AN: T71E-04
TI: Terranes and Terrane Boundaries Along the West Flank of the Coast Mountains Near Portland Canal, SE Alaska
AU: RF Butler
AF: Dept. of Geosciences, University of Arizona, AZ 85721; 520-621-6026
EM: butler@pmag.geo.arizona.edu
AU: GE Gehrels
AF: Dept. of Geosciences, University of Arizona, AZ 85721; 520-621-6026
EM: ggehrels@geo.arizona.edu

AB:
Previous paleomagnetic studies conducted near and south of Prince Rupert, B.C. revealed discordant paleomagnetic directions from Cretaceous plutons (Captain Cove, Stephen's Island, Ecstall, and Butedale) west of the Coast Shear Zone. These discordant directions have been interpreted to indicate either: (1) systematic NE-side-up tilting of Cretaceous plutons during their uplift history or; (2)-3000 km northwards translation from lower mid-Cretaceous paleolatitudes accompanied by clockwise vertical-axis rotation. Concordant paleomagnetic directions were observed from Late Paleocene and Eocene plutons (Ponder, Kasiks, and Quottoon) east of the shear zone. These data suggest that the major tectonic disturbance responsible for paleomagnetic discordance in the older plutons (either NE-side-up tilting or latitudinal transport) occurred between mid-Cretaceous and Eocene time and is located on or near the Coast Shear Zone. To test the regional significance of this pattern of paleomagnetic observations in the Coast Mountains of southeast Alaska and western B.C., detailed paleomagnetic investigations are being undertaken on plutonic rocks along the ACCRETE transect. Paleomagnetic sampling during the 1996 field season has covered the full range of pluton ages along one quasi-continuous exposure, from Cretaceous plutons west of Portland Canal, through the Coast Shear Zone, to Cenozoic plutons near Stewart, B.C. Samples were collected from: 47 paleomagnetic sites (> 8 samples/site) in the mid-Cretaceous Dundas Island pluton; 49 sites in the Paleocene Quottoon pluton in Steamer Passage and Portland Inlet: and 36 sites distributed amongst the Russeau Range, Davis River, and unnamed Cenozoic plutons in the cental and eastern portions of Portland Canal. In conjunction with the ACCRETE seismic, petrologic, structural, and geochronologic studies of these plutonic rocks which will provide independent control on the emplacement, cooling and uplift history, the current paleomagnetic and rock-magnetic studies may yield observations critical to solution of the "tilt versus translation" debate.