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Two- and three-dimensional velocity structure of the southwestern Canadian Cordillera from seismic refraction data Zelt, Barry Curtis


Seismic refraction/wide-angle reflection data recorded on a triangular array in the southwestern Canadian Cordillera in 1989 as part of the Lithoprobe Southern Cordillera transect are analyzed to determine the two- and three-dimensional velocity structure of the crust and upper mantle. In-line data recorded along two sides of the triangle are interpreted for 2-D structure using an iterative combination of traveltime inversion and forward modelling of amplitudes. An algorithm for the inversion of wide-angle seismic data to determine 3-D velocity structure and depth to reflecting interfaces is developed. The algorithm is based on an existing procedure for the inversion of first arrival traveltimes which includes: (i) forward modelling of traveltimes using a 3-D finite-difference algorithm; and (ii) a simple velocity model parameterization for the inversion which eliminates the need to solve a large system of equations. The existing procedure is extended to allow: (i) fast and accurate forward modelling of reflection times; (ii) the inversion of reflection times to solve for depth to a reflecting interface and/or velocity structure; (iii) the inversion of first arrival traveltimes to solve for depth to a refracting interface; and (iv) layer stripping. Application of the algorithm for southern Cordillera data uses Pg to constrain upper crustal velocity structure, PmP to constrain lower crustal velocity structure and depth to Moho, and Pn to constrain upper mantle velocities and depth to Moho. Results for a line running along-strike in the southern Intermontane Belt (Quesnellia terrane) reveal low average vertical velocity gradients, average depth to Moho of 32 km, and an upper mantle reflector ~ 16 km below the Moho that may represent the base of the lithosphere. The upper and middle crust of the refraction model comprise the upper part of the Quesnellia terrane; the lower crust probably comprises parautochthonous and cratonic North America, but does not show the division into two components that is inferred from reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. The lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. Results for a line running across the strike of the southernmost Coast Belt and eastern Insular Belt reveal large lateral variations in velocity. The most significant of these variations is a decrease in upper and middle crustal velocities to the east of the surface trace of the Harrison Fault, which likely represents the transition from Insular to Intermontane superterrane crust. Depth to Moho is 34-37 km beneath most of the Coast Belt, indicating a small crustal root associated with the Cascade magmatic arc. The Moho decreases to 30 km beneath the eastern Insular Belt, which is much less than previous estimates. The inferred crustal velocity structure beneath the Western Coast Belt (WCB) is consistent with the three layer conductivity structure for this area, and suggests that the upper 8-12 km represents the massive cover of plutonic rocks which characterizes the WCB. The middle and lower crust beneath the WCB is interpreted as Wrangellia, which may extend at depth eastwards as far as the Harrison Fault. The 3-D velocity model for the southwestern Canadian Cordillera is characterized by (i) significant lateral velocity variations at all depths that do not, in general, strongly correlate with surface geological features or gravity data; (ii) higher average crustal velocities in the Coast Belt in comparison with the Intermontane Belt; (iii) relatively high velocity middle and lower crust in the southwest which correlates with a strong relative gravity high and may outline the eastern extent of lower Wrangellia; (iv) average upper mantle velocity of 7.85 km/s; (v) depth to Moho of 33-36 km in the Intermontane Belt and 36-38 km throughout most of the Coast Belt, shallowing in the west to 33 km near the Insular-Coast contact. A correlation between thick crust and low heat flow suggests that a significant portion of total heat flow is sub-crustal in origin, possibly associated with (a recent) upflow of mantle convection.

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