THERMOCHRONOMETRIC RECONSTRUCTION OF GEOTHERMAL AND DEFORMATIONAL HISTORY OF LEWIS THRUST SHEET, SOUTHEASTERN CANADIAN CORDILLERA FORELAND BELT

The Lewis thrust, >225 km long and with a maximum displacement of >100 km, forms a major structure in the southeastern Canadian Cordillera foreland. Using thermochronometric indicators [vitrinite reflectance (VR) and apatite and zircon fission track (AFT, ZFT) data] we investigate the time of deformation of the Lewis thrust sheet (LTS), its geothermal gradient history and thickness. AFT temperature history models (ATHM) show rapid cooling of the LTS in late Campanian time (~75±5 Ma). This cooling constrains the timing of onset of displacement on the Lewis thrust and its subsequent denudation. Paleotemperatures derived from VR in Mesozoic Kootenay Group and ZFT data in underlying Mesoproterozoic Appekunny and Grinnell Formations indicate that the LTS succession was overlain by an additional 4-6.5 km of Upper Cretaceous strata. Along with the preserved succession of ~8 km, this suggests that the LTS was at least ~12 km thick immediately prior to thrusting.

Subsequent to folding of the LTS, a fossil AFT partial annealing zone was superimposed on the Akamina syncline. AFT data from this zone, east of the Flathead normal fault, record a subsequent cooling event during Middle Eocene and later time that was coeval with extension on the Flathead fault. AFT data from Lower Oligocene sediments within the Flathead graben preserve the paleotemperature history of their sediment source regions in the LTS, without significant thermal overprinting. Similar styles of ATHM that are consistent with the regional structural history can account for observed variations in AFT parameters at various levels in the LTS and underlying rocks.

The thermochronometric data constrain the late Campanian LTS predeformational geothermal gradient to ~20°-~32°C/km and indicate a significant paleogeothermal gradient decrease to ~8.6°-12°C/km, during thrusting. The present geothermal gradient is ~17°C/km. Geothermal gradient changes are attributed to advective heat transfer by tectonically induced, topographically driven, meteoric water flow. This suggests that complicated heat transfer mechanisms can operate in overthrust belts with important implications for organic maturation history and petroleum systems.