Tectono-metamorphic history of the eastern Taureau shear zone, Mauricie area, Québec: Implications for the exhumation of the mid-crust in the Grenville Province

Abstract

International audienceThis study investigates the tectono-metamorphic history and exhumation mechanisms of the mid-crustal Mékinac-Taureau domain of the Mauricie area, central Grenville Province. Macro- and micro-structural analyses reveal the top-down-to-the-ESE sense of shear on the eastern Taureau shear zone, a major extensional structure that exhumed the mid-crustal Mékinac-Taureau domain and juxtaposed it against the lower grade rocks of the Shawinigan domain. Peak metamorphism in the Mékinac-Taureau domain, inferred to be the result of northwestward thrusting and regional crustal thickening, took place under P–T conditions of 1000–1100 MPa and 820–880 °C prior to 1082 ± 20 Ma. Retrograde conditions varying from 775 to 675 °C and from 800 to 650 MPa were registered in its upper structural levels prior to and/or during shearing along the eastern Taureau shear zone that was active at 1064 ± 15 Ma. The Shawinigan domain records P–T conditions ranging from 850 to 625 MPa and from 775 to 700 °C, P–T values that are similar to or slightly lower than those for retrogressed samples from the upper structural levels of the Mékinac-Taureau domain, but clearly lower than the peak metamorphism values of the latter domain. Finally, the area cooled below 550–600 °C at ∼1000–1030 Ma and below 450 °C at ∼900–970 Ma on the basis of 40Ar/39Ar geochronology on amphibole and biotite. Structural and metamorphic characteristics of the Mauricie area are similar to those expected from a metamorphic core complex formed during post-convergence orogenic collapse in a gravity-driven fixed-boundary mode. The Mékinac-Taureau and Shawinigan domains were thus probably exhumed by a similar process, which supports the orogenic collapse model recently proposed to explain the exhumation of mid-crustal metamorphic core complexes in the Grenville Province

Aeromagnetic signature of an exhumed double dome system in the SW Grenville Province (Canada)

“…The WTSZ could correspond to the median high-strain zone; a dextral-thrust displacement has been proposed (Martignole & Friedman, 1998), but an oblique-normal displacement would be more consistent with exhumation of a metamorphic core complex (Figure 4b). The double dome system occurs in an overall extensional or transtensional setting (Rey et al, 2011(Rey et al, , 2017 consistent with orogenic collapse, which was proposed by Rivers (2012) and Soucy La Roche et al (2015) as the mechanism for exhuming mid-crustal core complexes in the Grenville Province. In this system, the M ekinac-Taureau dome is a metamorphic core complex that has similarities to the Qazaz metamorphic core complex (Saudi Arabia), with a principal axis broadly parallel to adjacent strike-slip shear zones, and the "Montagne noire" (France), which formed along a transtensional fault system during post-orogenic collapse (Rey et al, 2017;Roger et al, 2015).…”

“…Overall, the gneissic foliation suggests the presence of a regional scale dome bounded to the W by the LSZ and to the E by the TSZ (Figure 4). I thus interpret the Morin terrane as a regional-scale dome that is referred to here as the Morin dome (Figures 3 and 4 gan, 1995;Soucy La Roche et al, 2015) and may correspond to the double dome ( Figure 3). The WTSZ could correspond to the median high-strain zone; a dextral-thrust displacement has been proposed (Martignole & Friedman, 1998), but an oblique-normal displacement would be more consistent with exhumation of a metamorphic core complex (Figure 4b).…”

“…Close to the WTSZ, the gneissic foliation generally dips gently away from the contact with the Mékinac‐Taureau dome (Nadeau & Brouillette, ). In the easternmost part of the Morin terrane (ie, the Shawinigan domain), the gneissic foliation dips to the SE (Soucy La Roche et al, ). Overall, the gneissic foliation suggests the presence of a regional scale dome bounded to the W by the LSZ and to the E by the TSZ (Figure ).…”

“…Rey, Teyssier, Kruckenberg, and Whitney (2011), Meyer, Passchier, Abu-Alam, and St€ uwe (2014), and Roger et al (2015) suggest that core complexes may be present, but unrecognized, along worldwide crustal-scale strike-slip shear zones. In the south-west Grenville Province, inherited deep-crustal strike-slip shear zones have been recognized from gravity data (Dufr echou & Harris, 2013), and Soucy La Roche, Gervais, Tremblay, Crowley, & Ruffet (2015) described the role of regional shear zones in exhuming mid-crustal rocks during post-orogenic collapse. Although regional shear zones widely affect the Grenville Province (Dufr echou, Harris, & Corriveau, 2014;Dufr echou, Harris, Corriveau, & Antonoff, 2015), their role in the exhumation of deep crustal units and gneiss domes remains generally undocumented.…”

“…Bedrock is widely masked by vegetal and glacial covers, local observations are generally extrapolated over significant distances, and the interpretations of exhumation processes and associated structures are somewhat uncertain because of the difficulty in linking local observations to a more regional tectonic framework. Aeromagnetic data covering an area in which the exhumation of domes has been documented (Peck, 2012;Soucy La Roche et al, 2015) provides a means to extrapolate geological information obtained at scattered outcrops to the regional scale. This analysis helps the recognition of concealed domes and the interpretation of regional shear zones that played a role in the tectonic evolution and exhumation of Grenvillian crust.…”