Paleoproterozoic transpressional shear zone, eastern Black Hills, South Dakota: Implications for the late tectonic history of the southern Trans-Hudson Orogen

Abstract

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(ca. -1715. 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.

Crustal structure of the Bighorn Mountains region: Precambrian influence on Laramide shortening and uplift in north‐central Wyoming

“…Aeromagnetic anomalies within this belt are shown as dashed ovals. Deformation and metamorphic ages within the belt range from 1830 to 1715 Ma [ Dahl et al ., ; Krugh , ] and include 1740–1715 Ma strike‐slip couplet in the Black Hills [ Allard and Portis , ] and 1715 Ma thrust fault and shear zones in the Hartville Uplift [ Krugh et al ., ; Krugh , ]. Various names have been invoked for phases of this complex history including Trans‐Hudson orogen (THO) [ Lewry and Collerson , 1990], Black Hill (BH) orogen [ Dahl et al ., ], Dakotan orogen (DO) [ Chamberlain et al ., ], and Central Plain orogen [ Sims and Peterman , 1986].…”

“…This orientation requires that the Black Hills subsequently moved north to their current position. Proterozoic (~1.75–1.715 Ga) northwest‐southeast trending strike‐slip shear zones surrounding the Black Hills [ Allard and Portis , ] and northwest directed ~1.715 Ga thrust faults in the Hartville Uplift [ Chamberlain et al ., ; Krugh , ; Krugh et al ., ] fit this model, providing a mechanism for transport of a continental fragment that includes the Black Hills, separate from the Archean Wyoming craton.…”

Wyoming on the run—Toward final Paleoproterozoic assembly of Laurentia: REPLY

“…Northward shortening in the BH at 1.78 Ga is modeled to relate to Cheyenne belt accretion, but the BH may have been several 100 km south of their current position, along strike of the Cheyenne belt ca. 1.78 Ga-an idea supported by 1740-1715 Ma north-south strike-slip shear zones along the eastern and western flanks of the BH (Allard and Portis, 2013).…”