DISHARMONIC FOLDING AND FAULTING IN A PROTEROZOIC PHYLLITE, CUSTER, SD

A Proterozoic phyllite unit that crops out 3 miles north of Custer, South Dakota in the Black Hills records deformation dissimilar to typical regional map-scale folding. At this location the unit is a thinly laminated phyllite with significantly less quartzo-feldspathic component than the typical meta-greywacke that comprises most of the detrital supracrustal rocks in the hills. This research investigates if the structural character at this outcrop is due to an atypical local strain environment or to rheological characteristics distinctive to phyllite.

The outcrop was sketched on the scale of 1:36, photographed to complete a photo mosaic, and structural features were measured in fine detail. In this outcrop, NNW-striking folds range 0.5—5.0m in amplitude with >90% of the axial planes dipping within 10 degrees of vertical. Hinge lines are horizontal to shallow plunging, 0-15^o north or south, often appearing as en echelon groups. Crenulations are well developed and oriented parallel to the fold’s axial planes.

The west side of the outcrop contains shallow east-dipping thrust faults and associated fault-propagation folds with sub-meter offset. Near the east end of the outcrop a steep fault with reverse sense dips 75 degree west. East of this fault, hinge lines are more commonly south plunging whereas west of the fault they are more commonly horizontal to north plunging; presumably due to minor modification during faulting. 150 meters east of this location a 25 meter wide, NNW-striking, near-vertical, granitic dike crops out containing xenoliths of the phyllite.

The overall structural style in this outcrop is geometrically similar to a regional-scale fold and thrust belt. NNW-striking folds with shallow west-dipping thrust faults in the “foreland” and an antithetic back thrust towards the “internal zone” supports ESE-directed compression. This compression direction is consistent with regional stress during D₂, which led to regional NNW striking F₂ folds and the earliest fabric, S₂. Folding at this location, however, refolds an earlier fabric (presumably S₂) and therefore it must be a younger event. We interpret ballooning perpendicular to the NNW-striking granite dike during intrusion as providing a post D₂ localized strain regime responsible for the folding, faulting, and crenulation development at this location.