Collision with Yakutat pushes Yukon northward

The St. Elias mountain belt is created by the convergence of a small crustal fragment (Yakutat microplate) which is tectonically forced against other terranes in southern Alaska. At least 10 million years ago this fragment split off the coast of BC or Washington state and was transported north along the Queen Charlotte transform system. For the last 5 million years it has been jammed into the corner of the Aleutian subduction zone because its rocks are less dense than the oceanic volcanic rock, and are too buoyant to subduct (see Pavlis et al., 2004). This collision dramatically uplifts and deforms in the adjacent St. Elias Mountains; it also results in deformation and thrust faulting more than 600 km to the northeast– in the Mackenzie Mountains along the Yukon-Northwest-Territories border. This surprising result is confirmed by precision GPS tracking of survey monuments in southern Yukon, and the orientations of first motions during small earthquakes in the Wernecke, Richardson and Mackenzie Mountains.
Oblique satellite image of northwestern North America.
relative tectonic motion and areas of uplift
Credit: R. Hyndman, Geological Survey of Canada

Repeat surveys of benchmark bedrock sites across the Yukon are made every few years to track gradual movement. The first motions of small earthquakes are to the north and east and subhorizontal, indicating east- and north-directed thrusts. Both indicate to-the-north-northeast movement of southern Yukon of about 5 mm/year relative to the Canadian Shield. They imply that beneath southern Yukon the continental crust is unusually thin (15 instead of 50 km) and gliding on a weaker layer. Although these movements are small in terms of human lifetimes, they are surely changing the shape of northwestern North America, resulting in the continued uplift of our youngest mountain ranges.