NEW INSIGHTS ON THE CENOZOIC EXTENSION OF THE SOUTHEASTERN CANADIAN CORDILLERA

The long narrative of southeastern Canadian Cordilleran geology mostly ends in the Eocene with the development of metamorphic core complexes in the Omineca belt (Columbia Mountains) and the last phase of thrusting in the eastern Foreland belt. But what happened after the Eocene? To answer this question, we acquired low temperature thermochronology data crossing large structures such as the Rocky Mountain Trench (RMT) and the Columbia River Fault.

The RMT is a 1500 km long valley that coincides with normal and strike-slip faults separating the Foreland belt (Rocky Mountains) from the Omineca belt in the southern Cordillera. At depth the RMT coincides with a sharp change in lithospheric thickness that possibly represents the western limit of the ancient continental margin. We studied a 100-km-long section of the RMT that coincides with the northernmost exposure of high-grade basement rocks in the Malton Gneiss Complex. To the south is the Columbia River Fault, a brittle-ductile normal fault that bounds the eastern margin of the Monashee Core Complex in the Omineca belt.

New apatite (U-Th)/He and fission-track data reveal several phases of fault reactivation and rock exhumation following the Eocene. We integrate our new results with published thermochronology data and field observations that collectively suggest exhumation continued throughout the Miocene. We show that the Malton Complex exhumed as a horst structure 20–10 Ma bounded by the North Thompson Albreda Fault to the west and the RMT to the east. Normal faulting along both faults continued in the late Miocene, but west side down motion is implied along the RMT. Similarly, the Columbia River Fault was reactivated in the early Miocene (~20 Ma) and continued to exhume the Monashee Complex an additional ~2 km.

We propose that late Cenozoic deformation was driven by the high gravitational potential of the southeastern Canadian Cordillera and the tectonic reorganization along the Pacific–North American margin that resulted in increased transtension. We suggest the Columbia River Fault and the North Thompson Albreda Fault reactivated as splay faults due to dextral motion along the RMT. In contrast, the shallower dipping thrusts of the Foreland belt are unfavourable to slip and explain the general lack of young extension and the higher topography in the Rocky Mountains.