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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. 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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  1. Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska

  2. Marti L. Miller1Dwight C. Bradley1Thomas K. Bundtzen2Richard J. Goldfarb3 • U.S. Geological Survey, Anchorage • Pacific Rim Geological Consulting • U.S. Geological Survey, Denver

  3. 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

  4. SouthwesternAlaska—tectonostratigraphic terranes After Decker and others, 1984

  5. Geology--central Kuskokwim Mineral Belt Kuskokwim Group

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

  7. Deposits ofthe central KuskokwimMineral Belt • Epizonal Hg-Sb and Au • Precious metal-bearing intrusion related

  8. Shotgun Kuskokwim Group Granodiorite stock Mineralized qtz-feldspar porphyry • 70 Ma qtz-feldspar porphyry • Veins, breccias, stockworks • Au, As, B ± Cu, Mo, Bi, Te

  9. Epizonal Hg-Sb deposits Red Devil Cinnabar Creek

  10. Epizonal Au-bearing deposits Donlin

  11. Iditarod-Nixon Fork fault—at least 90 km dextral offset • Denali fault—at least 134 km dextral offset

  12. Along strike-slip faults: • Fortyseven Creek • Nixon Fork Dike-bearing ridge west of Fortyseven Cr

  13. Red Devil Between master faults: • Donlin • Red Devil

  14. 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

  15. 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?

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

  17. ~55 Ma: Dextral motion was possibly accentuated by “megakinking” during oroclinal bending Plate ?

  18. 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

  19. ~70 Ma: Dextral motion driven by oblique convergence prior to ridge subduction

  20. A witches brew: • Curved margin • Oblique subduction • Escape to free face

  21. Possible tectonic scenarios Low angle subduction Slab break off Ridge subduction Escape tectonics

  22. 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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