Early Miocene or Younger Normal Faults and the ACCRETE Seismic Survey, Northwest Canadian Cordillera

Evenchick, C A (1), Crawford, M (2), McNicoll, V J (3)

(1) Geological Survey of Canada, 101-605 Robson Street Vancouver, BC V6B 5J3 Canada; (2)Bryn Mawr College Bryn Mawr, PA 19010 United States; (3) Geological Survey of Canada 601 Booth Street Ottawa, ON K1A 0E8 Canada

Steeply dipping (N10W) brittle faults cut early Tertiary and older rocks in the Coast Belt of the Canadian Cordillera along and east, and west of, Portland Canal, in the vicinity of the ACCRETE seismic project. Faults are common, but few can be demonstrated to have accommodated significant displacement. Movement was focused on two north-trending faults. The Big Dam Fault is a brittle fault zone 2 km wide which cuts ca. 53-61 Ma granite in Observatory Inlet. Offset contacts in a pendant within the Coast Belt indicate west-side-down displacement. Farther west, the Portland Canal Fault separates migmatitic granitoid rocks and ca. 22 Ma massive granite on the west from greenschist facies rocks, massive ca. 58-60 Ma granite, and ca. 22 Ma volcanic rocks on the east. The difference in metamorphic grade, and position of volcanics 1700 feet above a coeval, and assumed comagmatic pluton, requires that Portland Canal Fault is an east-side-down fault of post-22 Ma age. Parallel faults in early Tertiary granite between northerly projections of the two faults are steeply east and west dipping, and extensional. Dip directions of the Big Dam and Portland Canal faults are unknown, but the faults are assumed to be related to the other post-early Tertiary faults of similar character, and therefore extensional. The Portland Canal Fault is approximately in the plane of the ACCRETE seismic line for at least 20 km, perhaps 50 km, and crosses the line in at least 2 places. Similar faults have been documented farther west which obliquely intersect the seismic line.