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Box, S.E., 1985

Mesozoic tectonic evolution of the northern Bristol Bay region, southwestern Alaska

Bibliographic Reference

Box, S.E., 1985, Mesozoic tectonic evolution of the northern Bristol Bay region, southwestern Alaska: University of California, Santa Cruz, Ph.D. dissertation, 163 p., illust., maps.


The northern Bristol Bay region can be divided into three tectonostratigraphic terranes (from southeast to northwest): (1) the Togiak terrane, comprising a Late Triassic through Early Cretaceous intraoceanic volcanic arc edifice and flanking basins, (2) the Goodnews terrane, a chaotic melange of mostly oceanic lithologies of late Paleozoic and Mesozoic age, and (3) the Kilbuck terrane of Precambrian metamorphic rocks of continental origin. Igneous growth of the Togiak arc terrane was interrupted in the late Early Jurassic and the earliest Cretaceous by unconformity-producing deformation events. Each event correlates in time with structural emplacement of parts of the unrelated Goodnews terrane beneath the northwest flank of the Togiak arc terrane. Blueschist metamorphism is associated with the first of these collisional events. Final emplacement of the amalgamated Togiak-Goodnews terranes onto the continental Kilbuck terrane in the middle Early Cretaceous resulted in: (1) cessation of arc volcanism, (2) northwest-directed thrusting, and (3) low-grade (locally blueschist facies) metamorphism of the Kilbuck and basal Goodnews terranes. The amalgamated Kilbuck-Goodnews-Togiak terranes were displaced (right-laterally?) against the volcanic Nyac terrane to the northwest in Late Cretaceous time. The northern Bristol Bay region occurs at the southeastern end of a sinuous 4,000-km-long arc-continent collisional belt in northwestern Alaska and northeastern USSR. The belt is divided across strike into five fault-bounded structural components (from highest to lowest): (1) an Early Cretaceous andesitic terrane, (2) a structurally complex terrane of upper Paleozoic and Mesozoic mafic igneous rocks, (3) an Early Cretaceous dynamothermal metamorphic belt, (4) a belt of imbricated upper Paleozoic and Mesozoic continental slope sedimentary rocks, and (5) a deep Early Cretaceous foreland basin. Collisional emplacement of the intraoceanic volcanic arc was completed by middle Early Cretaceous time and is correlated in space and time with the Eurasian-North American continent-continent collision. Analogy with modern arc-continent collisions in the western Pacific Ocean suggests that much of the sinuousity of the belt resulted from bending of the impinging volcanic arc around irregularities in the passive North American margin.

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