Lithospheric structure across the craton-Cordilleran transition of northeastern British Columbia

The lithospheric structure of the transition from the craton to the Cordill era in northeastern British Columbia is interpreted from inversion of seism ic refraction - wide-angle reflection data along a 460-km profile, and from 3-d (3-dimensional) inversion and 2.5-d forward modelling of Bouguer gravi ty data. The seismic profile extends westward from the sediment-covered edg e of cratonic North America across the Foreland and Omineca morphogeologica l belts to the eastern boundary of accreted terranes, beyond the Tintina Fa ult. Across the ancient cratonic margin, the resultant models reveal a west ward-thickening package of low upper crustal velocities (6.2 km/s and less) and low densities to almost 20 km depth below the Western Canada Sedimenta ry Basin, overlying a west-facing ramp of higher velocities and densities i n the middle and lower crust. These features are inferred to represent pass ive-margin sediments deposited on the ancient rifted margin during the mid- to-late Proterozoic and early Paleozoic. A wedge-shaped high-velocity (7.3 km/s) crustal layer at the base of the crust beneath the edge of cratonic N orth America is interpreted to be the result of magmatic underplating durin g rifting. In the Cordilleran Foreland Belt, high velocities (6.4 km/s) in the upper 5 km of the crust indicate rocks upthrust from the middle crust. A narrow trench of low velocities in the near-surface, which is imaged simi lar to 20 km to the west of the inferred location of the Tintina Fault, is interpreted to represent the actual location of the fault or a major splay. From east to west, the Moho decreases in depth from similar to 40 km to si milar to 34 km below the rifted margin of ancestral North America, then def ines a small root at similar to 38 km depth below the high topography and u pper crustal velocities of the eastern Foreland Belt, and gradually shallow s to similar to 34 km beneath the Omineca belt. An enigmatic laterally hete rogeneous upper mantle has anomalously high velocities (up to 8.3 km/s) ben eath the Foreland Belt, flanked by regions of low velocities (7.7-7.8 km/s) . Results indicate that the location of the Cordilleran deformation front w est of the ramped cratonic margin directly affected the tectonic evolution of the region.