2‐D Tomographic Imaging Across the North American Mid‐Continent Rift System

Abstract

2‐D traveltime tomography was used to model the seismic‐velocity structure of the Mid‐Continent Rift System underneath Lake Superior. the data set came from Line A of the 1986 Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) experiment. In‐line data analysed in this study were recorded on four land stations and on five ocean‐bottom seismometers. Shot spacing on the 240 km long line was approximately 333m. Both direct and refraction arrival times were used to image the upper crust before expanding the process to cover greater distances and depths. Crustal thickness was determined using reflection (PmP) data from the Moho. Several known near‐surface structures were resolved. an extensive high‐velocity zone (7.0‐7.2kms‐1) under the central basin of the lake was found to be composed of a plume‐like structure rising to a depth of approximately 10km. This plume originates from a broader‐based (approximately 50km) mantle bulge, and probably indicates an area of crustal weakness along the rift. A smaller, less complex intrusion appears under the northern basin of the lake. Moho depths increase from about 36 km under the southern flank to about 55 km under the central basin, decreasing again to about 45 km in the north. There is some evidence in the PmP seismic record that the Moho under the central basin is a disrupted transition zone. the velocity model presented here corroborates this. A stability test applied to the tomography velocity model‐along with comparisons to other Line A models‐indicate that the result is a reliable and a reasonable representation of the Mid‐continent Rift System under Lake Superior. These findings support the theory that this complex structure is part of a failed Keweenawan (1100 Ma) tectonic rifting event. Copyright © 1993, Wiley Blackwell. All rights reserved