Metamorphic evolution of the Great Slave Lake shear zone

Brendan Dyck, Rellie M. Goddard, David Wallis, Lars N. Hansen, Edith Martel

Research output: Contribution to journalArticlepeer-review

Abstract

The Palaeoproterozoic Great Slave Lake shear zone (GSLsz) is a crustal-scale strike-slip structure, with a total length of >1,000 km and a width of ~25 km, that separates the Archean Rae and Slave cratons. The range of metamorphic rocks now exposed at the surface encompasses granulite facies mylonite through to lower-greenschist facies ultramylonite and cataclasite, providing a potential type example of fault-zone structure in the middle and lower crust. However, the metamorphic evolution of the units remains poorly quantified, hindering detailed structural and tectonic interpretations. Here, we use phase equilibria modelling and thermobarometry to determine the metamorphic conditions recorded by pelitic, mafic and felsic GSLsz mylonites. Samples from the entire range of granulite–greenschist facies units preserve evidence for nested clockwise pressure–temperature paths that are consistent with a single orogenic cycle. Our findings indicate that the northern Rae margin underwent pervasive crustal thickening with peak pressures in metasedimentary rocks reaching ~1.1 GPa. The crustal thermal gradient at the onset of thickening was ~650°C/GPa, whereas the final stages of equilibrium recorded by fine-grained matrix minerals in all samples collectively define a metamorphic field gradient of ~1,000°C/GPa. Deformation microstructures are consistent with the main phase of dextral shear having been synchronous with or following peak metamorphism. The history of metamorphism and exhumation of the GSLsz is consistent with the Sibson–Scholz model for shear zones, with a narrowing of the deforming zone and the progressive overprinting of higher-grade assemblages during exhumation through shallower crustal levels.

Original languageEnglish (US)
Pages (from-to)567-590
Number of pages24
JournalJournal of Metamorphic Geology
Volume39
Issue number5
DOIs
StatePublished - Oct 24 2020

Bibliographical note

Funding Information:
Funding for this research was provided by a Natural Sciences and Engineering Research Council of Canada Discovery Grant to Dyck, Natural Environment Research Council Environmental Doctoral Training Grant to Goddard and Utrecht University start‐up funds to Wallis. Additional fieldwork funding and support was offered by University College Oxford, The Geological Society and the Northwest Territories Geological Survey (NTGS contribution #0122). We thank Kyle Larson and Lorraine Tual for insightful reviews and Julia Baldwin for editorial handling.

Funding Information:
Funding for this research was provided by a Natural Sciences and Engineering Research Council of Canada Discovery Grant to Dyck, Natural Environment Research Council Environmental Doctoral Training Grant to Goddard and Utrecht University start-up funds to Wallis. Additional fieldwork funding and support was offered by University College Oxford, The Geological Society and the Northwest Territories Geological Survey (NTGS contribution #0122). We thank Kyle Larson and Lorraine Tual for insightful reviews and Julia Baldwin for editorial handling.

Publisher Copyright:
© 2021 The Authors. Journal of Metamorphic Geology published by John Wiley & Sons Ltd

Keywords

  • mylonite
  • phase equilibria modelling
  • shear zone
  • thermometry

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