Yellowknife Geoscience Forum

The eastern margin of the Northern Canadian Cordillera is characterized by many earthquakes occurring ~1000 km east of the western North American margin. The ongoing mountain-building processes along the active plate margin in Alaska have been suggested to have cause reactivation of old structures at the far eastern side of the Cordillera. In the Mackenzie Mountains the fold-and thrust belt continues propagating to the east, and dextral strike-slip deformation occurs in the Richardson Mountains farther north. 

We investigated the thermal history of rock exposed in the Mackenzie Mountains and the Mackenzie Plain to quantify the timing and amount of cooling and heating related to deformation that caused rock exhumation, erosion and burial. We use (U-Th)/He thermochronology on apatite and zircon grains, which records cooling between 180–40ºC. We found three phases of cooling. The two oldest phases, found only in rocks from the Mackenzie Mountains, record the Cordilleran deformation during the Late Cretaceous (100–75 Ma) and Paleocene–Early Eocene (65–40 Ma). These phases of rock exhumation correlate with the deposition, burial and heating of the Devonian and Cretaceous strata collected in the Mackenzie Plain. The thermal history models of our Mackenzie Plain samples suggest burial and heating up to 140ºC during Paleocene–Eocene time. The third and youngest phase of cooling occurred during Oligocene and early Miocene time (33–20 Ma). Thermal history modeling suggests cooling from 120–80ºC started 33 Ma and by 20 Ma rock was below 40ºC. This young phase of deformation and cooling is recorded in Devonian and Cretaceous strata from the Mackenzie Plain and rock in the eastern Mackenzie Mountains. We recently collected new samples from the Richardson Mountains and preliminary results may suggest a similar Oligocene cooling. 

This new finding of post-Cordilleran deformation in the Oligocene time is unexpected and raises questions regarding the driving force. At that time, the western margin of North America was dominated by strike-slip motion and thus unlikely to cause compression towards the east. We therefore suggest that the change in the North American plate motion may have caused deformation. The final opening of the North Atlantic Ocean to the Arctic caused North America to move west and southwest, which may have caused the underthrusting of the ridged craton underneath the weak sediment cover and the fold-and thrust belt.