Indenter tectonics in the Canadian Shield: A case study for Paleoproterozoic lower crust exhumation, orocline development, and lateral extrusion

Highlights

• Indenter tectonics was the main deformation process in eastern Trans-Hudson orogen.

• Lower crust exhumed at leading edge of indenter fed foredeep – molasse basins.

• Deformation in the upper plate was dominated by orocline development.

• Material flow along edge of the indenting continent involved lateral extrusion.

• Mechanisms of convergence similar to those observed in modern orogens.

Abstract

There are lingering questions about how far back in geologic time plate tectonic processes began. In the Paleoproterozoic of eastern Laurentia, accretion of intra-oceanic juvenile terranes along the leading edge of the Superior craton apex (Ungava indenter) during the interval 1.87–1.83 Ga was followed by collision with the Churchill plate at ca. 1.83–1.79 Ga. Orthogonal shortening along the indenter led to early obduction of the juvenile terranes including the ca. 2.0 Ga Watts Group ophiolite, followed by out-of-sequence thrusting at ca. 1.83 Ga of granulite-facies crystalline basement of the Sugluk block (Churchill plate) along the Sugluk suture. Exhumation and erosion of the Sugluk block led to deposition of a foreland/delta fan sequence in the Hudson Bay re-entrant (Omarolluk and Loaf formations of the Belcher Group), with detritus sourced exclusively from the Sugluk block. Continued collision led to critical wedge development and orocline formation in the Hudson Bay re-entrant, forming a strongly arcuate fold-thrust belt. On the other (eastern) side of the indenter, material flow during crustal shortening was accommodated by lateral extrusion of microplates towards a then open ocean basin, in a manner similar to present-day extrusion of Indochina as a response to India – South China craton convergence. In the Churchill plate hinterland W-NW of the indenter, propagating strike-slip faults resulted in the far-field extrusion and oblique exhumation of Archean crustal slices of the Rae crustal block. The 1.83–1.79 Ga Superior-Churchill collision accommodated a minimum of 500 km of continent–continent convergence, with resulting style and mechanisms of orogenic growth and material flow similar to those observed in the Alpine-Himalayan orogenic system.

Introduction

Indenter tectonics is a widely documented feature of Phanerozoic orogenic belts, with some of the most spectacular and best documented examples occurring in the Himalayas (Molnar and Tapponnier, 1977, Morley, 2001, Schellart et al., 2019), and in the Eastern Alps and Mediterranean regions (e.g., Ratschbacher et al., 1991, Schattner, 2010). The shape, width, and rheology of the indenting plate (rigid indenter) form the principal factors contributing to the accommodation mechanisms in the overriding weaker plate (England and Houseman, 1986, Davy and Cobbold, 1988, Ellis, 1996). The amount of crustal thickening in the overriding plate is limited by time-dependant features such as thermal gradient, rheology, viscosity, and rate of convergence, as well as pre-existing structural parameters such as faults, sutures, and other zones of weakness (England and Houseman, 1988). Removal or displacement of crust by subduction and crustal thickening, rigid block extrusion (Molnar and Tapponnier, 1975, Peltzer and Tapponnier, 1988, Leloup et al., 2001), block rotation (England and Molnar, 1990), lower crustal flow (Bird, 1991, Royden et al., 1997), or channel flow (Beaumont et al., 2001, Godin et al., 2006), are among some of the mechanisms that have been proposed for the accommodation of continuous or semi-continuous continent–continent convergence.

Although there has been (and still is) considerable debate as to when ‘modern-type’ plate tectonic processes began (e.g., Rollinson, 2007, Hamilton, 2019), there is evidence that at least some form of continent–continent collision can be recognized in the Neoarchean (Gray and Pyslkywec, 2010, Zibra et al., 2017), and that accretionary and collisional processes similar in scale and style to present-day orogenic systems had been achieved by the Paleoproterozoic (Hoffman, 1988, St-Onge et al., 2006, Lahtinen et al., 2009, Corrigan et al., 2009), and definitely by the Meso- to Neoproterozoic (e.g., Jacobs and Thomas, 2004, Jamieson et al., 2010).

Gibb (1983), drawing analogy with the Cenozoic India-Eurasia collision, was the first to recognize the effects of Paleoproterozoic collision between the India-size Superior craton and a collage of juvenile terranes and reactivated Archean plates, the Churchill Province, postulating that some of the geophysical and geological features observed along that plate boundary could be explained by ocean opening and closure, subduction and obduction of oceanic crust, and double indentation along the leading (northern) edge of the Superior Craton (Fig. 1). He also speculated on possible crustal extrusion of hinterland terranes towards the central Hudson Bay embayment. Along the leading edge of the Ungava indenter (eastern apex of the Superior craton; “U” in Fig. 1), St-Onge et al., 2000a, St-Onge et al., 2006 reported on the history of orthogonal collision between the Superior craton and the Meta Incognita microcontinent, drawing analogies in first-order structural, metamorphic, thermochronological, and magmatic rates and processes between the Trans-Hudson and Himalayan orogens. On the eastern flank of the Ungava indenter, within the southeastern Churchill Province (SECP) (Fig. 1), Paleoproterozoic-age structures are dominated by orogen-parallel strike-slip shear zones that have been interpreted as the result of oblique dextral convergence between the Superior and North Atlantic cratons (Girard, 1990a, Hoffman, 1990a, Van Kranendonk et al., 1993, Wardle and Van Kranendonk, 1996).

This paper, based on new and existing bedrock mapping, structural analysis, and U-Pb geochronology (Fig. 2), as well as regional aeromagnetic (Fig. 3a) and Bouguer gravity anomaly (Fig. 3b) surveys (Miles and Oneschuck, 2016, Jobin et al., 2017), focuses on the effects of the ca. 1.83–1.79 Ga collision of the Ungava indenter on terranes and microplates in the surrounding foreland and hinterland. We expand on earlier models that suggest that the bulk of shortening orthogonal to the Ungava indenter was accommodated by accretion of intra-oceanic terranes (Hoffman, 1985, Picard et al., 1990, St-Onge et al., 1992), as well as thrusting and exhumation of Churchill plate lower crust along a first-order tectonic boundary, the Sugluk suture (Corrigan et al., 2009). We propose that curvilinear, doubly-plunging folded terranes now outcropping mainly in the Belcher Islands (Fig. 2, Fig. 3) represent a fold-thrust nappe that has undergone oroclinal bending, perhaps by a process of radial flow, within the Hudson Bay re-entrant. In the Churchill plate hinterland far-field region more or less orthogonal to the Ungava indenter, we concur with previous authors (Henderson and Roddick, 1990, Van Kranendonk et al., 1993) and expand on their models, which suggest that Churchill-Superior shortening was accommodated in part by westward (present-day coordinates) lateral extrusion and oblique exhumation of lower-crustal slices of the Archean Rae Province. On the eastern side of the Ungava indenter, the SECP was affected by an array of anastomosing, ductile strike-slip shear zones thought to have accommodated bulk dextral shear due to oblique collision (Girard, 1990a, Hoffman, 1990a, Van der Leeden et al., 1990, Wardle and Van Kranendonk, 1996). We present evidence suggesting that the array of shear zones accommodated rigid block extrusion, perhaps preceded, or assisted, by lower crustal flow of the SECP towards what was then an open ocean basin, in a manner analogous to present-day lateral extrusion of Indochina as a consequence of plateau development and India – South China craton convergence (e.g., Searle et al., 2011 and references therein).