POLYGENETIC ORIGIN FOR METAMORPHIC CORE COMPLEXES AS EXEMPLIFIED BY THE RAFT RIVER-ALBION-GROUSE CREEK METAMORPHIC CORE COMPLEX

The development of ideas regarding the tectonic significance of metamorphic core complexes (MCCs) has included root zones to Mesozoic thrusts, Mesozoic normal faulting and gravity sliding linked to foreland thrusts, rooted Tertiary extensional detachments, and gneiss dome diapirism during extension. These sometimes-polarized perspectives have often obscured the underpinnings of MCCs as polygenetic tectonic features whose histories share attributes with many, if not all, of these tectonic processes: sites of focused crustal shortening, syn-thrusting Cretaceous extension (nearly ubiquitous), protracted Tertiary low-angle normal faulting, and modification by doming. The Raft River-Albion-Grouse Creek metamorphic core complex serves as an exemplar of their polygenetic nature, and of the complexities in the growth and collapse of the Sevier orogenic wedge.

Focused Late Jurassic to Early Cretaceous burial beneath thrusts (nappes) now internal to the Sevier orogenic wedge and recorded in garnet PT paths dated by the Lu-Hf method, brought Neoproterozoic strata from the base of the passive margin sequence to kyanite-grade metamorphic conditions of ~8 kbar. Mid-Cretaceous orogen-parallel flow led to relaxation of gravity potential energy gradients resulting from focused shortening. Significant and rapid shortening at ~85 Ma along the Basin-Elba fault was shortly followed by Late Cretaceous extensional exhumation along the stacked Mahogany Peaks and Emigrant spring normal faults and decompression. Minor renewed Laramide shortening and metamorphism was accomplished by reactivation of the Basin-Elba fault and isoclinal folding of Late Cretaceous normal faults. Large magnitude Cenozoic extension along stacked low-angle detachment faults occurred episodically in Eocene, Oligocene, and Miocene time, responding to different geodynamic triggers, and spanned Oligocene crustal melting and intrusion. Intrusion of Oligocene plutons into the Eocene Middle Mountains shear zone, whose inception at 53-51 Ma was recently established by Lu-Hf dating of synkinematic garnets, led to doming, around which the distribution of strain is incompatible with diapirism. Thus, many general aspects of historical and current alternative models for MCC formation remain relevant to their polygenetic origin.