravina arc, in both the Kahiltna terrane and the Gravina-Nutzotin belt, is MT surveys were done in 1985 across the northeastern part of the interpreted to have been thrust under the Yukon-Tanana and other in- large flysch complex of the Kahiltna terrane in the central Alaska Range board terranes. Flysch of similar age and lithology to the Kahiltna terrane east of Mount McKinley (Figs. 1,2, and 3) in order to study the subsurface and Gravina-Nutzotin belt occurs to the southeast across the Denali fault configuration of the large volume of Jurassic and Cretaceous shale in this in the Dezadeash Formation (Eisbacher, 1976). terrane. Subsequently, during 1986 and 1987, two MT surveys were con- Extensive Mesozoic flysch has been mapped in the Windy terrane, ducted across the eastern Alaska Range as part of the TACT crustal re- which occurs as a narrow, fault-bounded lens that extends for several search program. These two 1986-1987 MT survey lines will be discussed hundred kilometers along the Denali fault (Fig. 1). In addition to mainly in more detail in a subsequent paper. Jurassic and Cretaceous argillite, volcanic graywacke, conglomerate, and andesite volcanic flows and tuffs, the Windy terrane contains rootless INTERPRETATION PROCEDURES FOR MT SOUNDINGS lenses of lower Paleozoic, calcareous sedimentary rocks and limestones (Nokleberg and others, 1985; Jones and others, 1987). The Mesozoic Individual MT soundings can be interpreted in terms of Earth struc- flysch and volcanic rocks in the Windy terrane are interpreted to be tures with varying degrees of complexity assumed for the geology. One- remnants of the Kahiltna terrane and associated Gravina arc. dimensional interpretations are the normal starting point for the The flysch of the Kahiltna terrane and Gravina-Nutzotin belt is interpretation process. MT measurements consist of tensor determinations strongly deformed, and the predominant structural style is thrust faulting of resistivity and phase in which the data are mathematically rotated into

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Figure 3. Detailed geologic map of part of the central Alaska Range in the Healey quadrangle, covering the main part of the survey area of profiles AA' and BB'. Modified from Csejtey and others (1986). A large klippe of Jurassic and Cretaceous flysch is outlined by the Honolulu thrust.

| | QUATERNARY & TERTIARY SEDIMENTARY ROCKS MT SOUNDING

tSSi MESOZOIC FLYSCH 0 10 km

TRIASSIC SEDIMENTARY & VOLCANIC ROCKS evidence for structures in the central Alaska Range, as they relate to the sutured Jurassic-Cretaceous flysch complex (Stanley, 1986). These two PALEOZOIC SEDIMENTARY ROCKS profiles are formed from 40 soundings performed in 1985. Access to the MT survey locations was provided by the main highway between Anchor- PALEOZOIC METAMQHPHIC ROCKS age and Fairbanks (Parks Highway) and the Denali Highway between

MELANGE Paxson and Cantwell (Fig. 2). The location of soundings in the central part of the two profiles is indicated on the more detailed geologic map in Figure A CRETACEOUS-TERTIARY GRANITES 3. The geologic setting of MT profiles CC' and DD' is shown in Figure 4. Discrete layered interpretations for profiles AA' and BB' are shown in Figure 5. Interpreted resistivities for the individual layers have been maximum and minimum resistivity directions (Vozoff, 1972). In most grouped as indicated in the legend. The connection between individual instances, these directions can be assigned to geologically equivalent, strike sounding models was made arbitrarily, and in some instances, with geo- and across-strike directions. The direction of rotation of the electric-field logic prejudice. For instance, between soundings 21 and 22, the contact data along electrical (and presumed geologic) strike is referred to as "E- portrayed between units of 600-5,000 and 170-600 ohm-m could have parallel" (also as "transverse electric," TE), and the direction across strike any configuration, based solely upon the one-dimensional interpretations; is referred to as "E-perpendicular" (also as "transverse magnetic," TM). thus, a probable geologic portrayal has been used. The relatively consistent The Earth can be simulated as a horizontally layered medium by either a trend line connecting the interfaces at soundings 36-21, however, is rea- small number of discrete layers (usually three to six layers) or a pseudo- sonably good assurance that this is a legitimate representation. For com- continuous model in which the number of layers is equal to the number of parison with the discrete layered interpretation, where the models have data points on the sounding curve (Bostick, 1977). Such one-dimensional been manually stitched together using geologic guidelines, computer- models can be connected or "stitched" together to form cross sections generated sections were also made. In these cross sections, an automatic along selected profiles. These one-dimensional interpretations generally gridding program was used to grid pseudo-continuous, one-dimensional indicate the gross structural details of the survey area, but more compli- models for the complete profile and resistivity intervals denoted by gray cated, two-dimensional models usually are required to fit both the E- scale patterns. The main features of the computer-generated sections are parallel and E-perpendicular observed data. The two-dimensional models quite similar to those of the stitched-together, discrete-layer model. The are generally derived by trial and error fitting of calculated values to the sense of dip implied in our discrete-layer inversions for profiles AA' and observed data. For regions with adequate sounding densities, three- BB' is supported by the computer-gridded continuous models. Subsequent dimensional modeling methods are sometimes employed as a final inter- discussion of two-dimensional models will further address constraints pro- pretive step. vided by the MT interpretations. Rock types for the highest and lowest resistivity ranges are easier to GEOLOGIC INTERPRETATION OF MT CROSS SECTIONS identify than for the intermediate 15-75, 100-170, and 170-600 ohm-m ranges. For these intermediate ranges, the typical resistivities of several Cross sections have been constructed from the one-dimensional mod- rock types overlap. Volcanic rocks, some sandstones, and carbonate rocks els for profiles AA' and BB' for the central Alaska Range region and for frequently have resistivities in these ranges. High-grade metamorphic rocks CC' and DD' in the eastern Alaska Range (Figs. 2 and 4). In addition, and plutonic rocks generally fall in the higher ranges, and shales, brine- two-dimensional models for the Alaska Range part of profiles AA', CC', saturated sandstones, and glacially derived Quaternary sediments normally and DD' have been constructed. Profiles AA' and BB' provided key fall into the lowest range.

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141° pr .64° c'a < 100 200 250 KILOMETERS » c YT )' \X z

STK

o. 'mm (Nutzotin Sequence)

i^y'.ij-

KJf- ^Kluane < Figure 4. Locations of TACT MT profiles CC' (Dezadeash^ iSchist and DD', along with TACT deep reflection profile Fm.) across the Alaska Range. Two subunits of Jurassic BRITISH COLUMB^ and Cretaceous flysch (KJf) are indicated as the Nutzotin and Dezadeash Formations. Terranes and figure adapted from Nokleberg and others (1985). f \ Terranes: MG, Maclaren Glacier metamorphic belt;

ES, East Susitna batholith; ALX, Alexander; P, Pe- ALX ninsular; Tu, Taku; W, Windy; YT, Yukon-Tanana. (See Fig. 1 for other abbreviations.)

CENTRAL ALASKA RANGE MT CROSS SECTIONS Kahiltna terrane that form massive cliffs to the east of the highway near the profile. These weakly metamorphosed shales are highly fissile and have Sections AA' and BB' of Figure 5 depict several key tectonic and coatings of carbon, possibly graphitic, between fissile planes. Organic anal- plutonic features. yses indicate a carbon content of 3-5% and also indicate that most of the (1) Soundings 21-36 (from Paxson west on the Denali Highway) carbonaceous material has been metamorphosed beyond the gas- are on the Wrangellia (Fig. 2) terrane (Jones and others, 1977; Nokleberg producing stage (Rick Stanley, 1986, personal commun.). Metamorphism and others, 1985); the terrane is interpreted to thin to the northwest of carbonaceous material to higher grades (ultimately to graphite) de- toward the Talkeetna thrust that along with the Broxon Gulch thrust (Fig. creases resistivity of rocks in which such material is a significant constitu- 4), separates Wrangellia to the south from the Kahiltna and Maclaren ent (Duba and Shankland, 1982; Duba and others, 1989). Near sounding terranes to the northwest. The MT interpretation provides evidence for 6 is the Honolulu thrust (Fig. 3) (Csejtey and others, 1986, 1988). This thrusting of Wrangellia over rocks of the Kahiltna flysch terrane as de- sinuous thrust outlines a large klippe of black shale and also occurs near scribed by Csejtey and others (1982 and 1988). sounding 42 on profile BB', where the MT interpretation shows a similar (2) Resistive rocks at MT soundings 34-32, extending westward on section of low-resistivity rocks also interpreted as black shale. the Denali Highway, may be highly metamorphosed argillite and gray- (5) Low-resistivity rocks are interpreted to occur between soundings wacke, analogous to the Maclaren terrane that occurs west of the Tal- 26 and 37 on profile AA' in a region north of the McKinley strand of the keetna and Broxon Gulch thrusts (Figs. 3 and 4; Nokleberg and others, Denali fault. The rocks occur within 3-5 km of the surface just north of 1985) to the northeast of MT sounding 34. Alternately, much of this the fault and extend to one-dimensional-interpreted depths of at least 30 section may be made up of Cretaceous and Tertiary plutonic rocks, which km. Structures with a wavelength of about 20 km are evident on the upper crop out at numerous nearby locations. surface of the conductive package; such structures appear in a very similar (3) Resistive rocks at soundings 5 and 8 south of the Honolulu thrust manner on profiles CC' and DD' and may be due to folds or to imbricated on profile AA' probably represent a Tertiary pluton similar to those fault slices. Additional depth and resistivity constraints on the conductive mapped on either side of the profile in this area (Csejtey and others, 1986, section are provided by the two-dimensional modeling discussed below. 1988). The resistive rocks at soundings 9, 7, and 17 are probably smaller This low-resistivity section may consist partially of (a) Paleozoic shales in separate masses of intrusive rock. Resistive rocks at soundings 1 and 3 may the McKinley terrane (Jones and others, 1982), (b) upper Mesozoic flysch be associated with the Peninsular terrane part of the Talkeetna superter- of the McKinley terrane, or (c) black shale of the Kahiltna terrane that rane (Fig. 4). occurs in massive exposures south of the Denali fault. This interpretation (4) Low-resistivity rocks beneath soundings 6-10 on profile AA' implies that the metamorphic rocks of the Yukon-Tanana terrane, as (north of the Honolulu thrust) probably represent black shales of the discussed below, and other pre-Jurassic units represent a thin-skinned

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HONOLULU THRUST^ 1 2 4 5 9 7 10 28 17 27 19 23 Figure S. Discrete one-dimension- T T * * T ttttt t TT al cross sections for profiles AA' and BB'. Interpreted resistivities for different parts of the profiles are indicated in 15-75 the explanation. The positions of the Denali fault strands and the Honolulu thrust (Fig. 3) are indicated on the DENALI FAULT sections.

EXPLANATION

Surficial materials, highly variable

< 12 ohm-m

15-75 ohm-m

100-170

170-600

600-5000

thrust sheet overlying conductive Upper Jurassic and Cretaceous sedimen- than in the central Alaska Range, although scalar AMT (audiomagnetotel- tary rocks for several tens of kilometers north of the McKinley strand of luric) soundings were used to add detail in some areas. Initial reconnais- the Denali fault. Both the Yukon-Tanana and Wrangellia terranes may sance soundings in 1986 were done using a truck-mounted system. These have overridden flysch basin rocks of the Kahiltna terrane and Gravina- studies were designed to obtain initial data on the nature of the geology in Nutzotin belt and associated Gravina igneous arc during convergence in this region and to study the effect of the Trans-Alaska Pipeline on MT the Cretaceous. According to the MT model for profile AA', the Hines measurements. Calculations by Campbell and Zimmerman (1980) dem- Creek fault does not appear to be a major resistivity discontinuity. onstrated that magnetic fields caused by geomagnetically induced currents (6) Resistive rocks occur north of the Denali fault on profile A A' at in the pipeline could cause problems in MT measurements at distances as soundings 29 and 30, southwest of Fairbanks. Sounding 29 was measured far as 50 km from the pipeline. MT measurements in 1986 suggested that on metasedimentary schist and marble of the Yukon-Tanana terrane; the this may be a more severe restriction than necessary; nevertheless, MT high resistivities are typical of high-grade metamorphic rocks. Sounding 30 measurements were made far away from the pipeline with the aid of indicates a much thinner section of the high-resistivity rocks, but the poor helicopters and float planes. A portable low-frequency MT system was data quality for sounding 30, and the occurrence of thick glacial sediments developed by one of us (VFL) for these surveys using low-noise flux-gate at the surface, make the interpretation of the data for this sounding suspect. magnetometers and a battery-operated data logger controlled by a small, Most of the section between soundings 15 and 30 consists of moderately hand-held computer. This system was combined with existing AMT in- conductive units (15-75 ohm-m) that are more typical of sedimentary strumentation for use from the aircraft. The north-south profile across the than of metamorphic rocks. Scattered Paleozoic sedimentary rocks of the Denali fault and parallel to the Richardson Highway (CC, Fig. 4) was Nixon Fork terrane (Fig. 1) crop out from beneath the glacial cover accomplished using a helicopter. The other southwest-northeast-trending between soundings 15 and 30; thus, the subsurface units on this portion of profile (DD', Fig. 4) to the east across the Totschunda and Denali faults the profile may be composed of Paleozoic sedimentary rocks rather than was completed using the truck-mounted system. metamorphic units associated with the Yukon-Tanana terrane. MT soundings from over the very conductive rocks in the Alaska Range are shown TWO-DIMENSIONAL MODELS FOR ALASKA in Figure 6. In part A, data from profile AA' are indicated, and in part B, RANGE MT SECTIONS data from the TACT profile DD' are shown. A two-dimensional forward modeling program (Vozoff, 1972) was EASTERN ALASKA RANGE MT CROSS SECTIONS used to study the structure along profiles AA', CC', and DD' from just south of the Denali fault to the northern ends of the profiles. This program The locations of the eastern Alaska Range MT profiles are indicated calculates responses for both the E-parallel and E-perpendicular resistivi- in Figure 4. The soundings were generally spaced somewhat wider apart ties and phases in order to fit the observed data. In some instances, the data

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The two-dimensional interpretations of data from profiles CC' and DD' in the eastern Alaska Range show remarkable similarities to features observed in the MT survey of the central Alaska Range (profile AA'). The leading edge (southern margin) of the highly conductive rocks has a similar shape on all three MT models in Figure 7, but the lateral extent of the conductive units on profiles CC' and DD' is much greater. Features on profile AA' used in the two-dimensional model do not differ greatly from those discussed in relation to the one-dimensional model of Figure 5. The conductive rocks on all three profiles occur quite near the surface close to the Denali fault. The simplicity of the upper 10 km of the model on profiles CC' and DD' may be a result of the clearer association of the upper part of these profiles with the Yukon-Tanana terrane metamorphic rocks. Conductive units on profile CC' cannot be directly correlated with surface geology as was demonstrated for similar units on profile AA'; however, some clues exist for such a correlation on profile DD'. Conduc- tive units on DD' are encountered just north of the Denali fault near carbonaceous flysch of the Windy terrane. As previously described, these rocks are similar to the flysch of the Kahiltna terrane and Gravina- Nutzotin belt. TACT reflection data from an 80-km-long profile across the Alaska Range (Fisher and others, 1988) is shown in simplified form in the CC' model panel of Figure 7. A north-dipping basal reflector occurs at depths from 8 to 22 km northward from the Denali fault. This basal reflector forms the bottom of a wedge of reflective material that may represent underthrust Mesozoic flysch units correlative with the highly conductive section. The upper surface of this reflective wedge is formed by reflections that have an undulatory character with a wavelength similar to that of the MT model. These structures may be due to folding, to stacked nappes, to successive underplating episodes, or to other mechanisms. The similarity of the northward-dipping structures observed on the seismic and MT data suggests thrusting beneath the Alaska Range with a consistent style that FREQUENCY (HZ) extends along the range for a distance of greater than 350 km. Figure 6. Example MT soundings from profiles AA' and DD'. The major conductive units underlying the Alaska Range appear to The soundings were made using two different MT systems, with com- be correlated with outcrops of weakly metamorphosed Mesozoic black pletely different processing programs. The data in part A are from shales and possibly sub-Mesozoic carbonaceous rocks. Additional evi- sounding 25 over the conductive part of the Alaska Range on profile dence for widespread occurrence of conductive Mesozoic flysch in Alaska AA', and those in part B are from the deep conductive region north of is revealed by magnetotelluric surveys in the Manley terrane (Fig. 1) the Denali fault on profile DD'. Triangles and squares denote the northwest of Fairbanks by Carl Long (1988, personal commun.). This E-parallel (TE) and E-perpendicular (TM) data, and the solid curves narrow, northeast-trending terrane and a similar southeasterly dispersed in part A are computed responses for the one-dimensional model of terrane (the Kandik River terrane, Fig. 1) contain Jurassic-Cretaceous Figure 5. In part B, the vertical bars represent 68% confidence inter- black shale units analogous to those in the Kahiltna terrane and the vals for the resistivity values. Gravina-Nutzotin belt. MT soundings on outcrops of the black shale in the Manley terrane show that these units have resistivities of less than 5 ohm-m at depths of greater than 1-2 km. Preliminary interpretations of appeared mildly three-dimensional, but in general, the fit of the field data MT profiles across the Manley and Yukon-Tanana terrane margins suggest with the two-dimensional model is reasonable, considering the complexity that metamorphic rocks of the Yukon-Tanana overlie conductive rocks of the geology. The two-dimensional modeling process greatly aided in that may be similar to those underneath the Alaska Range. understanding certain key constraints on the geology provided by the MT data, beyond those obtained from the one-dimensional modeling. CONDUCTION MECHANISMS RELATING TO ROCKS The best-fitting models derived from the two-dimensional modeling IN THE ALASKA RANGE are shown in Figure 7, and the comparison of computed to observed data is shown in Figure 8. These models require conductive units (1-3 ohm-m) Most marine sedimentary rocks are conductive, with sandstones and extending to 20-23 km depth. The main conductive section starts north of shales typically falling in the range from 2-20 ohm-m (Parkhomenko, the Denali fault for all three profiles, but on profile CC', these conductive 1967). Carbonate rocks generally have relatively high resistivity (100-500 rocks begin some 15 km north of the Denali fault. The data from all three ohm-m) unless saturated with brines. Conduction in sandstones is domi- profiles require a northward dip of the base of the conductive section, but nated by a combination of ionic conduction through saline pore waters modeling indicates a low sensitivity to the actual amount of dip. The and surface conduction in clays and zeolites coating the matrix grains. The required north dip implies possible thrusting, if the base of the conductor is ionic-conduction component is directly related to temperature and poros- a suture between terranes. ity. Conduction in low-porosity marine shales is somewhat less influenced

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SOUTH NORTH DENALI FAULT 27 19 2^2625 14 12 16 ^ y 38

Figure 7. Two-dimensional models for CENALI FAULT Alaska Range parts of profiles AA', CC', and \ V VP 'S 1J W V • Of V DD' (Figs. 1 and 4). MT sounding locations are denoted by inverted triangles, and AMT ...... ,,,..p^ ^ nJS sounding locations by solid squares. Model resistivity ranges are shown by the four patterns. The upper 3 km of the actual model are not shown. Key reflections from the TACT Richardson Highway seismic profile are shown by the dashed lines.

DENALI FAULT

° 8 27 9 11 25 26172lT 22 18 19 20 16 , » > T Y • T» • SL_

cn 10- cc

UJ 1 20 2

30-

REFLECTION ' 8-30 ^ 60-300 101Ì0-10000 T MT SITE MODEL RESISTIVITIES • AMT SITE

50 100 HOR. SCALE (KILOMETERS)

by the ionic-conduction parameters of temperature and porosity, and move demonstrated that the decrease in porosity in nonshales would lead to an directly related to surface conduction, because of the higher percentage of increase in resistivity at depths greater than a few kilometers. As shales are clay occurring in shales. These surface-conduction mechanisms disappear metamorphosed during burial and/or horizontal compression, however, with increasing pressure and temperature as the clay minerals are meta- contained organic matter is converted to conductive carbon (Duba and morphosed. Decreases in porosity with increasing depth further reduce the others, 1989) and mobilized into the planes of fissility. This process is ionic-conduction component. accentuated with higher degrees of metamorphism until the rocks become Another important conduction mechanism occurs in shales, espe- recrystallized and/or carbonaceous films between fissile planes are de- cially when they are moderately metamorphosed and contain significant stroyed. In addition, if the shales contain large amounts of iron sulfides or percentages of carbon and/or metallic minerals. Duba and Shankland other metallic minerals, as is common in black shales, burial diagenesis can (1982) and Duba and others (1989) have discussed the role of carbon in lead to formation of metallic mineral films along the fissile planes to crustal and mantle conductors. Stanley (1989) outlined the occurrence of augment or supplant the carbonaceous films. Thus, these mechanisms can carbon and sulfide mineral films as a major conductive mechanism in provide continuous conduction paths for the metamorphosed shales that metamorphosed shales and pointed out the importance of this mechanism are effective until pressures and temperatures necessary for complete re- in causing deep conductivity anomalies in suture zones such as that in the crystallization are reached. Alaska Range. In regions of high heat flow, it is possible to have deep The preceding discussion outlines conduction mechanisms that may conductors in the Earth's crust caused by partial melting or other effects of be responsible for the low resistivities beneath the Alaska Range but does free water at depths of greater than 10-15 km (Wannamaker, 1986). not constrain the age of the rocks. Black shale epochs have been common Areas exist, however, where very conductive rocks extend in a continuous in the Earth's geologic history, partially owing to world-wide anoxic fashion from as shallow as 1-2 km to depths of more than 20 km (Stanley, conditions in the oceans (Jenkyns, 1986) and partially owing to the occur- 1989). It is difficult to explain how rocks can maintain their low resistivi- rence of numerous deep marine basins during favorable tectonic episodes. ties over such a large depth range. With low heat-flow values, it can be The deep marine basin that existed between the northward-migrating Tal-

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keetna superterrane and proto-Alaska was probably such a large-scale in continuous form. Submicroscopic graphite may still be present in me- basin with anoxic conditions, as evidenced by the large volumes of black tamorphosed shales such as that in Figure 9C, but the percentage is too shale in the Kahiltna terrane, Gravina-Nutzotin belt, and Windy terrane. small to provide continuous conduction paths. From outcrop measure- Carbonaceous or mineralized rocks of ages other than Mesozoic may ments, the Mesozoic shales are known to be conductive, and the Paleozoic also compose conductive rocks beneath the Alaska Range and Yukon- units with similar carbon films can be assumed to have similar low resistiv- Tanana terrane. Some Ordovician and Devonian shales occur in the vicin- ities. The Paleozoic shales are of very limited extent in the survey area, and ity of the Alaska Range. In particular, the Dillinger terrane (Fig. 1) is part regionally, the Dillinger and McKinley terranes in which they occur are of a narrow belt of shale fades that Churkin and Eberlein (1977) corre- thin thrust slices of limited extent (Jones and others, 1982). Consequently, lated with the shale-out facies of the Nixon Fork terrane (Fig. 1). Scattered the much larger volumes of the Mesozoic black shales may be the primary outcrop patterns of carbonaceous rocks of Paleozoic age occur north of the rock type in the thick conductive zone; however, significant amounts of Denali fault in the central Alaska Range, but Mesozoic carbonaceous Paleozoic shale or other conductive rocks in the subsurface cannot be ruled flysch is predominant. If the assumption is made that there has been no out, especially as very little is known about the subsurface nature of the duplexing of crust, then one might expect the Paleozoic rocks to form the Yukon-Tanana terrane. basement rocks in the central Alaska Range region. This, however, is an assumption that probably is not correct in light of tectonic development of GEOLOGIC EVIDENCE FOR CRETACEOUS Alaska. UNDERTHRUSTING Thin-section study of central Alaska Range Paleozoic shales (Fig. 9A) shows a high density of carbon films, similar to that in the Mesozoic In support of the geophysical data presented in this study, several black shales (Fig. 9B). In a thin section from a sample of a higher-grade geologic observations indicate a period of major Early to middle Creta- Mesozoic metashale (Fig. 9C), the carbon has mostly disappeared, at least ceous oblique-slip underthrusting along the ancestral Denali fault. First, a

RICHARDSON STH T45 RICHFLRDSON STFL T66 RICHARDSON STFL T69

3 ••••: ;•••• 3 +_ J

: X : 2 ' : 2

X X x+ \

"' + B ; ;•••• a CC1 cc ; ; : ; ; ; CC' : 3 2 1 0-1-2-3 3 2 1 0-1-2-3 1 3 2 10- 1 -2 -3 TE-X-X- LOG FREQUENCY (HZ) TM-+-+- TE-X-X- LOG FREQUENCY (HZ) TM-+~T— TE-X-X- LOG FREQUENCY (HZ) TM- + -H—

Figure 8. Observed data and computed values from two-dimensional models for selected soundings on profiles AA', CC', and DD' (models in Fig. 7).

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Figure 9. Thin sections of carbonaceous shales from near the Denali fault. (A) Sample of Paleozoic black shales from the Yannert Fork sequence, McKinley terrane (Csejtey and others, 1986), north of the Denali fault. (B) Sample of Mesozoic black shale from north of the Denali fault (Windy terrane). (C) Sample of recrystallized Mesozoic flysch from north of the Denali fault.

zone of low-grade retrogressive metamorphism, with K-Ar mica ages of 100 to 110 m.y. (Nokleberg and others, 1989), associated ductile deformation, and south-verging structures, occurs along the southern margin of the Yukon-Tanana terrane for several hundred kilometers adjacent to the fault in a zone as much as 20 km wide (Nokleberg and Aleinikoff, 1985; c Nokleberg and others, 1986). This zone of low-grade, retrograde metamorphism is observed from along the southwestern margin of the Yukon-Tanana terrane in the central Alaska Range to the United States- Canada border. Fourth, ultramafic bodies occur in several places along the Denali Second, the intensity of low-grade retrograde metamorphism and fault; for example, two klippen of ultramafic rocks occur (Nokleberg and ductile deformation increases toward the fault, with lowest-greenschist- others, 1985) south of the Denali fault just east of MT profile CC' (Figs. 1 facies rocks and strongly deformed, abundant mylonitic schist near the and 4). Berg and others (1972) have documented 24 such bodies in the fault. To the north, sparse relict hornblende, biotite, and garnet occur in Gravina-Nutzotin belt. These ultramafic bodies may be related to pieces of metasedimentary and metavolcanic rocks. Progressively to the south, sheared oceanic crust formed during oblique underthrusting. hornblende, biotite, and finally garnet are successively replaced by chlorite Last, Early to middle Cretaceous high-grade regional metamorphism and associated minerals along a progressively more intensely developed and deformation has been documented in the deep structural levels of the schistosity. This schistosity dips gently north or south in the north and is Yukon-Tanana terrane (Aleinikoff and others, 1986; Nokleberg and oth- folded into a major west-northeast-trending antiform adjacent to the De- ers, 1986, 1989; Foster and others, 1987; Dusel-Bacon and others, 1989). nali fault. Locally abundant tightly appressed to sub-isoclinal folds are This upper-amphibolite- to eclogite-facies regional metamorphism associated with the intense schistosity. Where not dismembered along (Brown and Forbes, 1986; Foster and others, 1987) has been dated as be- axial-plane schistosity, these folds display south vergence (Nokleberg and tween approximately 105 and 128 Ma (Wilson and others, 1987) and has Aleinikoff, 1985). not previously been related to specific thermal events. Exposed Yukon- Third, lenses of dark argillite, metagraywacke, and andesite flows Tanana terrane units represent rocks metamorphosed at depths of as much with Cretaceous and Jurassic ammonites occur in mélange of the Windy as 15 km (Brown and Forbes, 1986). The uplift of the terrane since terrane along the Denali fault in the central and eastern Alaska Range metamorphism may be related to crustal duplexing from underplating of (Jones and others, 1987; Nokleberg and others, 1989). These outcrops are the extensive Mesozoic flysch system. We speculate that the subthrusting interpreted as relict lenses of flysch similar to that of the Kahiltna terrane of the Gravina arc ahead of most of the flysch basin may have contributed that remained after oblique underthrusting and Cenozoic dextral strike-slip to high-temperature metamorphism in deep levels of the Yukon-Tanana movement along the Denali fault. terrane.

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Figure 10. Schematic reconstruction of A MID-CRETACEOUS possible development of conductive rocks beneath the Alaska Range. Depicted are the suturing of the Talkeetna superterrane to continental Alaska during late Mesozoic and early Cenozoic time, closing of intervening regional flysch basin (short dashed pattern), and emplacement of flysch beneath the margin (inboard limit shown by dot-dashed line). Miniterranes swept along and then dispersed are shown by dot pattern. Metamorphic rocks of the Maclaren terrane and Kluane Schist shown by wiggly pattern, and the proposed regional metamorphic front indicated by the continuous wavy lines. Intrusions of 70-50 m.y. in age indicated by solid black pattern. The regional Gravina arc is indicated by the asterisks. WR, Wrangellia; AX, Alex- ander; PE, Peninsular; YT, Yukon-Tanana terranes. Adapted from Plafker and others (1989a).

B MID-TERTIARY

.-Nutzotin sequence

\ A new piece of supporting evidence for underplated flysch beneath the Yukon-Tanana terrane has recently been developed. Aleinikoff and

others (1989) have analyzed Pb isotope ratios in K-feldspar from Kluane schist Devonian-Mississippian metaplutonic rocks and Cretaceous-Tertiary gran- Range batholith ites in the Yukon-Tanana terrane and in samples of Jurassic-Cretaceous Dezadeash sequence flysch from the Kahiltna terrane and Gravina-Nutzotin belt. The Pb iso- V" tope data indicate a well-developed mixing curve between post-115 Ma granites and the Mesozoic flysch, with as much as 50% mixing of the Denali fault radiogenic (continental source, which may be the metaplutonic protoliths) and the less radiogenic (oceanic source, satisfied by the Mesozoic flysch). Granites older than 115 m.y. have Pb isotope ratios identical to those of the metaplutonic rocks of the Yukon-Tanana terrane, suggesting that underplating of the flysch (or other similar oceanic component) occurred between about 115 and 95 Ma. In the main parts of the MT models for profiles AA', CC', and DD', the ratios of conductive units assumed to be underplated flysch constitute about 20%-50% of the crust. The time-space- EXPLANATION isotope relationships favor the model of flysch underthrusting, rather than ^7777 '//// Jurassic-Cretaceous flysch previously proposed "oceanic" sources (Aleinikoff and others, 1987), such as mafic and ultramafic rocks that crop out in some locations in the Oceanic crust Yukon-Tanana terrane, and also favor underplating of Mesozoic flysch Met amorphics rather than sub-Mesozoic, autochthonous carbonaceous rocks as the cause of the MT conductor. Metamorphic front The geologic observations above support a middle Cretaceous period Intrusion of south-verging, oblique underthrusting along the ancestral Denali fault at Miniterrane the southern margin of the Yukon-Tanana terrane. The oblique thrusting and flysch-basin collapse occurred in response to accretion of the amal- Limit of undertucked flysch gamated Wrangellia, Peninsular, and Alexander terranes during the mid- Andesite volcanoes

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Late Jurassic "PROTO" ALASKA

10 -

30 - Early and Mid-Cretaceous / j3' 40 ^ Gravina Arc

Figure 11. Paleo- tectonic cross sections constructed according to preferred model of underthrust Mesozoic flysch beneath the Alaska Range. Symbols: TS, Talkeetna superterrane; YT, Yukon- Tanana terrane; MI, miniterrane. Sections based upon geologic mapping in the Healey quadrangle (Csejtey and others, 1988).

100 KILOMETERS _J

EXPLANATION

Anoxic basin Oceanic crust sediments Off-scraped sediments Miniterranes C/';] Granitic intrusions Proto Alaska terranes

die Cretaceous. The suture zone between this docked superterrane and the superterrane and the Alaskan mainland (Csejtey and others, 1982; Nokle- Alaska margin was instrumental in forming the ancestral Denali fault and berg and others, 1985). was modified by subsequent Cenozoic dextral strike slip. We also interpret Paleotectonic cross sections in Figures 10 and 11 illustrate a possible that the low-temperature, retrogressive metamorphism and intense ductile development of the conductive section underneath the Alaska Range. The deformation along the southern margin of the Yukon-Tanana terrane oc- cross sections illustrate consumption of a regional flysch basin and mag- curred in response to oblique thrusting of cold, wet flysch of the Kahiltna matic arc between the arriving superterrane and the Alaskan margin. terrane, after subthrusting of the major part of the Gravina arc. Although the presumed motion of the Kula plate carrying the superterrane was probably oblique (Engebretson, 1982), the dip component was ade- PROPOSED UNDERPLATING MODEL quate to imbricately shear thick flysch sequences under the Alaskan mar- FOR STUDY AREA gin. Figure 10A depicts the amalgamated Wrangellia, Alexander, and Peninsular terranes, the regional flysch basin, and various miniterranes The geologic and geophysical data and interpretations presented in such as the Pingston, Mystic, West Fork, McKinley, and Windy terranes. this paper provide new constraints for the regional tectonics of south- This figure also indicates our interpretation that a regional metamorphic central and southeastern Alaska and for the fundamental nature of struc- front (wiggly lines) developed in the flysch on the leading edge of the tures in the Alaska Range. The MT cross sections show evidence for rapidly closing superterrane, adjacent to the Kluane arc. Oblique motion extensive underthrusting, somewhat thin-skinned in nature, in the central of the superterrane resulted in dextral strike-slip transport of various mini- and eastern Alaska Range. Large volumes of conductive rocks occur un- terranes. Figure 10B depicts most of the flysch and associated igneous arc derneath thinner units of the resistant Yukon-Tanana terrane in the Alaska emplaced beneath the Yukon-Tanana terrane, as well as formation of the Range over a strike distance greater than 350 km. Our geophysical inter- Cenozoic Denali fault. The dot-dashed line indicates the inboard limit of pretations and geologic evidence suggest to us that a broad flysch basin and the underthrust flysch interpreted from the MT profiles. Most of the met- associated igneous-arc lavas may have been telescoped and emplaced be- amorphic belt associated with the Gravina arc disappeared along with the neath older tectonic flakes constituting proto-Alaska. As previously dis- flysch, but relatively younger remnants may be in evidence in the Mac- cussed, this deep flysch basin developed between the converging Talkeetna laren terrane and Kluane Schist. Remnants of an original, broader flysch

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basin occur in the Kahiltna terrane, in the Windy terrane, in the Gravina- tures of Alaskan tectonics; however, ongoing geophysical and geologic Nutzotin belt, and in the Dezadeash Formation. The late Mesozoic and studies as part of the TACT program need to be continually evaluated to Cenozoic dextral strike-slip component of motion contributed to dispersal test this model. Modeling of the geophysical data can be tested and refined; of the miniterranes and caused thrusting of the Wrangellia terrane over for instance, we have not integrated the seismic refraction and gravity- flysch of the Kahiltna terrane. The combination of north-directed move- magnetic data from the TACT transect with the MT and seismic reflection ment and possible early Tertiary development of oroclinal bending of data. Alertness for problems with high-latitude source fields is needed for western Alaska (Plafker and others, 1989a) resulted in additional deforma- Alaska surveys; although we have presently ruled out major source-field tion of unconsumed flysch in the Kahiltna terrane and terranes to the south problems for the MT surveys reported herein, the issue still needs con- of the Denali fault system (Fig. 10B). A cross-sectional view of this tec- tinued study with magnetic arrays. Thus, we caution that although unifica- tonic model is better illustrated in Figure 11. There, we portray lower tion exists in our tectonic model, the real situation might be related to Tertiary anatectic granites (Fig. 11C) intruding the northern Wrangellia pre-Mesozoic continental-margin features or other geology. We particu- and Kahiltna terranes (Lanphere and Reed, 1984) that may have been larly need to conduct additional regional MT surveys over known Paleo- generated from compressional heating of the lower part of a great thick- zoic, nonmetamorphic complexes in Alaska for study of electrical ness of duplexed flysch. During the Cenozoic, the Denali fault is inter- signatures of these units, as well as conduct a thorough laboratory study of preted to have been the locus of slip displacement, with interterrane rock properties from the various terranes. friction reduced by the abundant carbon/graphite purged from the metamorphosed carbon-rich flysch. Continuing Cenozoic oblique-slip move- ACKNOWLEDGMENTS ment may have consisted of a combination of dextral strike-slip movement along a vertical plane and oblique underthrusting along original northeast- We are indebted to W. J. Gwilliam of the Morgantown Energy dipping sole thrusts at the base of the underplated flysch or along its top Technology Center, U.S. Department of Energy, who arranged funding for surface (Fig. 11C). Local compression north of the Denali fault in the the 1985 MT surveys and provided encouragement toward their comple- Mount McKinley region caused folding and faulting in the Tertiary sedi- tion. D. W. McNair, U.S. Geological Survey, has been a consistent and mentary and volcanic rocks of the Cantwell formation (Csejtey and others, efficient facilitator in getting major parts of the instrumentation assembled 1982). for the Alaska surveys and assisted with most of the field work. The members of the TACT program have been extremely helpful in sharing SUMMARY AND CONCLUSIONS ideas regarding ongoing studies in Alaska. D. L. Jones and David Howell were very helpful in providing conceptual background for the central Extensive MT surveys in the central and eastern Alaska Range region Alaska Range and other areas of Alaska. Constructive reviews by John indicate that large volumes of conductive rocks occur beneath the range Booker, David Campbell, Wyatt Gilbert, Thomas P. Miller, and Norman and extend for large distances under the Yukon-Tanana terrane. These J. Silberling greatly improved the manuscript. Also, we thank the National rocks are hypothesized to be largely tectonically emplaced, weakly to Park Service for providing access to gravel pits in Denali National Park for moderately metamorphosed Upper Jurassic and Lower Cretaceous shale MT measurements. flysch. Low resistivities in the rocks are probably caused by carbon and/or metallic mineral films between fissile planes. The occurrence of such a REFERENCES CITED regional conductivity feature is evidence for remnants of a collapsed re- Aleinikoff, J. N., Dusel-Bacon, Cynthia, and Foster, H. L., 1986, Geochronology of augen-gneiss and related rocks, Yukon-Tanana terrane, east-central Alaska: Geological Society of America Bulletin, v. 97, p. 626-637. gional flysch basin that occurred between the accreting Wrangellia, Penin- Aleinikoff, J. N., Dusel-Bacon, C., Foster, H. L., and Nokleberg, W. J., 1987, Lead isotopic fingerprinting of tectono- stratigrapbic terranes, east-central Alaska: Canadian Journal of Earth Sciences, v. 24, p. 2089-2098. sular, and Alexander terranes and Mesozoic proto-Alaska. We theorize Aleinikoff, J. N., Stanley, W. D., and Nokleberg, W. J., 1989, Pb isotopic evidence for underthrusted Mesozoic flysch that this basin was mostly consumed during the collision of the superter- beneath the Yukon-Tanana terrane, east-central Alaska [abs.]: American Geophysical Union, Fall Meeting, San Francisco, California (in press). rane and proto-Alaskan, and substantial volumes of these rocks are inter- Berdichevsky, M. N,, and Dimitriev, V. I., 1976, Basic principles of interpretation of magnetotelluric sounding curves, in Adam, Antal, ed., Geoelectric and geothermal studies, KAPG Monograph: Budapest, Hungary, Akademiai Kiado, preted to be tectonically underplated at the base of the Yukon-Tanana p. 165-221. terrane. Remnants of this consumed flysch basin occur in the outcrops of Berg, H. C., Jones, D. L., and Richter, D. L., 1972, Gravina-Nutzotin belt—Tectonic significance of an upper Mesozoic sedimentary and volcanic sequence in southern and southeastern Alaska: U.S. Geological Survey Professional the Kahiltna terrane, Windy terrane, Gravina-Nutzotin belt, and Deza- Paper 800-D, p. D1-D24. Bostick, F. X., Jr., 1977, A simple, almost exact method of MT analysis, in Workshop on electrical methods in geothermal deash Formation (Fig. 4). Compression and metamorphism during amal- exploration: U.S. Geological Survey Contract No. 14080001-8-359. gamation of the large terranes caused recrystallization of some sedimentary Brown, E. H., and Forbes, R. B., 1986, Phase petrology of eclogitic rocks in the Fairbanks district, Alaska, in Evans, B. W., and Brown, E. H., eds., Blueschists and eclogites: Geological Society of America Memoir 164, p. 155-167. rocks in the flysch basin adjacent to the suture. The bulk of the andesitic Campbell, W. H., and Zimmerman, J. E., 1980, Induced electric currents in the Alaska oil pipeline measured by gradient fluxgate and SQUID magnetometers: Institute for Electrical and Electronic Engineers, Transactions on Geoscience Gravina arc, which developed in the flysch basin, may have also been and Remote Sensing, v. GE-18, no. 3, p. 244-250. swept under the Yukon-Tanana terrane, contributing to high-temperature Churkin, M., Jr., and Eberlein, G. 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