CHALLENGING ENTRENCHED PARADIGMS: THE INCREASING IMPORTANCE OF CA. 1730 MA SHEAR DEFORMATION ALONG THE EASTERN BLACK HILLS UPLIFT, SD

The Black Hills of South Dakota lie within the southern arm of the Trans-Hudson orogen, which separates the Archean Wyoming and Superior provinces. The Precambrian core of the uplift exposes 2480–1883 Ma supracrustal rocks deformed during suturing at 1780–1715 Ma. In the NE, two terranes expose rocks interpreted as the rifted basement these rocks were deposited on: the ≥2480 Ma supracrustal rocks in the Nemo group and the underlying ca. 2560 Ma granitic gneisses in the Little Elk Terrane (LET). Published models interpret map patterns in these older terranes by proposing deformation prior to rifting. Our detailed mapping and microstructural analysis refute any evidence for >2480 Ma deformation, and elevate D3 (ca. 1730 Ma) from a minor reworking of the main suturing fabrics (S2/D2) to the dominant deformation related to the suturing event.

A strong NW-striking fabric crosscuts all units in the Nemo group. Microstructural analysis established this as a mylonitic fabric, and determined folding in the area is modeled best as passive folding along that fabric. This fabric and associated folds record the same shear sense as the similarly oriented D3 shear structures in younger rift deposits across the uplift. Shear structures are mapped along strike continuing across the mapped unconformity, confirming the Nemo group structures are D3-related, not pre-rifting. Quartz veining and breccia in the Nemo group rocks, previously used to delineate rift-related faults, crosscuts the mylonitic fabric requiring it be ≤1730 Ma and not rift related.

Prior to our work, fabrics in the granitic gneisses exposed in the LET were interpreted as Archean, and the terrane was subdivided into two units. Our mapping, microstructural and geochemical analysis determined it is a single unit with all variation in the terrane a function of strain partitioning during the development of a single mylonitic shear fabric. Furthermore, this fabric is mapped continuously into the overlying supracrustal rocks, and is identical in orientation and shear sense to D3 fabrics elsewhere, supporting a ca. 1730 Ma age.

The recognition that the oldest terranes contain no evidence for >2480 Ma deformation or post 2480 Ma rifting, and in fact record the youngest suture-related structures in the uplift, demands a new model reflecting the significance of D3 in the NE Black Hills.