Precambrian rocks in the Black Hills record multiple tectonic processes, including suturing of the Wyoming and Superior cratons from ca. 1.740-1.715 Ga. To date, studies focused on this suturing event have primarily focused on prograde metamorphism and structures that record shortening between the cratonic blocks. However, a strike-slip shear zone subparallel to the shortening structures, named the Dakota Tectonic Zone (DTZ), has also been documented but is poorly understood. We examined intracrystalline deformation and associated microstructures in oriented thin sections of the Little Elk Granite (2.560 Ga mylonitized augen gneiss) within the mapped domain of the DTZ to further document how the strike-slip deformation fits into the Precambrian structural evolution of the Black Hills. At the outcrop scale, the Little Elk Granite contains two types of fabrics. Fabric type 1 is an augen gneiss fabric characterized by alignment of ~1-5 cm K-feldspar crystals that is interpreted to have formed during emplacement of the Little Elk Granite. Fabric type 2 cross-cuts the augen gneiss fabric and is characterized by comminution of the large K-feldspar grains within mylonitic shear zones. Whereas the type 1 fabric is folded throughout the field area, the type 2 shear fabric is consistently oriented at ~150/70°SW and contains a down-dip stretching lineation. Oriented thin sections cut perpendicular to foliation and parallel to lineation contain broken feldspar crystals that in some cases also exhibit undulose extinction. Domains between paired fragments of broken feldspar crystals are filled in with equant polycrystalline quartz aggregates and are regularly oriented at a high angle (>45°) to the shear foliation. Quartz-rich domains in the type 2 fabric generally display undulose extinction and dynamic recrystallization textures. Kinematic indicators from asymmetric strain shadows associated with feldspar porphyroclasts and asymmetrically folded micas yield dominantly top-to-the-left sense of motion, but top-to-the-right shear sense is also common (46%). The sum of microstructural data from the Little Elk Granite suggests that the DTZ is an upper greenschist facies (~300-450°C) left-lateral pure shear dominated transpression zone that likely formed late in the suturing of the Wyoming and Superior cratons.
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Distributed crustal shortening followed by transpressional shearing in the Superior Province, northeastern Canada: A Late Archean analogy to modern accretionary plate margins?
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 Qu´ebec, which intruded the southern, outer edge of the
presumed magmatic arc (Bienville domain) along its ~WNW–ESE-trending tectonic boundary with the protocratonic
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 ~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 (~NNE–SSW shortening and boundaryparallel
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 arclike
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.
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- Award ID(s):
- 1918357
- NSF-PAR ID:
- 10296956
- Date Published:
- Journal Name:
- Precambrian research
- ISSN:
- 0301-9268
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/doi.org/10.1016/j.precamres.2021.106322
