THERMOMECHANICS OF A METAMORPHIC CORE COMPLEX: RAFT RIVER MOUNTAINS (NW UTAH)

The Raft River metamorphic core complex in NW Utah is bounded to the east by a Miocene detachment that localized in a Proterozoic quartzite which rests unconformably on Archean crystalline basement. Well-exposed sections of this quartzite allow detailed sampling of the approximately 100 m thick quartzite mylonite that defines the detachment shear zone. Microstructural analysis of this quartzite shear zone provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes.

Microstructural evidence of high-stress microstructures, such as deformation bands, undulatory extension, and deformation lamellae that indicate a lack of recovery in the dislocation creep regime, suggests that the detachment shear zone evolved at peak strength, close to the dislocation creep/exponential creep transition. Isotopic evidence suggests that meteoric fluids played an important role in strain hardening, embrittlement, and probable seismic failure. To evaluate the flow stress history of the detachment shear zone, we combine two paleopiezometers based on quartz recrystallized grain size (RX) and a deformation lamellae spacing (DL). Both piezometers show overlapping results: flow stresses are 40-60 MPa for RX and 20-55 MPa for DL. Using the Hirth et al. (2001, Int. J. Earth Sciences) empirical flow law for quartzite dislocation creep we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10^-12-7 * 10^-15 s^-1. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone.