COMMON JURASSIC TO EOCENE TECTONIC EVOLUTION OF THE SOUTHERN CANADIAN ROCKY MOUNTAINS FOLD-AND-THRUST BELT AND OMINECA BELT; EVIDENCE FROM 40AR/39AR DATING

⁴⁰Ar/³⁹Ar illite dating of thrust-fault gouge and muscovite, biotite and feldspar ⁴⁰Ar/³⁹Ar dating of phyllonite show that eastward propagation of the southern Rocky Mountain fold-and-thrust belt (RM-FTB) in Canada occurred in four orogenic pulses (Late Jurassic, mid-Cretaceous, Late Cretaceous and early Eocene), which correlate with changes in depositional patterns in the foreland to the east and with tectonothermal events in the Omineca belt to the west.

Thin-skinned deformation and uplift in the SE Omineca is partly contemporaneous with the Late Jurassic Pyramid (163 Ma), Simpson Pass (162 Ma) and Johnson Creek (146 Ma) thrusts in the Main Ranges, and with the first clastic wedge in the foreland basin (upper Fernie and Kootenay/Nikanassin/ Minnes sequences). The Early Cretaceous tectonothermal event of the SE Omineca corresponds to the 136-123 Ma tectonic phase along SRMT shear zone and Walker Creek fault zone, but is only weakly expressed in the westernmost RM-FTB (135 Ma Moose Pass and Monarch thrusts, and 139 Ma Chancellor fault); in the foreland, this tectonic phase appears to have triggered the “sub-Cretaceous unconformity” followed by the extensive deposition of the Cadomin conglomerate. The mid-Cretaceous tectonothermal event in SE Omineca is quasi-contemporaneous with the 111-96 Ma transpressional tectonic phase identified along SRMT shear zone and Walker Creek fault zone, with the emplacement of the Greenock thrust (103 Ma) and Broadview/Snake Indian thrust (99 Ma) in the Front Ranges, and with the deposition of the deltaic Dunvegan Formation in the foreland. In the Front Ranges, the Rocky Pass (75 Ma), Sulphur Mountain (76 Ma), Clearwater (74 Ma), Rundle (73) and Lewis (72 Ma) thrusts, define a Campanian phase of tectonic loading, contemporaneous with the 74-72 Ma peak of transpressional tectonism along SRMT shear zone and Valemount strain zone, and with the Bearpaw transgression in the foreland. Along the eastern margin of the Front Ranges, the McConnell thrust (54-51 Ma), together with the Brule (54 Ma), Muskeg (52 Ma) and Nikanassin (52 Ma) thrusts in the Foothills, record an early Eocene, last phase of regional contraction, whereas the latest Cretaceous to early Eocene tectonothermal events in the Omineca belt and along the SRMT, and the depositional pattern in the foreland appear more continuous.