104 Structural Geology (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

THE OKANAGAN VALLEY FAULT AND OKANAGAN GNEISS: NEW HIGH-PRECISION AGE CONSTRAINTS on REGIONAL EXTENSION, METAMORPHISM, AND EXHUMATION

Paper number: 104-13
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

The Eocene Okanagan Valley shear zone (OVSZ) delineates the western margin of the Shuswap metamorphic complex (SMC), juxtaposing sillimanite-grade crystalline rocks against dominantly non-metamorphosed sedimentary and volcanic rocks. The boundary between these two disparate packages is a 1.5 km thick, moderately-west-dipping ductile shear zone bounded by a brittle detachment surface, the Okanagan Valley fault (OVF). Linear fabric elements and kinematic criteria indicate that the hanging wall moved to the WNW; structural features include a strong, penetrative mylonitic fabric, an array of high-angle normal faults, and locally, foliation-parallel pseudotachylites. Previous estimates of extension across the OVSZ range from 32-90 km; however, this has been challenged in adjacent study areas where the absence of a major detachment is proposed. Extension across the southern OVSZ (this study) is estimated at 29-86 km (probably >50 km) based on the shear zone geometry and new P-T estimates (non-retrograded garnet- pelites equilibrated at ~20 km depth). This study confirms the regional importance of the OVSZ as a major, Eocene, crustal-scale detachment.

The contemporaneous Okanagan gneiss is only within the shear zone where it is folded, stretched, transposed, and mylonitized. It is composed of both orthogneiss and paragneiss domains; the paragneiss domain is mainly composed of amphibolite-facies (sillimanite-grade), probably marine, siliciclastic metasedimentary rocks, with lesser volumes of metabasic rocks (~160 Ma U/Pb zircon crystallization ages) and migmatite. Leucosomes within the migmatite have crystallization ages of 53-50 Ma. Some zircons within the paragneiss domain record regional metamorphism at ~98-93 Ma. The protoliths of the orthogneiss were Jurassic to Eocene granodiorite plutons. The Okanagan gneiss is cross-cut by a suite of felsic intrusions; many are strongly, ductily deformed, while others are demonstrably post-ductile deformation and only exhibit a weak mylonitic fabric. The range of ages of all the felsic intrusions are indistinguishable at 52–50 Ma. Contrary to previous studies, the development of the Okanagan gneiss is demonstrably Eocene and genetically-related to the OVSZ; the gneiss is not an exposed slice of Precambrian cratonic basement, unlike others within the SMC.