Abstract

Apatite (U-Th)/He ages reveal three distinct periods in the exhumation history of the northern Coast Mountains (∼54°N). A well-developed helium partial retention zone indicates little or no exhumation between ca. 30 and 10 Ma. Beginning at 10 Ma and extending to at least 4 Ma the range underwent steady but slow exhumation of ∼0.22 mm/yr, after which the exhumation rate increased by at least 70%. An 85-km-long He age traverse across the orogen at sea level shows that vertical offsets on post-10 Ma faults are minor. Furthermore, the sea-level He ages (14–2.8 Ma) inversely correlate with local mean elevation along the traverse. These data suggest that the Coast Mountains first appeared as a significant topographic feature only within the past few million years. This history is not consistent with suggestions that uplift of the range resulted from formation of the Queen Charlotte basin in early-middle Miocene time. We speculate that intense erosion by alpine and continental glaciation after 2.5 Ma triggered isostatic uplift and creation of the modern topography of the Coast Mountains.

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