Paleomagnetic Evidence Showing Large-scale Rotation and InternalDeformation of the Paleocene Quottoon Pluton Near Douglas Channel, British Columbia
* Bogue, S W (1), Dodson, K E (1), Rusmore, M E (1), Farley, K A (2)
(1) Dept of Geology, Occidental College, Los Angeles, CA 90041 United States
(2) Cal. Inst. Tech. , Div. Geol. Planet. Sci., Mail Stop 170-25,Pasadena, CA 91125 United States
New paleomagnetic results from rock units in the Canadian Cordilleranear 54N, combined with structural and (U-Th)/He geochronologic studies, constrain the post-Paleocene deformational history of the Coast Mountains. We collected paleomagnetic samples from 19 sites in the tonalitic Quottoon pluton, a unit that postdates all the large-scale tectonic transport of the "Baja-BC" model, and from 12 sites within a widespread NE-striking Miocene dike swarm. All sites are located within the WNW-trending limb of an orogen-scale bend known as the Hawkesbury Warp. It is clear from our new results that the Quottoon pluton underwent significant internal deformation after magnetization. Fifteen sites in the Quottoon pluton yielded well-defined site mean directions. The characteristic remanence exhibited very high coercivity and blocking-temperatures exceeding 600 degrees C. The carrier is likely high-Fe ilmenohematite. The site-mean directions are very well determined; excluding one site with anomalously low precision, the average 95\% confidence angle was less than 5 degrees. The mean direction (from 9 normal and 6 reverse sites; D=299.2, I=66.1) is rotated 50 degrees CCW from the expected 60 Madirection for NAM. This result confirms the pioneering work by Symons who documented the paleomagnetic expression of the Hawkesbury Warp over two decades ago. On a 10-km long, sea level transect roughly perpendicular to the trend of the orogen, site-mean inclinations vary systematically (shallowing by 50 degrees from nearly vertical and then steepening again) and spatially correlate with a similar v-shaped pattern in (U-Th)/He ages. These data implyshort-wavelength deformation that is not readily apparent in outcrop scale structures and could easily be missed by less dense paleomagnetic sampling. Preliminary paleomagnetic results plus regionally consistent orientations of dikes and related topographic lineaments provide evidence that formation of the Hawkesbury warp predated intrusion of the dikes. Structural and (U-Th)/He data suggest that post-22 Ma east-west directed crustal extension documented farther north in the CoastMountains has been minimal along Douglas Channel. We speculate that the Hawkesbury Warp may reflect this orogen-parallel variation in extension.
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