Late Cenozoic upper-crustal cooling history of the Shuswap Metamorphic Complex, southern Canadian Cordillera, British Columbia: new insights from low-temperature multi-thermochronometry and inverse thermal modeling

Abstract

The Shuswap Metamorphic Complex (SMC), located in the southern Canadian Cordillera, transitioned from a contractional orogen in the Early Jurassic to Late Cretaceous to an extensional tectonic province in the Eocene. Normal faulting of the brittle upper crust and ductile thinning of the mid- to lower crust accommodated crustal extension and led to the development of metamorphic core complexes, such as the SMC, along the length of the Cordillera. Several models have been proposed to explain the onset of Eocene extensional deformation, including post-orogenic collapse, delamination of the lithospheric mantle, and channel flow. Determining the timing of activity of the main extensional shear zones bounding the metamorphic core complexes is a first step to discriminate between these different geodynamic models.

The study area encompasses the SMC, which includes some of the most deeply exhumed rocks of the southern Canadian Cordillera, and the underlying Monashee complex, where North American basement gneisses are exposed. At the latitude of this study, the SMC is bounded to the west by a low angle normal fault, the Eocene WNW-dipping Okanagan Valley shear zone (OVsz), and to the east by the Columbia River fault, a brittle-to-ductile Eocene east-dipping extensional fault zone. Twenty bedrock samples were strategically collected along a 50 km-long transect across the SMC in order to characterize the upper-crustal cooling history of the SMC and constrain the timing of activity of its bounding structures, including the OVsz and the Columbia River fault, using low-temperature multi-thermochronology combined with inverse thermal modeling.

We present a new dataset consisting of 12 apatite and five zircon (U-Th)/He (AHe and ZHe, respectively) and 15 apatite fission track (AFT) cooling ages. Inversion of this dataset using HeFty software suggest that along the OVsz and across the SMC, rapid cooling at rate of 15-30°C/Ma lasted from ~60 Ma until the Late Eocene (~37 Ma) and slowed down at rates of 2-5°C between ~37 and ~10-5 Ma. The structural distribution of AFT and AHe cooling ages suggest that displacement on the OVsz continued until 10-5 Ma, which is much younger than the 45 Ma ages previously proposed in the literature. This long protracted cooling of the SMC and sustained displacement on the OVsz can support a model of post-orogenic extensional collapse of the Canadian Cordillera.