Preliminary Paleomagnetic Results from Early Tertiary Plutons of the British Columbia Coast Ranges, Mt. Waddington Area.

Abstract

New paleomagnetic results from two early Tertiary granodioritic-to-tonalitic plutons suggest that NE-side-up tilting is not ubiquitous in the Coast Ranges of British Columbia. The study area, located ~350km NNW of Vancouver, extends northeastward across the Coast Ranges from Mt. Waddington. At each of 11 sites we used a gasoline-powered drill to extract 7 one-inch diameter core samples. The orientation of each core was measured using either a sun compass or a magnetic compass. After a preliminary thermal demagnetization experiment, 7 of 11 sites exhibited scattered remanence directions. More careful work is needed to establish the primary paleomagnetic direction of these sites. Four sites yielded consistent directions. Three of these exhibited site-mean directions (two normal; one reverse) close to the expected early Tertiary field direction. The fourth site exhibited a normal polarity remanence that is ~45 degrees clockwise and 15-20 degrees shallow compared to the expected direction. Combined with previous results from Douglas Channel (400 km. to the NW) we see no evidence of widespread NE-side-up tilting within the Coast Ranges of British Columbia. This finding indirectly supports the "Baja BC" hypothesis, which interprets the shallow remanence of older plutons as the result of tectonic transport rather than local tilting.