Structural and Metamorphic Relations in the Southwest Seward Peninsula, Alaska: Crustal Extension and the Unroofing of Blueschists
Structural and metamorphic relations in the southwest Seward Peninsula, Alaska: Crustal extension and the unroofing of blueschists Kimberly A. Hannula* Elizabeth L. Miller Department of Geology, Stanford University, Stanford, California 94305 Trevor A. Dumitru } Jeffrey Lee* Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 Charles M. Rubin Department of Geology, Central Washington University, Ellensburg, Washington 98926 ABSTRACT role in the exhumation of the blueschist-fa- these interpretations of similar structures is cies rocks but may have followed an earlier an indication of the difficulty of distinguish- An oblique crustal section on the south- period of partial erosional unroofing of ing structures formed during subduction western Seward Peninsula, Alaska, exposes these rocks. from those formed during exhumation of polydeformed, polymetamorphosed rocks blueschists. Careful determination of the that record extensional thinning of the crust INTRODUCTION age of structures and associated metamor- in the Late Cretaceous. The rocks experi- phic grade, along with consideration of tim- enced an early (M1a) high-pressure/low- The preservation, uplift, and exhumation ing of regional events (i.e., brittle supra- temperature metamorphism (pumpellyite- of high-pressure/low-temperature metamor- crustal faulting, basin formation, igneous actinolite facies in the upper part of the phic rocks have long been considered prob- activity, etc., synchronous with blueschist ex- section; blueschist facies at deeper struc- lematic (e.g., Ernst, 1988). Numerous mod- humation), is necessary to understand the tural levels) followed by a greenschist-facies els have been proposed to explain how significance of different generations of struc- overprint (M1b) and accompanied by D1 de- blueschist-facies rocks might be returned to tures.
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