W.A. Sullivan

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 ∼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 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Deformation path partitioning within the transpressional White Mountain shear zone, California and Nevada

\mathrm{L}\,> \mathrm{S}\,

Comparative anatomy of core-complex development in the northeastern Great Basin, U.S.A.

\end{document} 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.

Asymmetrical quartz crystallographic fabrics formed during constrictional deformation

The type locality of the Archean–Paleoproterozoic suture zone in the southern Rocky Mountains is marked by a series of subvertical shear zones collectively called the Cheyenne belt. The Cheyenne belt is a key structure for developing models for 1780–1740 Ma tectonism along the southern margin of the Archean Wyoming Province, which heralded a rapid period of continental amalgamation. This paper tests existing structural and plate-tectonic models for the Cheyenne belt with detailed geologic mapping, kinematic analyses, quartz crystallographic fabric analyses, and deformation mechanism analyses of the northern mylonite zone of the eastern Medicine Bow Mountains. Mylonites of this zone record a complex deformation history, but the main deformation phase was sinistral/northwest-side-up oblique transpression. Evidence for southeast-side-up, dip-slip motion that characterizes many other areas of the belt is confined to ultramylonites immediately adjacent to the terrane boundary. Hence, fabrics related to sinistral transpression were likely overprinted by southeast-side-up motion. Sinistral strike-slip motion is recorded in at least two other localities in the Cheyenne belt. Because synmetamorphic fabrics on both sides of the suture zone record sinistral strike-slip and northwest-side-up motion, this was probably the dominant deformation style in the field area and may have been the dominant deformation style throughout the Cheyenne belt. Based on these data and regional constraints, we interpret the Cheyenne belt as a subvertical transpressional stretching fault system that simultaneously accommodated sinistral strike-slip motion, penetrative horizontal shortening, and dip-slip motion related to differential crustal thickening between the relatively cold Wyoming Province and younger, hotter rocks to the south.