Cooling and exhumation of the Shuswap Metamorphic Core Complex constrained by 40Ar/39Ar thermochronology

Abstract

The exhumation history of the Shuswap Metamorphic Core Complex (Shuswap MCC) is deciphered from structural analysis and 4 0 Ar/ 3 9 Ar thermochronology. With the exception of a few samples contaminated by excess argon, the analytical results indicate a consistent range of early Tertiary 4 0 Ar/ 3 9 Ar ages throughout the area. Hornblende yields ages ranging from ∼59-54 Ma; muscovite and biotite ages are clustered between 49.5 and 48 Ma, and are consistent with the timing of closure of K-feldspars, typically ranging from 50 to 43 Ma, except for samples in the immediate footwall of the Columbia River fault that yield ages as young as 26 Ma. These 4 0 Ar/ 3 9 Ar ages, combined with existing U/Pb ages on zircons and monazites, constrain the cooling history at several localities in the Shuswap MCC between Trans Canada Highway 1 and the Thor-Odin dome. The migmatitic core of the complex is affected by a rapid cooling event (∼700 °C to ∼300 °C), from crystallization of zircons through closure of argon diffusion in hornblende and micas, between ∼56 and ∼48 Ma. Based on our previous structural analysis, we propose that this first period of rapid cooling follows an exhumation event associated with the formation of the Shuswap MCC by activation of a low-angle detachment and ductile thinning of a previously thickened and partially molten crust. Multidiffusion-domain thermal modeling of K-feldspar constrains the lower temperature history from ∼350 °C to ∼150 °C and indicates a period of thermal stability from ∼48 to ∼45 Ma, and a cooling event at ∼45 Ma, except for the K-feldspars sampled in the immediate footwall of the Columbia River fault, which indicate rapid cooling at ∼33 Ma. These cooling events are attributed to exhumation accommodated by local activation of high-angle normal faults. Therefore, we distinguish two periods of exhumation in the Canadian Cordillera: (1) Exhumation in Paleocene time related to crustal thinning and tectonic denudation by activation of detachment zones at ∼60-50 Ma, which corresponds to gravitational collapse of the thickened and partially molten Cordilleran crust; and (2) Exhumation related to Eocene regional extension, Basin-and-Range style, at ∼45 Ma, with reactivation of the eastern boundary fault of the core complex, the Columbia River fault, in Oligocene time at ∼33 Ma.