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 varying small amounts of oblique extension or compression, assuming that the Pacific-America-Farallon triple junction was to the south of the region. A late Miocene or early Pliocene (about 4 Ma) to present period of oblique convergence is well resolved. The present Queen Charlotte fault 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 change at about 20 Ma may have resulted in the transfer of the Yakutat terrane from the North America plate to the Pacific plate and the resulting motion of the terrane northwestward to its present position along the margin of Alaska. The onset of Tertiary Masset volcanism that is extensive on the Queen Charlotte Islands corresponds within a few million years to the time of major plate reorganization at 43 Ma that has tectonic expression around the entire Pacific basin. The period of most extensive Masset volcanism and plutonism appears to correlate with the model oblique extension in the mid-Tertiary for one model. The geochemistry and physical volcanology of the Masset volcanic s are indicative of an extension regime. The main syntectonic deposition in the Queen Charlotte Basin (Skonun Formation) as inferred from seismic reflection and well data also appears temporally correlated with this time interval 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 crustal extension of at least 20% in the Queen Charlotte region, with up to 150% (β up to 2.5) in the main Queen Charlotte Basin. Posttectonic 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 seismic sections of the northern parts of the basin may correlate with plate model oblique convergence from 4 Ma to the present. The latter convergence is associated with underthrusting that formed a trough or trench and an accretionary sedimentary prism off the west coast of the Queen Charlotte Islands and with inferred uplift and erosion of the western part of the islands.