CRUSTAL-SCALE DUCTILE BOUDINAGE IN THE SOUTH-CENTRAL CANADIAN CORDILLERA: TECTONIC EXHUMATION OF METAMORPHIC INFRASTRUCTURE BY PALEOGENE DEXTRAL TRANSTENSION

At the south end of the Tintina-Northern Rocky Mountain Trench dextral intra-continental transform fault, and adjacent to its westerly en echelon equivalents--- the Yalakom-Ross Lake and Fraser River-Straight Creek faults, the Cordilleran metamorphic infrastructure is exposed in a series of en echelon, elongate, N to N-NE trending structural culminations. The structure, metamorphic petrology, thermochronology, shape, and orientation of these culminations show that they were exhumed by E-W ductile, crustal-scale boudinage during a Late Paleocene (58 Ma) to late Middle Eocene (~40 Ma) episode of regional dextral transtension. The boudins have been exhumed from depths of up to 25 km or more, along shallowly dipping extension faults. Locally they include Archean or Paleoproterozoic parautochthonous North American basement rocks as well as allochthonous metamorphic tectonites from the deepest levels of the thrust and fold belt. The adjoining “neck zones” contain generally unmetamorphosed supracrustal rocks, and also, locally, Eocene volcanic rocks and extensional growth-fault clastic deposits. Lithoprobe seismic reflection profiles show that beneath the boudinaged crust the Moho is approximately planar and sub-horizontal. Although there is locally >25 km of structural relief between the highest sedimentary rocks in the “neck zones” and the top of the middle crustal metamorphic rocks in adjacent boudins, the total thickness of the crust is relatively uniform (~30 km). Moreover, pre-Cordilleran, Paleoproterozoic metamorphic structures and fabrics are preserved at the deepest exposed levels in the core of at least one of the boudins (the Monashee complex). The boudins comprise zones in which thick (~30 km) relatively unstretched middle and lower crust is overlain by very strongly attenuated upper crust; conversely, the “neck zones” comprise zones in which strongly attenuated middle and lower crust overlain by relatively unstretched upper crust. The combination of a planar Moho with this large-scale reciprocal stretching between the upper crust and the middle and lower crust may be explicable in terms of an episode of more vigorous retrograde westward upper mantle flow in the back-arc region of the Cordilleran subduction zone during Eocene transtension.