Post-10 Ma uplift and exhumation of the northern coast mountains, British Columbia

Apatite (U-Th)/He ages reveal, three distinct periods in the exhumation his tory of the northern Coast Mountains (similar to 54 degreesN). A well-devel oped helium partial retention zone indicates little or no exhumation betwee n ca. 30 and 10 Ma. Beginning at 10 Ma and extending to at least 4 Ma the r ange underwent steady but slow exhumation of similar to0.22 mm/yr, after wh ich the exhumation rate increased by at least 70%. An 85-km-long He age tra verse 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) invers ely correlate with local mean elevation along the traverse. These data sugg est that the Coast Mountains first appeared as a significant topographic fe ature only within the past few million years. This history is not consisten t with suggestions that uplift of the range resulted from formation of the Queen Charlotte basin in early-middle Miocene time. We speculate that inten se erosion by alpine and continental glaciation after 2.5 Ma triggered isos tatic uplift and creation of the modern topography of the Coast Mountains.