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Liam T. Fry, Eckerd College, Marine Science Discipline
Laura R. Wetzel, Eckerd College, Marine Science Discipline
Trevor S. Waldien, South Dakota School of Mines & Technology, Department of Geology and Geological Engineering
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). 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. 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 support previous interpretations of transpressional deformation within the LEG and metasedimentary rocks sheared along its western margin. Results suggest that the type 1 fabric formed prior to suturing of the Wyoming and Superior Cratons and the type 2 fabric formed during craton suturing.
For More Information: ltfry@eckerd.edu | wetzellr@eckerd.edu | Trevor.Waldien@sdsmt.edu
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