HR: 16:30h
AN: T24A-03    [Abstracts]
TI: Comparison of Lithospheric-Scale Structures Across the Alaskan and Canadian Cordillera and Their Tectonic Implications
AU: * Hammer, P T
EM: phammer@eos.ubc.ca
AF: Department of Earth and Ocean Sciences, University of British Columbia 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AU: Clowes, R M
EM: clowes@lithoprobe.ubc.ca
AF: Department of Earth and Ocean Sciences, University of British Columbia 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AU: Clowes, R M
EM: clowes@lithoprobe.ubc.ca
AF: Lithoprobe Secretariat, University of British Columbia 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AB: The Cordillera of Alaska and western Canada has been traversed by three multidisciplinary transects: a) the Lithoprobe Southern Cordillera transect (1000 km; 48^o-50^o N), b) the overlapping ACCRETE and Lithoprobe SNORCLE transects (1800 km; 54^o-63^o N), and c) the Alaskan TACT program (1200 km; 145^o-150^o W). Carried out over the last two decades, these studies integrate coincident and coordinated geological, geochemical and geophysical studies. Seismic reflection and refraction experiments contribute profiles of crustal and lithospheric mantle structure, providing a unifying framework for integrating the investigations and for advancing models of orogen structure and evolution. The resulting lithospheric-scale models enable along-strike comparison of large-scale structures, the style of accretion and deformation, and the influence of post-orogenic tectonics. Primary observations include the following three points. 1) A characteristic feature of the Canadian and Alaskan Cordillera is a crustal-scale decollement that acts as a tectonic accretion surface and separates the accreted terranes from the underlying, ancestral North American crust. All three trans-Cordillera profiles exhibit an unconformity or decoupling zone that dips outboard (westward beneath the Canadian Cordillera and southward beneath the Alaskan Cordillera) from the thrust and fold belt to the lower-most crust or Moho. Ancestral North American crust extends below 30-50% of the Alaskan and Canadian Cordillera. The observations require that the Mesosoic accretionary phase involved substantial delamination and subduction or subcretion of the allochthonous lithosphere. 2) The Moho remains remarkably flat and shallow (33-35 km) despite the variety of ages, orogenic styles and tectono-magmatic deformations that are spanned by the seismic corridors. The only major deviations are: a) just inboard of the transcurrent portion of the margin where the crust shallows to 20-25 km depth; and b) two crustal roots in the Alaskan Cordillera (50 km beneath the Brooks Range, and 60 km associated with the deformation and tectonic underplating that result from the attempted subduction of the Yakutat terrane). The shallow, flat Moho must be an active, near-solidus, deformation zone that represents a young, re-equilibrated crust-mantle boundary beneath much of the orogen. 3) Convergence and transcurrent motions have dominated Cordilleran development. The active subduction zones in Cascadia and southern Alaska exhibit similar structure. However, contradictory interpretations exist for teleseismic and controlled-source profiles. Resolution of these discrepancies will provide an improved understanding of dehydration metamorphism in these regimes and may influence analyses of megathrust earthquake hazard. The along-strike displacements of the accreted terranes remain one of the primary unsolved problems in Cordilleran development. The transects image the subsurface geometry of four major strike-slip faults that have played an important role for extended periods of time. They include: a) the Queen Charlotte-Fairweather fault system, b) the Denali fault, c) the Coast Shear Zone, and d) the Tintina fault system. All are distinguished as being crustal-penetrating by characteristic features of the seismic data and their interpretation.
DE: 7205 Continental crust (1219)
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8110 Continental tectonics: general (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005