The Southern Kahiltna Terrane: Implications for the Tectonic Evolution of Southwestern Alaska

The southern Kahiltna terrane: Implications for the tectonic evolution of southwestern Alaska

WESLEY K. WALLACE* \ CATHERINE L. HANKS* | ARCO Alaska, Inc., P.O. Box 100360, Anchorage, Alaska 99510-0360 JOHN F. ROGERS* j

ABSTRACT in approximately its present location by latest floored by oceanic crust, that existed between Cretaceous to Paleocene time. the Talkeetna superterrane and the continent Regionally extensive and highly deformed with which it ultimately collided (Coney, 1981; Jurassic to Cretaceous basinal turbidite de- INTRODUCTION Csejtey and others, 1982; Pavlis, 1982; Jones posits occur along most of the inboard and others, 1982, 1986). Accretion of the Tal- boundary of the Talkeetna superterrane in Alaska has been subdivided into many tecton- keetna superterrane to North America is thought southern Alaska and western Canada. The ostratigraphic terranes, based on apparent differ- by many workers to have occurred in middle southern Kahiltna terrane of southwestern ences in geologic history across faulted or Cretaceous time (Coney, 1981; Csejtey and oth- Alaska consists largely of such basinal depos- covered terrane boundaries (Fig. 1; Jones and ers, 1982; Monger and others, 1982; Pavlis, its. Regional and detailed studies in and adja- Silberling, 1979; Jones and others, 1981,1987). 1982; Jones and others, 1982, 1986), based on cent to this terrane suggest several new Paleomagnetic, paleobiogeographic, and other the age of the youngest highly deformed rocks conclusions regarding the origin of the basin evidence suggests that some of the largest ter- within these basins, as well as the age of regional and its role in the tectonic evolution of ranes of southern Alaska, including the Peninsu- magmatic and metamorphic overprints inter- southwestern Alaska. Stratigraphic, sedimen- lar, Wrangellia, and Alexander terranes (Fig. 1), preted to be related to collision. tologic, compositional, and structural evi- were far south of their present location during Little is actually known, however, about the dence suggests that the Upper Jurassic to early Mesozoic time and were translated north- stratigraphy, depositional geometry, and base- Lower Cretaceous strata of the southern Ka- ward to their present location during late Meso- ment of these basins, or about the character and hiltna terrane were derived from and depos- zoic to earliest Tertiary time (evidence summa- timing of their deformation. An improved united on rocks of the adjacent Talkeetna rized in Coe and others, 1985; Panuska and derstanding of these basins is necessary to con- superterrane. Deposition of these clastic Stone, 1985; Jones and others, 1986; Stone and strain the timing and mode of emplacement of rocks apparently postdated arc magmatism in McWilliams, 1989). Similarities in overlap as- the Talkeetna superterrane. the Talkeetna superterrane, suggesting depo- semblages and stratigraphy suggest that these In southwestern Alaska, the southern part of sition in a basin developed on the suture be- three terranes were juxtaposed to form a single the Kahiltna terrane consists mainly of the de- tween the Talkeetna superterrane and North composite terrane, the "Talkeetna superterrane," posits of one of these basins (Fig. 1; Jones and America. If true, this observation implies that at least by Late Jurassic time (Berg and others, others, 1984,1987). In contrast with suggestions collision of the Talkeetna superterrane with 1972; Jones and Silberling, 1979; Csejtey and by previous workers, our regional and detailed North America began prior to latest Jurassic others, 1982; Monger and others, 1982). [Note studies lead us to conclude that basinal deposi- time, probably at a more southerly location. that we have expanded the definition of Tal- tion in the southern Kahiltna terrane began after The superterrane has since been translated keetna superterrane (Csejtey and others, 1982) the onset of collision. The Upper Jurassic and northward, probably by coast-parallel strike- to include the Alexander terrane, along with the Lower Cretaceous turbidite deposits probably slip displacement, to its present location, Wrangellia and Peninsular terranes.] were derived from and deposited on rocks of the where it was structurally juxtaposed with adjacent Talkeetna superterrane. Post-deposi- middle to Upper Cretaceous rocks of the A series of basins containing highly deformed, Jurassic to Cretaceous flysch-like turbidite de- tional dismemberment of the basin and defor- Kuskokwim group. A magmatic overprint in- mation of its deposits may have occurred during dicates that the Talkeetna superterrane was posits occurs along much of the present landward boundary of the Talkeetna superterrane in early Late Cretaceous northward strike-slip Alaska and western Canada (Fig. 1; Berg and translation of the Talkeetna superterrane from others, 1972; Eisbacher, 1974,1976,1985; Csej- its original site of collision. Magmatism and con- Present addresses: (Wallace and Hanks) Geophysi- tey and others, 1982; Jones and others, 1982, tinued contractional and strike-slip deformation cal Institute and Department of Geology and Geo- 1983, 1986; Coney and Jones, 1985). These further overprinted the basin after most of its physics, University of Alaska, Fairbanks, Alaska northward transport was complete. These inter- 99775-0760; (Rogers) 7125 Jill Place, Anchorage, clastic rocks have been interpreted as pre- to Alaska 99502. syn-collisional deposits in basins, perhaps pretations have major implications for the em-

Additional material (tables) for this article may be obtained free of charge by requesting Supplementary Data 8915 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 101, p. 1389-1407, 8 figs., 1 table, November 1989.

1389

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Figure 1. Simplified terrane map of Alaska, modified from Jones and others (1987) and Monger and Berg (1987). The terranes interpreted to have acted as the backstop for the more seaward terranes of southern and western Alaska are mainly of continental affinity. Most are presumed to have originated as parts of North America, although some have since migrated from their sites of origin. Only those "backstop" terranes mentioned in the text are identified. Terranes shown without a pattern were added successively to the continental margin, according to the scenario illustrated in Figure 7. Jurassic-Cretaceous basinal terranes are interpreted to have been deposited during or following collision of the Talkeetna superterrane along the site of the suture. Successor basin and post-accretionary deposits overlap boundaries across which large displacements have occurred.

placement history of the Talkeetna superterrane, ern and central Alaska Range (Fig. 1; Jones and complex that includes Jurassic plutonic rocks of and we have used them as essential elements in a others, 1984, 1987). These highly deformed the Peninsular terrane as well as Cretaceous and new synthesis of the tectonic evolution of rocks are regarded as a terrane because their Tertiary plutonic rocks that postdate emplace- southwestern Alaska. relationships to rocks of adjacent terranes are ment of the Talkeetna superterrane. The north- unknown (Jones and others, 1984,1986,1987). ern part of the Kahiltna terrane surrounds and is THE SOUTHERN KAHILTNA The Kahiltna terrane lies landward of the Tal- bounded on the northwest by numerous small TERRANE keetna superterrane, including the Wrangellia terranes (Jones and others, 1982, 1983); how- terrane in the north and the Peninsular terrane in ever, its southern part is bounded to the north- The Kahiltna terrane consists predominantly the south. Much of its southeastern boundary is west by the middle to Upper Cretaceous of Jurassic to Cretaceous turbidite deposits that defined by the Alaska-Aleutian Range batholith Kuskokwim Group, a clastic sequence that dep- are exposed over an extensive area in the west- (Reed and Lanphere, 1973), a huge plutonic ositionally overlaps many of the terranes

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Figure 1. (Continued).

EXPLANATION

ARC AND OCEANIC TERRANES OF WESTERN POST-ACCRETIONARY ROCKS

AND INTERIOR ALASKA r;Cz':| Cenozoic sedimentary, volcanic, and plutonic rocks | t | Togiak arc complex, consists of Goodnews, Tikchik, and Togiak terranes MID TO UPPER CRETACEOUS SUCCESSOR BASIN DEPOSITS | yk | Jurassic to Cretaceous arc terranes associated with the EfKk'^ Kuskokwim Group Yukon-Koyukuk basin, includes the Koyukuk and Nyack fcKyjl Deposits of the Yukon-Koyukuk basin terranes ACCRETIONARY TERRANES OF SOUTHERN ALASKA | o | Terranes of oceanic affinity peripheral to the Yukon- I y | Yakutat Koyukuk basin, includes the Angayucham, Innoko, and I pw | Prince William Tozitna terranes I c | Chugach

TERRANES OF CONTINENTAL AFFINITY OF INTERIOR TALKEETNA SUPERTERRANE AND NORTHERN ALASKA I A | Alexander n"| Terranes which have acted as a backstop against which I w | Wrangellla more seaward terranes in southern and western Alaska I P | Peninsular have been tectonically juxtaposed. Includes North America (NA), Nixon Fork-Dilllnger-Mystic-Minchumina JURASSIC-CRETACEOUS BASINAL TERRANES (N), Ruby (R), Kilbuck (Ki), Yukon-Tanana (Y), Por- l;jj§N|| Gravlna-Nutzotin cupine, Arctic Alaska, and Seward terranes. liiNKajll Northern Kahiltna MAJOR RIGHT-LATERAL FAULTS WHICH ARE IN PART iiiSKaii! Southern Kahiltna DISCORDANT WITH TERRANE BOUNDARIES D Denali SMALL TERRANES OF THE MT. McKINLEY REGION T Tintina | m | Includes Broad Pass, Chulitna, Clearwater, Maclaren, K Kaltag McKinley, Nenana, Pingston, Susitna, West Fork, and Windy terranes

northwest of the Kahiltna terrane (Wallace, comprise an extensive sequence of Upper Juras- lar blocks separated by slip surfaces, and litho- 1983,1984). sic to Lower Cretaceous clastic deposits that are logic units or bodies of rock are laterally A number of studies have addressed the best exposed in the area drained by the Kok- discontinuous. Most structural surfaces and fab- northern part of the Kahiltna terrane and its role setna River. rics dip steeply to the northwest. in the tectonic evolution of the region (Csejtey Diverse metasedimentary rocks locally are and others, 1978,1982; Reed and Nelson, 1980; The Chilikadrotna Greenstone spatially associated with the volcanic rocks, al- Jones and others, 1982, 1983, 1986). The though the exact relationship between them is southern Kahiltna terrane, however, is isolated The Chilikadrotna Greenstone (Eakins and obscure. Well-exposed contacts with the vol- from the remainder of the Kahiltna terrane by a others, 1978; Bundtzen and others, 1979) occurs canic rocks are uncommon but generally are tec- portion of the Alaska-Aleutian Range batholith. in a series of discontinuous, northeast-trending tonic or tectonically modified. The metasedi- In the vicinity of Lake Clark, the southern Ka- antiformal exposures near the northwestern mentary rocks include limestone, chert, cherty hiltna terrane has been partially mapped (Eakins boundary of the southern Kahiltna terrane (Fig. shale and tuff, and various metaclastic rocks and and others, 1978; Detterman and Reed, 1980; 2). Exposures are generally quite poor, and thus occur mainly in narrow, structurally concordant Nelson and others, 1983), but its role in the little is known about the sequence. The main zones around or between exposures of volcanic tectonic evolution of the region is difficult to constituent of the Chilikadrotna Greenstone is rocks. The sedimentary rocks generally display a interpret because of a lack of information about massive and featureless altered basalt. It com- fabric that dips steeply, mainly to the northwest, its character and relationships. monly is amygdaloidal and locally is pillowed, and ranges from a weak slaty cleavage to a phyl- In the Lake Clark area, the southern Kahiltna with rare interpillow limestone and chert. In the litic schistosity. Highly schistose zones com- terrane can be divided into two major litho- northeast, pyroxene-bearing andesitic flows, tuff monly enclose lenses of relatively more coherent, stratigraphic units (Figs. 2,3, and 4). A sequence breccia, and crystal-lithic lapilli tuff are an im- less deformed rocks. Common faults and tec- of volcanic deposits, the Chilikadrotna Green- portant constituent of the sequence. tonic fabrics, combined with tectonic mixing of stone of Bundtzen and others (1979), is at least The volcanic rocks of the Chilikadrotna the various lithologies, suggest that much of the partially Late Triassic in age and is best exposed Greenstone commonly are metamorphosed to Chilikadrotna Greenstone has been highly in low hills adjacent to the Chilikadrotna River prehnite-pumpellyite facies. Penetrative defor- imbricated. (Eakins and others, 1978). The overlying rocks, mational fabrics are developed only locally, but At one location, however, basaltic greenstone informally referred to herein as the "Koksetna fault surfaces and shear zones are abundant. is depositionally overlain by lenticular beds of River sequence" (Hanks and others, 1985), Consequently, the rocks are broken into lenticu- limestone, which in turn are interbedded with

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Figure 2. Generalized geologic map of the Lake Clark region. Substantially modified from Beikman (1980), Detterman and Reed (1980), and Nelson and others (1983). Note that isolated plutons and volcanic rocks of Late Cretaceous to Tertiary age have not been shown, for purposes of clarity. Right-lateral displacements across faults in Lake Clark-Iliamna Lake area are interpreted to be small, based on apparent continuity of Tlikakila complex. Displacement on unnamed fault between Farewell and Mulchatna faults may be as much as 45 km, based on offset facies trends in the Kuskokwim Group.

and depositionally overlain by andesite flows (map number 9, Table B1 and Fig. 5). The origi- lent exposures occur in river gorges and the and tuffs. This relationship suggests that ande- nal collection cannot be located to confirm the more rugged foothills west of the Alaska Range sitic rocks may postdate basaltic rocks through- Silurian age determination (R. B. Blodgett and (Fig. 2). The sequence consists almost entirely of out the Chilikadrotna Greenstone. Fossils from T. K. Bundtzen, 1984, verbal commun.), and so complexly deformed, volcanic-lithic turbidites the lenticular limestone beds have been inter- the limestone must be assumed to be of Late lacking in fossils or distinctive marker horizons. preted to be of Silurian age (Eakins and others, Triassic age. No other fossil localities have been Consequently, no definite stratigraphic succes- 1978; Bundtzen and others, 1979); however, the discovered in the Chilikadrotna Greenstone. sion could be determined during the course of locality was recollected during this study, and no this study. The Koksetna River Sequence Silurian fossils were found. Instead, conodonts The depositional environments of the turbid- from the limestone were assigned Late Triassic In general, the Koksetna River sequence ites in the KJkr were interpreted using the tur- (Norian) ages by two independent investigators (KJkr) is poorly exposed, although a few excel- bidite facies and facies associations of Mutti and (T. R. Carr, 1983, written commun.; N. M. Sav- Ricci Lucchi (1978). Interpreted environments

age, 1983, written commun.), and brachiopods 1 range from slope and inner fan in the southeast, from the limestone are also of probable Norian Tables A and B may be obtained free of charge by requesting Supplementary Data 8915 from the GSA to middle fan and outer fan in the northwest age (P. R. Hoover, 1984, written commun.) Documents Secretary. (Fig. 5). Deposits interpreted to represent a

Figure 2. {Continued).

EXPLANATION

POST-ACCRETIONARY SEDIMENTARY ROCKS SOUTHERN KAHILTNA TERRANE

I Q I Surficial deposits (Quaternary) KJkr Koksetna River sequence: Marine clastic rocks (Up- per Jurassic (Kimmeridgian) to Lower Cretaceous (Valanginian)) Ts Clastic rocks (Probably Paleogene)

Chilikadrotna Greenstone: Basaltic pillows and flows, Kk Kuskokwim Group: Marine clastic rocks (Lower andesitic breccia, chert, and limestones (Limestone is Cretaceous (Albian) to Upper Cretaceous (at least to Upper Triassic (Norian)) Coniacian, probably to Maastrichtian))

POST-ACCRETIONARY IGNEOUS ROCKS PENINSULAR TERRANE

;its-'(. Plutonic rocks (Mainly Eocene to Oligocene) Js Tuxedni Group, Chinitna Formation, and Naknek Formation: Marine clastic rocks (Middle to Upper Jurassic) VTKv Volcanic rocks (Includes both Eocene to Oligocene rocks, and uppermost Cretaceous to Paleocene rocks of the "Alaska Range magmatic belt") Plutonic rocks (Mainly Lower to Middle Jurassic)

TKtT Plutonic rocks (Mainly uppermost Cretaceous to TVrm; Cottonwood Bay greenstone. Kamishak Formation, and JTws. i™sJ Paleocene rocks of the "Alaska Range magmatic Talkeetna Formation: Mafic volcanic rocks, mainly belt") andesitic, and interbedded carbonates, siliceous clastic rocks, and tuffs (Upper Triassic to Lower Jurassic) CONTINENTAL BACKSTOP Basaltic pillows and breccias, and minor clastic rocks, raSTJTTT, Tlikakila complex: Metamorphosed and tectonically capping the Mystic terrane (Triassic to Jurassic) •fiPztc disrupted clastic rocks, limestones, cherts, basalts, Pzm Mystic terrane: Clastic rocks, carbonates, and chert gabbros, and peridotites. Local Triassic limestone;other- (Upper Devonian to Permian) wise succession and age uncertain

Dillinger terrane: Basinal carbonate and clastic Roof pendants, probably derived largely from J"Svs deposits (Cambrian to Middle Devonian) or older rocks

, . Nixon Fork terrane: Platformal deposits, mainly dominantly plagioclase), volcanic quartz, and h 1 1'"' carbonates (Cambrian to Middle Devonian), probably intraformational sedimentary rock fragments deposited on Precambrian metamorphic basement (Fig. 6). Quartzofeldspathic intrusive rock fragments and chert fragments are rare. Metamor- phic clasts are only a minor constituent but are slope environment are well exposed in the lower casts and ripples, suggest transport to the north- conspicuous in their contrast with the predomi- Koksetna River. These consist of a thick, mo- east in the few locations where they could be nantly volcanogenic composition of the rocks.

notonous sequence of thin (<4 cm) Bouma Tc measured. The distribution of interpreted depo- Metamorphic clasts include polycrystalline de- and Tc^. sandstones interbedded with pelagic sitional environments suggests regional sediment formed quartz, quartz-mica schist, amphibole- shales. Coarse, unsorted, and chaotic matrix- transport to the northwest or north, although it bearing quartzite, amphibolite, and rare scapo- supported conglomerate occurs sporadically must be emphasized that the interpretations are lite-diopside-bearing fragments suggestive of through this interval. Farther north along the preliminary and that the detailed stratigraphic a contact-metamorphic origin. Mafic mineral Koksetna River and on adjacent ridges, the relationships among the individual sites are assemblages are dominated either by clino- KJkr consists of thick (15 cm-1.5 m), fining- unknown. pyroxene and hornblende or by epidote. Detri- upward sandstone beds with scoured bases and Isolated outcrops of pebble and cobble con- tal clinopyroxene is extremely abundant at rippled and/or contorted tops. Depending on glomerate possibly correlative with the KJkr some locations. Micas are notably absent. the location, these sandstone beds consist of occur south of Lake Clark (Fig. 2); however, Sandstones of the KJkr plot predominantly in thinning- and fining-upward sequences, inter- these rocks contain no fossils, are poorly and the magmatic-arc and recycled-orogen fields of a preted as inner-fan deposits, or coarsening- and sporadically exposed, and cannot be proven Q(mono)-F-[L + Q(poly)] ternary diagram, ac- thickening-upward sequences, interpreted as conclusively to be part of the KJkr. Clast types cording to the classification of Dickinson and middle- to outer-fan deposits. To the west and in the conglomerate include intermediate to Suczek (1979) (Fig. 6). Qualitatively, the clast northeast, the proportion of sand greatly de- silicic volcanic and plutonic rocks and subord- compositions of the sandstones suggest deriva- creases, and the majority of the KJkr consists of inate chert and argillite. tion from a magmatic-arc provenance, domi- siltstones, shales, and thin, fine-grained turbidite Petrographic study shows the sandstones of nated by volcanic rocks but with local exposures sandstone interbeds. These have been inter- the KJkr to consist predominantly of volcanic of plutonic rocks, fine-grained clastic rocks, dy- preted as outer-fan and basin-plain deposits. Pa- rock fragments of mafic and intermediate com- namothermally metamorphosed rocks, and con- leocurrent indicators, such as groove and flute position, with varying amounts of feldspar (pre- tact-metamorphosed rocks.

EXPLANATION Greenstone. Although their stratigraphic position within the KJkr is unknown, and they can- - Siltstone & shale not be demonstrated to rest depositionally upon VALANGINIAN Sandstone MEGAFOSSILS the Chilikadrotna Greenstone, these sandstones Conglomerate almost certainly were derived from distinctive pyroxene-porphyritic flow rocks of the Chili- ° Andesitic tuff breccia tr LU kadrotna Greenstone. Thus, we suggest that the 5= Limestone Chilikadrotna Greenstone was both the deposi- Basaltic flows < 2 tional basement and a local sediment source for Z LU the overlying Koksetna River sequence. Basaltic pillows K 3 UJ O CO UJ We suggest a stratigraphic sequence for the Convolute bedding * CO o southern Kahiltna terrane (Fig. 4) based on this Peiecypod debris relationship, the basalt to andesite succession, and the fossil ages. The proposed sequence consists, from bottom to top, of Upper Triassic ba- Figure 4. Schematic stratigraphic col- salts, Upper Triassic(?) to Upper Jurassic(?) umn for the southern Kahiltna terrane. andesites, and Upper Jurassic to Lower Cre- The column is a composite, based on taceous turbidites. observations throughout the southern Ka- < I hiltna terrane, because no intact strati- Z UJ THE SOUTHERN KAHILTNA graphic sequence was located. Points of z TERRANE AND ITS RELATIONSHIP o o TRIASSIC age control are indicated. The strati- DC I- MEGAFOSSILS TO THE PENINSULAR TERRANE O CO graphic succession within the Chilikad- < z & CONODONTS * UJ rotna Greenstone is well defined. The Ij w The Peninsular terrane lies to the southeast of d oc succession within the Koksetna River se- X O o , the southern Kahiltna terrane (Fig. 1), but the quence is uncertain because of complex nature of the relationship between the two ter- deformation, lack of marker units, and ranes is uncertain. This is largely because the incomplete exposures. The succession shown is based on gross lithologic trends and the major exposures of the two terranes are sepa- character of intervals within which fossils were found. The Koksetna River sequence struc- rated by extensive latest Cretaceous and Tertiary turally overlies the Chilikadrotna Greenstone. The relationship probably was originally deposi- volcanic and plutonic rocks, including post- tional because detritus from the Chilikadrotna Greenstone occurs in the Koksetna River Jurassic elements of the Alaska-Aleutian Range sequence. batholith (Fig. 2). A comparison of the two terranes, however, suggests some stratigraphic similarities and potential depositional relationships. Only two localities with datable megafossils probably are a roof pendant of the KJkr within Furthermore, the igneous rocks separating the have been discovered in the K Jkr. A new collec- the pluton. Biotite from these schists has yielded two terranes contain isolated remnants of rocks tion from a previously reported locality (Eakins a K-Ar age of 63.2 ±1.9 m.y. (map number 4, that probably are parts of the Peninsular terrane, and others, 1978) yielded a Late Jurassic Table A and Fig. 5). The KJkr is progressively thus indicating little spatial separation between (Kimmeridgian) age (map number 10, Table B covered toward the southeast by unconformably the two terranes. and Fig. 5), and a newly discovered locality overlying, predominantly latest Cretaceous to Character of the Peninsular Terrane yielded an Early Cretaceous (Valanginian) age Paleocene volcanic rocks preserved in a regional (map number 11, Table B and Fig. 5). No mi- northeast-trending structural low. Stratified and plutonic rocks of the Peninsular crofossils were recovered from the KJkr, despite terrane (Fig. 3) occur southeast of the southern sampling of shales for forams and palyno- Stratigraphic Sequence of the Southern Kahiltna terrane in the Lake Clark region (Fig. morphs, and siliceous and limy beds for Kahiltna Terrane 2; Detterman and Hartsock, 1966; Detterman radiolarians. and Reed, 1980). The oldest exposed rocks are Beds in the KJkr generally dip steeply to the Contacts between the Chilikadrotna Green- Upper Triassic (?) altered mafic flows and tuffs, northwest. The presence of both upright and stone and the Koksetna River sequence gener- overlain by a well-dated Upper Triassic (Nor- overturned beds indicates that the unit has been ally are not exposed, and in the few places ian) unit consisting of limestone, calcareous isoclinally folded and faulted, although only a where the contact is visible, its original character siltstone, siliceous mudstone, and tuff. The Tri- few faults and large fold closures could be identi- cannot be determined because of overprinting assic rocks are disconformably overlain by a re- fied in the field. Interestingly, beds more com- by a strong tectonic fabric. The Chilikadrotna gionally extensive and thick sequence of Lower monly are overturned than upright in those Greenstone, however, is exposed in a series of Jurassic basaltic to andesitic flows, pyroclastic places where facing direction was determined. antiforms, indicating that it structurally underlies rocks, and volcaniclastic rocks. These volcanic The KJkr locally is intruded and metamor- the Koksetna River sequence (Fig. 2). The age rocks have been interpreted to be deposits of a phosed by latest Cretaceous to Paleocene plu- data, albeit limited, indicate that the Koksetna Lower Jurassic arc in the Peninsular terrane tons. Strongly foliated and lineated upper River sequence is younger than the Chilikad- (Jones and Silberling, 1979; Moore and Con- greenschist-facies biotite schists (MzPzs unit of rotna Greenstone. Massive amalgamated sand- nelly, 1979). Rocks of the Jurassic part of the Eakins and others, 1978) at the southeastern stones rich in clinopyroxene detritus are abun- Alaska-Aleutian Range batholith have intruded limit of the KJkr are closely associated with a dant in the Koksetna River sequence immediate- and metamorphosed all three of these Triassic to partially foliated quartz monzonite pluton and ly adjacent to exposures of the Chilikadrotna Lower Jurassic units.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/11/1389/3380407/i0016-7606-101-11-1389.pdf by guest on 30 September 2021 Figure S. Map of depositional environment interpretations for the Koksetna River sequence EXPLANATION and the Kuskokwim Group and locations of K-Ar and fossil samples. Base as in Figure 2, but the only units identified are the Koksetna River sequence and Kuskokwim Group. Key to Kuskokwim Group depositional environments: N, nonmarine; L, littoral to neritic; S, slope; I, turbidite inner fan; M, turbidite middle fan; O, turbidite outer fan; B, basin plain. Note that some interpretations Koksetna River Sequence apply to outcrops too small to be shown at scale of map. Most interpretations by Rogers, Hanks, and Wallace; additional interpretations by T. E. Moore and J. A. Pacht, particularly to MAP LOCATIONS northwest in Kuskokwim Group. Map numbers 1-4 refer to K-Ar dates in Table A; map numbers 5-14 refer to fossil dates in Table B. • K-Ar Age Locality • Fossil Locality

On the south side of the Peninsular terrane, western edge of the Cretaceous and Tertiary part 1979; Detterman and Reed, 1980). Although Middle and Upper Jurassic marine clastic rocks of the Alaska-Aleutian Range batholith (Fig. 2; many of the lithologic components of the Tli- locally unconformably overlie the Lower Juras- Nelson and others, 1983). This belt of rocks, the kakila complex suggest a dismembered ophio- sic mafic volcanic rocks. An upward composi- Tlikakila complex (Carlson and Wallace, 1983), lite, it is not safe to assume that they represent tional transition in these clastic units, from marks the boundary between unroofed batholith oceanic lithosphere because many of the sedi- chiefly volcanogenic graywackes to arkoses, on the southwest, and a relatively less uplifted mentary components are not of oceanic affinity suggests progressive unroofing of the Jurassic belt to the northwest consisting predominantly and the igneous components could be parts of magmatic arc to the northwest (Detterman and of uppermost Cretaceous and Paleocene vol- the Peninsular terrane arc or its basement. Hartsock, 1966; Detterman and Reed, 1980). canic rocks. The Tlikakila complex consists of a The northwestern and southeastern bounda- Early to Middle Jurassic plutonic rocks of the diverse assemblage of rocks, the protoliths of ries of the complex are faulted in most places, Alaska-Aleutian Range batholith are a signifi- which include calcareous to siliceous clastic but rocks of the Tlikakila complex have been cant component of the northwestern part of the rocks, limestone, thin-bedded chert, massive ba- intruded by plutonic rocks of the batholith and Peninsular terrane (Reed and Lanphere, 1973; salt, massive to layered gabbro and pyroxenite, locally are overlain depositionally by volcanic Detterman and Reed, 1980; Reed and others, and ultramafic rocks. Most contacts are faults, rocks. A clinopyroxene-hornblende andesite 1983). This composite plutonic complex has and the different lithologies of the complex have dike cutting metamorphic rocks of the Tlikakila yielded ages ranging from 174 to 158 m.y. and is been tectonically mixed. Most of the complex complex has yielded a K-Ar age of 79.6 ± 2.4 interpreted to be at least in part cogenetic with was metamorphosed to greenschist facies, but m.y. on hornblende (map number 1, Table A the Lower Jurassic volcanic rocks. Widespread essentially unmetamorphosed to amphibolite- and Fig. 5). roof pendants derived in part from sedimentary facies rocks occur in the complex. Age control The occurrence of the complex along the and volcanic rocks of the Peninsular terrane on the protoliths of these rocks is poor and con- boundary between the Cretaceous and Tertiary occur locally within the Jurassic plutons (Det- sists only of Late Triassic megafossils and cono- part of the Alaska-Aleutian Range batholith and terman and Reed, 1980). donts from limestones (map numbers 5 and 8, coeval volcanic rocks to the northwest suggests Table B and Fig. 5). The lithologic similarity of that its deformation and metamorphism may be The Tlikakila Complex: A Northwestern many rocks of the Tlikakila complex with those related to localization of uplift in a region of of the Upper Triassic to Lower Jurassic section Extension of the Peninsular Terrane intense magmatism. The possibility that at least of the Peninsular terrane, combined with the some of the metamorphism and deformation Late Triassic ages, suggests that at least part of A narrow belt of variably metamorphosed predated the local onset of magmatism in Late the Tlikakila complex represents a northwestern and highly deformed rocks probably related to Cretaceous time cannot be ruled out, however. extension of the Peninsular terrane (Stanley, the Peninsular terrane occurs along the north- The Peninsular Terrane: Sediment Source and Depositional Basement for the Koksetna River Sequence

The Late Triassic to Early Jurassic histories of the Peninsular terrane and the Chilikadrotna Greenstone of the southern Kahiltna terrane are remarkably similar, suggesting a genetic relationship between the two (Fig. 3). Marine basalts, limestones, and cherts were deposited in both terranes until latest Triassic time, although the relative proportions of these components differ considerably between the two. An abrupt shift to andesitic volcanism occurred in both terranes by earliest Jurassic time. The Jurassic arc of the Peninsular terrane included both Lower Jurassic volcanic and Lower to lower Upper Jurassic plutonic elements. The andesitic rocks of the southern Kahiltna terrane have not yet been directly dated, but their age range has been stratigraphically bracketed and corresponds with the age range of magmatism in the Peninsular terrane. Sandstone petrography suggests that the Kok- setna River sequence was derived from a Figure 6. QFL plot for sandstone samples from the Koksetna River sequence (KJkr) and magmatic-arc/recycled-orogen source (Fig. 6). from the Kuskokwim Group in the Nushagak Hills area. The 25 KJkr and 4 Kuskokwim Volcanic detritus dominates, suggesting that plu- Group samples were counted by C. L. Hanks; 23 additional Kuskokwim Group samples tonic and metamorphic elements were only lo- (crosses) were counted by K. Fischer; data courtesy of J. A. Pacht (unpub. data). Provenance cally being eroded in the source area at the time fields are from Dickinson and Suczek (1979). of deposition. Although part of the Koksetna

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Figure 7. Schematic paleogeographic reconstructions of south- MIDDLE TO LATE JURASSIC ern and western Alaska, Middle Jurassic to middle Tertiary time. (BAJOCIAN TO CALLOVIAN) These diagrams are meant to illustrate the general tectonic setting 180-1B3 MYBP of the major geologic elements for each time period shown, and the relationships among those elements. The diagrams are not to scale, and northward displacements of the Talkeetna superter- NORTH AMLRICA rane and outboard terranes have been foreshortened for pur- "GONTINLNTAL BACKSTOP' poses of illustration. Plate motion vectors show approximate YUKON-KOYUKUK ARC direction and magnitude of relative motions between oceanic plate and North America (based on plate motion reconstructions of Engebretson and others, 1985). Each diagram is discussed in TOGIAK ARC detail in the text. TALKEETNA SUPERTERRANE

PLATE MOTION VECTOR

MID-CRETACEOUS (APTIANTO AL8IAN) 119-100 MYBP

D E

River sequence was derived locally from the un- gests that the Koksetna River sequence was 1985b, 1985c) occurs along the southeast mar- derlying Chilikadrotna Greenstone, the distribu- derived from the southeast by erosion of the gin of the Togiak terrane and is superficially tion of turbidite facies suggests that the main Peninsular terrane. very similar to the Koksetna River sequence. source terrane for the sediments was to the Alternatively, the Koksetna River sequence The two units, however, do not overlap in age, southeast (Fig. 5). The combination of sediment could have been derived from another Jurassic as the youngest fossils in the graywacke of Ku- source-terrane character, apparent direction of arc complex to the west, the Togiak terrane (Fig. lukak Bay are of Oxfordian age, whereas the sediment derivation, and proximity of an ap- 1; Box, 1985b, 1985c). The "graywacke of Ku- oldest in the Koksetna River sequence are of propriate source in the Peninsular terrane sug- lukak Bay" (Hoare and Coonrad, 1978; Box, Kimmeridgian age.

MIDDLE TERTIARY (LATE EOCENE TO OLIGOCENE) 43-24 MYBP

TEC TONOS TRA TIGRAPHIC CRETACEOUS & TERTIARY TERRANES MAGMATIC ARCS

PWT-PRINCE WILLIAM TERRANE Tv- UPPER EOCENE TO OLIGOCENE Paleocene to Eocene Accretionary Rocks Arc Volcanic & Intrusive Rocks

CT-CHUGACH TERRANE TKv- UPPER CRETACEOUS TO PALEOCENE Lower Cretaceous & Upper Cretaceous Arc Volcanic & Intrusive Rocks Accretionary Rocks TS -TALKEETNA SUPERTERRANE Paleozoic and Mesozoic Sedimentary SUCCESSOR BASIN DEPOSITS and Volcanic Sequences Characterized

pLi"^^ To™1'¡fnnlr pK EM! Kk-KUSKOWIM & YUKON-KOYUKUK BASINS Peninsular Terrane upper Paleozoic y er Lower t0 Upper cretaceous Magmatic Arc Rocks in Wrangellia C^tic Rocks Terrane, and lower to mid-Paleozoic Arc Rocks in Alexander Terrane I;;;;;;;;;; I KJkr-JURASSIC-CRETACEOUS BASINAL |:;:iii|iiil TERRANES (Koksetna River Sequence & T-TOGIAK ARC Lateral Equivalents) Upper Jurassic to Lower Middle Jurassic to Lower Cretaceous Cretaceous Deep-Water Clastic Rocks Island Arc Rocks

YK-YUKON-KOYUKUK ARC Plate motion vector indicates motion of Middle Jurassic to Lower Cretaceous Oceanic Plate (FA-Farallon, KU-Kula, Island Arc Rocks PA-Pacific) with respect to North America (NA)

NA-NORTH AMERICA CONTINENTAL Length of plate motion vector proportional BACKSTOP to convergence rate

FAULTS - Solid during times of major displacement-, Dashed during times of minor displacement. , SUBDUCTION ZONE-Dashed where existence is inferrred from model & plate motion vectors

Figure 7. (Continued).

The Kimmeridgian fossils of the Koksetna This difference can be attributed to greater uplift duction during Jurassic time occurred along the River sequence are only slightly younger than to the east, along the Bruin Bay fault, and conse- northwestern margin (present coordinates) of the youngest dated plutonic rocks (158 Ma, Ox- quent unroofing of the batholithic portion of the the Peninsular terrane, implying that an ocean fordian, Reed and Lanphere, 1973) from the arc in that region (Fig. 2; Moore and Connelly, basin of unknown size lay to the northwest Peninsular terrane arc. The Upper Jurassic ma- 1979). As can be seen, the latest Triassic through (present coordinates) of the Peninsular terrane rine clastic rocks in both the southern Kahiltna Jurassic geologic histories of the southern Ka- (Fig. 7 A). Late Jurassic cessation of arc magma- terrane and the Peninsular terrane were derived hiltna terrane and the Peninsular terrane are re- tism in the Peninsular terrane might then mark from a magmatic-arc source, although the Pe- markably similar, with the differences in their the onset of collision of the Peninsular terrane ninsular terrane rocks contain a major plutonic stratigraphic sequences being perfectly consist- with the North American continental margin. detrital component (Detterman and Hartsock, ent with the fades changes to be expected from Modern examples in which magmatism has 1966), in contrast with the Koksetna River se- one side of an arc axis to the other. ceased in an intra-oceanic arc because of its col- quence, which contains little plutonic detritus. Reed and others (1983) suggested that sub- lision with a continental margin include the col-

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lision of Taiwan with Asia, and the Banda arc ranes were depositionally overlapped by Albian Group, but Albian microfossils and a Turonian (at Timor) with Australia (McGeary and others, and younger deep-marine, shallow-marine, and megafossil were found during this study (map 1985). Deposition of the Koksetna River se- nonmarine deposits of the Kuskokwim Group numbers 12-14, Table B and Fig. 5), both of quence would then have occurred in the result- (Hoare and Coonrad, 1978; Wallace, 1984). To which are significantly younger than the Kim- ing collisional foredeep as convergence either the northeast at the same time, the Kuskokwim meridgian and Valanginian fossils from the slowed or ceased as a consequence of collision. Group depositionally overlapped several major Koksetna River sequence. Such foredeeps exist in both the Taiwan (Covey, terranes (Fig. 1), including the Ruby, Innoko, The contact between the KJkr and the Kus- 1986) and Timor (Audley-Charles, 1986) ex- Nixon Fork, Dillinger, and Mystic terranes kokwim Group is sharp and steeply dipping. amples, but in the case of the Koksetna River (Wallace, 1983, 1984; Pacht and Wallace, There are no direct indications of a depositional sequence, only that part of the foredeep which 1983, 1984; Decker, 1984; Jones and others, relationship, such as interbedding, gradational or was deposited on the leading edge of the colli- 1987; Decker and others, 1989). Thus, begin- unconformable depositional contacts, or detritus sional upper plate is exposed. ning in Albian time, terranes over an extensive from the southern Kahiltna terrane in the sedi- The tectonic significance of the Chilikadrotna area in southwestern Alaska were linked to- ments of the Kuskokwim Group. This contact Greenstone and Tlikakila complex remains un- gether by deposits of a single major basin sys- can be located within tens of meters (Fig. 2) but certain. They may include parts of the Peninsu- tem, although the original basin configuration is not directly exposed; however, rocks adjacent lar terrane arc and its basement, as we propose. probably has been significantly disrupted since to the contact display characteristics suggesting On the other hand, if they do not include parts that time. This assemblage of diverse terranes that it is structural. Rocks of the lowest struc- of the arc, they may be parts of the subduction formed a continental margin, or "backstop," tural level of the southern Kahiltna terrane, the complex formed at the convergent plate bound- against which the Talkeetna superterrane and Chilikadrotna Greenstone, are localized at or ary associated with the arc. In this case, it is Kahiltna terrane were later structurally em- near the contact in a series of discontinuous, possible that the Koksetna River sequence was a placed. The nearest elements of this backstop to elongate, and probably antiformal exposures. part of this subduction complex and that its de- the southern Kahiltna terrane are the Nixon The contact itself is marked in many places by a formation began prior to the onset of collision. Fork, Dillinger, and Mystic terranes, which have belt of phyllitic rocks which have been derived been interpreted as tectonically displaced parts from rocks of the southern Kahiltna terrane by THE SOUTHERN KAHILTNA of the Paleozoic North American passive con- low-grade dynamothermal metamorphism. Lo- TERRANE AND ITS RELATIONSHIP tinental margin (Decker and others, 1989). Pa- cally, the metamorphic grade and intensity of TO THE KUSKOKWIM GROUP leomagnetic and paleobiogeographic evidence metamorphic fabric increases progressively to (Coe and others, 1985; Plumley and Coe, 1982, the northwest toward the contact, but in most 1983; Vance-Plumley and others, 1984; Blod- places, changes in grade and fabric are abrupt, The southern Kahiltna terrane is bounded to gett, 1983; Blodgett and Clough, 1985) indi- suggesting that rocks of the phyllitic belt were the northwest by the Kuskokwim Group (Figs. cates that they have been a part of North faulted after they were metamorphosed and 1 and 2). The Kuskokwim Group depositionally America at least since middle Paleozoic time penetratively deformed. Faulting is further indi- overlapped most of the terranes of interior and that they have been in approximately their cated by local imbrication of rocks of the southwestern Alaska by middle Cretaceous time present location at least since deposition of the Chilikadrotna Greenstone with metaclastic (Wallace, 1983, 1984), including some which Kuskokwim Group. rocks possibly derived from both the Koksetna appear not to have been transported northward River sequence and the Kuskokwim Group. Al- any significant amount (Coe and others, 1985). though this evidence suggests that the contact Thus, the nature of the contact between the Evidence for a Faulted Contact between the between the Kuskokwim Group and the south- Kuskokwim Group and the southern Kahiltna Southern Kahiltna Terrane and the ern Kahiltna terrane is a fault, the possibility terrane has important implications for the travel Kuskokwim Group remains that it is a depositional contact that was history of the Peninsular terrane if the southern structurally modified. We refer to this structural Kahiltna terrane is a northwestern extension of In the Lake Clark region, rocks of the contact as the "Chilchitna fault" because it fol- the Peninsular terrane, as we suggest it is. Kuskokwim Group and the Koksetna River se- lows the Chilchitna River for much of its length. quence of the southern Kahiltna terrane are su- The Kuskokwim Group and the perficially very similar. Consequently, the two The available data allow no more than specu- "Continental Backstop" units have not been differentiated on previous lation about the sense of displacement on the maps of the area (Eakins and others, 1978; Nel- Chilchitna fault and associated structures. Struc- A number of diverse terranes of both island- son and others, 1983). The two units, however, tures near the fault and probably oogenetic with arc and continental affinity occur to the north can be distinguished based on composition, de- it are predominantly contractional, but a strike- and west of the southern Kahiltna terrane. The gree of induration, age, and to a certain degree, slip component of displacement is possible. Goodnews, Tikchik, and Togiak terranes (Fig. facies. Rocks of the Kuskokwim Group gener- Most beds and fold axial surfaces dip steeply to 1) constitute parts of an intra-oceanic Jurassic to ally appear less indurated and contain more the northwest on both sides of the Chilchitna Early Cretaceous arc system and associated sub- quartz, lithic clasts, and white mica than do fault, suggesting that the fault itself is steeply duction complex that was assembled by Early those of the adjacent Koksetna River sequence, northwest dipping, emplacing the Kuskokwim Cretaceous (Valanginian) time (Box, 1985b, reflecting derivation from a complex continental Group over the KJkr. Antiforms exposing older 1985c). This arc complex collided with the Kil- source (Fig. 6). Facies patterns suggest a north- and more-metamorphosed rocks adjacent to a buck terrane, which consists of Precambrian erly source for the Kuskokwim Group marine thrust fault generally are found in its hanging high-grade metamorphic rocks of continental clastic rocks (Moore and Wallace, 1985a, wall; however, in this instance, the older and composition (Turner and others, 1983; Decker 1985b), with the sediments closest to the KJkr more-deformed rocks of the southern Kahiltna and others, 1989). Collision occurred between interpreted as basin-plain or outer-fan turbidite terrane are found in the footwall. In addition, Valanginian and Albian time, when the two ter- deposits. Fossils are sparse in the Kuskokwim northwest-dipping overturned beds dominate

over upright beds in the KJkr. These observa- chatna fault appears to be one of the numerous lapped by extensive belts of Upper Cretaceous tions suggest that originally southeast-dipping, northeast-trending right-lateral strike-slip faults and Tertiary plutonic and volcanic rocks. The northwest-vergent folds and thrust faults have found throughout southwestern Alaska and plutonic rocks constitute the northwestern por- been rotated through the vertical and now dip to coincides with the abrupt northwestern bound- tion of the Alaska-Aleutian Range batholith the northwest. The regional predominance of ary of the latest Cretaceous-Paleocene Alaska (Fig. 2; Detterman and Reed, 1980; Nelson and northwest-dipping beds and structures suggests Range magmatic belt (Wallace and Engebret- others, 1983; Reed and Lanphere, 1973). Mag- that the latest deformation was southeast ver- son, 1984). In the Lake Clark area, Upper Cre- matism in the region appears to have occurred gent, which is how any shortening following ro- taceous to lower Tertiary volcanic rocks and during two distinct episodes: latest Cretaceous- tational dip reversal is likely to have been isolated plutons are abundant within both the Paleocene (74-55 Ma) and late Eocene-early accommodated. Rotation of pre-existing struc- southern Kahiltna terrane and the Kuskokwim Miocene (42-21 Ma), corresponding to the tures and southeast-vergent deformation may Group southeast of the fault; however, little evi- Alaska Range and ancestral Aleutian arc mag- have occurred either as a late phase of a single dence for extensive magmatism within the Kus- matic belts, respectively, of Wallace and Enge- progressive deformation event or in a separate kokwim Group northwest of the fault is seen on bretson (1984). event postdating the inferred northwest-vergent the surface or on aeromagnetic data (U.S. Geo- An extensive belt of volcanic rocks and local deformation that juxtaposed the Kuskokwim logical Survey, 1978). The truncation of the lat- plutons occurs to the northwest of the Alas- Group and the southern Kahiltna terrane. Most est Cretaceous to Paleocene Alaska Range ka-Aleutian Range batholith and the Tlikakila of this deformation probably occurred between magmatic belt by the Mulchatna fault suggests complex (Eakins and others, 1978; Detterman the deposition of the youngest Kuskokwim that there has been significant strike-slip and/or and Reed, 1980; Nelson and others, 1983) (Fig. Group turbidites in the vicinity (Turonian, map dip-slip displacement on the fault since latest 2). The volcanic rocks range in composition number 14, Table B and Fig. 5) and the oldest Cretaceous to Paleocene time. from basalt to rhyolite and generally are unde- plutons in the vicinity which postdate the re- formed, displaying only local gentle folds. The gional deformational overprint (71 Ma/Maas- LATEST CRETACEOUS TO isolated plutons in the belt also are quite diverse trichtian, Eakins and others, 1978). TERTIARY MAGMATISM: A LINK in composition, ranging from diorite to granite, The structural contact between the Kuskok- BETWEEN THE TALKEETNA and commonly are intrusive into overlying vol- wim Group and the southern Kahiltna terrane SUPERTERRANE, THE SOUTHERN canic and sedimentary rocks. The northwestern probably has been modified by at least one epi- KAHILTNA TERRANE, THE boundary of the belt is very poorly defined be- sode of strike-slip faulting. A major linear topo- KUSKOKWIM GROUP, AND THE cause it is controlled by the local depth of ero- graphic low, in part along the Mulchatna River, "CONTINENTAL BACKSTOP" sion. Isolated plutons and volcanic rocks pre- occurs to the northwest of the Chilchitna fault served in local structural lows are common within the southern Kahiltna terrane and the (Fig. 2). The Chilchitna fault appears to merge The Peninsular terrane, the Kahiltna terrane, Kuskokwim Group as far northwest as the Mul- with this lineament to the northeast and south- the Kuskokwim Group, and the inboard "con- chatna fault (Fig. 2). west and may be truncated by it. This lineament tinental backstop" upon which the Kuskok- has been mapped as the Mulchatna fault (Beik- wim Group is deposited were linked together by Most published K-Ar ages for volcanic and man, 1980). Although not exposed, the Mul- latest Cretaceous time because they are over- plutonic rocks of the belt fall within a range from 71 to 56 m.y. (Eakins and others, 1978),

TABLE 1. COMPARISON OF THE ESSENTIAL FEATURES OF END-MEMBER MODELS FOR COLLISION AND EMPLACEMENT but new ages ranging from 44 to 34 m.y. OF THE TALKEETNA SUPERTERRANE IN SOUTHWESTERN ALASKA (Thrupp and Coe, 1986) indicate that at least some younger rocks occur in the belt. New ages Age of events A. Early collision to the south B. Late collision near present locatioQ ranging from 61 to 54 m.y. have been obtained

Late Triassic-Early Jurassic Origin of ¡ntra-oceanic Peninsular terrane arc Origin of intra-oceanic Peninsular terrane arc from a granite and a dacite in the southwest- (subduction polarity uncertain) (subduction polarity uncertain) ernmost exposures of the belt (map numbers Middle-Late Jurassic Intra-oceanic Peninsular terrane arc, subduction Intra-oceanic Peninsular terrane arc, subduction 2-3, Table A and Fig. 5). The age data for the zone on continental side (Fig. 8,1C) zone presumably on oceanic side (Fig. 8, IB) volcanic and plutonic rocks of this belt suggest Late Jurassic-Early Cretaceous Onset of collision (south of present location) Magmatic quiescence in Peninsular terrane (Fig. 8, 2C) arc that the same latest Cretaceous-Paleocene and Cessation of magmatism in Peninsular terrane arc Deposition of Kahiltna terrane turbidites in oceanic Deposition of Koksetna River sequence in collisional basin between arc and continent (to Cenomaoian time) late Eocene-early Miocene magmatic episodes foredeep (to Valanginian time) occurred in this region as in the Alaska-Aleu- Middle Cretaceous Shift of subduction rone to oceanic side of Collision (near present location): tian Range batholith (Wallace and Engebretson, Talkeetna superterrane By subduction beneath North America Continued convergence ± right-lateral (?) strike slip (Fig. 8,2B) (Csejtey and others, 1982) 1984). By subduction beneath both North America and continental side of Talkeetna superterrane (Fig. 8, 2B and C combined) (Nokleberg and others, 1985) IMPLICATIONS FOR THE TECTONIC OR EVOLUTION OF SOUTHWESTERN Late Cretaceous Major right-lateral strike slip Collision (near present location): Transpressive emplacement of Talkeena By subduction of unspecified polarity (Jones and ALASKA superterrane near present location others, 1982,1986) By subduction beneath North America (Fig. 8,2B) (Pavlis, 1982) Our observations in southwestern Alaska Latest Cretaceous-Paleocene Formation of Alaska Range magmatic belt due to Shift of subduction zone to oceanic side of Talkeetna suggest some significant conclusions regarding subduction on oceanic side of Talkeetna superterrane and consequent formation of Alaska superterrane (Fig. 8,3) Range magmatic belt (Fig. 8,3) the origin of the southern Kahiltna terrane and its role in the tectonic evolution of southwestern

Nine: a generalized sequence of events is shown for two models. A. Early collision of the Talkeetna superterrane far to the south of its present location (this paper). Alaska. B. Late collision of the Talkeetna superterrane near its present location (based on models proposed by Csejtey and others, 1982; Pavlis, 1982; Jones and others, 1982, 1986; Nokleberg and others, 1985). (1) In the vicinity of Lake Clark, the available evidence suggests that the turbidites of the

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1. BEFORE EMPLACEMENT A. Continental arc and back-arc rift B. Intra-oceanic arc, subduction zone on C. Intra-oceanic arc, subduction zone oceanic side on continental side 5

NA NA

2. MODE OF EMPLACEMENT A. Collapse of continental rift B. Collision by subduction C. Collision by subduction beneath beneath North America Talkeetna superterrane

EXPLANATION Intra-oceanic * magmatic arc 3. AFTER EMPLACEMENT ftii Oceanic SMI TS ' lithosphère llll

Intra-oceanic arc may be built on oceanic lithosphère or continental llthospheric fragment

Figure 8. Schematic diagrams showing range of possible modes of emplacement of Talkeetna superterrane against North America. (1) Shows possible settings for Peninsular terrane arc before emplacement of the Talkeetna superterrane against North America. (2) Shows various ways in which the Talkeetna superterrane could have been emplaced against North America. Possible additional complications are shown by using dashed lines. (3) Shows configuration after juxtaposition of the Talkeetna superterrane and North America. Events are shown only in relative sequence because ages of the various events vary to some extent from model to model. In this paper, collision is proposed to have occurred by mode 2C, but the configuration prior to Middle Jurassic time could have been either IB or 1C.

southern Kahiltna terrane, the Koksetna River River sequence was deposited on and derived Kuskokwim Group and the "continental back- sequence, were derived from a magmatic-arc partially from the Chilikadrotna Greenstone. stop" upon which it was deposited, suggesting source. Evidence suggests, albeit not conclu- (4) The present contact between the Kus- that the Peninsular terrane was in approximately sively, that most of the sediments were derived kokwim Group and the southern Kahiltna ter- its present location by latest Cretaceous time. from the southeast from the Peninsular terrane. rane is a fault, and no evidence has been found (6) Rocks of both the southern Kahiltna ter- (2) Fossil ages from the Koksetna River se- to indicate that the Kuskokwim Group ever de- rane and the Kuskokwim Group are strongly quence indicate a Kimmeridgian to Valanginian positionally overlapped the southern Kahiltna deformed, whereas the volcanic and plutonic age range, postdating Jurassic arc magmatism in terrane. Thus, the two may have been spatially rocks of the magmatic belt overprinting them the Peninsular terrane. Extinction of the arc sug- and genetically unrelated at least until Turanian are not. This suggests a major deformational gests the possibility that collision of the terrane time, the age of the youngest dated fossils from event between Turonian and Campanian time, began prior to deposition of the Koksetna River the Kuskokwim Group in the vicinity. probably coincident with final emplacement of sequence. (5) Uppermost Cretaceous (Campanian) to the Peninsular terrane into its present location. (3) The Chilikadrotna Greenstone and the Paleocene plutonic and volcanic rocks define a These conclusions have important implica- Tlikakila complex are interpreted to represent magmatic belt that overprints the Peninsular ter- tions for the tectonic evolution of southwestern parts of the Peninsular terrane. The Koksetna rane, the southern Kahiltna terrane, and the Alaska. In particular, if deposition of the Kok-

setna River sequence began after the onset of acknowledge that some aspects of our model are Plate reconstructions suggest that the Farallon collision, then collision of the Peninsular terrane, speculative. We offer this model not as the only plate lay offshore of North America at this time and hence the Talkeetna superterrane as a possible solution, but rather as an end-member and that it displayed westward to northwest- whole, with North America must have begun no alternative to be tested against existing models. ward convergence with respect to northwestern later than Late Jurassic time. This casts doubt on The model differs in two important respects North America until about 85 Ma (Wallace and the middle Cretaceous or younger age of colli- from most models for the evolution of the re- Engebretson, 1984; Engebretson and others, sion proposed in most previous interpretations, gion. First, collision of the Talkeetna superter- 1985). This would provide the mechanism by based on deformational metamorphic, and rane is assumed to have occurred far south of its which the intra-oceanic arcs migrated toward magmatic overprints attributed to collision present location and to have begun in Late Ju- and eventually collided with North America. (Coney, 1981; Silberman and others, 1981; rassic time, in contrast with other interpretations The continental margin may already have had Csejtey and others, 1982; Monger and others, involving a later collision closer to the present an embayment in the future location of the 1982; Pavlis, 1982; Jones and others, 1982, location of the superterrane (Table 1). Second, Yukon-Koyukuk basin, and a salient in the fu- 1986; Lanphere and Reed, 1985). No clear link, subduction is assumed to have occurred on the ture site of the Ruby geanticline, a regional however, can be established across the boundary continental side of the Talkeetna superterrane in structural high corresponding approximately between the southern Kahiltna terrane and the Late Jurassic time, as suggested by Reed and with the Ruby and Nixon Fork terranes. Both of Kuskokwim Group until latest Cretaceous time. others (1983). Although closure of the hypothe- these features, however, may have originated, This suggests the possibility that the Talkeetna sized basin separating the Talkeetna superter- and at least were dramatically modified and ex- superterrane collided with North America south rane from North America could have occurred aggerated, during later deformation, particularly of its present location and that a significant por- in a variety of ways (Fig. 8), we favor this as strike-slip faulting. tion of its northward motion to its present posi- the simplest interpretation that is consistent with tion was by strike-slip displacement along the the observations. Although other controversial Late Jurassic to Early Cretaceous North American continental margin after colli- choices were necessary in order to construct a (Oxfordian to Valanginian, 163-130 Ma) sion began. If this is true, then the Peninsular complete model, these are the two assumptions terrane arc, including the southern Kahiltna ter- most directly relevant to the southern Kahiltna By Late Jurassic time, cessation of magma- rane and the Peninsular terrane itself, has been terrane and its role in the tectonic evolution of tism within the Peninsular terrane suggests that separated from its forearc region and the colli- southwestern Alaska. it had begun to collide with the North American sional suture. Thus, much of the evidence for continental margin somewhere to the south of its Middle to Late Jurassic collision of the Talkeetna superterrane may represent location (Fig. 7B), forming a collisional (Bajocian to Callovian, 180-163 Ma) main to the south, at the original site of collision. foredeep. The site of collision may have re- We suggest that the post-Turonian, pre- The first reconstruction (Fig. 7A) shows the mained a topographic low because of loading by Campanian penetrative deformational overprint Peninsular terrane to be an intra-oceanic arc and the upper plate (Stockmal and others, 1986) and on the southern Kahiltna terrane and the Kus- represents the time of emplacement of the Juras- if the crust of the colliding plates were thin be- kokwim Group formed during the final em- sic part of the Alaska-Aleutian Range batholith. fore collision (Murrel, 1986), leading to deposi- placement of the Talkeetna superterrane into its Subduction occurred on the side of the arc fac- tion of the Koksetna River sequence and similar present position, likely under conditions of ing North America (Reed and others, 1983), deposits on the landward side of the Talkeetna transpression. This emplacement event would and a marine clastic sequence (Detterman and superterrane (northern Kahiltna terrane, Csejtey account for the deformation, metamorphism, Hartsock, 1966; Detterman and Reed, 1980) and others, 1982; Jones and others, 1982,1983, and magmatism attributed to collision by others. was deposited in the back-arc region (Wang and 1986; Coney and Jones, 1985; Gravina-Nutzotin Structural juxtaposition of the two units would others, 1988). The Togiak arc complex (Box, belt and Dezadeash Formation, Berg and others, have been completed by the time a belt of Cam- 1985b, 1985c), another intra-oceanic arc, oc- 1972; Eisbacher, 1974, 1976,1985). panian to Paleocene volcanic and plutonic rocks curred farther to the northwest, and the Yukon- A new subduction zone would be expected to was superimposed on the two units. Koyukuk arc may have been active even farther form on the seaward side of the accreted Penin- to the northwest (Patton, 1984). Examples of sular terrane. There appears, however, to have SYNTHESIS OF THE TECTONIC similar intra-oceanic arcs facing a continent been no arc magmatism within the Peninsular EVOLUTION OF SOUTHWESTERN occur in the modern southwestern Pacific, in- terrane during this time interval, suggesting that ALASKA cluding the northern Philippines arc (Ham- the inferred arc should be sought within the burger and others, 1983) and the New Britain- North American continental margin at the orig- We have constructed a model for the tectonic Solomon-New Hebrides arc (Hamilton, 1979). inal site of collision of the Peninsular terrane. To evolution of southwestern Alaska (Fig. 7) that It is possible that the various arc segments repre- the east and southeast, the Gravina-Nutzotin belt incorporates our observations and interpreta- sented a single continuous arc system (Box, (Berg and others, 1972) occupies the same tec- tions. We have attempted to make this model as 1985a), but it seems more likely that they tonic position as the Koksetna River sequence consistent as possible with what is known about formed as separate arcs in a similar tectonic set- with respect to the Talkeetna superterrane, oc- the geology of southwestern Alaska and about ting, as is true for the arcs found in the south- curring landward and depositionally overlap- the relative motion between the North Ameri- western Pacific today (Taylor and Karner, ping the Wrangellia and Alexander terranes can continent and adjacent oceanic plates since 1983). These arcs may have originated within (Jones and others, 1987; Monger and Berg, Late Jurassic time (Engebretson and others, oceanic crust, or could have been built on con- 1987). Andesitic rocks in the Gravina-Nutzotin 1985); however, too little is known about the tinental fragments rifted from the North Ameri- belt could represent continental arc magmatism geology of southwestern Alaska to determine a can margin or perhaps the margins of other associated with a new subduction zone seaward unique solution for its evolution, and we readily circum-Pacific continents. of the Talkeetna superterrane. Alternatively, the

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Gravina-Nutzotin volcanic rocks might repre- Yukon-Koyukuk basin, extensive deposits on tle, if any, magmatism affected the Togiak arc sent continued pre-collisional, intra-oceanic-arc the seaward side of the extinct arc have also complex; the Nixon Fork, Dillinger, and Mystic magmatism if the Talkeetna superterrane col- been preserved (Nilsen and Patton, 1984). The terranes; the Kuskokwim basin; the Kahiltna lided obliquely with North America, starting tectonic environment for the voluminous deposi- terrane; or the Peninsular terrane during this in- first in the north and not colliding to the south tion of this time is uncertain. It likely was domi- terval, although scattered magmatism still con- until middle Cretaceous time (Monger and oth- nated by continued convergence at a new tinued farther inland. ers, 1982). Both the Togiak and Yukon- subduction zone formed seaward of the collided The Kuskokwim basin and the southern Ka- Koyukuk intra-oceanic arcs remained active arcs, but local extensional and strike-slip envi- hiltna terrane were structurally juxtaposed, and during this time, although collision of the ronments may have existed within this complex rocks in a broad zone on either side of the fault Yukon-Koyukuk arc with the continental mar- convergent continental margin. The transition were complexly folded and thrust faulted. This gin late during this interval is reflected by exten- from a middle Cretaceous predominantly con- deformation probably was the product of right- sive thrust-faulting and metamorphism around vergent to a Late Cretaceous predominantly lateral strike-slip displacement along a zone near the edges of the Yukon-Koyukuk basin in the transform environment may have been similar the suture between the Talkeetna superterrane Ruby geanticline, the Brooks Range, and the to that which occurred along the Californian and North America. This interpretation is con- Seward Peninsula (Patton, 1984; Turner, 1984; continental margin in late Mesozoic to Cenozoic sistent with a probable major change in relative Box, 1985a; Dillon and others, 1985; Armstrong time (for example, Howell and others, 1980; plate motion along the northwestern North and others, 1986; Patrick, 1988). Crouch, 1981). American continental margin that began with a Modern examples of similar collisions of Middle to Late Cretaceous (about 115-65 major plate reorganization at about 85 Ma landward-facing intra-oceanic arcs with conti- Ma) plutonism occurred in a diffuse and wide- (Engebretson and others, 1985). Birth of a new nents include the Banda arc (at Timor) (Audley- spread belt (not shown in Fig. 7C) now spreading center within the Farallon plate led to Charles, 1981) and the northern Philippines arc extending from the Gravina-Nutzotin belt on the formation of the Kula plate, which displayed a (at Taiwan) (Suppe, 1987). Both of these exam- southeast through the Yukon-Tanana terrane, large component of transform motion to the ples display a cessation of magmatism, forma- the northern part of the Ruby geanticline, the north with respect to the northwestern North tion of a collisional foredeep, and incipient Yukon-Koyukuk basin, and westward across American margin (Wallace and Engebretson, relocation of the subduction zone to the other the Seward Peninsula (Dadisman, 1980; Miller, 1984; Engebretson and others, 1985). side of the arc (Audley-Charles, 1981, 1986; 1985; Wilson and others, 1985; Armstrong and At about the same time, east-vergent thrusting Silver and others, 1983; McGeary and others, others, 1986; Patton and others, 1987). Magma- occurred along the boundary between the 1985; Covey, 1986; Price and Audley-Charles, tism within this area is unevenly distributed and Seward Peninsula and the Yukon-Koyukuk 1987). It is also worth noting that these arcs displays considerable local variation in composi- basin and along strike to the northwest, proba- interact in a complex way with the facing plates tion and age but may represent some combina- bly as a result of a period of convergence be- as a result of pre-existing embayments and tion of magmatism resulting from arc collision tween Eurasia and North America (Patton and promontories in the continents on those plates. and later establishment of a subduction zone on Tailleur, 1977; Engebretson and others, 1985). the seaward side of the accreted arcs. The exis- This may have accentuated an already-existing Middle Cretaceous tence at this time of a subduction zone seaward westward bend or salient in the generally (Aptian to Albian, 119-100 Ma) of the Talkeetna superterrane is suggested by the northwest-trending North American margin, presence of a middle Cretaceous subduction thereby providing a larger and more effective By middle Cretaceous time, the Yukon- complex, consisting largely of mélange, along backstop against which northerly migrating ter- Koyukuk and Togiak arcs had collided with the the seaward margin of the superterrane (Plafker ranes would eventually accumulate. continent, and intra-oceanic arc magmatism had and others, 1977; Moore and Connelly, 1977, ceased (Fig. 7C). The latest Jurassic to earliest 1979). Latest Cretaceous to Early Tertiary Cretaceous main phase of deformation in the (Maastrichtian to Paleocene, 74-56 Ma) Brooks Range may mark collision of the Yukon- Late Cretaceous Koyukuk arc (Box, 1985a). Thrusting and (Santonian to Campanian, 85-74 Ma) A major new tectonic regime was established metamorphism along the southeastern margin in latest Cretaceous to Paleocene time (Fig. 7E), of the Yukon-Koyukuk basin (Patton and Moll, Late Cretaceous time (Fig. 7D) appears to marking clear consolidation of the Talkeetna 1982) and landward of the Togiak arc complex represent a critical juncture in the evolution of superterrane with the rocks which now lie (Box, 1985b) probably also were products of southwestern Alaska, corresponding with the landward of it. At about this time, a major, pre- collision but are not as well documented and did time of a major plate reorganization in the Pa- sumably subduction-related magmatic zone not produce a major orogenic belt as in the cific Ocean basin (Wallace and Engebretson, formed parallel with the continental margin in Brooks Range. 1984; Engebretson and others, 1985). southern Alaska and western Canada (Hudson, Beginning by Albian time, the intra-oceanic Voluminous, deep-water turbidite deposition 1979, 1983; Brew and Morrell, 1983; Wallace arc complexes and the continental rocks that lay in the Kuskokwim basin appears to have been and Engebretson, 1984). Development of this landward of them were sources for and were completed by this time, although shallow-water magmatic zone coincided with an increase in the unconformably overlapped by the clastic depos- to nonmarine deposition may have continued. convergent component of motion of the Kula its of the Yukon-Koyukuk and Kuskokwim ba- There is no evidence that the Kuskokwim plate with respect to North America. Magma- sins (Hoare, 1961; Patton, 1973; Wallace, Group ever depositionally overlapped the tism was extremely widespread throughout the 1984). As in the case of the earlier Koksetna southern Kahiltna terrane, although a deposi- southern half of Alaska during this time interval, River sequence, the arc-continent suture marked tional relationship cannot yet be ruled out to the perhaps as a result of a gently dipping subduc- a major depocenter, although in the case of the northeast, in the northern Kahiltna terrane. Lit- tion zone as has been suggested for Laramide

magmatism in the southwestern United States Cessation of magmatism in south-central and For a brief period prior to formation of the (Coney and Reynolds, 1977; Cross and Pilger, southwestern Alaska may have been the result of Aleutian arc, relative motion was parallel to the 1987; Keith, 1982). A well-defined seaward belt a very low subduction dip angle due to high ab- northwest-trending continental margin of the within this zone, the Alaska Range magmatic solute and relative plate velocity combined with Bering Sea shelf. This motion may have caused belt, overprinted the landward boundary of the the predominance of transform motion along the formation of a series of deep pull-apart basins, Talkeetna superterrane, thereby indicating that continental margin. Major strike-slip displace- including St. George and Navarin basins, on the the composite terrane was in approximately its ment may also have occurred during this time site of the former forearc basin near the shelf present location by latest Cretaceous time (Wal- on the Tintina fault, landward of the Denali edge (Whitney and Wallace, 1984). lace and Engebretson, 1984). fault, but the timing of displacement is not well Slowing of convergence allowed renewal of Shallow-marine to nonmarine deposition constrained (Tempelman-Kluit, 1979; Gabrielse, arc magmatism in southwestern Alaska. This may have continued in the Kuskokwim basin 1985). Significant counterclockwise rotation oc- may have been the time of formation of the during this time, although the upper part of the curred at this time in western Alaska, as docu- ancestral Aleutian arc (Wallace and Engebret- Kuskokwim Group is only locally exposed and mented by paleomagnetic data (Coe and others, son, 1984), although it has been argued that the is poorly dated. Volcanic rocks are locally inter- 1985), probably resulting from a combination of arc originated as early as 55 Ma (Scholl and calated near the top of the Kuskokwim Group rigid rotation due to strike-slip faulting (Stout others, 1986). Formation of the new, intra- (Bundtzen and Laird, 1980,1983; Bundtzen and and Chase, 1980; Wallace and Engebretson, oceanic arc segment resulted in entrapment of a Gilbert, 1983) and probably are related to the 1984) and indentation resulting from convergent piece of the former Kula plate, forming the pres- Kuskokwim Mountains magmatic belt of Maas- coupling and the collision of northward-moving ent Aleutian basin (Scholl and others, 1975; trichtian to Paleocene age (Wallace and Enge- terranes against the growing westward salient in Marlow and Cooper, 1983). bretson, 1984). the continental margin of western Alaska. This While magmatism was renewed in south- Meanwhile, trench-fill turbidites (Nilsen and may be analogous, although on a smaller scale, western Alaska, transform motion may have Zuffa, 1982) of the seaward portion of the to the tectonic extrusion and block rotation re- continued in western Canada and southeastern Chugach terrane were being deformed into an sulting from collision of India with Eurasia (for Alaska. Strike-slip motion on the Denali fault accretionary wedge along an active continental example, Tapponnier and others, 1986). The has continued to the present, but its rate proba- margin (Plafker and others, 1977; Sample and combination of strike-slip slivering and rotation bly decreased dramatically beginning at this Moore, 1987). Although these rocks could have resulted in narrowing and lengthening of the time (Lanphere, 1978). Strike-slip to thrust dis- been deposited and deformed essentially in embayments and salients in the Early Creta- placements may have occurred on other major place, the nature of their relationship to more ceous collisional continental margin, thereby faults in interior Alaska, but their displacement landward terranes, notably the Peninsular ter- further accentuating the major westward salient history is poorly constrained. Closer to the con- rane, remains uncertain (Coe and others, 1985; in the continental margin in western Alaska. tinental margin, however, significant northward Dumoulin, 1988), and paleomagnetic data Major northward movement of the Prince Wil- motion of several terranes may have occurred, (Gramme and Hillhouse, 1981) suggest about liam terrane, and likely the Chugach terrane, probably predominantly by coast-parallel strike- 24° northward transport since latest Cretaceous also occurred at this time (Moore and others, slip faulting, including the Chugach (Cowan, time. Existence of an accretionary wedge in the 1983; Coe and others, 1985), probably distrib- 1982; Moore and others, 1983), Prince William present position of the Chugach terrane is likely uted along numerous inter- and intra-terrane (Moore and others, 1983), and, most recently, because remnants of similar accretionary wedges faults with strike-slip to thrust displacements. Yakutat (Lahr and Plafker, 1980; Plafker, 1983; of this age occur along much of the western Bruns, 1983) terranes. North American margin (Jones and others, Middle Tertiary The basic elements of the tectonic regime es- 1978), extending at least as far as the Koryak (Late Eocene to Oligocene, 43-24 Ma) tablished beginning at about 43 Ma have existed Peninsula of the northeastern USSR (Fujita and ever since, although magmatism and deforma- Newberry, 1983). A major change in plate motions occurred at tion have varied episodically. about 43 Ma, corresponding with the marked Early Tertiary bend in the Hawaii-Emperor seamount chain in ACKNOWLEDGMENTS (Early to Middle Eocene, 56-43 Ma) the Pacific Ocean basin (Engebretson and others, 1985). The resulting changes in relative We would like to thank ARCO Alaska, Inc., Major plate reorganization occurred between plate motion led to establishment of a new tec- for providing us with the opportunity and sup- about 56 and 43 Ma (Byrne, 1979; Engebretson tonic environment in Alaska (Fig. 7G), the basic port to pursue this study and the permission to and others, 1985). Engebretson and others elements of which have persisted to the present. present the results publicly. We particularly (1985) postulated that during this time, relative By this time, the Kula plate had ceased to exist thank David Hite, who originated the idea for motion between the Kula and North American as a separate plate and was welded to the Pacific the study and encouraged us at many steps along plates increased dramatically in speed and had a plate (Engebretson and others, 1985). The speed the way. Tom Moore and Jory Pacht contrib- somewhat greater transform component. Signif- of motion between the Pacific and North Amer- uted both their insights in the field and helpful icant geologic changes along the Alaskan con- ican plates decreased significantly, and the direc- data. Other valuable participants in the field tinental margin may reflect these changes in tion shifted northwestward. This resulted in included R. Blodgett, C. Carlson, M. Churkin, relative plate motion (Fig. 7F). Magmatism in continued transform motion along the north- B. Hall, J. Helwig, M. Kremer, C. Millage, and the Alaska Range belt apparently ceased during west-trending continental margin of western D. Wetherell. Geochronologic results were this time (Wallace and Engebretson, 1984), and Canada and southeastern Alaska and continued provided by D. Turner (University of Alaska, about 400 km of right-lateral displacement oc- convergence along the continental margin of Fairbanks), and paleontologic interpretations curred along the Denali fault (Lanphere, 1978). southwestern Alaska, now northeast trending. were provided by R. Brooks (ARCO), T. Carr

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Carlson, C., and Wallace, W. K., 1983, The Tlikakila Complex, a disrupted Hamburger, M. W., Cardwell, R. K., and lsacks, B., 1983, Seismotectonics of (ARCO), A. Harris (U.S. Geological Survey), terrane in the southwestern Alaska Range: Geological Society of Amer- the northern Philippine island arc, in Hayes, D. E., ed., The tectonic and P. Hoover (Paleontological Research Institu- ica Abstracts with Programs, v. 15, no. 5, p. 406. geologic evolution of Southeast Asian seas and islands, Part 2: Ameri- Coe, R. S., Globerman, B. R., Plumley, P. W„ and Thrupp, G. A., 1985, can Geophysical Union Geophysical Monograph 27, p. 1-22. tion), J. Miller (U.S. Geological Survey), and Paleomagnetic results from Alaska and their tectonic implications, in Hamilton, W., 1979, Tectonics of the Indonesian region: U.S. Geological Sur- N. Savage (Oregon State University). Discus- Howell, D. G., ed., Tectonostratigraphic terranes of the circum-Pacific vey Professional Paper 1078, 345 p. region: Circum-Pacific Council for Energy and Mineral Resources Hanks, C. L., Rogers, J. F., and Wallace, W. K., 1985, The Western Alaska sions with other geologists working in south- (AAPG), Earth Science Series, no. 1, p. 85-108. Range Flysch terrane: What is it and where did it come from?: Geologi- Coney, P. J., 1981, Accretionary tectonics in western North America, in Dick- cal Society of America Abstracts with Programs, v. 17, no. 6, p. 359. western Alaska, including S. Box, T. Bundtzen, inson, W. R., and Payne, W. D., eds., 1981, Relations of tectonics to ore Hoare, J. M., 1961, Geology and tectonic setting of lower Kuskokwim-Bristol J. Decker, and G. Thrupp, helped put our ob- deposits in the southern Cordillera: Arizona Geological Society Digest, Bay region, Alaska: American Association of Petroleum Geologists Bul- v. 14, p. 23-37. letin, v. 45, p. 594-611. servations in perspective. We appreciate the Coney, P. J., and Jones, D. L., 1985, Accretion tectonics and crustal structure Hoare, J. M., and Coonrad, W. L., 1978, Geologic map of the Goodnews and in Alaska: Tectonophysics, v. 119, p. 265-283. Hagemeister Islands region, southwestern Alaska: U.S. Geological Sur- thoughtful reviews by David Stone, Tom Miller, Coney, P. J., and Reynolds, S. J., 1977, Cordilleran Benioff zones: Nature, vey Open-File Report 78-9B, scale 1:250,000. Tim Byrne, and two anonymous reviewers. v. 270, p. 403-406. Howell, D. G., Crouch, J. K., Greene, H. G., McCulloch, D. S., and Vedder, Covey, M., 1986, The evolution of foreland basins to steady state: Evidence J. G., 1980, Basin development along the late Mesozoic and Cainozoic Although the data and interpretations pre- from the western Taiwan foreland basin, in Allen, P. A., and Home- California margin: A plate tectonic margin of subduction, oblique sub- wood, P., eds., Foreland basins: International Association of Sedimen- duction and transform tectonics, in Ballance, P. F.,and Reading, H. G., sented herein are largely the result of work done tologists Special Publication Number 8, p. 77-90. eds., Sedimentation in oblique-slip mobile zones: International Associa- Cowan, D. S., 1982, Geological evidence for post-40 m.y. B.P. large-scale tion of Sedimentologists Special Publication Number 4, p. 43-62. for ARCO, the conclusions and interpretations northwestward displacement of part of southeastern Alaska: Geology, Hudson, T., 1979, Mesozoic plutonic belts of southern Alaska: Geology, v. 7, are our own and do not necessarily reflect the v. 10, p. 309-313. p.230-234. Cross, T. A., and Pilger, R. 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