What are accretionary terranes in the Boundary Ranges?

In the Boundary Ranges of southeastern Alaska and northwestern British Columbia, accretionary terranes are massive zones of exotic rock types that were rafted into their current positions due to tectonic stresses along plate boundaries.1

These distinctive rock formations represent pre-Cenozoic blocks that were shifted northward along the trace of ancient island arcs. They are found at significant distances inland, including on the interior Canadian flanks of the Juneau and Stikine ice fields. This process of accretion is a fundamental aspect of the region's complex geological history, where fragments of crust were independently added to the North American continent over time.12