MESOZOIC BASIN DEVELOPMENT AND DEFORMATION ALONG THE OUTBOARD MARGIN OF THE WRANGELLIA COMPOSITE TERRANE

Mesozoic strata in the Talkeetna, Wrangell, and Chugach Ranges of southern Alaska provide a long-term record of basin development, arc construction, and collisional tectonics along the outboard (southern) margin of the Wrangellia composite terrane (WCT). New sedimentologic, petrologic, and geochronologic data document the following stages associated with collision of the WCT with the former continental margin of North America: (1) A northward-dipping subduction zone and associated intraoceanic arc (Chitina arc) formed along the outboard margin of the WCT during the Middle to Late Jurassic. Marine volcaniclastic strata record development of forearc (Tuxedni and Chinitna Fms.) and backarc (Nizina Mountain and lower Root Glacier Fms.) basins. (2) During the latest Jurassic, a narrow thrust belt and foreland basin formed inboard of the Chitina arc. Gravel-rich fan-deltas transported sediment cratonward in the foreland basin (Kotsina Cong.) and trenchward in the forearc basin (Naknek Fm.). Plutonic and metavolcanic detritus deposited in the forearc basin record exhumation of the roots of remnant Jurassic arcs. Sedimentary and igneous detritus deposited in the foreland basin document uplift of deep levels of the WCT and the Chitina arc. An angular unconformity developed throughout the outboard margin of the WCT during the Late Jurassic-Early Cretaceous. We interpret the evidence for late Jurassic-early Cretaceous crustal-scale shortening as recording initial collision of the WCT. (3) Following orogenesis, Cretaceous volcanic arcs (Chisana and Kluane arcs) were constructed inboard of the remnant Jurassic arcs and basinal deposits. Lower to Upper Cretaceous strata were deposited in nonmarine and marine deposystems within a continental margin forearc basin. Sediment was derived from remnant arcs and recycled basinal strata along the arcward margin of the basin and from subduction complex deposits and remnant arcs along the trenchward margin of the basin. (4) Latest Cretaceous strata (MacColl Ridge Fm. and uppermost Matanuska Fm.) record regional uplift along the trenchward margin of the forearc basin. Cratonward-prograding submarine fan systems developed along the Border Ranges fault, prior to latest Cretaceous-Paleocene uplift and unconformity development.