Eva Zaleski

Seismic reflections from subvertical diabase dikes in an Archean terrane

A 20 km seismic reflection profile in the Superior province of northwestern Ontario imaged polydeformed Archean terrane, including the Manitouwadge greenstone belt and the adjacent Wawa-Quetico subprovince boundary. In addition to reflectivity from Archean lithostructural elements, we recognize discrete reflections that correlate with undeformed subvertical Proterozoic dikes. Moderately to steeply inclined linear events in the stacked section are related to Matachewan dikes that intersect the seismic profile at 50°–75°. The reflections project to the surface at points corresponding to dike-profile intersections and have apparent dips in the seismic section consistent with the intersection angles in map view. Subhorizontal, linear to curvilinear reflections originate from Marathon dikes that are subparallel to the seismic line. On the basis of forward modeling studies, the events occur at two-way traveltimes for horizontally propagating waves. We conclude that a number of dipping and subhorizontal events represent refracted waves propagated along horizontal ray paths near the surface and reflected from subvertical dikes. In many other seismic profiles in crystalline crust, reflections originating from subvertical dikes have likely been overlooked or misinterpreted, either because the presence of dikes was not recognized, or because their seismic response was not evaluated.