LITHOPROBE-southern Vancouver Island: Cenozoic subduction complex imaged by deep seismic reflections:• Discussion

Clowes et al. presented an informative article on the results of seismic reflection experiments conducted along four lines for LITHOPROBE on southern Vancouver Island. The various reflections indicated in the profiles, which are distributed as deep as the Mohorovicic discontinuity, formed the basis for proposing various scenarios for the tectonic development of the crust. Two of these hypotheses, outlined in Fig. 9, were discarded. In Fig. 10, schematic illustrations on the possible crustal development were presented. The selection of a pre­ ferred model was based entirely on reflection and refraction data, but no consideration was given to other parameters such as density, which influences the gravity field. Usually there is a strong relationship between densities and crustal seismic­ wave velocities. Analysis of gravity data would have provided an independent means of evaluating the various evolutionary models. The gravity field is mapped over the area of Fig. 1 (Riddi­ hough 1979), and presumably significant density differences exist between the continental framework (Wrangellia-Paleo­ zoic and Mesozoic rocks), Leech River schist, Eocene basalt, and core rocks (sandstone and shales). A gravity high is noted over southwestern Vancouver Island, with a peak value (over 50 mGal ( 1 mGal = 10 μm s-2) ) over the exposed Eocene basalt (Riddihough 1979). This high extends inland, indicating that the basalts probably extend inland below the surface. A trend of highs and relative highs also extends southward into Washington State, roughly following the trend of exposed basalts of Fig. 1. In addition, a gravity low (minimum -75 mGal) overlies the Olympic Peninsula of Washington State, where the low-density sedimentary core rocks are exposed. In fact, excellent correlation exists between the gravity and surface geology noted in Fig. 1. It is likely, there­ fore, that significant density differences exist between the various lithologic units. These relationships could be utilized to derive reliable gravity-density models.