A schematic crustal cross-section is presented for the southwestern Grenville Province based on reprocessed Lithoprobe near-vertical incidence seismic reflection data and compiled seismic refraction - wide-angle velocity models interpreted with geological constraints. The schematic crustal architecture of the southwest Grenville Province from southeast to northwest comprises allochthonous crustal elements (Frontenac-Adirondack Belt and Composite Arc Belt) that were assembled prior to ca. 1160 Ma, and then deformed and transported northwest over reworked rocks of pre-Grenvillian Laurentia and the Laurentian margin primarily between 1120 and 980 Ma. Reworked pre-Grenvillian Laurentia and Laurentian margin rocks are interpreted to extend at least 350 km southeast of the Grenville Front beneath all of the Composite Arc Belt. Three major structural boundary zones (the Grenville Front and adjacent Grenville Front Tectonic Zone, the Central Metasedimentary Belt boundary thrust zone, and the Elzevir-Frontenac boundary zone) have been identified across the region of the cross-section based on their prominent geophysical signatures comprising broad zones of southeast-dipping reflections and shallowing of mid-crustal velocity contours by 12-15 km. The structural boundary zones accommodated southeast over northwest crustal stacking at successively earlier times during orogeny (ca. 1010-980 Ma, 1080-1060 Ma, and 1170-1160 Ma, respectively). These shear zones root within an interpreted gently southeast-dipping regional décollement at a depth of 25-30 km corresponding to the top of a high-velocity lower crustal layer.
The 1992 Lithoprobe Abitibi–Grenville Seismic Refraction Experiment was conducted using four profiles across the Grenville and Superior provinces of the southeastern Canadian Shield. Delay-time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another, with the largest velocity values occurring underneath the Central Gneiss and the Central Metasedimentary belts south of the Grenville Front. The Grenville Front Tectonic Zone is imaged as a southeast-dipping region of anomalous velocity gradients extending to the Moho. The velocity-anomaly maps suggest an Archean crust may extend, horizontally, 140 km beneath the northern Grenville Province. Near-surface velocity anomalies correlate well with the known geology. The most prominent of these is the Sudbury Structure, which is well mapped as a low-velocity basinal structure. The tomography images also suggest underthrusting of the Pontiac and Quetico subprovinces beneath the Abitibi Greenstone Belt. Wide-angle PmP signals, indicate that the Moho varies from a sharp discontinuity south of the Grenville Front to a rather diffuse and flat boundary under the Abitibi Greenstone Belt north of the Grenville Front. A significant crustal thinning near the Grenville Front may indicate post-Grenvillian rebound and (or) the extensional structure of the Ottawa–Bonnechere graben. Crustal thickening resulting from continental collision may explain the tomographic images showing the Moho is 4–5 km deeper south of the Grenville Front.
Summary. During the summer of 1982 the Canadian Consortium for Crustal Reconnaissance using Seismic Techniques (COCRUST) conducted a major long‐range seismic refraction and wide‐angle reflection experiment across the Grenville province of the Canadian Shield. Three seismic lines each approximately 300 km in length were located (i) along the Ottawa–Bonnechere graben, (ii) perpendicular to the graben and (iii) perpendicular to the Grenville Front. Geological evidence indicates that the graben may have originated from a failed arm of the St Lawrence rift system, and the Grenville Front marks the boundary between the Grenville province and the much older Superior province. Other geological features of the Grenville province that were traversed by the profiles were the Central Metasedimentary belt, and the Central Gneiss belt. Analysis of the data involved conventional travel‐time procedures coupled with the use of synthetic seismogram analysis using programs that were written to handle laterally heterogeneous structures. Results from the survey indicate a variation in near surface seismic velocity from 5.8 to 6.4 km s−1 with the highest values occurring in the Central Metasedimentary belt just north of Marmora, Ontario. Near Mont Laurier, Quebec, a zone of low velocity near‐surface material was found which is probably related to a nearby gravity low. Upper crustal velocity gradients differed from one profile to another but there was little evidence for any significant intermediate velocity discontinuity such as the Conrad. A study of wide angle reflected waves from the Mohorovickić discontinuity (Moho) showed that all the major tectonic features in the region have an expression at depth. The Moho is a very well defined sharp discontinuity beneath the Gneiss belt and there is strong evidence for a significant thickening of the crust by at least 5 km in the vicinity of the Grenville Front. The boundary between the Central Metasedimentary and Gneiss belts is characterized by a 2 to 3 km fault‐like step in the Moho with the thinner part being under the Gneiss belt. The Moho is very disturbed and poorly defined along major portions of the Ottawa graben and in some localities there is evidence for a rise in high velocity material at its base. This gives added support to the theory that the graben is similar to major rift structures on other continents.
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.
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