COMPARING DEFORMATION STYLE AND MECHANISMS OF RHEOLOGICALLY DIFFERENT ROCKS NEMO, BLACK HILLS, SD

The Nemo group, is a package of clastic and volcanic supracrustal rocks deformed during the suturing of the Wyoming and Superior cratons. The Nemo group is dominantly thick-bedded quartzite intruded by a ~800 meter thick mafic sill, with lesser interlayered carbonates and pelitic rocks in the Roberts Draw area in the southeast portion of the group. Previously interpreted to have deformed by buckle folding, ongoing research by Winona State University (WSU) now interpret this entire package to have deformed under transpressional shearing. This provides an excellent opportunity to compare how rheologically different rocks deform under transpression, with this project comparing the carbonate and pelitic units in the Roberts Draw area to other rock types nearby.

Recent research by WSU documented the quartzite and gabbroic sill to be folded passively by slip along a NW-striking S-C mylonitic fabric, consistent with the shear folds and s-c mylonites associated with left-lateral east side up movement, described for the Dakotan Tectonic zone along the eastern margin of the Black Hills. The Roberts Draw area, where a carbonate unit is interlayered with a metapelite, is recognized to contain similar structural features, but prior to this research, a transpressional model has not been considered.

The Roberts Draw area is composed primarily of strongly foliated metapelites with a NW striking shear fabric, locally crenulated in tight 5-10 meter hinge areas of shear folds. In the southern portion of Roberts Draw the carbonate unit has a strong NW-striking planar fabric and locally contains evidence for tight isoclinal hinges. The unit crops out in narrow 10 meter wide layers elongate parallel to the shear fabric in discontinuous lenses from 150 to 250 meters long. In the central and northern portion, the carbonate is exposed in massive, low-strain, resistant knobs with only minor fabric development. This outcrop pattern is interpreted as isolated carbonate fold hinges surrounded by metapelite. Carbonate outcrops are difficult to connect across the entire area, with metapelite filling areas between them. This pattern is interpreted to reflect boudin-like disarticulation of the carbonate layer with flow in the metapelite. Ongoing microstructural analysis compares deformation in the carbonate and pelitic units to the quartzite.