QUEEN CHARLOTTE AREA CENOZOIC TECTONICS AND VOLCANISM AND THEIR ASSOCIATION WITH RELATIVE PLATE MOTIONS ALONG THE NORTHEASTERN PACIFIC MARGIN

Cenozoic relative plate interactions along the Queen Charlotte margin, based on published models for the North America and northeast Pacific plates, are related to the tectonic and igneous history of the region . Plate motion models indicate convergence and subduction prior to the Eocene. Since that time, there has been transcurrent motion with vary ing small amounts of oblique extension or compression, assuming that t he Pacific-America-Farallon triple junction was to the south of the re gion. A late Miocene or early Pliocene (about 4 Ma) to present period of oblique convergence is well resolved. The present Queen Charlotte f ault zone along the west coast of the islands may have initiated more recently. Within the Cenozoic transcurrent regime, a period of oblique extension in the mid-Tertiary (36-20 Ma) in one model is supported by a variety of volcanic and tectonic evidence. A small plate motion cha nge at about 20 Ma may have resulted in the transfer of the Yakutat te rrane from the North America plate to the Pacific plate and the result ing motion of the terrane northwestward to its present position along the margin of Alaska. The onset of Tertiary Masset volcanism that is e xtensive on the Queen Charlotte Islands corresponds within a few milli on years to the time of major plate reorganization at 43 Ma that has t ectonic expression around the entire Pacific basin. The period of most extensive Masset volcanism and plutonism appears to correlate with th e model oblique extension in the mid-Tertiary for one model. The geoch emistry and physical volcanology of the Masset volcanics are indicativ e of an extension regime. The main syntectonic deposition in the Queen Charlotte Basin (Skonun Formation) as inferred from seismic reflectio n and well data also appears temporally correlated with this time inte rval of oblique extension. A variety of data, including plate models, dikes, normal faults, basin subsidence, crustal thickness from seismic refraction, and present and paleo-heat flow, indicate mid-Tertiary cr ustal extension of at least 20% in the Queen Charlotte region, with up to 150% (beta up to 2.5) in the main Queen Charlotte Basin. Posttecto nic basin subsidence and deposition may correlate with the model time interval of general transcurrent or oblique convergence motion from 20 to 4 Ma. More recent shortening deformation observed in outcrop and s eismic sections of the northern parts of the basin may correlate with plate model oblique convergence from 4 Ma to the present. The latter c onvergence is associated with underthrusting that formed a trough or t rench and an accretionary sedimentary prism off the west coast of the Queen Charlotte Islands and with inferred uplift and erosion of the we stern part of the islands.