Transamazonian “Cusp Tectonics” in the Guiana Shield

The applicability of plate tectonic concepts to Precambrian Earth is still contentious. However, from the identification of 2.2-2.0 Ga blueschist facies metamorphism in Eburnean greenstones in the West African craton, temporally and tectonically equivalent to the Transamazonian orogen in the Guiana Shield, Ganne et al. (2019) concluded that modern-style plate tectonics existed in Palaeoproterozoic time (see also Cawood et al., 2018, and Brown et al., 2022). Then, identification of plate tectonic setting of Palaeoproterozoic magmatic suites by ‘chemo-tectonics’ based on present day settings is equally justified. In the Guiana Shield, Fraga et al. (2009) first identified subduction of the high-grade supracrustal Cauarane-Kanuku-Coeroeni belt (Fig. 1) to the north along the southern margin of the western granite-greenstone belt, starting around 2.04 Ga. Mahabier and De Roever (2018) showed that the Caicara-Dalbana felsic metavolcanics, part of the ca. 1700 km long and 1.99-1.95 Ga old volcano-plutonic Orocaima belt between the high-grade belt and the granite-greenstone belts (Fig. 1), have volcanic arc chemistry. McFarlane et al. (2019) concluded similarly for magmatic suites of the 2300-2070 Ma Sefwi greenstone belt of SW Ghana.
Cusped lithospheric plate boundaries, also known as ‘syntaxes’, occur in present day Earth where structural arcs are joined end to end (Hoffman, 2021). Beunk et al. (2021) identified a fossil syntax in the Guiana Shield, traceable by the map pattern of the 1.99-1.95 Ga old granites and felsic volcanics (Fig. 1), and posited that the syntax played a crucial role in the exhumation of the Orosirian lower crustal Bakhuis granulite belt in western Surinam amidst the upper crustal greenstone belts (Fig. 1). Here we explore tectonic consequences of this ‘Bakhuis syntax’ in the high-grade, 2.08-2.02 Ga Kanuku migmatites and granulites, which transect southern Guyana from west to east, and extend into northernmost Brazil (west) and Surinam (east), see Fig. 1. We summarize their compositional and structural characteristics from Berrangé (1977) and Fraga et al. (2009):
Migmatitic paragneisses form the bulk rock type, with subordinate calc-silicate rocks, dolomitic marbles, amphibolites, metacherts, quartzites, BIFs and metamorphosed mafic and ultramafic rocks. Foliation or gneissic layering is considered to be relic supracrustal stratification. Most of the foliation dips more than 75 ̊. Lineations on the foliation plane are well developed, often better than foliation, and plunge steeply. Axial planar cleavage is absent (note: a common characteristic of migmatite terrains), with foliation-forming biotite flakes and sillimanite needles wrapping around fold hinges. Chaotic (polyclinal) and intrafolial F1 folds, occasionally accompanied by F2 chevron-type folds, considered to be synchronous to F1, have steeply plunging hinges, (sub)parallel to the lineations. Poles to foliation form gently dipping girdles orthogonal to fold axes and lineations.
The predominance of vertically plunging folds (‘vertical folds’) over large orogenic stretches is unusual. A common setting for their occurrence are oroclines, curved orogens that originate from tectonic bending of originally straight(er) belts, as for example in the Palaeoproterozoic oroclines in the Svecofennian orogen of Sweden (Beunk and Kuipers, 2012). There, apparently similar to the Kanuku mountains of Guyana, steeply dipping (volcano-)sedimentary stratification was rotated along vertical axes into vertical folds on the scales of the regional map down to thin sections for microscopy. A prerequisite for the development of vertical folds of sedimentary layering is their initial rotation from horizontal into the vertical; commonly a first phase of tight, upright ‘horizontal folding’ (rotation along horizontal axes) is responsible for the steepening. Although crossed girdles of foliation poles in Berrangé’s map (1977) imply the presence of near-horizontal folds, these appear to postdate, not predate the vertical rotations. We expect that initial horizontal folding has steepened the sedimentary protoliths of the Kanuku migmatites and granulites.
The map pattern of the high-grade Cauarane-Kanuku-Coeroeni belt forms an broad arch, trending east-west in its western parts, and changing to southeast in its eastern branch in SW Surinam (Fig. 1). Whether such an open oroclinal bend would by itself be sufficient to generate belt-wide (oroclinal) vertical folds is uncertain, but we propose a dominant role for the Bakhuis syntax in the folding of the Kanuku migmatites and granulites. That role is a priori suggested by the map pattern: The axial plane of the bent Kanuku belt strikes right into the center of the syntax. Our proposed model is sketchily illustrated in Fig. 2. A plate subducting from the SW moves directly into the cusp and experiences lateral constriction and folding due to the cusp geometry (see also Hoffman, 2021).
Original (D1) steepening of sedimentary stratification in the lower plate, by folding on horizontal axes parallel to the plate boundary prior to syntax-induced vertical folding (D2), would be a logical consequence of the compressional nature of the plate boundary when the incoming plate carries buoyant continental crust. The geometry of Fig. 2 also suggests a transition of Z-type D2-folds to the left of the syntaxial axis, to M-type folds along its axis, to S-type folds on the opposite side.