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

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142 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 Karsten, J.L., Delaney, J.R., Rhodes, J.M., and Liios, R.A., 1990, Spatial and temporal evolution of magmatic systems beneath the Endeavor Segment, Juan de Fuca Ridge: Tec- tonic and petrologic constraints: Journal of Geophysical Research, v. 95, no. B12, p. 19,235-19,256. Ludden, J.N., and Thompson, Geoffrey, 1978, Behavior of rare earth elementsduring submarine weathering of tholeiitic basalt: Nature, v. 274, p. 147-149. Lull, J.S., and Plafker, George, 1990, Geochemistry and paleotectonic implications of metabasaltic rocks in the Valdez Group, southern Alaska, in Dover, J.H., and Gallo- way, J.P., eds., Geologic studies in Alaska by the U.S. Geo- logical Survey, 1989: U.S. Geological Survey Bulletin 1946, p. 29-38. Miyashiro, Akiho, 1975, Classification, characteristics, and origin of ophiolites: Journal of Geology, v. 83, p. 249-281. Moore, J.C., Byrne, T., Plumley, P.W., Reid, M., Gibbons, H., and Coe, R.S., 1983, Paleogene evolution of the Kodiak Is- lands, Alaska-consequences of ridge-trench interaction in a more southerly latitude: Tectonics, v. 2, p. 265-293. Mullen, E.D., 1983, Mn0/TiO2/P2O5: A minor element dis- criminant for basaltic rocks of oceanic environments and its implications for petrogenesis: Earth and Planetary Science Letters, v. 62, p. 53-62. Nelson, S.W., 1992, Ophiolitic complexes of the Gulf of Alaska: U.S. Geological Survey Open-File Report 92-20C, 9 p. Nelson, S.W., and Blome, C.D., 1991, Preliminary geochemistry of volcanic rocks from the McHugh Complex and Kachemak terrane, southern Alaska: U.S. Geological Sur- vey Open-File Report 91-134, 14 p. Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Geologic map of the Chugach National Forest, Alaska: U.S. Geo- logical Survey Miscellaneous Field Studies Map MF-1645- B, scale 1:250,000, 15 p. Nelson, S.W., Miller, M.L., and Dumoulin, J.A., 1987, Resurrec- tion Peninsula and Knight Island ophiolites and recent fault- ing on Montague Island, southern Alaska: Geological Society of America Centennial Field Guide-Cordilleran Section, p. 433-438. Pearce, J.A., 1982, Trace element characteristics of lavas from destructive plate boundaries, in Thorpe, R.S., ed., Andes- ites: orogenic andesites and related rocks: Chichester, Wiley, p. 525-548. 1983, The role of sub-continental lithosphere in magma genesis at destructive plate margins, in Hawkesworth, C.J., and Norry, M.J., eds., Continental basalts and mantle xeno- liths: Nantwich, Shiva, p. 230-249 Pearce, J.A., and Cann, J.R., 1973, Tectonic setting of basic volcanic rocks determined using trace element analyses: Earth and Planetary Science Letters, v. 19, p. 220-300. Plafker, George, Jones, D.L., and Pessagno, E.A., 1977, A Cre- taceous accretionary flysch and melange terrane along the Gulf of Alaska margin, in Blean, K.M., ed., United States Geological Survey in Alaska: Accomplishments during 1976: U.S. Geological Survey Circular 75 1-B, p. B41-B43. Plafker, George, Keller, Gerta, Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Summary of data on the age of the Orca Group, Alaska, in Bartsch-Winkler, Susan, ed., The United States Geological Survey in Alaska: Accomplish- ments during 1984: U.S. Geological Survey Circular 967, p. 74-76. Plafker, George, Moore, J.C., and Winkler, G.R., [in press], Ge- ology of the southern Alaska margin: Boulder, Geological Society of America, Decade of North American Geology (DNAG) Series. 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. Plafker, George, Nokleberg, W.J., Lull, J.S., Roeske, S.M., and Winkler, G.W., 1986, Nature and timing of deformation along the Contact fault system in the Cordova, Bering Gla- cier, and Valdez quadrangles, Alaska: U.S. Geological Sur- vey Circular 978, p. 74-77. Plumley, P.W., and Plafker, George, 1985, Additional estimate of paleo-latitude for the Paleocene/Eocene Prince William terrane Orca volcanics: Geological Society of America Ab- stracts with Programs, v. 17, p. 401. Richards, T.A. and McTaggart, K.C., 1976, Granitic rocks of the southern Coast Plutonic Complex and northern Cascades of British Colulmbia: Geological Society of America Bulletin, v. 87, no. 6, p. 935-953. Richter, D.H., 1965, Geology and mineral deposits of central Knight Island, Prince William Sound, Alaska: Alaska Divi- sion of Mines and Minerals Geologic Report 16, 37 p. Sem, Giancarlo, 1981, The petrochemistry of ophiolite gabbroic complexes: A key for the classification of ophiolites into low-Ti and high-Ti types: Earth and Planetary Science Let- ters, v. 52, p. 203-212. Sun, Shen-Su, and Nesbitt, R.W., 1978, Geochemical regulari- ties and genetic significance of ophiolitic basalts: Geology, V. 6, p. 689-693. Tysdal, R.G., Case, J.E., Winkler, G.R., and Clark, S.H.B., 1977, Sheeted dikes, gabbro, and pillow basalt in flysch of coastal southern Alaska: Geology, v. 5, p. 377-383. Wilson, Marjorie, 1989, Igneous petrogenesis: London, Unwin Hyman, 466 p. Winkler, G.W., 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 environments in Alaska: Anchorage, Alaska Geological Society Symposium Proceedings, p. R1-R20. Wood, D.A., 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to estab- lishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province: Earth and Plan- etary Science Letters, v. 50, p. 11-30. Wood, D.A., Joron, Jean-Louis, and Trevil, Michel, 1979, A re- appraisal of the use of trace elements to classify and dis- criminate between magma series erupted in different tectonic settings: Earth and Planetary Science Letters, v. 45, p. 326-336. Reviewers: Peter Haeussler and William W. Patton, Jr.
STRUCTURAL EVOLUTION OF THE CHUGACH-PRINCE WILLIAM TERRANE AT THE HINGE OF THE OROCLINE IN PRINCE WILLIAM SOUND, AND IMPLICATIONS FOR ORE DEPOSITS ABSTRACT The Chugach-Prince William terrane is a Mesozoic through Tertiary accretionary complex that lies along coastal southern and southeastern Alaska. In Prince Will- iam Sound, the regional structural fabric bends about 90°, forming an orocline. Rocks at the hinge of the orocline consist of turbidites, conglomerate, and minor volcanic rocks and limestone. The structural geology in the hinge region defines a number of domains (each >15 km2) con- sisting of kilometer-scale tight folds. Adjacent domains may have up to a 90" difference in the strike of bedding and trend of fold axes. Four granite to tonalitic or gabbro plutons are dated or inferred to be about 35 Ma in age, and all were emplaced after regional folding. Base-metal sulfide mineral occurrences, barren quartz veins, and smke- slip late faults locally cut the plutons and generally strike north-south. The mineral occurrences often have a den- dritic network of quartz veinlets adjacent to the mineralized zone and brecciated wall rock within the zone. Oroclinal bending, in the style of bending a bar, is consistent with the origin of the complicated domainal geometry of the structures. The Contact fault, thought by some workers to juxtapose two parts of the accretionary prism, truncates one of these 35-Ma plutons with strike-slip off- set, but previous reverse motion cannot be ruled out. A magmatic source for the ore-forming fluids is consistent with the structural and mineralogical data. The north-south orientation of mineralized zones suggests that east-west extension occurred possibly during release of fluids from the plutons, which locally hydrofractured the wall rocks and allowed migration of ore-forming fluids along pre- existing fractures, INTRODUCTION The geology of coastal south-central Alaska is dominated by rocks of a vast accretionary prism of Late By Peter J. Haeussler and Steven W. Nelson Cretaceous to Eocene(?) age (Plafker and others, 1989). In Prince William Sound, the regional strike of bedding and cleavage changes by about 90" from the east side to the west side (Nelson and others, 1985). This change in orientation of the structural grain defines the orocline of southern Alaska (Carey, 1955) (fig. 1). At the hinge of the orocline, previous studies recognized (1) regions where the strike of bedding was discordant from the regional strike, and (2) Zn-, Cu-, Pb-rich minera! occurrences that are not found elsewhere in the accretionary prism (Nelson and others, 1984, 1985; MacKevett and Hoiloway, 1977a, 1977b). Most mineral occurrences in the prism consist of syngenetic volcanogenic massive sulfide deposits associ- ated with ophiolites (Crowe and others, 1992), or, turbid- ite-hosted epigenetic lode-gold deposits in structurally simple regions of harmonic folds with subhorizontal fold axes (for example, see Mitchell, 1979). Because of the presence of the unique Zn-, Cu-, Pb-rich veins described above and to support the Chugach National Forest man- agement plan, the U.S. Geological Survey and the U.S. Bureau of Mines recommended additional study in this area. Funds for this work were provided by the U.S. Forest Service. This paper presents the structural geology of the Chugach National Forest Special Study Area based upon 1:63,360-scale (1 inch = 1 mile) mapping in the area between Columbia Glacier and Unakwik Inlet (fig. 2) and discusses the structural and tectonic development at the hinge area of the orocline in addition to the relationship between structure, magmatism, and mineral occurrences. REGIONAL GEOLOGY Rocks in the vicinity of Prince William Sound largely consist of siliciclastic turbidites (Dumoulin, 1987) of the Orca and Valdez Groups (figs. 1 and 2). The rocks of the Valdez Group are thought to have originated as trench-fill deposits at the margin of an Andean-type arc (Nilsen and Zuffa, 1982; Plafker and others, 1989), and the rocks of
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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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Table 3. Summary of geochemical sig
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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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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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