The Pacific Rim Terrane in British Columbia is a group of fault-bound forearc metasedimentary and metaigneous rocks subcreted to Wrangellia, comprising three lithological units: the Leech River Complex (LRC), the Pandora Peak Unit (PPU), and the Pacific Rim Complex. Of these three, the LRC and PPU were subject to an elevated thermal metamorphic event which locally overprinted typical low temperature, medium pressure forearc assemblages with low greenschist through amphibolite facies assemblages. The field study shows that biotite, garnet and staurolite isograds occur concentrically in the LRC, centered on the Leech River fault, which separates the Pacific Rim Terrane from the underlying Metchosin Igneous Complex of the Crescent terrane. Local thermal overprint in the PPU is sub-biotitic and is characterized by local replacement of prehnite-pumpellyite and lawsonite-bearing assemblages with muscovite ± chlorite. Multi-method geothermobarometry shows peak metamorphic temperatures from ~230 °C in the northern PPU to ~600 °C near the Leech River fault at ~4 kbar, and isotherms are continuous across the LRC-PPU boundary. The interfoliated Tripp Creek metabasites and Eocene Walker Creek intrusions do not control the distribution of isotherms, and syn-metamorphic felsic sills rarely have contact aureoles. Intercalated metabasites show two distinct rare earth element (REE) patterns, including NMORB-like light REE depletion among most Tripp Creek metabasites, and light REE enrichment in PPU metabasites. The lack of thermal aureoles associated with metabasites, and interlayered garnetite bands with negative Ce-anomalies attributed to seafloor hydrothermal processes, suggest the Tripp Creek metabasites are not syn-metamorphic sills and formed prior to accretion. The subcretion of then recently formed oceanic crust belonging to the Crescent terrane is identified as the probable cause of anomalously high temperature forearc conditions, as well as possible proximity to an Eocene mid ocean ridge. The high temperature metamorphic rocks in the Pacific Rim Terrane document the conversion of inherited primary pyrite to pyrrhotite in carbonaceous metasediments. S-inclusive pseudosections for LRC protoliths predict a low temperature (<420 °C) narrow pyrite desulfidation window that produces pyrrhotite and releases negligible S to the fluid phase. Conversely, sulfide petrography in the LRC shows pyrite can persist up to ~550 °C as inclusions in andalusite and staurolite porphyroblasts, as well as possibly in the rock matrix. S contents in carbonaceous pelites show a marked reduction at medium grade, associated with a dearth of visible sulfide in LRC phyllites. Sluggish pyrite desulfidation, pyrrhotite desulfidation, and terrane-scale S mobility are interpreted as the driver for mobility of intra-terrane sourced Au, leading to the formation of a hypozonal orogenic Au deposit in the central LRC. / Graduate / 2022-06-11

The primary focus of this thesis is to examine the detailed seismic structure of the northern Cascadia margin, including the Cascadia basin, the deformation front and the continental shelf. The results of this study are contributing towards understanding sediment deformation and tectonics on this margin. They also have important implications for exploration of hydrocarbons (oil and gas) and natural hazards (submarine landslides, earthquakes, tsunamis, and climate change). The first part of this thesis focuses on the role of gas hydrate in slope failure observed from multibeam bathymetry data on a frontal ridge near the deformation front off Vancouver Island margin using active-source ocean bottom seismometer (OBS) data collected in 2010. Volume estimates (∼ 0.33 km^3) of the slides observed on this margin indicate that these are capable of generating large (∼ 1 − 2 m) tsunamis. Velocity models from travel time inversion of wide angle reflections and refractions recorded on OBSs and vertical incidence single channel seismic (SCS) data were used to estimate gas hydrate concentrations using effective medium modeling. Results indicate a shallow high velocity hydrate layer with a velocity of 2.0 − 2.1 km/s that corresponds to a hydrate concentration of 40% at a depth of 100 m, and a bottom simulating reflector (BSR) at a depth of 265 − 275 m beneath the seafloor (mbsf). These are comparable to drilling results on an adjacent frontal ridge. Margin perpendicular normal faults that extend down to BSR depth were also observed on SCS and bathymetric data, two of which coincide with the sidewalls of the slump indicating that the lateral extent of the slump is controlled by these faults. Analysis of bathymetric data indicates, for the first time, that the glide plane occurs at the same depth as the shallow high velocity layer (100±10 mbsf). In contrast, the glide plane coincides with the depth of the BSR on an adjacent frontal ridge. In either case, our results suggest that the contrast in sediments strengthened by hydrates and overlying or underlying sediments where there is no hydrate is what causing the slope failure on this margin. The second part of this dissertation focuses on obtaining the detailed structure of the Cascadia basin and frontal ridge region using mirror imaging of few widely spaced OBS data. Using only a small airgun source (120 cu. in.), our results indicate structures that were previously not observed on the northern Cascadia margin. Specifically, OBS migration results show dual-vergence structure, which could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Understanding the physical and mechanical properties of the basal layer has important implications for understanding earthquakes on this margin. The OBS migrated image also clearly shows the continuity of reflectors which enabled the identification of thrust faults, and also shows the top of the igneous oceanic crust at 5−6 km beneath the seafloor, which were not possible to identify in single-channel and low-fold multi-channel seismic (MCS) data. The last part of this thesis focuses on obtaining detailed seismic structure of the Vancouver Island continental shelf from MCS data using frequency domain viscoacoustic full waveform inversion, which is first of its kind on this margin. Anelastic velocity and attenuation models, derived in this study to subseafloor depths of ∼ 2 km, are useful in understanding the deformation within the Tofino basin sediments, the nature of basement structures and their relationship with underlying accreted terranes such as the Crescent and the Pacific Rim terranes. Specifically, our results indicate a low-velocity zone (LVZ) with a contrast of 200 m/s within the Tofino basin sediment section at a depth 600 − 1000 mbsf over a lateral distance of 10 km. This LVZ is associated with high attenuation values (0.015 − 0.02) and could be a result of over pressured sediments or lithology changes associated with a high porosity layer in this potential hydrocarbon environment. Shallow high velocities of 4 − 5 km/s are observed in the mid-shelf region at depths > 1.5 km, which is interpreted as the shallowest occurrence of the Eocene volcanic Crescent terrane. The sediment velocities sharply increase about 10 km west of Vancouver Island, which probably corresponds to the underlying transition to the Mesozoic marine sedimentary Pacific Rim terrane. High attenuation values of 0.03 − 0.06 are observed at depths > 1 km, which probably corresponds to increased clay content and the presence of mineralized fluids. / Graduate / 0373 / 0372 / 0605 / subbarao@uvic.ca