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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This content will become publicly available on May 16, 2025
Kinematic analysis of Little Elk Granite shear zones, Black Hills South Dakota
The Black Hills of western South Dakota and eastern Wyoming were uplifted as a result of the Laramide orogeny occupying the suture between the Wyoming and Superior Cratons. Two exposures of Archean orthogneiss, the Bear Mountain Terrane and the Little Elk Granite (LEG), represent the oldest rocks exposed in the Precambrian core of the Black Hills and offer the opportunity to study tectonic processes involved in forming the Laurentian craton. This study presents new structural field data (orientation of foliation planes, stretching lineations, and cross cutting relations; n=270 measurements) along a ~5 km transect that record the deformation history of the LEG. Two dominant fabric types were found in outcrop: augen gneiss (type 1) and mylonitized granite (type 2). The type 1 fabric is characterized by 1-5 cm K-feldspar crystals aligned to give top-down or “normal” sense of shear, small-scale folding of the fabric, and is cross-cut by aplite dikes in multiple sites. The type 2 mylonitic fabric overprints the type 1 fabric and intensifies from east to west along the transect, resulting in a loss of the type 1 fabric. The stretching lineation in the type 2 fabric plunges down dip with shear sense indicators observable in outcrop. Both fabrics display a NW/SE striking and ~70°SW dipping foliation at every site. Yet, subtle folding of the type 1 fabric at some sites causes it to be crosscut by the type 2 fabric. Based on the high-temperature deformation features in the type 1 fabric and the cross-cutting relationship with aplite dikes, we interpret that the type 1 fabric formed during emplacement of the granite. Assuming the LEG has not experienced significant tilting since emplacement, the top-down shear sense recorded by alignment of K-feldspar may suggest emplacement of the LEG into an extensional setting. Our observations of the type 2 fabric, including down-plunge stretching lineations and opposing shear sense indicators support previous interpretations of transpressional deformation within the LEG and metasedimentary rocks sheared along its western margin. With the new data describing shear zone kinematics in the LEG, we interpret that the type 1 fabric formed prior to suturing of the Wyoming and Superior Cratons and the type 2 fabric formed during craton suturing.
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- Award ID(s):
- 2050697
- NSF-PAR ID:
- 10508871
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Volume:
- 56
- Issue:
- 4
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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