Magnetism and age of the Porteau Pluton, southern Coast Belt, British Columbia: evidence for tilt and translation

Abstract

The Porteau Pluton is a variably foliated quartz diorite to granodiorite intrusion in the southern Coast Belt of the Canadian Cordillera (49.6°N, 123.2°W). 40Ar/39Ar 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 emplaced, uplifted, and cooled rapidly in mid-Cretaceous time. The rocks contain high coercive force (hard) remanent magnetizations with unblocking temperatures between 500 and 600 °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 elongate distribution, in agreement with the predictions of M.E. Beck regarding magnetization acquired during tilting, uplift, and cooling of plutons. No part of the distribution agrees with the direction expected from observations from rocks of mid-Cretaceous age from cratonic North America. The elongate distribution defines the axis of tilt (347° east of north) but not its direction; tilt could have been down toward the east or down toward the west. The former yields an inclination that is 29.0 ± 4.9° 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 anomalously shallow by 14.8 ± 3.9°, corresponding to a displacement 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.