MAGNETISM AND AGE OF THE PORTEAU PLUTON, SOUTHERN COAST BELT, BRITISH-COLUMBIA - EVIDENCE FOR TILT AND TRANSLATION

The Porteau Pluton is a variably foliated quartz diorite to granodiori te intrusion in the southern Coast Belt of the Canadian Cordillera (49 .6 degrees N, 123.2 degrees W). Ar-40/Ar-39 ages are 95 +/- 5 Ma from biotite and 101.5 +/- 0.7 Ma from hornblende, which, together with an earlier U-Pb zircon age of 100 +/- 2 Ma, indicate that the body was em placed, uplifted, and cooled rapidly in mid-Cretaceous time. The rocks contain high coercive force (hard) remanent magnetizations with unblo cking temperatures between 500 and 600 degrees C, close to those of Ar in hornblende, indicating that remanence was acquired at or close to the hornblende plateau age. The hard remanence directions have an elon gate distribution, in agreement with the predictions of M.E. Beck rega rding magne tization acquired during tilting, uplift, and cooling of p lutons. No part of the distribution agrees with the direction expected from observations from rocks of mid-Cretaceous age from cratonic Nort h America. The elongate distribution defines the axis of tilt (347 deg rees east of north) but not its direction; tilt could have been down t oward the east or down toward the west. The former yields an inclinati on that is 29.0 +/- 4.9 degrees shallower than expected from cratonic observations, corresponding to a displacement from the south of 3200 /- 500 km. The latter reconstruction yields an inclination that is ano malously shallow by 14.8 +/- 3.9 degrees, corresponding to a displacem ent from the south of 1600 +/- 400 km, which is a minimum estimate. It is argued, therefore, that the Porteau Pluton has undergone both tilt and displacement from the south by distances substantially in excess of 1000 km.