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

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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 un- ited 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 land- ward 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 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 1390 WALLACE AND OTHERS <5=>C 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 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 KAHILTNA TERRANE: IMPLICATIONS FOR TECTONIC EVOLUTION OF ALASKA 1391 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
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