UBC Theses and Dissertations

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UBC Theses and Dissertations

Teleseismic imaging : field study in southern Alberta and numerical simulations of inverse scattering Shragge, Jeffrey Chilver


This thesis consists of two parts. Part I presents results from a Lithoprobe teleseismic experiment undertaken across southern Alberta. Relative P-wave delay-times from 293 earthquakes have been inverted for upper mantle velocity perturbations. The recovered model reveals a high velocity anomaly underlying a substantial portion of the southern Hearne Province to depths of 200-250 km which is interpreted as the signature of deep-seated lithospheric structure. This result suggests that, contrary to recent tectonic models, the bulk of the lithosphere in this region has remained essentially intact. In particular, it appears unlikely that evidence for extensive lower crustal melting is due to wide-scale lithospheric delamination. However, observed high mantle conductivity may be the result of small volumes of connected hydrous minerals or some other conductive species introduced during subduction that contributed to the construction of a root. Multi-event SKS-splitting results yield an average delay-time of 0.82±0.30 s and fast polarization direction of 45°±8° which broadly coincides with both the presumed orientation of fossil strain fields related to the ca. 1.8 Ga NW-SE shortening of the Hearne Province and absolute North America plate motion. Processing of receiver functions yields Moho depth estimates which are fairly uniform (~38 km) beneath northern stations, but show crustal thickening (>40 km) within the Medicine Hat Block. Part II investigates the formal inversion of synthetic teleseismic P-coda waves for subsurface elastic properties using an asymptotic method which assumes single-scattering. The model comprises an idealized lithospheric suture zone. Two dimensional, pseudo-spectral synthetic seismograms representing a plane P-wave incident upon this structure are preprocessed to extract an estimate of the scattered wavefield. These data are employed in a series of experiments that examine the dependence of multi-parameter inversion on a range of input parameters and demonstrate: i) the contrasting sensitivity which forward- and back-scattered waves display to structural recovery; ii) the diminution of the problem null-space accompanied by increased source coverage; iii) improvements in model reconstruction achieved through simultaneous treatment of multiple scattering modes; and iv) the robustness of the approach for data sets with noise levels and receiver geometries that approach those of field experiments.

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