HR: 08:00h
AN: T41C-01    [Abstracts]
TI: Provenance Constraints on the Mesozoic-Cenozoic Tectonic Evolution of the Queen Charlotte Islands Region
AU: * Mahoney, J
EM: mahonej@uwec.edu
AF: University of Wisconsin, Dept of Geology , Eau Claire, WI 54701, United States
AU: Haggart, J W
EM: JHaggart@nrcan.gc.ca
AF: Geological Survey of Canada, Robson Street, Vancouver, BC , Canada
AU: Kimbrough, D
EM: dkimbrou@geology.sdsu.edu
AF: San Diego State University, Geological Sciences, San Diego, CA , United States
AU: Grove, M
EM: marty@ess.ucla.edu
AF: University of California-Los Angeles, Earth and Space Sciences, Los Angeles, CA , United States
AB: The medial Cretaceous magmatic arc system of western North America was flanked by a series of forearc basins extending from Mexico to Alaska. Cretaceous strata in the Queen Charlotte Islands of northwest British Columbia are unique in this series of basins, as these strata have been displaced from the arc system by formation of the extensional Queen Charlotte basin in Cenozoic time. This displacement complicates reconstruction of the forearc basin, and makes it difficult to evaluate the controls on basin evolution. Sedimentologic, paleontologic, and detrital zircon analyses of forearc strata represented by the Valanginian- Campanian Queen Charlotte Group (QCG) constrain basin evolution and provide a framework for an interpretation of the Mesozoic-Cenozoic tectonic evolution of the Queen Charlotte Islands region. Basin subsidence initiated in Valanginian time with a marine transgression over irregular topography consisting of extensional fault blocks of pre-Cretaceous strata. Locally derived conglomerates at the base of the Longarm Formation are overlain by shallow marine shelf deposits that represent a westward-deepening, fining-upward transgressive succession with an eastern depositional edge that migrated eastward during Valanginian to Aptian time. West-directed paleocurrents and a unimodal detrital zircon population of 120-175 Ma grains provide the first linkage between the Cretaceous QCG and unroofed Jura-Cretaceous plutons of the Coast Plutonic Complex to the east. This initial transgressive sequence is superseded by a second pulse of clastic detritus in early Albian time, characterized by an easterly-derived, fossiliferous shallow-shelf sandstone (Haida Formation), fine-grained, outer shelf to upper slope strata (Bearskin Bay Formation), and mass-sediment gravity flows (Skidegate Formation). The unimodal zircon population (ca 140-175 Ma) in the lower Haida Formation is interpreted to reflect renewed uplift of Jura-Cretaceous arc plutons by contractional tectonism in the arc system to the east. A significant subsidence event in Late Turonian to Coniacian time resulted in accumulation of a thick, westward- prograding fan-delta/submarine fan complex, the Honna Formation, reflecting a tight linkage between orogenic exhumation and basin subsidence. The distinctly bimodal zircon signature of the Honna and upper Skidegate formations strongly suggests rapid exhumation of both syndepositional Late Cretaceous plutons (ca. 90 -110 Ma) and older arc plutons (ca 140-170 Ma), potentially driven by a major magmatic pulse within the Coast Plutonic Complex. This bimodal zircon signature persists through Late Santonian time, represented by fine-grained distal shelf deposits of the Tarundl Formation. This fine-grained succession may represent a decrease in the rate of basin subsidence and final basin infilling resulting from a decrease in exhumation within the arc system associated with the transition from orthogonal convergence to dextral translation in Late Cretaceous to Paleocene time.
DE: 8108 Continental tectonics: compressional
DE: 8169 Sedimentary basin processes
DE: 8175 Tectonics and landscape evolution
DE: 9610 Cretaceous
SC: Tectonophysics [T]
MN: 2007 Joint Assembly