OROGENIC SUTURES IN THE ARCHEAN SUPERIOR PROVINCE, CANADA – A COUNTEREXAMPLE FROM NORTHERN QUEBEC

Orogenic sutures mark the consumption of oceanic lithosphere by subduction. They are underlined by major faults within collisional orogens, between terranes of contrasting geological evolution. Archean cratons consist of greenstone belts juxtaposed to deeper crustal rocks dominated by TTG’s. The greenstone belts show steep folds and localised shear zones, whereas plutonic belts display dome structures. These rocks usually preserve MT/HT-LP/MP metamorphism that varies from greenschist- to granulite-facies, from upper to lower crustal domains. The nature of Archean tectonism remains debated: were Archean processes similar to those of present-day plate tectonics? Archean greenstone belts thus recording plate convergence and subduction/collision, or was Archean tectonics different? in which case, burying and exhumation of crustal rocks can be mainly attributed to vertical tectonics.

We present a structural and metamorphic study of the Abitibi Greenstone Belt (AGB) and Opatica Plutonic Belt (OPB) of the Archean Superior Province in Quebec. The AGB-OPB contact is currently interpreted as an archetype example of Archean subduction, based on a LITHOPROBE seismic profile showing a North-dipping lithospheric-scale reflector interpreted as the vestige of subduction of an AGB plate. However, our mapping indicates that the AGB overlies the OPB. Moreover, that contact does not show evidences of significant shear deformation, as expected if it is a major upper plate-lower plate boundary. Furthermore, it does not show any metamorphic break but a progressive increase of metamorphism toward the OPB, from greenschist- to amphibolite-facies. Thus, we think that the OPB exposes the deepest part of a composite AGB-OPB crustal sequence. ⁴⁰Ar/³⁹Ar ages from the study area suggest that from ~2685 Ma to ~2632 Ma, the deepest level of the AGB and underlying OPB reached amphibolite-facies conditions, and that progressive cooling was accompanied by strain localisation along strike-slip shear zones, when lateral flow of the lower crust became predominant over vertical tectonics after ~2600 Ma. Comparison with adjacent areas suggests that regional metamorphism has been coeval over a large region, which is consistent with pervasive deformation and slow cooling as expected in Archean vertical tectonic models.