U.S. Department of the InteriorU.S. Geological Survey
Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska
Marti L. Miller1
Dwight C. Bradley1
Thomas K. Bundtzen2
Richard J. Goldfarb3
1 U.S. Geological Survey, Anchorage
2 Pacific Rim Geological Consulting
3 U.S. Geological Survey, Denver
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
SouthwesternAlaska—tectonostratigraphic terranes
After Decker and others, 1984
Geology--central Kuskokwim Mineral Belt
Kuskokwim Group
Geology--central Kuskokwim Mineral Belt
Volcanic-plutonic complex
Felsic porphyritic dike
Deposits ofthe central KuskokwimMineral Belt
Epizonal Hg-Sb and Au
Precious metal-bearing intrusion related
Shotgun
Mineralized qtz-feldspar porphyry
Kuskokwim Group
70 Ma qtz-feldspar porphyry
Veins, breccias, stockworks
Au, As, B ± Cu, Mo, Bi, Te
Granodiorite stock
Epizonal Hg-Sb deposits
Red Devil
Cinnabar Creek
Epizonal Au-bearing deposits
Donlin
Iditarod-Nixon Fork fault—at least 90 km dextral offset
Denali fault—at least 134 km dextral offset
Along strike-slip faults: Fortyseven Creek Nixon Fork
Dike-bearing ridgewest of Fortyseven Cr
Between master faults: Donlin Red Devil
Red Devil
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
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
Present: Dextral motion related to tectonic escape in collisional foreland, despite local sinistral sense of oblique subduction
Escape
~55 Ma: Dextral motion was possibly accentuated by “megakinking” during oroclinal bending
Plate ?
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
~70 Ma: Dextral motion driven by oblique convergence prior to ridge subduction
A witches brew: Curved margin Oblique
subduction Escape to free
face
Low angle subduction Slab break off
Ridge subduction
Escape tectonics
Possible tectonic scenarios
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.