Pop-ups and related damage in granite at the Medika Pluton in southeastern Manitoba

Abstract

Stress relief in the form of small displacements and a pop-up has occurred in a line-drilled, blast-free quarry in Archean granite 100 km east of Winnipeg. A 60 m by 19 m and up to 1.5 m thick block, forming the east wall of the quarry, has undergone deformation and displacement following quarrying. The pop-up base is a low-dipping exfoliation fracture that predates the quarry. Block movement is complex, with compressional, strike-slip, and extensional features observed around the damaged zone perimeter. Overall, block displacement and expansion are dominated by upslope thrusting. The northeast–southwest displacement of the postexcavation damage zone mimics that of a nearby pre-quarry pop-up. The orientation of the maximum horizontal in situ compressive stress, σh max, inferred from the postquarry pop-up is consistent with on-site overcoring results. The compressional origin of the quarry-induced low-dip thrust fault and pop-up and the predominant orientations of these structures are compatible with the ambient stress field in eastern North America. This indicates that, although the quarry initiated and localized the damage zone, the regional in situ stress field defines its geometry and relative displacement.