THE THERMOCHRONOLOGICAL RECORD OF TECTONICS AT THE PACIFIC – NORTH AMERICAN PLATE CONVERGENT MARGIN: VANCOUVER ISLAND, CANADA

Plate motions and tectonics along convergent boundaries influence the uplift and erosion processes along the North American Cordilleran margin. Over the past 60 Myr, this active margin has experienced oblique and convergent motions of the Kula, Farallon and Pacific plates, accompanied by terrane accretion, subduction and translation. The exhumation response to these tectonic events has been examined on the Coast and Olympic Mountains and Haida Gwaii, but the record in the intervening region of Vancouver Island remains unclear. We use low-temperature thermochronology of bedrock on Vancouver Island to determine the exhumation patterns and response to the Cenozoic tectonic events along the boundary. Here we report 28 apatite (U-Th)/He (AHe) mean ages, 7 zircon (U-Th)/He (ZHe)mean and 19 apatite fission track (AFT) central ages. We see mainly Oligocene AHe ages (12 to 45 Ma) across the island. Older ZHe ages of 136 to 147 Ma are recorded in the northern end of the island in contrast with much younger ages of 31 to 36 Ma in the central-southern part. The AFT central ages range from 17 to 95 Ma, mainly reflecting cooling processes during Paleocene to Eocene. Based on these data, we estimate thermal history models and discuss the drivers for the temporal and spatial patterns of exhumation of Vancouver Island. The results suggest that the central-southern part of the island experienced a faster exhumation during late Cretaceous to Eocene than the northern part. The island has experienced consistent and slow exhumation since around 20 Ma, indicating a steady uplift history since then. There is no apparent thermal effect of slab window development nor post-accretionary magmatism on the cooling history of the crust of Vancouver Island. Our thermal history models explore reasons for the different age spectrum and exhumation history for Vancouver Island compared with surrounding regions such as Haida Gwaii and the Olympic or Coast Mountains.