HR: 11:35h
AN: U32A-06    [Abstracts]
TI: The Cordilleran Cretaceous Apparent Polar Wander Path : Implications for the paleogeographic evolution of Saybia and Laurentia
AU: * Johnston, S T
EM: stj@uvic.ca
AF: Earth and Ocean Sciences, University of Victoria PO Box 3055 STN CSC, Victoria, BC V8W 3P6 Canada
AB: Late Cretaceous paleopoles from stratified sequences in the Canadian Cordillera are discordant with respect to autochthonous North America and require 3000 km of dextral translation of much of the Cordillera between 85 and 55 Ma. The region characterized by discordant paleopoles extends, in Yukon, as far east as the Omineca Crystalline belt (OCB) where 70 Ma volcanic rocks of the Carmacks Group overlie the OCB and yield paleopoles requiring 2000 km of dextral offset. The OCB is stitched together and to more westerly geological belts by mid-Cretaceous plutons. These data are consistent with a model of Laramide orogeny resulting from a transpressional collision between a northward translating ribbon continent (Saybia) that included the OCB and more westerly portions of the Cordillera. Northward translation in the north was accommodated by buckling of Saybia giving rise to the Alaskan salient. Some mid-Cretaceous stratified sequences in Saybia yield paleopoles that require smaller degrees of northward translation relative to North America and imply southward translation of portions of the ribbon continent between 120 and 90 Ma. Southward translation was coeval with voluminous, eastward-younging granitoid magmatism including minor S-type granites, and large scale dextral shearing within the OCB. These data are consistent with oblique southwestward-directed subduction of oceanic crust that lay between Saybia and North America beneath the east (inboard) margin of Saybia in the mid-Cretaceous. Oblique subduction explains the voluminous OCB arc magmatism. Southward (dextral) translation resulted from entraining of orogenic float along Saybia's inboard margin. S-type granites and younging of the arc toward the east are attributable to the progressive accretion of sedimentary orogenic float to the upper plate, continually displacing the active arc to the west and placing accreted sedimentary rocks above upward-rising gabbroic arc magmas that underplated and then melted the sediments. Doubling up of the Quesnellia-Stikinia magmatic arc may have occured at this time by southward translation of Quesnellia. The current inability to resolve the geometry of the Meso- to Neoproterozoic Rodinian supercontinent may be in part the result of the false assumption that the OCB is autochthonous North America. Proterozoic paleopoles from the OCB should not be used to constrain the North American - Laurentian APWP, and neither should OCB stratigraphy and structures be used to aid in constraining Rodinian reconstructions.
DE: 8155 Plate motions--general
DE: 1724 Ocean sciences
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1620 Climate dynamics (3309)
SC: Union [U]
MN: 2004 AGU Fall Meeting