Synmetamorphic structural evolution of the Seward Peninsula blueschist terrane, Alaska

Abstract

Blueschist-facies rocks of the central Seward Peninsula cropout over 8000 km 2. Protoliths were Lower Paleozoic-Precambrian(?) shallow-water miogeoclinal sediments that were metamorphosed during the Middle Jurassic. Thermobarometric estimates yield ‘peak’ metamorphic conditions of 10–12 kbar at 460 ± 30° C. Crystallization of blueschist-facies minerals was synkinematic with development of a transposition foliation. This foliation is parallel to lithologic contacts and is axial planar to recumbent mesoscopic isoclinal folds. These folds are refolded by larger scale recumbent tight to isoclinal folds. Both fold sets have hinges parallel to a well-developed N—S stretching lineation. Sheath folds are also present. The long axes of the sheath folds also parallel the stretching lineation. This deformation was non-coaxial as indicated by microstructures and quartz c-axis fabrics. Folds nucleated, then rotated into parallelism with the stretching direction. Kinematic indicators show unequivocal top-to-the-north shear sense, compatible with blueschist formation during mid-Jurassic collision between the Brooks Range continental margin and a N-facing island arc (Yukon-Koyukuk). Convergence of these two plates is believed to have been nearly N—S (in present co-ordinates).