Paper No. 4
Presentation Time: 8:50 AM


BOX, Stephen E.1, KARL, Susan M.2, JONES III, James V.3, LEASE, Richard O.2, BRADLEY, Dwight C.4, HAEUSSLER, Peter J.5, GRAHAM, Garth6, SALTUS, Richard W.7 and HULTS, Chad P.8, (1)U.S. Geological Survey, 904 W. Riverside Ave, Room 202, Spokane, WA 99201, (2)USGS, 4210 University Dr, Anchorage, AK 99508-4626, (3)U.S. Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, AK 99508, (4)U.S. Geological Survey, 11 Cold Brook Rd, Randolph, NH 03593, (5)U.S. Geological Survey, 4210 University Dr, Anchorage, AK 99508, (6)U.S. Geological Survey, DFC MS973, Lakewood, CO 80225, (7)U.S. Geological Survey, Box 25046, Mail Stop 964, Denver Federal Center, Denver, CO 80225-0046, (8)USGS, 4210 University Dr, Anchorage, AK 99508,

The Kahiltna basin in the western Alaska Range consists of deformed, dominantly turbiditic Lower to Upper Cretaceous strata. The basin is flanked by the Farewell terrane to the NW and by the Peninsular-Wrangellia terrane to the SE. A magnetic boundary separates the two terranes beneath the center of the basin. Kahiltna strata are divided into two distinct compositional facies: a NW facies with abundant quartz and and low-grade metasedimentary rock fragments, and a SE facies with abundant feldspar and volcanic rock fragments. The contact between these facies is sharp and generally parallel to but 10-20 km NW of the magnetic boundary.

U-Pb ages of detrital zircons (DZ) were analyzed from 24 sandstone samples in a NW-SE transect across the Kahiltna basin. Typical samples from the NW facies have DZ peaks at 105-120, 195, and 340-360 Ma. Precambrian ages make up 10-40% of DZ grains, with peaks similar to those of Farewell strata (Dumoulin, this meeting). The SE facies typically has one or two peaks between 140 and 170 Ma; pre-Mesozoic grains are rare. The NW facies is derived from both Yukon-Tanana (115, 195 and 360 Ma age peaks) and Farewell (Early Paleozoic and Precambrian grains) terranes to the north, while the SE volcanic-sourced facies is derived solely from the Peninsular-Wrangellia terrane to the south. The general lack of common peaks between the two facies indicates discrete detrital sources; however, near the contact between the two facies belts, both contain an additional zircon population. The SE part of the NW belt has an additional peak between 87-99 Ma; the NW part of the SE belt has a peak between 92-99 Ma. These shared ~90 Ma DZ peaks in the central basin correlate in age with tuffs and plutons within the SE basin flank, suggesting airfall tuff sources.

Post-kinematic plutons as old as 75 Ma cut the deformation fabric of the young central facies, constraining deformation between 90 and 75 Ma. Kahiltna strata are deformed into overturned folds verging to the NW near the NW basin margin and SE near the SE margin. Steeply plunging folds near the contact of the two facies belts are tentatively interpreted to indicate transpressive tectonics along that boundary. Several mineralized TK plutonic systems (Whistler Cu-Au, Mt. Estelle Au, Copper Joe Cu, Terra Au-qtz veins, and Tired Pup REE) flank this contact on its N side for 150 km along strike.