Tectonic Sections Across the Insular Suture (IS), Southernmost SE Alaska

Saleeby, J B (1)

(1)Caltech G.P.S., Mail Code 100-23, 1200 E. California Blvd. Pasadena, CA 91125 United States

Abstract:
A series of tectonic sections is presented for an important segment of the IS. The IS is used here to denote the complex polyphase system of structures which juxtapose the Alexander terrane (AT) to rocks presumed to represent the Paleozoic-early Mesozoic North American continental margin (NA). This segment of the IS has been noted for some time to be characterized by mid-Cretaceous west-directed thrust faults commonly equated to the collisional accretion of the AT. The fundamental structure of the IS as depicted in the sections, however, is a regional flat lying tectonic boundary which originated in Jurassic or earlier times with AT extending eastward beneath NA, on the order of 100 km. The AT-NA structural sequence underwent high-magnitude horizontal extension, with major strain axis sub-parallel to the continental margin, between Late Jurassic and mid-Cretaceous time. Depositional overlap by the Upper Jurassic-Lower Cretaceous Gravina sequence occurred in a basinal setting presumed to have resulted from regional extension. Mid-Cretaceous time was characterized by the emplacement of ultramafic to tonalitic plutons, at least in part within the regional extensional environment. Within this interval of plutonism (~100 Ma), the region began to undergo regional crustal shortening. Much of the study region underwent generally west-directed thrusting, but displacement and strain patterns were complex with significant margin-parallel components. A critical feature of the west-directed thrusting was the out-of-sequence juxtaposition of regional lower plate AT rocks back above NA rocks, rendering the NA rocks as an isolated nappe sequence above AT rocks. East of this out-of-sequence structure, the emergent AT rocks fan over into an east-directed fold-ductile fault sequence which developed in latest Cretaceous-Paleocene time. Further east this structural sequence is steepened and overprinted by the Paleocene-Eocene Coast batholith.

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