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.