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Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska. M arti L. Miller 1 Dwight C. Bradley 1 Thomas K. Bundtzen 2 Richard J. Goldfarb 3. U.S. Geological Survey, Anchorage Pacific Rim Geological Consulting U.S. Geological Survey, Denver.

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Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska

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Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska


M arti l miller 1 dwight c bradley 1 thomas k bundtzen 2 richard j goldfarb 3

Marti L. Miller1Dwight C. Bradley1Thomas K. Bundtzen2Richard J. Goldfarb3

  • U.S. Geological Survey, Anchorage

  • Pacific Rim Geological Consulting

  • U.S. Geological Survey, Denver


Tectonic setting of the kuskokwim mineral belt

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


Southwestern alaska tectonostratigraphic terranes

SouthwesternAlaska—tectonostratigraphic terranes

After Decker and others, 1984


Geology central kuskokwim mineral belt

Geology--central Kuskokwim Mineral Belt

Kuskokwim Group


Geology central kuskokwim mineral belt1

Geology--central Kuskokwim Mineral Belt

Volcanic-plutonic complex

Felsic porphyritic dike


Deposits of the central kuskokwim mineral belt

Deposits ofthe central KuskokwimMineral Belt

  • Epizonal Hg-Sb and Au

  • Precious metal-bearing intrusion related


Shotgun

Shotgun

Kuskokwim Group

Granodiorite stock

Mineralized qtz-feldspar porphyry

  • 70 Ma qtz-feldspar porphyry

  • Veins, breccias, stockworks

  • Au, As, B ± Cu, Mo, Bi, Te


Epizonal hg sb deposits

Epizonal Hg-Sb deposits

Red Devil

Cinnabar Creek


Epizonal au bearing deposits

Epizonal Au-bearing deposits

Donlin


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

  • Iditarod-Nixon Fork fault—at least 90 km dextral offset

  • Denali fault—at least 134 km dextral offset


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Along strike-slip faults:

  • Fortyseven Creek

  • Nixon Fork

Dike-bearing ridge

west of Fortyseven Cr


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Red Devil

Between master faults:

  • Donlin

  • Red Devil


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

What we know

  • 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


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

What we don’t know

  • 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?


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

  • Present: Dextral motion related to tectonic escape in collisional foreland, despite local sinistral sense of oblique subduction

Escape


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

  • ~55 Ma: Dextral motion was possibly accentuated by “megakinking” during oroclinal bending

Plate ?


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

At ~60 Ma three possibilities for identity of subducting plate

--Resurrection Plate preferred

Preferred:

Miller et al.,2002

Haeussler et al., 2003

Engebretsenet al., 1985

Bradleyet al., 1993


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

  • ~70 Ma: Dextral motion driven by oblique convergence prior to ridge subduction


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

A witches brew:

  • Curved margin

  • Oblique subduction

  • Escape to free face


Late cretaceous tectonic evolution and metallogeny of southwestern alaska

Possible tectonic scenarios

Low angle subduction

Slab break off

Ridge subduction

Escape tectonics


References cited

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.


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