UPPER-MANTLE STRUCTURE OF THE NORTHERN CASCADIA SUBDUCTION ZONE

Previous knowledge of the structure of the Cascadia subduction zone no rth of the Canada - United States border has been derived from a varie ty of geophysical studies that accurately delineated the downgoing Jua n de Fuca plate from the offshore deformation front to depths of appro ximately 50-60 km beneath south-central Vancouver Island and the Georg ia Strait. Little is known, however, of the structure of the Cascadia subduction zone farther westward and to greater depths in the upper ma ntle. We have assembled a set of some 1100 teleseismic traveltimes fro m events recorded on the Western Canadian Telemetered Network to augme nt a previously existing data set recorded on the Washington Regional Seismograph Network. The composite data set is inverted for upper mant le structure below Washington, Oregon, and southwestern British Columb ia. We analyze the new northern portion of the model between 48.5-50-d egrees-N and 118-127-degrees-W, which provides the first images of the deep slab structure in this region. The model is parameterized using splines under tension over a dense grid of knots. The nonlinearity of the inverse problem is treated by iteratively performing three-dimensi onal ray tracing and linear inversion. Resolution tests performed with a synthetic slab model indicate that the deep structure is resolved b y the data north to at least 50-degrees-N. The inversions are characte rized by a quasi-planar, high-velocity body inferred to represent the thermal and compositional anomaly of the subducted Juan de Fuca plate. This body exhibits velocity deviations of up to 3% from the backgroun d reference model and extends to depths of at least 400-500 km. The de pth contours of the slab in the upper mantle mimic those of the shallo w slab by changing strike, in the latitude range 48.0-48.5-degrees-N, from north-south in Washington to northwest-southeast in southern Brit ish Columbia. This forces the development of two arch-type structures: a main arch observed in previous studies trending east-west over Puge t Sound and a possible second arch extending northeasterly from the Ge orgia Strait into the British Columbia interior. A steepening of the d eep slab dip from British Columbia south towards Puget Sound and compl exity in the evolution of the arches in depth may be the result of a c hange in plate motions at 3.5 Ma associated with the detachment of the Explorer plate.