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Teleseismic imaging of the southeastern Canadian Shield and Cascadia subduction zone Rondenay, Stéphane

Abstract

This thesis is divided into two parts that illustrate different approaches to multichannel analysis of broadband, teleseismic array data. In Part I, a range of established techniques is applied to data from a teleseismic experiment conducted across the southeastern Canadian Shield. Multi-event SKS-splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93° E± 18° for the region, which is consistent with an earlier interpretation of fossil strain fields in the lithosphere. Profiling of radial receiver functions reveals an abrupt, northward thinning of the crust, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major, pre-Grenvillian episode of lithospheric assembly. P and S-velocity models derived from teleseismic travel-time inversion outline a low-velocity, NW-SE striking corridor that crosses the array at latitude 46°N and lies between 50 and 300 km depth. This feature postdates stabilization of the craton and is postulated to represent interaction of the Great Meteor plume with zones of weakness developed during earlier rifting episodes. Part II demonstrates the application of a new technique for formal, multi-parameter, 2-D inversion of scattered waveforms. Upon discussing a range of practical and numerical considerations, the method is applied to data from the CASC93 experiment undertaken in central Oregon to investigate the structure of the Cascadia subduction zone. Two major features are imaged in the resulting model. The continental Moho appears at approximately 150 km from the coast as a positive velocity contrast near 40 km depth. The layers of the eastward dipping Juan de Fuca plate continue from 20 km depth below the coast to at least 100 km beneath the High Cascades. A disruption of oceanic crust is imaged near 40 km depth, below which crustal expression is considerably diminished. This depth coincides with an increase in plate dip, in addition to enhanced mantle conductivity and seismic reflectivity documented in previous geophysical studies. I interpret these observations to represent the effects of metamorphic reactions within the oceanic crust which lead to dehydration and eclogitization.

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