Multistage Cenozoic extension of the Albion–Raft River–Grouse Creek metamorphic core complex: Geochronologic and stratigraphic constraints

Abstract

Despite numerous studies on the structural evolution of metamorphic core complexes, there is still little consensus on the set and sequence of processes that bring deep levels of the crust to the surface during extension. This problem is partially related to the fact that core complexes expose polydeformed rocks, the history of which has been challenging to decipher. New geochronological and structural data combined with existing data provide improved insight into the Cenozoic extensional evolution of the Albion–Raft River–Grouse Creek (ARG) metamorphic core complex. The Cenozoic extensional history of the core complex can be divided into several distinct stages based on the geochronology and structure of igneous and metamorphic rocks in the lower plate of the complex combined with the geochronology and regional geologic context of sedimentary and volcanic rocks flanking the complex. Initial volcanism and plutonism was Eocene age (42–34 Ma), related to a regional southward-younging magmatic event. The development of high-temperature (sillimanite grade) metamorphic fabrics and mineral assemblages in footwall rocks was mostly Oligocene (ca. 32–25 Ma), synchronous with the diapiric rise and intrusion of evolved plutons to mid-crustal depths (∼10–15 km), formed by partial melting and remobilization of the deeper crust. There is no evidence for associated volcanism or basin development at the surface during this time span. The metamorphic and plutonic rocks of the core complex apparently remained at depth for ∼10–12 m.y. until the Middle Miocene (ca. 14 Ma), when they were exhumed by Basin and Range faulting.