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

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. Detrital zircon studies of continental basin sediments demonstrate that the synextensional Raft River Basin, bounded by the Albion–Raft River fault system, began to develop along the eastern side of the ARG metamorphic core complex ca. 13.5 Ma, synchronous with footwall cooling and uplift between 13.5 and 7 Ma recorded by apatite fission track ages. The evolution of sediment sources in the Raft River Basin help define three phases of Miocene tectonism. (1) Between 13.5 and 10.5 Ma, rapid slip on the Albion fault, which rooted into a ductile-brittle transition zone represented by the Raft River detachment, exhumed Paleozoic strata that, together with Miocene volcanic rocks, sourced the basin. (2) Between 10.5 and 8.2 Ma, continued slip resulted in a topographic depression filled with volcanic rocks and detritus derived from footwall metamorphic and crystalline rocks as well as prior sources. (3) After 8.2 Ma, the sedimentary basin was cut, rotated, and repeated by a set of younger north-south–striking normal faults that extended the basin in an east-west direction, structurally uplifting the basin sediments to erosion. These younger faults die out to the south and minimally displace the fault system that bounds the metamorphic core of the Raft River Mountains. The Cenozoic evolution proposed for the ARG metamorphic core complex indicates that the formation of Oligocene granite-cored gneiss domes and their high-temperature metamorphic carapace and overlying detachments, are distinctly older (ca. 10 Ma), and thus unrelated to the younger exhumation by high-angle faulting.