HOT AND COLD DETACHMENT ZONES: LINKING LISTRIC NORMAL FAULTING IN THE SOUTHERN CANADIAN ROCKIES FORELAND THRUST-FOLD BELT TO BACK-ARC DEXTRAL TRANSTENSION AND DUCTILE CRUSTAL BOUDINAGE

Late Paleocene - Eocene crustal extension in the SE Canadian Cordillera was part of an episode of dextral transtension within the area of en echelon overlap linking the Tintina-Northern Rocky Mountain trench fault zone, first to the Yalakom-Ross Lake fault zone, and then to the Fraser River-Straight Creek fault zone. It followed an episode of Late Cretaceous-Paleocene dextral transpression that produced most of the thrusting in the southern Canadian Rockies. The Monashee, Valhalla, and Priest River complexes, three north-trending, en echelon, left-stepping, doubling plunging, elliptical structural culminations, were exhumed as boudins of relatively unstretched (~35 km thick) Paleoproterozic continental crust along the footwalls of east-dipping, hot, extensional detachment zones that were located at mid-crustal depths within partly molten supracrustal rocks and were rooted in an adjacent neck zone of attenuated (~15 km thick) continental crust. The basal detachment thrust of the foreland thrust-fold belt was uplifted, with the Moho, during the extension. It is exposed within the Monashee and Priest River complexes and is interpreted to be close to the surface in the Valhalla complex. Cenozoic listric normal faults in the Canadian Rockies of SE British Columbia record 15-20 km of Eocene - Miocene horizontal extension above the planar SW-sloping basal detachment thrust of the foreland thrust-fold belt and the underlying ~35 km-thick layer of Paleoproterozoic continental crust that extends under the Canadian Rockies from beneath Alberta. But below the southern Rocky Mountain trench (SRMT) normal fault, which has a stratigraphic separation of <3 km, 15-20 km of horizontal crustal extension was transferred from the basal detachment thrust zone into ductile stretching of continental crust in the neck zone. As the basal detachment steepens to the SW, the Moho rises abruptly from >45 Km to <37.5 km. From the SRMT to the Coast Mountains the Moho is relatively planar and at ~35 km. Extension of supracrustal strata above the cold relatively shallow (<15 km) basal detachment thrust in the Rockies was balanced by equivalent hot ductile crustal extension in the adjacent deeper zone of crustal necking. Extension of supracrustal strata overlying the hot detachment above the boudins also was balanced by equivalent ductile crustal extension in that zone of crustal necking.