Imaging of the Sudbury Structure, Ontario, Canada, using the seismic reflection and refraction method
The Sudbury Structure in Ontario, Canada, has been extensively investigated since its discovery more than a hundred years ago. The genesis of the structure has been an object of controversy since its discovery. In this thesis, the 1992 Lithoprobe Abitibi-Grenville Transect high-resolution seismic wide-angle reflection and refraction data from the Sudbury subtransect (profiles AB and XY) are processed and modeled to provide geophysical constraints on addressing the controversy. Information on the subsurface structure of the crust and upper mantle will allow better understanding of the Sudbury Structure's geological evolution. Cerveny's ray tracing algorithm was employed to model the complicated crustal structure along profile AB. The RAYINVR ray tracing inversion algorithm was used to obtain the relatively simple crustal structure along profile XY. A 3-D ray tracing algorithm based on the Fletcher Reeves conjugate gradient technique was developed to model the fan-shot data. A 3-D weighted backprojection tomographic method was used to reconstruct the 3-D velocity image of the Sudbury and surrounding area. The modeling results using the seismic data have revealed a lenticular higher velocity body under the Sudbury Basin. The high-velocity body extends from 4 to 9 km depth and its velocity is approximately 6.31-6.4 km/s. Along profile XY the body dips toward northwest whereas along profile AB it dips slightly toward southeast. The lenticular high-velocity body was also imaged on the 3-D tomographic modeling. Comparison of the modeled velocities and the velocities measured from rock samples indicates that the high-velocity body is associated with the norite of the Sudbury Igneous Complex. According to both 2-D and 3-D ray tracing modeling results, interpretation of the seismic data indicates that there is neither an apparent central uplift in the crust and upper mantle, nor an igneous "feeding-root" in the lower crust beneath the basin. The seismic refraction study has, therefore, confirmed most geological and meteorite impact model constraints; however, the high-velocity body imaged in this research requires further petrological and high-resolution seismic investigation in 3-D.