EARLY TERTIARY EXTENSION IN THE CENTRAL KOOTENAY ARC

Early Tertiary extension resulted in formation of a narrow elongate metamorphic high parallel to the Purcell Trench in SE British Columbia. Metamorphic grade ranges from the biotite zone on its flanks to the sillimanite zone in the centre of the metamorphic high. The isograd pattern is controlled by the early Tertiary Gallagher and Purcell Trench normal faults. These faults juxtapose rocks with contrasting metamorphic, structural and cooling histories.

The western boundary of the northern part of the amphibolite-facies belt is marked by the Gallagher fault. Rocks in the footwall of the Gallagher fault were metamorphosed and deformed under conditions of 650-700 °C at ~7 kb in the mid-Cretaceous, and yield early Tertiary 40Ar/39Ar mica cooling ages. Rocks in the hanging wall underwent contact metamorphism at low pressure (3.5-4 kb) during the Middle Jurassic. They produce Jurassic-early Cretaceous mica 40Ar/39Ar ages, older than the age of Barrovian metamorphism and deformation in adjacent footwall rocks.

These differences reflect variation in the tectonic and thermal histories of rocks from different structural levels. Rock belonging to the hanging wall remained at upper crustal levels and were relatively unchanged during the mid-Cretaceous, while underlying rocks experienced penetrative deformation and metamorphic recrystallisation.