CENOZOIC DEFORMATION AND EXHUMATION OF THE COAST MOUNTAINS, MT. WADDINGTON REGION

We integrate new apatite (U-Th)/He ages with structural and paleomagnetic studies to evaluate the exhumation history of the Coast Mountains British Columbia along a 200 km long, northeast-trending transect (52 N latitude). In general, the orogen here shows only minor effects of post-Paleocene faulting, and the pattern of Cenozoic exhumation mimics topography; the high elevation Waddington massif records the youngest ages. NE of Mt. Waddington, ages at equal elevations gradually increase to the NE, and detailed mapping revealed only minor brittle faulting. 7 paleomagnetic sites in Eocene plutons appear to record an undisturbed Eocene field. SW of the Waddington massif, ages at sea level gradually increase toward Vancouver Island, but this age pattern and preliminary paleomagnetic directions are disrupted locally. The most significant disruption is that the age trend reverses 15 km NE of Vancouver Island, with ages gradually younging from 55 Ma to 27 Ma. NE- and NW-striking faulting is concentrated in this area and likely caused the anomaly.

An age-elevation plot of samples from Mt. Waddington suggest an exhumation rate of ~0.23 mm/y from 14 to 4 Ma. Within this period, exhumation rate appears to vary, with ~0.18 mm/yr from 14 to 7 Ma and a slightly higher rate of 0.3 mm/yr between 7 and 4 Ma. Exhumation rates for Mt. Waddington are roughly comparable to erosion rates calculated from 3D thermo-kinematic models, suggesting that glacial erosion could have contributed significantly to exhumation of the massif. Discrepancies between model and He ages at low-elevations also suggest significant, glacially induced, local changes in topographic relief after 7 to 5 Ma.

Overall, the He ages, distribution of faults, and preliminary paleomagnetic results show that Eocene and younger faulting did not contribute significantly to exhumation of the Waddington massif and development of the present day topography. In contrast to the northern Coast Mountains, the subduction zone margin at Mt. Waddington lacks the Miocene and younger extensional faulting prevalent inboard of the Queen Charlotte transform. Significant Late Miocene and younger exhumation did affect the Coast Mountains at both plate margins, and may reflect the intense glacial erosion of the orogen.