RETHINKING THE PROTEROZOIC ASSEMBLING OF THE NORTH-CENTRAL US

The crystalline core of the Black Hills (BH) uplift exposes Precambrian rocks near the Trans-Hudson Orogen (THO) where it is “truncated” by the Central Plains Orogen (CPO). Although this geometry is consistent with continental reconstructions interpreting the CPO as younger than the THO, structural orientations and age relationships identified in the BH support north-south compression prior to east-west suturing. This research investigates a basement-involved shearing event interpreted as final suturing of the Wyoming (WP) and Superior (SP) provinces and draws ties to similar-age deformation in the WP as far west as the Laramie Mountains.

Proterozoic rocks in the east-central BH and Archean basement in the Little Elk Terrane, northeast BH, both contain NW-trending shear zones indicating east-side up, left-lateral transpression. In both study areas shear fabrics are coupled with folds oriented parallel to the shear fabric with hinge lines parallel to shear lineations. Reconnaissance work between field sites identified several locations with similar folds and shear fabrics supporting a connection between the two. Identification of a NW-striking shear zone with similar shear sense along the Archean-Proterozoic contact in the Bear Mtn Terrane, western BH, suggests the basement-involved shear event affected rocks along both eastern and western margins of the uplift.

The structures described above postdate regional ENE-trending recumbent folds (ca. 1785) and subsequent NNW-trending upright folds and associate metamorphism (ca. 1750 Ma), and in the east-central BH predate intrusion of the post-tectonic Harney Peak Granite and associated metamorphism (ca. 1715 Ma). Timing for this event is similar to shearing in the Hartville Uplift, (ca. 1714) as well as post Cheyenne Belt (CB) deformation in the Eastern Laramie Mountains (ca. 1722). Therefore, I propose the Green Mountain Block, south of the CB in WY, extends eastward at least to the BH and was attached to the WP prior to the WP—SP suturing.