Across the spectacular south face of Mount Logan area lies the Border Ranges Fault. It separates the shale of Chugach formation which underlies the steep spurs and lower elevation nunataks on Seward Glacier, from light coloured granite of Mount Logan batholith that underlies most of the summit plateau. To some extent the dramatic topographic relief of Mount Logan can be attributed partly to the greater erosion by glaciers of the softer shale on the south flank of the mountain, compared to the more resistant igneous rock on the plateau (see photo).
This map shows the distribution of the rock units in the Mount Logan area. The heavy black line is the Border Ranges Fault, separating the Chugach shale (Kvm, because it formed during the Cretaceous period), from the Mount Logan batholith (JKg, which cooled earlier, during the Jurassic period). In two places the Border Ranges Fault is intruded by granite of the King Peak pluton (orange lTg, which cooled during the early Tertiary period). Because the younger pluton is not offset, fault motion must have ceased here more than 50 million years ago.
The Border Ranges Fault can be traced southeastward from nunatak, across the Seward Glacier, toward southeast Alaska where it merges with the Fairweather Fault, a locus of periodic large earthquakes (see later section). Neverthless the Border Ranges Fault is structurally important, because it marks the boundary between old sedimentary rocks formed in a deep tropical ocean (exposed as schist ‘islands’ in the Mount Logan batholith), and younger shale deposited off the west coast of North America. How did rock from these very different environment become juxtaposed in a mountain belt? Some geologists have proposed that the Border Ranges Fault represents an ancient subduction zone. To know that story, we need a plate tectonic perspective and to learn about terranes.