Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska
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Transcript of Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska
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U.S. Department of the InteriorU.S. Geological Survey
Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska
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Marti L. Miller1
Dwight C. Bradley1
Thomas K. Bundtzen2
Richard J. Goldfarb3
1 U.S. Geological Survey, Anchorage2 Pacific Rim Geological Consulting3 U.S. Geological Survey, Denver
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Tectonic setting of theKuskokwim Mineral Belt
Lies within a broad zone of dextral strike-slip faults
Occupies a backarc position ~400 km inboard of the present subduction zone
Occurs at the western end of a curved, continental-scale, strike-slip system
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SouthwesternAlaska—tectonostratigraphic terranes
After Decker and others, 1984
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Geology--central Kuskokwim Mineral Belt
Kuskokwim Group
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Geology--central Kuskokwim Mineral Belt
Volcanic-plutonic complex
Felsic porphyritic dike
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Deposits ofthe central KuskokwimMineral Belt
Epizonal Hg-Sb and AuPrecious metal-bearing intrusion related
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Shotgun
Mineralized qtz-feldspar porphyry
Kuskokwim Group
70 Ma qtz-feldspar porphyry
Veins, breccias, stockworks
Au, As, B ± Cu, Mo, Bi, Te
Granodiorite stock
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Epizonal Hg-Sb deposits
Red Devil
Cinnabar Creek
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Epizonal Au-bearing deposits
Donlin
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Iditarod-Nixon Fork fault—at least 90 km dextral offset
Denali fault—at least 134 km dextral offset
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Along strike-slip faults: Fortyseven Creek Nixon Fork
Dike-bearing ridgewest of Fortyseven Cr
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Between master faults: Donlin Red Devil
Red Devil
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Dextral strike-slip motion was taking place at the time of ~70 Ma deposit formation
Faults focused the fluids and accompanying mineralization
Some of the deposits are spatially associated with the master faults and others lie between these faults
What we know
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Why was there voluminous ~70 Ma magmatism over a wide area?
Why was the regional thermal gradient elevated across a broad region?
Why are both mantle- and flysch-derived intrusive rocks present?
What got the fluids and melts moving?
Are mineralization and magmatism both products of the same tectonic event?
What changed in the tectonic regime at ~70 Ma?
What we don’t know
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Present: Dextral motion related to tectonic escape in collisional foreland, despite local sinistral sense of oblique subduction
Escape
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~55 Ma: Dextral motion was possibly accentuated by “megakinking” during oroclinal bending
Plate ?
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At ~60 Ma three possibilities for identity of subducting plate--Resurrection Plate preferred
Engebretsenet al., 1985
Bradleyet al., 1993
Preferred:Miller et al.,2002Haeussler et al., 2003
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~70 Ma: Dextral motion driven by oblique convergence prior to ridge subduction
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A witches brew: Curved margin Oblique
subduction Escape to free
face
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Low angle subduction Slab break off
Ridge subduction
Escape tectonics
Possible tectonic scenarios
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References cited
Bradley, D.C., Haeussler, P.J., and Kusky, T.M., 1993, Timing of early Tertiary ridge subduction in southern Alaska: U.S. Geological Survey Bulletin 2068, p. 163-177.
Decker, J., Bergman, S.C., Blodgett, R.B., Box, S.E., Bundtzen, T.K., et al., 1994, The geology of southwestern Alaska, in Plafker, G., and Berg, H.C., eds., The geology of Alaska: Geological Society of America DNAG Series, v. G-1, p. 285-310.
Ebert, S., Miller, L., Petsel, S., Dodd, S., and Kowalczyk, 2000, Geology, mineralization, and exploration at the Donlin Creek project, southwestern Alaska: British Columbia and Yukon Chamber of Mines Special Volume 2, p. 99-114.
Engebretsen, D.C., Cox, Allan, and Gordon, R.G., 1985, Relative motions between oceanic and continental plates in the Pacific Basin: Geological Society of America Special Paper 206, 59 p.
Haeussler, P.J., Bradley, D.C., Wells, R.E., and Miller, M.L., 2003, Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Pacific in Paleocene-Eocene time: Geological Society of America Bulletin, v. 115, p. 867-880.
Miller, M.L., Bradley, D.C., Bundtzen, T.K., and McClelland, W., 2002, Late Cretaceous through Cenozoic strike-slip tectonics of southwestern Alaska: Journal of Geology, v. 110, p. 247-270.