Fried Schwerdtner (Emeritus)
University of Toronto
Department of Earth Sciences
22 Russell Str. Toronto
Ontario, Canada M5S 3B1
Tel.: 416 978 5080
Fax.: 416 978 3938
Late-orogenic cross-folds in the Ottawa River Gneiss Complex, western Grenville Province: Shear-induced sheath folds or products of superimposed deformation?
Field-based litho-structural study of the Ottawa River gneiss complex (Ontario portion), a large metamorphic core complex in the western Grenville Province
The Grenville Province of the Canadian Shield, a 2000-km-long segment of the Mesoproterozoic Grenville Orogen, includes several large metamorphic core complexes (LMCCs; Rivers 2012, Can. J. Earth Sci., 49: 1−42). Since 2011, Rivers, Schwerdtner and their junior assistants (undergraduate students) have investigated the well-exposed parts of one LMCC core (see references listed below), the Ottawa River gneiss complex (ORGC, see geological sketch map below the following paragraphs. The ORGC consists of parautochthonous and allochthonous high-grade metamorphic rocks with Nd model ages ranging from Archean to Mesoproterozoic (Dickin 2000, Can. J. Earth Sci., 37: 165−181). However, the northwestern limit of the ORGC remains uncertain, and some aspects of the core-detachment process are poorly understood and warrant additional investigation.
According to the model of Rivers (2012) and Rivers and Schwerdtner (2015), the ORGC comprises a large extensional bending fold produced by late-Ottawan (~1045−1020 Ma) vertical thinning of the early-Ottawan (1090−1050 Ma) thrust-sheet stack. Rigolet (1015−990 Ma) bulk distortion of the northwestern parautochthonous part of ORGC, known as the Grenville Front Tectonic Zone (GFTZ, see sketch map below), resulted in superimposed strain and a profound metamorphic overprint of its rocks, and led to major thrusting at the Grenville Front. Using structural and metamorphic criteria, we will investigate in 2018-19 whether the Rigolet net-slip along the Grenville Front thrust removed a slice of the original LMCC from the present erosion level and whether the associated ductile shearing of the hanging wall resulted in the marked structural grain of the GFTZ.
Results of our recent work suggest that the late-Ottawan extensional bending of the ORGC and its overlying detachment zone gave rise to orogen-perpendicular, axially-lineated, multi-scale folds that have been termed cross-folds (Rivers and Schwerdtner 2014, 2015, Schwerdtner et al. 2016, see reference lists added below). Comparable orogen-perpendicular structures have been observed in Phanerozoic metamorphic core complexes in the Cycladic (Alpine) orogenic belt (e.g., Jolivet et al. 2004, their Fig. 7; Geol. Soc. Amer. Spec. Paper, 380: 185−210) and the North American Cordillera (e.g., Singleton 2013, Geol. Soc. Amer. Bull., 125: 453−472; Brown et al. 2015, Lithosphere, 8: 412−421), and in the latter orogen they have been referred to as corrugations. In contrast to the topographic surface cutting through the upper parts of most Phanerozoic metamorphic core complexes, the Precambrian peneplain through the ORGC cuts the complex at a deep structural level. Thus, Rivers and Schwerdtner have been able to document that multi-scale cross-folds characterize the entire exposed core structure of the LMCC, and not only the detachment zone.
In general, where cross-folds occur in metamorphic core complexes, but especially where they affect moderately-strained granitoid and gabbroid sheets with deformed igneous megacrysts, they are characterized by L-S mineral shape fabrics that vary systematically with position, with the presence of L>>S fabrics in the hinge zones being indicative of a quasi-constrictional strain state (e.g., Schwerdtner et al. 2010, Geol. Soc. Amer. Memoir 20: 773−794; Singleton 2013, Geol. Soc. Amer. Bull., 125: 453−472). In the case of the ORGC, both the fabric-variation pattern and the presence of sinistral and sinistral-normal shear zones within and adjacent to the eastern ORGC in southwest Québec (see sketch map below), suggest that the cross-folds of the ORGC formed by transtensional deformation as the rocks were exhumed from mid- to upper-crustal levels (Rivers and Schwerdtner 2015; Schwerdtner et al. 2016, see reference lists added below).
In 2018-19, Rivers, Schwerdtner and their junior assistants will undertake further field-based research in order to answer the following questions: (1) What are the areal distribution, preferred orientation, level of bulk strain, structural style, and metamorphic grade of multi-scale cross-folds within and southeast of the GFTZ in Ontario? (2) Does the Grenville Front overprint the northwest boundary of the original LMCC? (3) Did late-Ottawan crustal extension create, or at least strongly affect, the L-S shape fabrics of high-strain zones marking the subdomain boundaries in the northern Muskoka Lakes region?
Schwerdtner, W.M., Rivers, T., Tsolas, J., Waddington, D.H., Page, S., and Jang, J. 2016. Transtensional origin of multi-order cross-folds in a high-grade gneiss complex, south- western Grenville Province: formation during postpeak gravitational collapse. Canadian Journal of Earth Sciences, 53: 1511-1538.
Rivers, T. and Schwerdtner, W.M. 2015. Post-peak evolution of the Muskoka Domain, western Grenville Province: ductile detachment zone in a crustal-scale metamorphic core complex. Geoscience Canada, 42: 403-436.
Schwerdtner, W.M.,Rivers, T., Zeeman, B., Wang, C.C., Tsolas, J., Yang, J., and Ahmed, M. 2014. Post-convergent structures in lower parts of the 1090-1050 Ma (early Ottawan) thrust-sheet stack, Grenville Province of Ontario, southern Canadian Shield. Canadian Journal of Earth Sciences, 51 243-265
Schwerdtner, W.M. 2013. Photograph of the month. Journal of Structural Geology, 48 (October issue): 1-2.
Schwerdtner, W.M. and Yang, J.F. 2011. Photograph of the Month: Pseudotachylite at the basal contact of the Parry Sound allochthon, Grenville Province of Ontario, Canada. Journal of Structural Geology, 33: 1714.
Schwerdtner, W.M., Lu, S. J. and Yang, J.F., 2010. Wall-rock structure at the present contact surfaces between repeatedly deformed thrust sheets, Grenville Orogen of central Ontario, Canada. Canadian Journal of Earth Sciences, 47: 875-899.
Schwerdtner, W.M., Lu, S.J. and Landa, D., 2010. S/Z buckle folds as shear-sense indicators in the ductile realm: Field examples from the Grenville Province of Ontario and the Appalachians of South Carolina. In ‘From Rodinia to Pangea: The Litho- tectonic record of the Applachian Region, Geological Society of America Memoir 206, Chapter 30, p. 773-794.
Schwerdtner, W.M. and Klemens, W.P., 2008. Structure of the Ahmic domain and its vicinity, southwestern Central Gneiss Belt, Grenville Province of Ontario (Canada). Precambrian Research, 167: 16 – 34.
Schwerdtner, W.M., Riller, U.P. and Borowik, A., 2005. Structural testing of tectonic
hypotheses by field-based analysis of distributed tangential shear: examples from major high-strain zones in the Grenville Province and other parts of the Canadian Shield. Canadian Journal of Earth Sciences, 42: 1927 – 1947.
Schwerdtner, W.M., Downey, M.W. and Alexander, S.A., 2004. L-S shape fabrics in the Mazinaw domain and the issue of NW-directed thrusting in the Composite Arc Belt, southeastern Ontario. Geological Society of America, Memoir 197, p. 183 – 2008.
Rivers, T. and Schwerdtner, W.M. 2014. New ideas on the post-peak development of the Central Gneiss Belt in the Muskoka Region. Field trip guide, Friends of the Grenville weekend in Gravenhurst, Ontario; October 3-5, 2014.
Schwerdtner, W.M., Robin, P.-Y. F., Alexander, S., Burke, J., Downey, M.W., and Serafini, G., 2003. Connection between Grenvillian deformation and granitoid pluton emplacement in the Composite arc Belt (Tweed-Kaladar area), southestern Ontario, Canada. Guidebook for Friends of the Grenville (Amis du Grenville) field trip, Department of Geology, University of Toronto.
Schwerdtner, W.M., Klemens, W.P., Robin, P.-Y. F., Vertolli. V.M., and Waddington, D.H., 2005. Geological structure and lithology in parts of the Muskoka region, southwestern Central Gneiss Belt, Grenville Province of Ontario. Field trip guide, 25th annual workshop of the Canadian Tectonics Group and Structural Geology-Tectonics Division of the Geological Association of Canada, Department of Geology, University of Toronto.
Schwerdtner, W.M., Dickin, A.P. and Robin, P.-Y. F., 2009. Rocks, structures and tectonic scenarios: Introduction to the Grenville Province of Ontario. Guidebook for a pre-sessions field trip (Geological Association of Canada), Joint Assembly (AGU- GAC-MAC Meeting), Department of Geology, University of Toronto.