Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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162 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY. 1992 ments during 1984: U.S. Geological Survey Circular 967, p. 74-76. Plafker, George, Nokleberg, W.J., and Lull, J.S., 1989, Bedrock geology and tectonic evolution of the Wrangellia, Peninsu- lar, and Chugach terranes along the Trans-Alaska Crustal Transect in the Chugach Mountains and southern Copper River basin, Alaska: Journal of Geophysical Research, v. 94, p. 4255-4295. Richter, D.H., 1970, Geology and lode-gold deposits of the Nuka Bay area, Kenai Peninsula, Alaska: U.S. Geological Survey Professional Paper 625G, 16 p. Sample, J., and Fisher, D., 1986, Duplex accretion and underplating in an ancient accretionary complex, Kodiak Is- lands, Alaska: Geology, v. 14, p. 160-163. Sample, J.C., and Moore, J.C., 1987, Structural style and kine- matics of an underplated slate belt, Kodiak and adjacent is- lands, Alaska: Geological Society of America Bulletin, v. 19, p. 7-20. Silbennan, M.L., Mitchell, P.A., and O'Neil, J.R., 1981, Isotopic data bearing on the origin and age of the epithennal lode gold deposits in the Hope-Sunrise mining district, northern Kenai Peninsula, Alaska: in Albert, N.R.D., and Hudson, T., eds., the U.S. Geological Survey in Alaska-Accom- plishments during 1979: U.S. Geological Survey Circular 823B, p. B81-B84. Stuwe, K., 1986, Structural evolution of the Port Wells gold mining district, Prince William Sound, south central Alaska: Implications for the origin of the gold lodes: Mineralium Deposita, v. 21, p. 288-295. Tapponnier, P., Peltzer, G., Le Dain, A.Y., Armijo, R., and Cobbold, P., 1982, Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine: Ge- ology, v. 10, p. 61 1-616. Tysdal, R.G., and Case, J.E., 1979, Geologic map of the Seward and Blying Sound quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series 1-1 150, 12 p., 1 sheet, scale 1:250,000. Westbrook, G.K., Mascle, A., and Biju-Duval, B., 1984, Geo- physics and the structure of the Lesser Antilles forearc, in Biju-Duval, B., Moore, J.C., and others, Initial Reports of the Deep Sea Drilling Project, v. 78A: Washington, U. S. Government Printing Office, p. 23-38. Winkler, G.R., 1976, Deep sea fan deposition of the lower Ter- tiary Orca Group, eastern Prince William Sound, Alaska, in Miller, T.P., ed., Recent and ancient sedimentary environ- ments in Alaska: Anchorage, Alaska Geological Society Symposium Proceedings, p. R1-R20. 1992, Geologic map and summary geochronology of the Anchorage 1" x 3" quadrangle, southern Alaska: U.S. Geo- logical Survey Miscellaneous Investigations Map 1-2283. Reviewers: Dwight C. Bradley and Richard J. Goldfarb
TIMING OF EARLY TERTIARY RIDGE SUBDUCTION IN SOUTHERN ALASKA By Dwight C. Bradley, Peter J. Haeussler, and Timothy M. Kusky ABSTRACT We present a new compilation of 158 isotopic ages from Tertiary plutons that intrude the accretionary prism (Chugach-Prince William composite terrane) of southern Alaska. Two broad plutonic age groups are present: Pal- eocene to Eocene (the Sanak-Baranof plutonic belt), and Oligocene to Miocene. Plutons of the Sanak-Baranof belt are broadly coeval with magmatism along the axis of an Andean-type arc that lay farther inboard. A plot of age versus position along strike shows that near-trench magmatism progressed from about 66-63 Ma in the west to about 50 Ma in the east. We interpret the near-trench magmatism as the product of subduction of the Kula- Farallon ridge (or another unidentified spreading center) and suggest that the age progression tracks a migrating trench-ridge-trench triple junction along the continental margin. INTRODUCTION Subduction of the Kula-Farallon ridge beneath the North American continent is a fundamental feature of all marine-based global plate reconstructions for early Tertiary time (Byrne, 1979; Engebretson and others, 1985; Stock and Molnar, 1988; Lonsdale, 1988; Atwater, 1989). In all reconstructions, however, the position through time of the trench-ridge-trench triple junction between the Kula, Farallon, and North American plates, where ridge subduction occurred, has been hypothetical. Although none of the published reconstructions have placed the triple junction farther north than Vancouver Island, evidence does exist in southern Alaska that a spreading ridge-perhaps the Kula-Farallon, or perhaps another ridge-was sub- ducted in early Tertiary time. The near-trench position and geochemistry of plutons of the Sanak-Baranof belt of Hudson and others (1979) (fig.1) have led several previous workers to link this mag- matic event to subduction of the Kula-Farallon ridge (Hill and others, 1981; Helwig and Emmet, 1981; Moore and others, 1983). In this paper, we discuss another aspect of Sanak-Baranof magmatism that supports the ridge-subduction hypothesis: the age pattern of magmatism along the belt. Several previous workers have commented, without thorough documentation, on the spatial distribution of ages along the belt. Hudson and others (1979). Hudson (1983), and Barker and others (1992) have contended that there were two pulses: at about 60 Ma in the west (Sanak Island to Kodiak Island) and at about 50 Ma in the east (Prince William Sound to Baranof Island). Others have argued, also without much documentation, that these same ages show a progression from 65-60 Ma in the west to 50-45 Ma in the east (Hill and others, 1981; Helwig and Emmet, 1981; Moore and others, 1983). An age progression of near-trench magmatism would be most readily interpreted in terms of ridge subduction, whereas a bimodal age pattern might have a number of possible causes. Accordingly, in this paper we present a compilation of isotopic ages from the Sanak-Baranof belt (table 1) and discuss some tectonic implications of these ages. The available data now span the entire Sanak-Baranof belt without major gaps; new 40~r/39~r and Uffb determinations fill what had been a crucial gap between Kodiak Island and Prince William Sound, where an age jump would occur if the age distribution were bimodal. The available data show that Sanak-Baranof magmatism progressed generally from west to east, in support of the ridge subduction model. This conclusion has implications for plate reconstructions, terrane displacement, orocline formation, gold mineralization, and deformation of the accretionary wedge. REGIONAL GEOLOGY Plutons of the Sanak-Baranof belt intrude a complexly deformed Mesozoic and Cenozoic accretionary prism-the Chugach-Prince William composite terrane (the term "composite" is omitted below for brevity) (fig.1)-along the seaward margin of the Peninsular-wrangellia- Alexander composite terrane. Plafker and others (in press) provide a thorough review of the regional geology. The inboard part of the prism is a melange of relatively compe- tent blocks and fault slices of basalt, chert, and graywacke, in an argillite matrix (Early Cretaceous Uyak Complex, Triassic to mid-Cretaceous McHugh Complex, and Late
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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DEEP-WATER LITHOFACIES AND CONODONT
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Table 1. Locality register for key
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Table 1. Locality register for key
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LITHOFACIES AND CONODONTS OF CARBON
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LITHOFACES AND CONODONTS OF CARBONI
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Table 1. Conodont faunules and lith
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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U-Pb AGES OF ZIRCON, MONAZITE, AND
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APPEAL FOR NONPROLIFERATION OF ESCA
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FAVORABLE AREAS FOR METALLIC MINERA
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GOLD AND CINNABAR IN HEAVY-MINERAL
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EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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EARLY CENOZOIC DEPOSITIONAL S YSTEM
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GEOCHEMICAL CHARACTER OF UPPER PALE
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RECONNAISSANCE GEOCHEMISTRY OF BASA
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OSTRACODE ASSEMBLAGES FROM MODERN B
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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US. GEOLOGICAL SURVEY REPORTS ON AL
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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