Preliminary description of the Late Silurian-Early Devonian Klakas Orogeny in the southern Alexander terrane, southeastern Alaska

The Klakas orogeny is a Late Silurian-Early Devonian deformational, metamorphic, and mountain-building event that marks a major change in the geologic history of the southern Alexander terrane. During Ordovician-Silurian time this region was a marine volcano-plutonic province in which volcani-clastic strata and shallow-water limestones were deposited adjacent to andesitic and dacitic volcanic centers. After the Klakas orogeny, shallow-marine sedimentation prevailed with only local volcanism. Manifestations of this orogenic event included: 1) shallow-level brecciation of Ordovician-Silurian rocks on southern Prince of Wales Island, 2) deformation along with greenschist- and perhaps amphibolite-facies metamorphism of Ordovician-Silurian rocks on Annette and Gravina Islands, 3) structural uplift of at least several kilometers during or shortly after the deformation, 4) uplift of mountainous areas with kilometer-scale topographic relief, and 5) deposition of a subaerial to shallow-marine elastic wedge that was shed from these uplifted areas. Reconstructions of the paleogeography and tectonic history of the Alexander terrane during Ordovician through Devonian time reveal that: 1) the eastern (Annette) and western (Craig) subterranes of the Alexander terrane are part of the same tectonic fragment, 2) the deformational fabrics in Paleozoic rocks in the southern part of the terrane are primarily Late Silurian-Early Devonian in age, and not a product of the Late Cretaceous accretion of the terrane, and 3) northeastern Chichagof Island may have been adjacent to southern Prince of Wales Island during Silurian-Devonian time, which suggests that the Chatham Strait fault and related fault systems may have approximately 350 km of post-Devonian right-slip displacement