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 Proterozoic Elba quartzite. Well-exposed sections of this mineralogically simple (quartz + white mica) quartzite allow detailed sampling of the approximately 100 m thick quartzite mylonite. Based on temperature calculated from oxygen isotope values for quartz-muscovite pairs and on stress estimated from recrystallized grain sizes, strain rate estimates range from 10^-12 to 5 * 10^-14 s^-1. Temperature and strain rate self-adjusted to maintain near-constant stress conditions. Quartz grains uniformly display extensive recrystallization by subgrain rotation and some grain boundary migration. Quartz crystallographic preferred orientation measured using EBSD (15 samples) shows nearly symmetrical patterns indicative of dominant pure shear. The main intracrystalline slip systems are basal-a, rhomb-a, and prism-a, as indicated by girdles developed normal to lineation.

Throughout the detachment shear zone, deformation lamellae affect 20 to 50% of quartz grains. Lamellae are straight to slightly curved, image well in cathodoluminescence, and crosscut subgrain boundaries, suggesting they formed late relative to quartz deformation-recrystallization. The poles to lamellae planes as measured on the universal stage form two cones centered on the lineation. The spacing of deformation lamellae, which was evaluated using a universal stage, is a calibrated paleopiezometer (Koch and Christie, 1981). Lamellar average spacing (determined by the intercept method on at least 20 grains per sample) ranges between 6.9 ± 1.4 and 11.3 ± 3.1 µm from the top to the base of the section. Using the Koch and Christie (1981) paleopiezometer we estimate the differential flow stress ranging from 53 to 20 MPa from the top to the base of the section. These values are slightly smaller and overlap with stress estimates based on recrystallized grain size. Using the same flow law parameters and a temperature estimate of 400°C, we obtain a strain rate ranging from 10^-13 s^-1 (top of section) to 7 * 10^-15 s^-1 (bottom of section). This upward flow stress increase is consistent with the final stages of ductile deformation in the detachment shear zone, as deformation migrated into the brittle detachment that overlies the mylonitic section.