Distributed crustal shortening followed by transpressional shearing in the Superior Province, northeastern Canada: A Late Archean analogy to modern accretionary plate margins?
Abstrakt
The Canadian Superior Province has become one of the key test pieces to discuss tectonic processes and mechanisms of crustal growth in the Late Archean. The Province consists of a >2.8 Ga proto-cratonic core intruded by voluminous arc-like plutons and surrounded by a series of narrow, elongate ca. 2.8-2.7 Ga juvenile belts, also referred to as terranes or domains.
The terranes seem to wrap around the proto-cratonic core and generally young outward, but the kinematics and geodynamic causes of their assembly remain debated. In this paper, we examine the Radisson pluton in northeastern Quebec, which intruded the southern, outer edge of the presumed magmatic arc (Bienville domain) along its similar to WNW-ESE-trending tectonic boundary with the proto-cratonic crust (La Grande domain).
The pluton, dominated by porphyritic monzogranite to quartz monzonite, was emplaced at around 2712 Ma and exhibits complex internal structure resulting from superposed magmatic to solid-state deformations. An early margin-parallel similar to WNW-ESE magmatic foliation containing a steep lineation, recognized by the anisotropy of magnetic susceptibility (AMS), is interpreted as recording vertical stretching and horizontal flattening of highly crystallized magma, either due to emplacement and/or pure shear dominated transpression.
More widespread, however, is a horizontal lineation within the same foliation that is interpreted as recording post-emplacement, but still syn-magmatic, tectonic strain (similar to NNE-SSW shortening and boundary-parallel stretching). Upon cooling, localized dextral S-C mylonite zones accommodated further shortening within the pluton whereas undeformed late-stage felsic dikes cross-cut the solid-state fabric at an angle to the pluton margins.
We suggest that this structural succession, also reproduced by numerical fabric modeling, is a local-scale signal of a two-stage assembly of the northeastern Superior Province: the frontal, NNE-directed terrane convergence and attachment to the cratonic nucleus, operating in a 'hot' regime with voluminous arc-like plutonism, was followed by more localized dextral shearing parallel to terrane boundaries. The latter phase is recorded at the proto-craton margin but also in the outboard Abitibi greenstone belt virtually at the same time (ca. 2700-2690 Ma).
In combination, the two-stage evolution and similar deformation distributed over a broad region resemble modern large hot orogens formed in a plate-tectonic regime.