UNVEILING THE NORTHERN CORDILLERAN PUZZLE: FROM THE ST. ELIAS TO THE MACKENZIE MOUNTAINS

The ongoing oblique collision of the Yakutat microplate with North America formed the highest coastal mountains on Earth – the heavily glaciated St. Elias Range. This natural beauty, riddled with geological extremes, once captured Terry’s curiosity and resulted in the STEEP project that brought together many of us. This presentation starts in the St. Elias Mountains but will quickly lead you to the less extreme, but still interesting regions located >800 km inboard of the Yakutat plate boundary. Here, the ongoing seismicity in the Mackenzie and Richardson mountains has long been debated. Is the Yakutat collision driving this deformation? Is it gravitation pushing from above, or the mantle dragging from below?

Low-temperature thermochronology measures when and how much rocks cooled in the past. This rock cooling is inferred to result from deformation and erosion. In the St. Elias, thermochronology data are extremely young, testifying that rock exhumation from great depths happened very rapidly. In contrast, the interior of the Northern Cordillera records very little cooling since Paleocene–Eocene times. However, the far eastern regions of the Mackenzie and Richardson mountains reveal Oligocene and Miocene cooling phases. This spatial and temporal pattern has been explained by the plate motion of North America that is driven by the North Atlantic opening. We present a new stress model that tests the various plate boundary forces, mantle traction and gravitational forces as potential drivers of the current deformation. We show that the Northern Cordillera is divided into distinct stress regions. The active regions at the northeastern edge of the Cordillera coincide with the superposition of stresses from both, the Pacific–North American transform boundary and the Arctic/Atlantic ridge push. Despite the large distance from the plate boundaries, the alignment of stress vectors results in deformation, whereby the misalignment explains the seismic quiescence in the interior regions. We review the Cenozoic rock cooling record across the Northern Cordillera which are consistent with the findings of the stress analysis. Our goal is to pay tribute to Terry’s scientific achievements and his excellence as a leader and mentor. We hope to convince him that the less extreme and less active regions are fascinating despite the more subtle geological signals.