ALBERT

Description: Paleoproterozoic rocks in the Black Hills of southwestern South Dakota record deformation associated with accretion of Proterozoic terranes from the south and suturing of the Wyoming and Superior cratons (ca. 1780–1715 Ma). Folding associated with suturing of these Precambrian terranes is well documented. However, penetrative fabrics related to shearing are characterized in only a few localities. The research presented herein identifies and characterizes a ≥4-km-wide, pervasive, thick-skinned shear zone, which deforms the regional-scale folding in the Black Hills portion of the Wyoming craton. Shearing is bracketed between ca. 1740 Ma (post regional-F 2 folding) and ca. 1715 Ma (intrusion of the Harney Peak Granite). This high-strain zone is characterized by narrow anastomosing shear zones (m-scale) in the Paleoproterozoic rocks in the east-central Black Hills near Rockerville, South Dakota. The shear zones converge to form a km-wide zone of deformation in Paleoproterozoic and Archean rocks to the north near Nemo, South Dakota. Kinematic indicators such as composite foliations, microfolding, and asymmetric mantled porphyroclasts from within the shear zone support left-lateral, east-side-up transpression. Furthermore, strain associated with the shearing is coupled to vertically plunging, isoclinal F 3 folds, commonly identified only as hinge areas preserved between strongly sheared limbs, within and adjacent to the shear zone in the study area. Correlation of this deformational event from the Paleoproterozoic rocks in the east-central Black Hills to older Paleoproterozoic and Archean rocks to the north has implications for these structures to have formed during a basement-involved deformational event. Evidence that stresses associated with this ~1740–1715 Ma event were transferred inboard to the Wyoming Province is present to the southwest in the Hartville Uplift and Laramie Mountains of southeastern Wyoming. In both locations, structures with similar timing and kinematics overprint Cheyenne-Belt deformation. The regional extent and thick-skinned nature of these structures are interpreted to signify the change to continent-continent collisional tectonics during the final suturing of the Wyoming and Superior Provinces.