Integrated geophysical modelling of the northern Cascadia subduction zone
Creators: name: Dehler, Sonya Astrid
Publisher: name: University of British Columbia
Year: 2011

description: The northern Cascadia subduction zone involves convergence of the Explorer Plate and northern part of the Juan de Fuca Plate with the North American Plate along a margin lying west of Vancouver Island, Canada. A wide accretionary complex which underlies the continental slope and shelf has been formed. Two allochthonous terranes, the Crescent Terrane of Eocene oceanic crustal volcanics and the Pacific Rim Terrane of Mesozoic melange sedimentary rocks and volcanics, lie against the Wrangellia Terrane backstop beneath the west coast of Vancouver Island and outcrop on the southern tip of the island. The intrusive Coast Plutonic Complex underlies the westernmost part of the British Columbia mainland east of Vancouver Island and marks the location of the historic and modern volcanic arcs. An integrated interpretation of geophysical and geological data has been conducted for the northern Cascadia subduction zone. Regionally extensive gravity and magnetic anomaly data have formed the basis of the interpretation, while surface geology, physical properties, and seismic reflection, refraction, heat flow, borehole, magnetotelluric, and seismicity data have provided constraints on structure and composition. Horizontal gradient and vertical derivative maps of the potential field data were calculated to provide additional control on the locations of major faults and lithologic boundaries. Iterative forward modelling of the gravity and magnetic anomaly data was conducted along three offshore multichannel seismic reflection lines and their onshore extensions. The two-and-a-half-dimensional (2.5-D) models extended from the ocean basin across the accretionary complex and Vancouver Island to the mainland along lines perpendicular to the major structural trends of the margin and revealed lateral changes in the location of several structural components along the length of the margin. The interpretations were extended laterally by moving the original models to adjacent parallel positions and perturbing them to satisfy the new anomaly profile data and other constraints. The models thus formed were moved to the next position and the process repeated until a total of eleven models was developed across the margin. A twelfth line across a gravity anomaly high on southern Vancouver Island was independently modelled to examine the source of this feature. An average density model for the southern half of the convergent margin was constructed by averaging the models and profiles for seven lines at 10 km spacings. This process removed anomalies due to small source bodies and concentrated on the larger features. Finally, a regional density structural model was developed by linearly interpolating between all eleven cross-margin lines to construct a block model which could then be 'sliced' open to examine the internal structure of the margin at any location. The final models allow the Pacific Rim and Crescent Terrane positions to be extended along the offshore margin from their mapped locations. The Pacific Rim Terrane appears to be continuous and close to the coastline along the length of Vancouver Island, while the Crescent Terrane either terminates halfway along the margin or is buried at a depth great enough to suppress its magnetic signature. The location of the Westcoast Fault, separating the Pacific Rim and Wrangellia Terranes, has been interpreted to lie west of Barkley Sound at a position 15 km west of its previously interpreted position. Beneath southern Vancouver Island and Juan de Fuca Strait, the Crescent Terrane appears to have been uplifted into an anticlinal structure, bringing high density lower crustal or upper mantle material close to the surface and thereby causing the observed gravity anomaly high. The western part of the Coast Plutonic Complex has been interpreted as a thin lower density layer extending from its surface contact with Wrangellia to a position 20 to 30 km further east where the unit rapidly thickens and represents the main bulk of the batholith. The complexity of the thermal regime and its effects on density in this region allows for other interpretations. Finally, a comparison of the models along the length of the margin reveals that the crust of Vancouver Island appears to thin toward the north above the shallower Explorer Plate and the complex low - high density banding used in the southern Vancouver Island models is replaced with a single high density unit on the northernmost line.