THICK-SKINNED PALEOPROTEROZOIC SHEAR-FOLD COUPLED DEFORMATION ALONG THE EASTERN MARGIN OF THE BLACK HILLS, SOUTH DAKOTA

Detailed structural mapping in the east-central Black Hills near Rockerville S.D. revealed a zone of transpression expressed as a shear-fold couple in Proterozoic supracrustal rocks. This 7–8 km wide zone includes a central high-strain domain (CHSD) bound on both sides by F₃ fold domains. The CHSD contains a NW-striking, near-vertical mylonitic fabric (S_m) with a steeply-plunging stretching lineation (L_m). East-side up, left-lateral transpression is interpreted from porphyroclast asymmetry visible in outcrop and in thin section. NE and SW of the CHSD are fold domains containing tight, upright folds (F₃) with axes plunging vertically or steeply to the SE, parallel to L_m. A spaced crenulation cleavage (S₃), oriented parallel to S_m, is well developed in the hinge areas of F₃ folds. Outside these domains, supracrustal rocks contain only an axial planar S₂ related to the upright, horizontal folds (F₂) that define the dominant structural across the hills.

Microtextures show M₁ (ca. 1750 Ma) garnet and staurolite overgrow S₂, yet asymmetric strain shadows and wrapping of S₂ around M₁ garnets within the CHSD requires a post-M₁ age for the transpressional event. Static replacement of M₁ garnets by quartz, biotite and chlorite and of S₂ biotite by chlorite, along with biotite growth across both S_m and S₃, requires cessation of transpression prior to biotite-chlorite grade M₂ metamorphism (ca.1715 Ma).

Our research to the north in the Little Elk Terrane identified a kinematically similar shear fabric in both Archean basement and Proterozoic supracrustal rocks, allowing this fabric and the deformation described above to have developed concurrently. Furthermore, deformational events of similar age have been identified in the Black Hills near Lead (ca.1730 Ma) and along the Grand Junction Fault adjacent to the Bear Mtn terrane (ca.1732 Ma). Prior to our research, folds with geometries similar to our F₃ were interpreted as local “cross-folds” that developed late during the Wyoming-Superior suturing. We propose that the structures described here are evidence that this was a more significant and wide-spread basement-involved event during the final suturing of these two Archean provinces. Additionally, this event may extend beyond the Black Hills and be related to deformation of similar age in the Hartville Uplift and in the southeastern Laramie Mtns.