Tertiary Exhumation and Tilting, Coast Mountains, British Columbia: Preliminary Results
Rusmore, M E (1), Farley, K A (2), Bogue, S W (1)
(1) Occidental College, Dept. Geology, Los Angeles, CA 90041 United States; (2) Cal. Inst. Tech., Div. Geol. and Planet. Sci. Pasadena, CA 91125 United States
Abstract:
Combined apatite (U-Th)/He dating, paleomagnetic, and structural studies seek to assess models of Tertiary tilting and exhumation in the Coast Mountains of British Columbia, ~lat 54. The measured closure temperature for He is ~73C on apatites from the Ecstall and Quottoon plutons, so ages constrain the most recent cooling. Apatite He ages of 18 samples on a 35 kmlong, orogen-perpendicular transect on Douglas Channel are uniformly $<$15 Ma. Sea-level samples yield almost invariant He ages of 5 +/- 0.6 Ma. In contrast, in a vertical transect He ages are correlated with elevation and increase by ~ 4 Myr/km. These observations require substantial post middle-Miocene exhumation, probably >3 km since 15 Ma. The data could be explained by steady and ongoing exhumation at a rate of ~ 0.25 mm/yr. The apatite ages show no evidence for either tilting or vertical faulting in the NE portion of Douglas Channel. In contrast, preliminary paleomagnetic results on the Quottoon show evidence of earlier rotation and tilting. As noted previously by Symons, declinations follow a large-scale, s-shaped bend ("Hawkesbury warp") in the trend of the orogen. Variable inclinations are inconsistent with either uniform or "venetian-blind" models of block tilting. Widespread brittle faults and mafic dikes record Miocene and younger deformation of the range. Fault kinematics and orientations are consistent with formation on a dextral transcurrent margin, rather than NE-directed low-angle normal faulting. The dikes dip steeply and strike generally NE. Their origin is unclear. Overall, our preliminary data suggest early Tertiary tilting and rotation, followed by slow exhumation of a broad, stable crustal block fractured by minor faults related to the transform margin.
