Structural and metamorphic relations in the southwest SewardPeninsula, Alaska: Crustal extension and the unroofing of blueschists

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An oblique crustal section on the southwestern Seward Peninsula, Alaska, exposes polydeformed, polymetamorphosed rocks that record extensional thinning of the crust in the Late Cretaceous. The rocks experienced an early (M1a) high-pressure/low-temperature metamorphism (pumpellyite-actinolite facies in the upper part of the section; blueschist facies at deeper structural levels) followed by a reenschist-facies overprint (M1b) and accompanied by D1 deformation. A second deformational event, D2, is responsible for the prominent, gently dipping foliation, northwest-southeast–trending stretching lineations, and abundant recumbent isoclinal folds throughout the section. Metamorphism during D2 (M2) varied from extremely low-grade at the shallowest structural levels to upper amphibolite- to granulite-facies within the Kigluaik gneiss dome. Apatite fission-track ages from the shallowest structural levels indicate that cooling below ∼120 °C took place at ca. 70–100 Ma, during the same time span as high-grade metamorphism and pluton intrusion at depth. Significant vertical attenuation of the crustal section apparently took place during D2 deformation, resulting in close spacing of both M1a and M2 isograds and overall thinning of the crust. This extensional deformation played an important role in the exhumation of the blueschist-facies rocks but may have followed an earlier period of partial erosional unroofing of these rocks.


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