Evolution of the Queen Charlotte Basin; further paleomagnetic evidence of Tertiary extension and tilting

Abstract

The Queen Charlotte Islands form the northwestern margin of the Queen Charlotte Basin. The basin, which was formed by extension, contains numerous mid-Tertiary lavas, plutons and dykes, referred to as the Masset Igneous Complex, and Eocene to Present marine and non-marine sedimentary rocks. Throughout its history, the basin has been bounded on the west by a north-northwest oriented transform plate boundary, the Queen Charlotte Fault. Initially, motion along this fault was transtensional (phase 1), then more purely transform (phase 2), and finally transpressive (phase 3). Most of the Masset Igneous Complex appears to have been formed under transtension (phase 1, Middle Eocene through Early Miocene). Previous paleomagnetic studies of the complex (129 sites mainly from the southern Queen Charlotte Islands) and new results herein (102 sites mainly from the northern islands) illustrate magnetizations with inclinations generally steeper than those expected from coeval rocks of continental North America, indicating north–south extension, segmentation and tilting of fault-bound slivers along the western margin of the basin. We argue that this north–south extension occurred in phase 2, during the Middle and/or Late Miocene, long after the main east–west extension of phase 1. Results from the northwest corner of the Queen Charlotte Islands, although commonly anomalous, are not oversteepened, and may delineate a structural domain separate from the rest of the archipelago. As a by-product, this study provides two reference paleopoles for North America – one for the Late Eocene–Early Oligocene from Ramsay Island lavas (78.1°N, 119.9°E, A° 95=7.0°), and one of lesser reliability for the Early Miocene from the lavas of Dinan Bay (83.3°N,137.8°E, A° 95=9.8°). A second by-product is that the magnetic polarities and radiometric ages are consistent with the Cande and Kent global geomagnetic polarity time scale.