2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 11
Presentation Time: 10:55 AM

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


ALLARD, Stephen T., Department of Geoscience, Winona State University, P.O. Box 5838, Winona, MN 55987 and PORTIS, Douglas H., Department of Geosciences, Winona State University, PO Box 5838, Winona, MN 55987, sallard@winona.edu

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 (Sm) with a steeply-plunging stretching lineation (Lm). 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 Lm. A spaced crenulation cleavage (S3), oriented parallel to Sm, 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 Sm 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.