A revised model for the crustal structure of the SW Grenville Province, Ontario, Canada

Abstract

The Grenville Province forms the exhumed remnants of a 1.1 Ga collisional orogeny that telescoped an older continental margin. Terranes with distinct crustal formation ages can be mapped using Nd isotopes, revealing a ramp–flat thrust structure. The ramp is identified by the presence of retrogressed eclogites, and its trajectory is refined using Nd model ages. The main allochthon is locally overlain by the Parry Sound klippe, but is also underlain by a tectonic duplex. Northwest-directed nappes represent remnants of a corrugated thrust sheet, but a ring-shaped remnant was also preserved where the thrust sheet was down-buckled under the dense rocks of Parry Sound domain.

Three-dimensional visualization of top-down superimposed thrust sheets in the SW Grenville Province, Ontario

“…4; Dickin et al 2014). Figure 4a is a map of the crustal structure of the SW Grenville Province, based on Nd isotope mapping summarized by Dickin et al (2017). This work showed that the surface trajectory of the Allochthon Boundary Thrust is close to the position originally proposed by Rivers et al (1989), along the basal shear zone of the Mesoproterozoic Muskoka domain.…”

“…The location of the ramp is constrained by windows through the allochthon, identified by Nd isotope mapping. The windows and overlying allochthonous domains are exposed as a repetitive antiform-synform pattern perpendicular to the orogen (Dickin et al 2017).…”

“…This structure is more clearly seen in the northeastern part of Figure 4a, where the duplex forms a narrow strip of Palaeoproterozoic crust (yellow shading) between Archean basement (pink) and the overlying allochthon (green). However, the duplex structure NW of Parry Sound has younger crustal formation ages than at Lac Watson because the more southerly crustal units around Parry Sound were generated outboard of the Archaean craton on the Proterozoic active continental margin (Dickin et al 2017). As a result, the deepest structural level around Parry Sound has a Nd model age range of 1.82-2.0 Ga (Dickin & McNutt, 1989;, while the duplex in this vicinity has a model age range of 1.65-1.79 Ga and the allochthon has a model age range of 1.35-1.65 Ga (Dickin & North, 2015).…”

“…This paper therefore presents a virtual 3D model of the Ontario gneiss belt using the software package SketchUp. The 3D model expands on the tectonic interpretation derived from seismic reflection profiles and detailed Nd isotope results of the Grenville Province in Ontario (Dickin et al 2017;Dickin & Strong, 2019). On the firmly demonstrated basis of this tectonic model, we go on to interpret the large-scale tectonic evolution of the Ontario Grenville Province, arguing that this involved top-down control of the locations of major thrusts which originated from the structure of a failed back-arc rift zone that forms parts of the Grenvillian Central Metasedimentary Belt.…”

Mountain Building Orogeny in Precollision Hot Backarcs: North American Cordillera, India‐Tibet, and Grenville Province

“…Tectonically driven lateral flow and expulsion of lower crust may have been triggered by underthrusting of a strong indenter. Northwest directed nappes represent remnants of a deep corrugated thrust sheet (e.g., Dickin et al, ; Jamieson & Beaumont, ). They and others interpreted that when tectonic convergence turned off, the orogenic plateau underwent gravitational spreading, in which ductile lower crust flowed laterally from the plateau toward the foreland, driving thrusting on the orogenic flanks and ductile thinning in the orogenic core.…”