STRUCTURAL AND PETROLOGIC EVOLUTION OF THE SOUTHERN BROOKS RANGE NEAR WISEMAN, ALASKA

Abstract

Three east-west trending, geologically distinct packages, separated by south-dipping low-angle faults, were examined in a transect through the southern Brooks Range fold and thrust belt near Wiseman, Alaska. The southernmost and structually highest package is the Angayucham ophiolitic terrane, made up of steeply dipping Carboniferous basalt flows and pillow basalt with intercalated bedded chert. Structures indicate approximately bedding-parallel, south-vergent shear, probably related to imbrication during obduction; the prehnite-pumpellyite facies metamorphism which affects the Angayucham terrane presumably accompanied imbrication. The Angayucham terrane is structurally underlain by penetratively deformed Lower Devonian metasandstone, phyllite and slate; superposed structures in these rocks are consonant with formation during north-directed contractional deformation. The structurally lowest and northernmost package is the Koyukuk Schist, composed chiefly of quartz-mica schist with minor interfoliated mafic schist, metagabbro, albite schist, marble, calc-schist, metachert (?) and eclogite, derived from pre-Devonian (?) epicontinental (?) protoliths. Metamorphic rocks are exposed in a large, asymmetric antiform, bounded on its northern limb by the south-dipping Minnie Creek thrust. Structural and textural relationships indicate that the Koyukuk Schist was affected by north-vergent ductile shear deformation concurrent with pumpellyite-actinolite to glaucophanitic greenschist facies metamorphism (D$\sb 1$ and D$\sb 2$); the metamorphic grade increases with apparent continuity from south to north. Later north-directed movement on the Minnie Creek thrust (D$\sb 3$) resulted in the uplift and broad-scale arching of the metamorphic terrane. Structural relationships in all three packages indicate a complex history of imbrication, folding, and metamorphism related to continuous north-directed contraction of Mesozoic age; contraction was sufficiently rapid to produce glaucophane- and crossite-bearing assemblages in the Koyukuk Schist. The emplacement of rocks metamorphosed at relatively low pressures above the high-pressure metamorphics of the Koyukuk Schist, and the presence of younger on older structural relationships, indicate that the imbricate stack has been modified by south-dipping low-angle normal faults. Normal faults may have formed synchronous with the uplift of the Koyukuk Schist, or result from a post-compressional period of crustal extension. Movement on normal faults may be responsible for the reorientation of imbrication-related structures in the Angayucham terrane.