Detrital blueschist‐Facies metamorphic mineral assemblages in Early Cretaceous sediments of the foreland basin of the Brooks Range, Alaska, and implications for orogenic evolution

Abstract

Detrital metamorphic minerals found in the foreland basin of the Brooks Range were derived from blueschist‐facies rocks that now occupy the range's metamorphic core. The metamorphic minerals occur in the Torok Formation and the Nanushuk Group; they are contained in sediments at least as old as middle Albian in age, and may have arrived earlier. The source of the minerals is the blueschist‐facies schist belt, the structurally highest, more internal of two structurally and metamorphically distinct belts that occupy the metamorphic core of the Brooks Range. The blueschist facies rocks of the schist belt were exhumed between Late Jurassic and middle Albian time, during the major pulse of contractional deformation in the Brooks Range orogeny, rather than during late stage extensional collapse or postcontractional isostatic rebound, as suggested by other workers. The processes by which the blueschist‐facies rocks were uplifted and denuded must have operated during contractional tectonism. Post peak metamorphic structures in the blueschist‐facies rocks support their uplift by thrust systems; their unroofing apparently occurred by erosion and possibly by small‐scale extensional faulting.