A seismic-based cross-section of the Grenville Orogen in southern Ontario and western Quebec

Abstract

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.

Tectonic Setting and Evolution of the Grenville Orogen: An Assessment of Progress Over the Last 40 Years

“…There is a case to be made that the impact of modern U-Pb geochronology was greater in the Grenville Province than in other orogens in Canada, in part because of the low level of prior knowledge and the cryptic nature of many units due to polyphase deformation and high grade metamorphism, but also because sampling was carefully integrated with the ongoing new mapping throughout the orogen, enhancing the impacts of both. In any event, in a relatively short time some key features of the lithologic make-up of the orogen came into focus, including the extent of reworked Archean crust, the existence of a previously unknown late Paleoproterozoic orogeny within the eastern Grenville Province (the ~1660-1610 Ma Green et al 1988, White et al 2000. B: Lines 52-54 in western Québec (after Martignole and Calvert 1996;Martignole et al 2000).…”

“…These included interpretations of all the LITHO-PROBE deep crustal seismic transects (Green et al 1988;Forsyth et al 1994a, b;Kellett et al 1994;White et al 1994White et al , 2000Eaton et al 1995;Martignole and Calvert 1996;Gower et al 1997;Hynes et al 2000;Martignole et al 2000), and a fence diagram with all the transects assembled in a single figure (Ludden and Hynes 2000), which provided the first constrained image of the crustal structure along the length of the province. In addition, regional syntheses that collectively covered most parts of the province had been published, i.e.…”

L-S shape fabrics in the Mazinaw domain and the issue of northwest-directed thrusting in the Composite Arc Belt, southeastern Ontario

“…Five of the six gravity profiles published by these workers transect the Mazinaw domain and thrusts I-VI. By contrast, only thrusts I and IV are crossed by Lithoprobe Line 33 and its reflection seismic profile (White et al, 1994(White et al, , 2000.…”

“…Lithoprobe Lines 32 and 33 and their reflection-seismic profiles cross the Composite Arc Belt in southeastern Ontario (White et al, 1994(White et al, , 2000. In the profiles, LTBs and high-strain zones or thrusts may correspond to shallow crustal reflectors, whose average true dip is <25° (cf.…”

Logan Medallist 2. Geophysics and Geology: An Essential Combination Illustrated by LITHOPROBE Interpretations–Part 1, Lithospheric Examples

“…17b, d). Figure 17c shows the model of Figure 17b with the grid removed and the outline of the Georgian Bay cross-section determined from Lithoprobe studies (Culshaw et al 1997;White et al 2000;Jamieson et al 2007) superimposed. Figure 17e shows the portion of the Georgian Bay cross-section corresponding to Figure 17d.…”