Transtensional origin of multi-order cross-folds in a high-grade gneiss complex, southwestern Grenville Province: formation during post-peak gravitational collapse

Abstract

The well-mapped western part of the Ottawa River Gneiss Complex (ORGC; new name), a large metamorphic core complex, hosts a system of gently plunging cross-folds of outcrop to regional scale situated in the ductile detachment zone between the lower grade cover and high-grade core of the complex. The cross-folds are buckle structures that deform the attenuated gneissic layering, plunge parallel to the regional elongation lineation, range from upright to recumbent, and exhibit distinctive hinge-parallel elongation lineations, all features of extension-dominated ductile transtension. Our L–S fabric data are consistent both with kinematic modelling that predicts progressive constrictional strain in the hinge zones of transtensional folds, and with dynamic modelling that predicts rotation of flattened fold limbs into moderately dipping attitudes. On the basis of petrologic data, we show that cross-folding postdated the peak Ottawan metamorphism and took place during retrogression and exhumation of the thrust-sheet stack. The cross-folds form an inclined system that is principally developed in retrograde melt-weakened amphibolite-facies rocks, with the transtensional origin implying that exhumation and retrogression of the high-grade core of the complex took place in an oblique extensional setting. A transtensional origin for the cross-folds removes the need to appeal to orogen-parallel regional shortening, an implausible requirement of previous interpretations, and is compatible with data indicating that much of the visible fabric and structure of amphibolite-facies domains of the ORGC developed during post-peak exhumation, retrogression, and gravitational collapse of the thrust-sheet stack.

Devonian–Mississippian collapse and core complex exhumation, and partial decoupling and partitioning of Eurekan deformation as alternatives to the Ellesmerian Orogeny in Spitsbergen

“…CC BY 4.0 License. 1998; Krabbendam and Dewey, 1998), California (Fletcher and Bartley, 1994), and eastern Canada (Schwerdtner et al, 2016). Notably, in eastern Canada and California, such folds are associated 1205 with the exhumation of metamorphic core complexes (Fletcher and Bartley, 1994;Schwerdtner et al, 2016), thus suggesting that folds in Devonian rocks in Spitsbergen (e.g., in Røkensåta; Figure 1a) might have formed during Devonian-Mississippian core complex exhumation.…”

“…1998; Krabbendam and Dewey, 1998), California (Fletcher and Bartley, 1994), and eastern Canada (Schwerdtner et al, 2016). Notably, in eastern Canada and California, such folds are associated 1205 with the exhumation of metamorphic core complexes (Fletcher and Bartley, 1994;Schwerdtner et al, 2016), thus suggesting that folds in Devonian rocks in Spitsbergen (e.g., in Røkensåta; Figure 1a) might have formed during Devonian-Mississippian core complex exhumation. A formation as transtensional folds of presumed (contractional-transpressional) Ellesmerian structures in southern Spitsbergen is further supported by their NW-SE to NNW-SSE trend (Bergh et al, 2011(Bergh et al, , their 1210 figure 8b), i.e., sub-parallel to slightly oblique to the WNW-ESE-to NW-SE-oriented extension direction in Mississippian times inferred by Koehl and Muñoz-Barrera (2018), which is typical for transtensional folds (Fossen et al, 2013), e.g., in western Norway where transtensional folds in Middle Devonian collapse basins formed parallel to the extension direction during a late phase of the extensional collapse of the Caledonides (Chauvet and Séranne, 1994;Osmundsen and 1215 Andersen, 1994).…”

Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

“…In the Rivers (2012) model, the CMBbz was near or at the boundary of the ductile mid-crust beneath a rigid orogenic plateau that developed during the peak Ottawan and later phases of the orogeny. Rivers and Schwerdtner (2015) and Schwerdtner et al (2016) recently extended this model to suggest that the Gneiss Belt in the CMBbz footwall was essentially a late Ottawan metamorphic core-complex-type shear zone with transtensional kinematics.…”

Late Ottawan orogenic collapse of the Adirondacks in the Grenville province of New York State (USA): Integrated petrologic, geochronologic, and structural analysis of the Diana Complex in the southern Carthage-Colton mylonite zone

“…Modeling studies of metamorphic core complex formation (Lavier et al, 1999;Platt et al, 2014) demonstrate that the mid-crustal mylonite zone can be folded and rotated to steep orientation during doming and may explain such characteristics found in the northern half of the CCMZ. However, an alternative explanation for the locally folded character of the CCMZ is that metamorphic core complexes or ductilely extended crust commonly exhibits corrugations or cross-folds (e.g., Fossen, 2016;Brown et al, 2016;Schwerdtner et al, 2016), produced by transtension (Fossen et al, 2013). Lyon Mountain Granite Gneiss intruded the Adirondack Highlands too during exhumation and locally along the CCMZ (ca.…”

“…0.98 Ga (Rivers, 2008;McLelland et al, 2010). However, recent work within the Grenville province also emphasizes the importance of tectonic collapse late in its history, as exemplified by work in the Adirondacks, the Morin terrane, the east side of the Mékinac-Taureau domain, and the Ottawa River Gneiss Complex Wong et al, 2011;Rivers and Schwerdtner, 2015;Soucy La Roche et al, 2015;Schwerdtner et al, 2016;Dufréchou, 2017;Regan et al, 2019).…”