We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore‐offshore large‐aperture seismic and gravity data. High velocities obtained within a highly reflective half‐graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide‐angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high‐velocity lower crust interpreted to result from syn‐rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half‐graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre‐rift Moho is probably within or above the high‐velocity lower crust, whereas the post‐rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.
From 1983 to 1984 the United States Geological Survey and the Geological Survey of Canada undertook a series of seismic experiments to determine the crustal structure of the Appalachian mountain belt in Maine and southeast Québec. These experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high‐resolution refraction/wide‐angle reflection profiles both parallel and perpendicular to strike. Here the data recorded in Québec and the northwest part of Maine are interpreted in conjunction with existing reflection profiles from Québec. Synthetic seismogram calculations in laterally varying media are used to model the refraction/wide‐angle reflection data, while reprocessing, migration, and attribute plotting are used in the interpretation of the reflection data. A major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the St. Lawrence Lowlands southeast to about 25 km depth beneath the southeast edge of the Chain Lakes Massif. This zone of reflections is interpreted as a master décollement separating autochthonous Grenville basement from overlying allochthonous rocks of the Appalachian Orogen, including foreshortened miogeoclinal rocks, the remnants of one or more magmatic arcs, the Connecticut Valley ‐ Gaspé Synclinorium, and the Chain Lakes Massif. In the vicinity of the Chain Lakes Massif, extension has caused Grenville basement to be thinned by as much as 40%. Reflections from the décollement show strong layering and evidence for basement faulting associated with the Guadeloupe and other faults. Normal incidence and wide‐angle reflections are observed to be almost continuous beneath the Baie Verte ‐ Brompton Line and Connecticut Valley ‐ Gaspé Synclinorium, indicating that the Baie Verte ‐ Brompton Line is confined to the upper crust. The Connecticut Valley ‐ Gaspé Synclinorium is characterized by P‐wave velocities of about 5.3 km s−1 and is shown to be a shallow structure with a maximum depth of about 3 km. Rock velocities immediately below the Connecticut Valley ‐ Gaspé Synclinorium are typical of Chain Lakes material (6.1 km s−1). A new model for the tectonic development of the region is proposed in which all pre‐Silurian units between the Baie Verte ‐ Brompton Line and the southeastern edge of the Chain Lakes Massif are allochthonous and confined to the upper crust. We infer that the Chain Lakes Massif, volcanics of the Ascot‐Weedon Formation, and associated deeper crustal rocks were thrust over Grenvillian basement as a composite unit and may now underlie parts of the Connecticut Valley ‐ Gaspé Synclinorium.
Eight reversed seismic refraction profiles have been recorded across the Phanerozoic Williston sedimentary basin that overlies the margins of the Superior and Wyoming Arcbean cratons and the Proterozoic Trans-Hudson orogen. These clata have been modeled using two-dimensional ray-tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals. Within the southern extension of the Superior craton margin (Thompson belt) a high-velocity (•6.4 km/s) lid overlies a narrow region of high electrical conductivity, and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge (Thompson fault) of the craton margin. Complex zones of low-velocity material and the North American Central Plains electrical conductivity anomaly are concluded to lie within the southern extension of the Reindeer-South Indian Lakes terrain, a component of the Trans-Hudson orogen. A high-velocity (•> 7.0 km/s) lower crustal layer, which represents a zone of complexity separating "normal" crust from the upper mantle, extends throughout the region. The crust is relatively thick, with depths to Moho varying smoothly between 41 and 48 km. There is no evidence in the relatively low-resolution seismic refraction data for abrupt changes in the level of the Moho, but there may be a minor (3-4 km) thinning of the crust near the center of the Williston Basin. iments were the eastern edge of the Superior craton, the Trans-Hudson orogen [Hoffman, 1981], the North American Central Plains (NACP) electrical conductivity anomaly [Camfield and Gough, 1977], and the Williston Basin. During the three field campaigns, approximately 2250 km of reversed inline refraction data were collected along eight profiles arranged in the following geometry: (1) four N-S lines (A, C, D, and E), three of which were parallel to the major tectonic axes of the buried Precambrian shield, (2) three lines (E, F, and G) in a triangular configuration (included one of the N-S lines), and (3) two E-W lines (B and H) that cut across the axis of the Trans-Hudson orogen and that connected most of the other lines. Fan shooting into the three sides of the equilateral triangle E, F, and G was also performed. Preliminary analyses of parts of these data have been published by Green et al. [1980], Delandro and Moon [1982], Hajnal et al. [1984], and Kanasewich and Chiu [1985]. The interpretation of lines A and B by Green et al. [1980] was based on inversions of first arrival times assuming simple dipping models and synthetic seismogram analyses using the Fuchs and Muller [1971] reflectivity algorithm. Delandro and Moon's [1982] interpretation of the same lines incorporated slightly more complex structures using the Whittall and Clowes [1979] two-dimensional ray-tracing program and the WKBJ method for calculating one-dimensional synthetic seimograms [Chapman, 1978]. The interpretation of lines C, D, • Now at Texaco Canada Resources, and H (western shot) by Hajnal et al. [1984] was based on one-dim...
Under the auspices of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE), coincident steep-and wide-angle seismic reflection data sets were acquired in Lake Huron. The wide-angle data were recorded at stationary onshore receivers during the shooting and recording of the conventional marine steep-angle survey. To extract information from the wide-angle data, a variety of processing techniques including automatic trace editing, amplitude balancing, F-K filtering, static corrections, and predictive deconvolution were applied to the wide-aperture commonreceiver gather before migration. The application of a slowness-weighted depth migration algorithm, which was first tested on synthetic data and which included a slowness filter, resulted in an image of the Grenville Front tectonic zone (GFTZ) remarkably similar to that observed in the multichannel steep-angle section. The migrated wide-angle data suggest that prominent bands of east dipping seismic reflections observed across the GFTZ extend unattenuated to depths of at least 45 km.
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