Geologists working in the western mountains of North America recognize groups of rocks with common backgrounds; for some of these the latitude at which they formed can be determined. Areas of rock with similar geological character and history that have subsequently been displaced are called terranes. Many of these terranes are separated by faults (most are no longer active). Lateral slip has moved some terranes tens to hundreds of kilometres; other faults indicate thrusting of one terrane over another, or the juxtaposed rock types indicate that intervening oceanic crust was subducted as two terranes converged. The map shows the terranes currently recognized in northwestern BC, southwestern Yukon and adjacent Alaska.
About seven rock packages, called terranes, became attached (accreted) to ancestral North America.
About seven rock packages, called terranes, became attached (accreted) to ancestral North America.
Credit: modified from Geological Survey of Canada

The southern St. Elias Mountains are underlain by the Alexander and Wrangellia terranes. They are bounded to the southwest by Chugach and Yakutat terranes which consist of younger seafloor sediments that have been pushed landward and subsequently uplifted. Beneath these are older rocks which may belong to other terranes.

Alexander terrane containes the oldest rocks in the southern St. Elias Mountains. It includes deformed sedimentary rocks with fossils akin to those now in the Canadian Arctic and Ural Mountains, as well as volcanic rocks that probably originated at the edge of a continent during the Cambrian period (about 500 million years ago). Separated from the parent continent by rifting or faulting, this fragment became surrounded by oceanic sedimentary rocks and volcanic islands about 310 million years ago. These younger rocks are now called Wrangellia and widely exposed on Vancouver Island and the Queen Charlotte Islands, but also north of Mount Logan. Wrangellian fossils originated in a tropical sea; its equatorial latitude can be determined by the orientation of minute crystals of iron-rich minerals, which align like compass needles as the rock forms (paleomagnetism). Together with Alexander terrane, Wrangellia moved northward and collided with westward-moving North America about 150 million years ago (Jurassic period).

Chugach terrane is exposed in the mountains south of Mount Logan. Deep-sea volcanic rocks and sediments transported by rivers draining western North America were scraped off the subducting oceanic plate and raised by the Border Ranges and other faults. The Yakutat terrane, also consists of alluvial sediments shed from the proto-St. Elias Mountains and underlie the fast-rising coastal hills and benches.