Muskoka Domain as a First-Order Detachment Zone during Exhumation and Orogenic Collapse: Support for a Core Complex Model for the Central Gneiss Belt, Western Grenville Province

Toby Rivers, Memorial University of Newfoundland, Earth Sciences, St John's, NF, Canada and Walfried Martin Schwerdtner, University of Toronto, Earth Sciences, Toronto, ON, Canada

Contact First Author: Toby Rivers; trivers@mun.ca

Previously Published Material: Paper is an overview. A small component of the information was reported by Schwerdtner et al (2014) in CJES. Other aspects were reported in a Field Trip Guide for Friends of the Grenville (FOG) in October 2014. This field trip guide has not been widely circulated. The overall message of the abstract has not been reported elsewhere.

Abstract ID#: 33352

 

English Abstract:
The Central Gneiss Belt (CGB) in the western Grenville Province of Ontario and Quebec is a large high-grade gneiss terrane with a surface area of >60,000 km2. It consists of a crustal-scale stack of thrust sheets assembled during the Ottawan phase of the Grenvillian orogeny at ~1080 Ma, and is juxtaposed on its southeastern side against the lower grade Composite Arc Belt (CAB). Our work has focussed on the southern margin of the CGB (Ontario segment), where the granulite-facies rocks of the CGB (Algonquin domain) are separated from the CAB by a several km-thick, predominantly upper amphibolite-facies sheet known as the Muskoka domain.

On the basis of local, but widespread relict granulite-facies mineralogy and decompression textures, we infer that peak Ottawan metamorphic conditions in the Muskoka domain were comparable to those in the underlying Algonquin domain, and that the main amphibolite-facies mineralogy and high-strain planar gneissic fabrics characteristic of the Muskoka domain developed after the metamorphic peak during exhumation and retrogression. Post-peak exhumation and retrogression in the Muskoka domain began at ~1060 Ma while the rocks were still hot, and was associated with important ductile crustal thinning, orogen-perpendicular extension, transtensional folding and boudinage on all scales. Related fluid influx promoted widespread migmatization, which caused rheological weakening, with the result that the Muskoka domain became a mobile zone akin to a ductile glide horizon between two more competent units.

Collectively this evidence supports a core complex model for the exhumation of the CGB, in which the Muskoka domain formed a weak extensional detachment zone as the underlying hot granulite-facies mid crust rose into the thinned neck regions of the boudinaged overlying cooler amphibolite-facies upper crust during prolonged extensional collapse.

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