HR: 17:45h
AN: GP54A-08    [Abstracts]
TI: Deformation History of the Paleocene Quottoon Pluton (British Columbia) at 54N: Constraints From Paleomagnetism and Miocene Dike Orientations
AU: * Bogue, S W
EM: bogue@oxy.edu
AF: Department of Geology, Occidental College, 1600 Campus Road, Los Angeles, CA 90041, United States
AU: Rusmore, M E
EM: rusmore@oxy.edu
AF: Department of Geology, Occidental College, 1600 Campus Road, Los Angeles, CA 90041, United States
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: GPS Division, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, United States
AB: New paleomagnetic results from the Paleocene Quottoon pluton (Douglas Channel, Coast Mountains, British Columbia) and the orientations of Miocene dikes that intrude it reveal two episodes of deformation that correlate with regional events. The very-well determined site-mean remanence directions from 15 sites in the tonalite intrusion form two spatially-distinct groups with declinations that are substantially (63 deg and 40 deg) CCW of the appropriate NAM reference direction. The trends of over 100 dikes intruding the Quottoon do not appear to have been dispersed by this differential vertical-axis rotation, strongly suggesting that the deformation (an oroclinal bending known as the Hawkesbury Warp) predated dike intrusion at approximately 20 Ma. The orocline may be the result of southward decreasing amounts of E-W extension related to development of the Queen Charlotte Basin Remanence directions from the Quottoon also show evidence of block tilting manifested as inclinations significantly steeper or shallower than expected. These tilt domains (a few km wide) are smaller in scale than the vertical-axis rotation domains (10s of km wide). Large jumps in site-mean inclinations between tilt-domains occur over distances as short as 0.5 km, suggesting that boundaries between the domains are sharp. Of the 7 domains, 6 are tilted W-side or NW-side up, a pattern that is at odds with alternatives to the "Baja BC" hypothesis that invoke ubiquitous NE-side-up tilting to explain anomalous paleomagnetic results from Cretaceous plutons in the Coast Mountains. Restoring the blocks to their pre-tilt orientations (and applying the same rotations to dikes that intrude them) steepens the average dip of the dikes by 4 deg to 85 deg, suggesting that block-tilting was post-mid-Miocene. Furthermore, tilt magnitudes along a transect perpendicular to the length of the orogen mimic a smooth, V- shaped trend in (U-Th)/He-in-apatite ages, with the location of maximum tilting corresponding to the youngest age (2.8 Ma). This correlation suggests that post-4 Ma erosionally-driven exhumation may have caused the tilting.
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8106 Continental margins: transform
DE: 9350 North America
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting