Structural significance of L tectonites in the eastern-central Laramie Mountains, Wyoming

The formation of L tectonites is little understood and scarcely studied; however, it is probably an important part of plastic deformation in the crust. To improve our understanding of this strain phenomenon, I present a detailed case study of a kilometer-wide domain of L tectonites developed in and around the approximately 2.05-Ga Boy Scout Camp Granodiorite (BSCG) in the Laramie Mountains, Wyoming. Detailed mapping and structural analyses allow for the reconstruction of the structural setting of this domain of apparent constrictional strain. Elongation lineations plunge moderately to the south-southwest and lie parallel with both the local fold hinge lines and regional fold axes, whereas poles to foliation generally cluster in the northwest quadrant, roughly defining fold axial surfaces. Map-scale folds are west-northwest vergent, but at the outcrop and thin-section scales, there is no evidence for a significant component of simple shear. Reconstruction of the orientation of contacts in and around the BSCG indicates that L tectonites have developed in the hinge zone of a large synform. Deformation fabrics die out to both the east and the west of the map area. These data indicate that the domain of L and L > S tectonites is accommodating oblique extrusion of material parallel with the axis of folding between two relatively rigid crustal blocks. Correlation with other deformation fabrics in the central Laramie Mountains indicates that this structure probably developed during northwest-directed contractional deformation during the 1.78-1.74-Ga Medicine Bow Orogeny.