Crustal seismic velocity structure of the intermontane and coastal belts, southwestern Canadian Cordillera
Date
1995
Authors
McLean, Nancy Ann
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Abstract
One phase of LITHOPROBE's seismic program for the Southern Cordillera included a seismic refraction experiment (SCoRE '89) focussing on the Coast and Intermontane Belts of the Southern Canadian Cordillera. Primary objectives of the refraction program include; i) determining 2-D velocity structure and interrelationships of the component erranes, ii) defining the depth and configuration of the Mohorovic discontinuity, and iii) determining the extent of vertical displacement of major faults within the region. In particular, it was hoped that any differences in structure across the boundaries of the Coast Belt, either in the west beneath Georgia Strait or in the east beneath the Fraser fault system, could be resolved.
In this study, seismic refraction data were recorded from 10 shot points along a 450 km profile, cross-strike the Fraser fault system, across the Intermontane, Coast and Insular Belts. From these data, a two-dimensional P-wave velocity structural model of the crust and upper mantle was interpreted using a combination of traveltime inversion and amplitude forward modelling. The structural model features a varying-thickness near-surface layer with large vertical and lateral velocity variations. This uppermost stratum overlies a crustal velocity structure which is divided into three crustal layers separated by wide-angle reflections. The velocity of the upper crustal structure changes abruptly at the Harrison fault. West of the fault, the average upper crustal velocity was 6.4 km/s, while east of the fault the average upper crustal velocity was 6.1 km/s. The mid-crust and lower crust have average velocities of 6.5 km/sand 6.6 - 6.8 km/s, respectively. A transition to lower velocities in the east is maintained in these crustal units, although it is less abrupt. The depth to the Mohorovic discontinuity was typically 34 km with variations up to 1.5 km. The interpreted depth to the Moho along SCoRE '89 Line 2 contrasts with interpretations of SCoRE surveys to the south. Analyses of these data indicate an abrupt thickening from 34 km to 37 km below the Central Coast Belt and a further decrease to 30 km beneath the Insular Belt. This variability is probably an indication of the true 3-D structure beneath the Cordillera. Upper mantle velocities were typically 7.9 km/s. An upper mantle reflector was interpreted at ~ 46 km depth below which lies the top of the asthenosphere.
In general, the crustal and upper mantle structure across the refraction line is well resolved with the exception of the deep structure in the Western Coast Belt and Insular Belt. The lower structure of the subduction complex interpreted from previous surveys could not be correlated with the rest of the region. This is largely due to an unusual disruption of lower crust and upper mantle energy propagating into the region. A striking feature of the velocity structural model is an abrupt transition to lower crustal velocities east of the Harrison fault. Interpretations of LITHOPROBE reflection data and other refraction surveys in the region display a similar characteristic. The location of this transition is interpreted as representing the suture zone between the Insular and Intermontane superterranes. It is inferred that the Harrison fault penetrates the upper crustal layer, perhaps to 10 km depth. Analysis of the Line 2 data indicate a structural discontinuity in the reflecting horizons of the lower crust beneath the surface location of the Fraser fault. This implies deep crustal penetration of the Fraser fault, consistent with magnetotelluric data and analysis of reflection profile 88-18.