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[1] A stochastic relationship between topography and Bouguer gravity is used to calculate high-resolution variations in effective elastic thickness, T e , of the lithosphere in western Canada. The topography-gravity coherence is calculated using a two-dimensional, maximum-entropy-based spectral estimator. This method allows for smaller data windows and provides T e determinations with higher spatial resolution than standard Fourier spectral estimators. Our analysis shows significant variations in T e in western Canada. T e increases from $20-40 km in the weak, young portions of the Cordillera to 100 km and greater in the strong, old Canadian Shield. T e estimates are in good agreement with lithospheric temperatures calculated from surface heat flow and radioactive heat generation data. Our calculated T e distribution also shows strong correlation with other thermally related geophysical parameters, such as lithospheric age, regional heat flow, seismicity, seismic properties, and the stress field. Consequently, we infer that lithospheric temperatures exert a primary control on large-scale variations in T e . Collectively, the correlations readily explain why the Craton continues to be stable and undeformed, whereas the Cordillera has continued to be deformed through the Cenozoic. An exception is the Wopmay Orogen, which includes the easternmost part of the northern Cordillera. There T e is $90 km, although the surface heat flow is $90 mW/m 2 . We infer that the high heat flow in this region is caused primarily by very high radioactive heat generation in the upper crust and that deep lithospheric temperatures are moderately low as expected from its age and long-term geological stability.

Summary The Strait of Georgia is a topographic depression straddling the boundary between the Insular and Coast belts in southwestern British Columbia. Two shallow earthquakes located within the strait (M = 4.6 in 1997 and M = 5.0 in 1975) and felt throughout the Vancouver area illustrate the seismic potential of this region. As part of the 1998 Seismic Hazards Investigation of Puget Sound (SHIPS) experiment, seismic instruments were placed in and around the Strait of Georgia to record shots from a marine source within the strait. We apply a tomographic inversion procedure to first‐arrival traveltime data to derive a minimum‐structure 3‐D P‐wave velocity model for the upper crust to about 13 km depth. We also present a 2‐D velocity model for a profile orientated across the Strait of Georgia derived using a minimum‐parameter traveltime inversion approach. This paper represents the first detailed look at crustal velocity variations within the major Cretaceous to Cenozoic Georgia Basin, which underlies the Strait of Georgia. The 3‐D velocity model clearly delineates the structure of the Georgia Basin. Taking the 6 km s−1 isovelocity contour to represent the top of the underlying basement, the basin thickens from between 2 and 4 km in the northwestern half of the strait to between 8 and 9 km at the southeastern end of the study region. Basin velocities in the northeastern half are 4.5–6 km s−1 and primarily represent the Upper Cretaceous Nanaimo Group. Velocities to the south are lower (3–6 km s−1) because of the additional presence of the overlying Tertiary Huntingdon Formation and more recent sediments, including glacial and modern Fraser River deposits. In contrast to the relatively smoothly varying velocity structure of the basin, velocities of the basement rocks, which comprise primarily Palaeozoic to Jurassic rocks of the Wrangellia Terrane and possibly Jurassic to mid‐Cretaceous granitic rocks of the Coast Belt, show significantly more structure, probably an indication of the varying basement rock lithologies. The 2‐D velocity model more clearly reveals the velocity layering associated with the recent sediments, Huntingdon Formation and Nanaimo Group of the southern Georgia Basin, as well as the underlying basement. We interpret lateral variation in sub‐basin velocities of the 2‐D model as a transition from Wrangellian to Coast Belt basement rocks. The effect of the narrow, onshore–offshore recording geometry of the seismic experiment on model resolution was tested to allow a critical assessment of the validity of the 3‐D velocity model. Lateral resolution throughout the model to a depth of 3–5 km below the top of the basement is generally 10–20 km.

No abstract

SUMMAR YThe Strait of Georgia is a topographic depression straddling the boundary between the Insular and Coast belts in southwestern British Columbia. Two shallow earthquakes located within the strait (M=4.6 in 1997 and M=5.0 in 1975) and felt throughout the Vancouver area illustrate the seismic potential of this region. As part of the 1998 Seismic Hazards Investigation of Puget Sound (SHIPS) experiment, seismic instruments were placed in and around the Strait of Georgia to record shots from a marine source within the strait. We apply a tomographic inversion procedure to ®rst-arrival traveltime data to derive a minimum-structure 3-D P-wave velocity model for the upper crust to about 13 km depth. We also present a 2-D velocity model for a pro®le orientated across the Strait of Georgia derived using a minimum-parameter traveltime inversion approach.This paper represents the ®rst detailed look at crustal velocity variations within the major Cretaceous to Cenozoic Georgia Basin, which underlies the Strait of Georgia. The 3-D velocity model clearly delineates the structure of the Georgia Basin. Taking the 6 km s x1 isovelocity contour to represent the top of the underlying basement, the basin thickens from between 2 and 4 km in the northwestern half of the strait to between 8 and 9 km at the southeastern end of the study region. Basin velocities in the northeastern half are 4.5±6 km s x1 and primarily represent the Upper Cretaceous Nanaimo Group. Velocities to the south are lower (3±6 km s x1 ) because of the additional presence of the overlying Tertiary Huntingdon Formation and more recent sediments, including glacial and modern Fraser River deposits. In contrast to the relatively smoothly varying velocity structure of the basin, velocities of the basement rocks, which comprise primarily Palaeozoic to Jurassic rocks of the Wrangellia Terrane and possibly Jurassic to mid-Cretaceous granitic rocks of the Coast Belt, show signi®cantly more structure, probably an indication of the varying basement rock lithologies. The 2-D velocity model more clearly reveals the velocity layering associated with the recent sediments, Huntingdon Formation and Nanaimo Group of the southern Georgia Basin, as well as the underlying basement. We interpret lateral variation in sub-basin velocities of the 2-D model as a transition from Wrangellian to Coast Belt basement rocks. The effect of the narrow, onshore±offshore recording geometry of the seismic experiment on model resolution was tested to allow a critical assessment of the validity of the 3-D velocity model. Lateral resolution throughout the model to a depth of 3±5 km below the top of the basement is generally 10±20 km.