U.S. Department of the Interior U.S. Geological Survey 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 1 U.S. Geological Survey, Anchorage 2 Pacific Rim Geological Consulting 3 U.S. Geological Survey, Denver

Tectonic setting of the Kuskokwim 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 After Decker and others, 1984

Geology--central Kuskokwim Mineral Belt Kuskokwim Group

Geology--central Kuskokwim Mineral Belt Volcanic-plutonic complex Felsic porphyritic dike

Deposits of the central Kuskokwim Mineral 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 ridge west of Fortyseven Cr

Between master faults: Donlin 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 Engebretsen et al., 1985 Bradley et al., 1993 Preferred: Miller et al.,2002 Haeussler 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 subductionSlab break off Ridge subductionEscape 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  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  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  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  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