STRUCTURAL ZONATION ASSOCIATED WITH PALEOPROTEROZOIC TRANSPRESSION IN THE LEAD ANTICLINORIUM, HOMESTAKE MINE AREA, BLACK HILLS, SD

The Precambrian rocks of the Black Hills,SD reveal a complex history of deformation associated with tectonic activity. The western limb of the Lead Anticlinorium is dominated by a series of SE-plunging folds in a high-strain zone coincident with garnet isograd and associated with Precambrian intrusions. A detailed structural analysis was conducted to better understand the structural evolution of this zone. Several domains containing structures of different orientation were identified across the high-strain zone. Changes in foliation, lineation, and fold orientations occur over short distances. In the westernmost domain, the dominant foliation strikes N to NW with moderately SE-plunging fold axes. Slightly to the west, NW foliation, shallow chert rod lineations, and steep fold axes occur. Near the Homestake Mine open cut, foliation ranges from NW to NE and has been secondarily folded by moderate SE-plunging folds with the same orientation as mineral lineations and chert rods. East of the anticlinorium, lineations are shallow SE to ESE. Throughout the study area, shallowly dipping crenulation cleavages can be observed. Kinematic indicators, such as fish structures, indicate sinistral, east-side-up shear.

Based on correlations of structures and timing within these domains, four major deformation events are apparent. D1 Proterozoic deformation produced S1 with steep foliation at a small angle to the dominant S2 foliation. L1 is a shallowly SE-plunging lineation. D2 is characterized by prominent, NNW striking, S2 foliation with near vertical, easterly dip, and L2 with shallow to steep, E-SE lineation. D3 Proterozoic deformation produced S3 with N-S crenulation cleavage and shallow dip. Finally, D4 is Laramide age, low to moderate angle, reverse faulting possibly associated with emplacement of Tertiary intrusions. Previous modeling of upright transpressional regimes indicates that foliation is vertical and lineation will be horizontal or vertical depending on the magnitude of the pure shear and simple shear components. S2, L2, and kinematic indicators are consistent with a transpressive regime with a significant simple shear component.