*** The Crustal Structure of The Grenville Front ***

The Grenville province


The Grenville province is the primary exposure of the Grenville orogen, probably the largest collisional orogen the world has ever seen. In Canada, it extends from Lake Huron northeastward to the coast of Labrador and exposes mainly crystalline rocks that were deformed and metamorphosed at lower to mid-crustal depths in the west, and middle to upper crustal depths in the east (Figure AG-1). The Grenville Front is the boundary between the Grenville province and the older structural provinces to the northwest (Wynne-Edwards 1972). At the crustal scale, it is a sharp, northeast-trending tectonic break that is well defined on regional gravity and magnetic maps. The Front marks the northwestern limit of tectonic reworking of rocks of the older provinces during the Grenville orogeny, a complex series of tectonic events generally considered to have taken place between 1300 and 1000 Ma.


The uniformly high grade of metamorphism on the Grenville side of the Front, in contrast to varying grades of the earlier orogens to the northwest, indicate that it was the site of considerable uplift. In the western Grenville, in Ontario, kinematic indicators and seismic reflection surveys (Green et al. 1988, Culshaw et al. 1994, White et al. 1994) suggest a tectonic development involving northwest-directed stacking of crustal segments to produce an extensive mountain belt and an overthickened crust. The Grenville Front and the adjacent parautochthonous belt in this region may represent a deep crustal, reverse-sense shear zone, whereas in the central Grenville, near the Quebec-Labrador border, the Front is a mid-crustal metamorphic foreland fold/thrust belt (Rivers et al. 1993). Both features may be reconciled with a single tectonic framework, in which the presently exposed east-west topographic surface of the Grenville province represents an oblique section through the crust.


The Grenville province has been subdivided into first-order belts that reflect the tectonic character of the orogen (Rivers et al. 1989). The most northerly, located adjacent to the Grenville Front, is known as the parautochthonous Belt, and consists of units that can be traced into the foreland north of the Front. The more southerly Allochthonous Belt, comprising thrust slices of both monocyclic rocks and older polycyclic pre-Grenvillian units, tectonically overlies the parautochthonous Belt. It contains most of the relics of recognizable supracrustal rocks and associated plutonic units. In the eastern Grenville province, the Allochthonous Belt can be subdivided into a thrust belt in the north and a syn-to post-tectonic granitoid magmatic belt to the south (Gower et al. 1991).


Recent LITHOPROBE and other tectonic, metamorphic and geochronological studies (Davidson 1986, Indares and Martignole 1990a, b, Dickin and McNutt 1990, Rivers et al. 1993, Culshaw et al. 1994, White et al. 1994, Eaton et al. 1995, Guo and Dickin 1996, Martignole and Calvert 1996) have recognized smaller lithotectonic divisions whose boundaries are marked by ductileductile shear zone. Some of these boundaries separate supracrustals from different environments and age ranges that may reflect lithological packages emplaced during the closure of an oceanic basin (Easton 1986, Martignole and Pouget 1994). A major period of magmatism late in the Grenville orogeny flooded lower and mid-crustal sections with mafic magma that differentiated into large anorthosite complexes (Figure AG-1).


The Grenville province represents a virtually unique window into the roots of a Precambrian mountain belt. Thus, the primary objective of transect studies in the Grenville province has been to formulate a generalized model of Precambrian mountain building that will provide fundamental insights into modern and pre-Grenvillian tectonics. Specifically, the role of the existing Archean craton and its roots in shaping the structural evolution of the orogen at depth must be assessed. The three study areas shown in Figure AG-1 provide three-dimensional control on the evolution of this extensive Precambrian mountain belt by determining the structure and crustal characteristics at different structural depths within the orogen, and have also been used to characterize internal sutures and boundaries and the lithotectonic domains that comprise the Grenville province.


Figure AG-1

Three-dimensional structure of the Grenville province


We now have seismic reflection data across the Grenville province in eastern Ontario, western Quebec and eastern Quebec. These provide compelling evidence for a crustal architecture consisting of large-scale, southeastward-dipping, duplex structures over a distance of 1500 km along tectonic strike. In eastern Ontario, the northwestern limit of Grenvillian deformation extends straight, through apparently the entire thickness of the crust, with regional dips of ca. 25o (Green et al. 1988). In western Quebec, dips near the limit of Grenvillian deformation are shallower (20o) but still extend through the entire crust. Our line in western Quebec does not span the orogenic front, but reflections on the ECSOOT line, off the eastern coast of Labrador (Hall et al. 1994), are consistent with a continuous, shallowly southeast-dipping, crustal-scale shear marking the northwestern limit of the Grenville orogen along its entire length. These generalizations, however, mask important differences, most notably in the extent to which Archean crust is involved in the orogen, in the thickness of presently preserved transported material and in the apparent importance of extensional tectonics.


In eastern Ontario, no Archean rocks are exposed at the surface within the Grenville orogen, and reflectors that can be confidently correlated to boundaries between major Proterozoic tectonic units like those of the Central Gneiss Belt and the Central Metasedimentary Belt delineate a 20 to 25 km-thick duplex. If Archean rocks are present in the subsurface, they occur only beneath this duplex, in the lowermost crust, a region dominated by subhorizontal reflectors (Fig. AG-2). In western Quebec, Archean rocks are exposed in a promontory (Baskatong Promontory) for 150 km SE of the Grenville Front. Distinctive northwest-pitching reflectors associated with these rocks extend through the full thickness of the crust, and pinch out only far to the SE, extending more than 100 km in the subsurface (Fig. AG-3a). Although Grenvillian metamorphism in the Archean rocks makes it clear that significant thickness of transported rocks once overlay the Archean rocks (Childe et al. 1993), only a thin klippe of transported material now overlies them, near the Grenville Front. In eastern Quebec, Archean rocks are exposed within the Grenville orogen only in a narrow zone near the Grenville Front. Seismic data make it clear, however, that Archean rocks occur in the subsurface at depths as shallow as 10 km, more than 100 km SE of the Grenville Front, corroborating the thin-skinned model for the geometry of the thrust belt in western Labrador proposed by Rivers and coworkers (1993). As in western Quebec, the Archean unit tapers towards the SE, but in the region sampled on Line 55 the trend of the ramp on the Archean is NNE rather than ENE (the trend of the Grenville orogen) and the ramp itself dips 50o, compared with only about 20o in eastern Quebec. Structure contours on one of the major crustal-scale shears imaged on Line 55 (Fig. AG-4a) illustrate both the steepness of the ramp and its obliquity to the regional strike. Both the obliquity and the steepness of the ramp may have been important to the uplift and rapid exhumation of the high-pressure metamorphic rocks that are so spectacularly developed above and to the northwest of this ramp in the Manicouagan region (the Manicouagan Imbricate Zone of Fig. AG-4a).




There are marked differences in the scale and disposition of the Archean blocks in western and eastern Quebec (Fig. AG-3). In western Quebec, the Archean block has a full crustal thickness of more than 40 km; that in eastern Quebec is only 30 km thick, with the balance of the crust being comprised of an upper, 10 km thick duplex of parautochthonous Meso-Proterozoic sedimentary rocks. We interpret the thinner Archean crust of eastern Quebec as part of the continental margin of the pre-Grenvillian continent that had been thinned by passive-margin formation. Unless there was substantial rethickening of the Archean crust in western Quebec during the Grenville orogeny, a possibility that is not supported by the apparent preservation of an early, northwest-pitching reflectivity, the Baskatong Promontory was unthinned Archean crust when it became involved in the Grenville orogeny. Regardless of the reason for it, the smaller thickness of Archean crust provides the simplest explanation for the greater thickness of preserved transported rocks in eastern Quebec than in western Quebec.


Kinematic indicators in the Grenville of both Ontario and Quebec are predominantly consistent with top-to-northwest transport, and the seismic results provide strong support for northwest-vergent duplexes in several places. There are, however, regions in which extensional ductile shear, with top-to-southeast transport, is reported (e.g., Anovitz & Chase 1990, Culshaw et al. 1994, Busch et al. 1996). In western Quebec, late extension is very clear in the seismic record, where it has produced normal-sense drag on reflectors in the Archean crust (Fig. AG-3a) and thinning of the crust to 35 km from more than 40 km. The shallow attitudes of reflectors associated with the Grenville Front Zone in this region may be due to isostatic rebound associated with this extension. Despite geological evidence for extensional tectonics in eastern Ontario and eastern Quebec, comparable evidence for extension on a crustal scale is absent. In eastern Ontario, seismic refraction surveys indicate slightly thickened (45 km) crust in the region beneath the Grenville Front Tectonic Zone (Epili & Mereu 1991). In eastern Quebec, there is no direct evidence for crustal thickness directly beneath the Grenville Front because the road network (and seismic surveys) do not reach it, but crustal thickness imaged at the northern termination of the line are 49 km, and the trough of the regional gravity low associated with the Grenville Front from eastern Quebec to the coast of Labrador (Hynes 1994) is not reached by the line (Fig. AG-4b); crustal thickness beneath this low are probably higher still (Fig. AG-3b). The LITHOPROBE studies indicate that a possible explanation for restriction of the Grenville-Front gravity low to the eastern half of the Grenville orogen is the absence of well-developed late extension there capable of reducing relief on the Moho.

Figure AG-2


Figure AG-3


Figure AG-4


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