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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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The influence of Precambrian fabric reactivation on formation of the Laramide White Gates monocline, Black Hills, South Dakota
The Black Hills, in western South Dakota and eastern Wyoming, were formed by Laramide orogeny deformation that was focused on a Precambrian suture along the eastern margin of the Wyoming Craton. Uplift of the Black Hills primarily took place by the development of monoclines on the eastern and western flanks of the Black Hills: west-vergent monoclines in the west and east-vergent monoclines in the east. Although the exhumed metamorphic core of the Black Hills contains abundant Precambrian structures that could have been reactivated during the Laramide orogeny, it remains unclear if the monoclines formed above reactivated basement structures. We present new balanced cross section modeling focused on the White Gates monocline along the eastern margin of the Black Hills to test whether it records reactivation of Precambrian basement structures. To better determine the geometry of the White Gates Monocline, we collected 22 bedding attitude measurements from the upper Deadwood and the Pahasapa formations along an 1,723-meter-long transect across the strike of the fold axis. We forward modeled monocline development related to slip on blind thrusts in three dip orientations: 30° (Andersonian thrust fault), 45° (maximum resolved shear stress), and 70° (orientation of nearby basement fabrics). Preliminary model results reveal that the 70° fault dip angle produces fold geometries most consistent with the geometry of the White Gate monocline. This result suggests that the reactivation of Precambrian fabrics during the Laramide orogeny influenced the formation of the White Gates monocline. Elsewhere in the Precambrian core of the Black Hills, conjugate thrust faults inferred to be Laramide age clearly cross-cut basement fabrics, which suggests that the role of structural reactivation in Laramide deformation varies spatially throughout the Black Hills.
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- Award ID(s):
- 2050697
- PAR ID:
- 10508869
- 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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