SHEAR DEFORMATION IN THE ARCHEAN BASEMENT IN THE BEAR MOUNTAIN TERRANE, BLACK HILLS, SOUTH DAKOTA

The Bear Mountain Terrane (BMT) in the southwestern Black Hills is one of two basement exposures of the Archean Wyoming Province of South Dakota. Within this terrane the pegmatitic, locally foliated, S-type Bear Mountain Granite (BMG) intrudes Archean schist, and crops out as both plutonic bodies and sill-like intrusions parallel to the foliation in the schist. This research investigates the nature of the contact between the Archean BMT and the Proterozoic supracrustal rocks adjacent to the east.

Near the contact between the BMT and Proterozoic supracrustals the BMG contains a well developed mylonitic fabric. This fabric strikes NW and dips moderately to the NE with a lineation plunging shallowly to the SE. Within the mapped area the contact between the BMT and adjacent Proterozoic rocks, and the foliation in these Proterozoic rocks are all oriented parallel to the mylonitic fabric in the BMG. Field evidence shows that the BMG and the supracrustal rocks contain asymmetric porphyroblasts of quartz and K-spar sigma grains on the millimeter to tens of centimeters scale. This asymmetry indicates left-lateral strike-slip coupled with east-up dip-slip movement. This supports the interpretation that the BMT and Proterozoic rocks are in sheared contact and that the deformation is Proterozoic in age.

Microstructural analysis of oriented samples of both lithologies supports outcrop evidence for sense of shear. This movement is indicated by quartz and K-spar sigma grains and mica orientation in both the supracrustals and BMG. Within the Proterozoic rocks, garnet-staurolite porphyroblasts show deformation associated with the shear event; therefore, assuming the metamorphism in the Proterozoic rocks is the same age as the regional metamorphism in the Black Hills, the shearing event must post-date this metamorphic event.

The shearing described for the BMT is oriented similarly and records the same shear sense as that described for the sheared rocks of both the Archean Little Elk Terrane (LET) and Proterozoic supracrustal rocks in the eastern Black Hills near Rockerville. Additionally, the post regional metamorphic timing for shearing in the BMT is also consistent with the relative timing near Rockerville. The similar orientation of shear sense and timing is an indication for a major regional shear event in the Black Hills.