New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone

Abstract

The Okanagan Valley shear zone delineates the SW margin of the Shuswap metamorphic complex, the largest core complex within the North American Cordillera. The Okanagan Valley shear zone is a major Eocene extensional fault zone that facilitated exhumation of the southern Shuswap metamorphic complex during the orogenic collapse of the SE Canadian Cordillera when convergence at the western margin of North America switched from transpression to transtension. This study documents the petrology, structure, and age of the Okanagan gneiss, the main lithology within the footwall of the Okanagan Valley shear zone, and constrains its history from protolith to exhumed shear zone. The Okanagan gneiss is an ~1.5-km-thick, west-dipping panel composed of intercalated orthogneiss and paragneiss in which intense ductile deformation of the Okanagan Valley shear zone is recorded. New U-Pb zircon ages from the gneiss and crosscutting intrusions constrain the development of the Okanagan gneiss to the Eocene, contemporaneous with widespread extension, intense deformation, high-grade metamorphism, and anatexis in the southern Canadian Cordillera. Thermobarometric data from the paragneiss domain indicate Eocene exhumation from between 17 and 23 km depth, which implies 64-89 km of WNW-directed horizontal extension based on an original shear zone angle of ~15°. Neither the Okanagan gneiss nor its protolith represents exhumed Proterozoic North American cratonic basement as previously postulated. New U-Pb data demonstrate that the protolith for the gneiss is Phanerozoic, consisting of Mesozoic intrusions emplaced within a late Paleozoic-Mesozoic layered sequence of sedimentary rocks.

Quartz Flow Law Revisited: The Significance of Pressure Dependence of the Activation Enthalpy

“…The slip rate along the San Andreas Fault is constrained between 5.7 and 35 mm/year (Fialko, ; Heermance & Yule, ; McGill et al, ; Titus et al, ; van Der Woerd et al, ). The thicknesses of exhumed mylonites vary from 0.2 km to more than 3.5 km, but most mylonite zones are under 2 km (e.g., Brown et al, ; Davis et al, ; Norris & Cooper, ; Schulz & Evans, ). It is unclear if this thickness range that we observe today can represent the active thickness at the time of deformation.…”

Corrugated architecture of the Okanagan Valley shear zone and the Shuswap metamorphic complex, Canadian Cordillera

“…Herein, we refer to the segment south of 51°N latitude as the Okanagan Valley shear zone, including a 1-2-kmthick distributed brittle to ductile shear zone ( Fig. 1; Brown et al, 2012).…”

“…98-92 Ma, followed by exhumation during ca. 60-48 Ma (Brown et al, 2012). The upper plate consists of (1) greenschist-to amphibolite-facies Paleozoic and Mesozoic marine metasedimentary and metavolcanic rocks, and (2) nonmetamorphosed Eocene terrestrial sedimentary and volcanic rocks deposited in supradetachment basins (McClaughry and Gaylord, 2005) and NNE-trending grabens (Suydam and Gaylord, 1997) on the upper plate.…”

A window into the Early to mid‐Cretaceous infrastructure of the Yukon‐Tanana terrane recorded in multi‐stage garnet of west‐central Yukon, Canada

“…Rocks presently in the upper exposed structural levels were buried, heated and exhumed in the Jurassic (Murphy et al ., ; Colpron et al ., ; Gibson et al ., ), while structurally deeper rocks were progressively buried and heated from at least Cretaceous to earliest Eocene (Carr, ; Parrish, ; Crowley & Parrish, ; Gibson et al ., , ; Crowley et al ., ). The deepest structural levels within the core of the orogen, which includes autochthonous and parautochthonous North American crust, were largely exhumed by extensional shear zones in the early Eocene (Parrish et al ., ; Brown et al ., ), marking a shift to a transtensional tectonic setting.…”