FAULT GEOMETRY AND MECHANICS IN IMPACT STRUCTURES: NEGATIVE INVERSION OF BOUNDING FAULTS IN THE FLYNN CREEK IMPACT STRUCTURE, TENNESSEE

Physical models on and field data from impact structures have provided insight into their surface geometry, mechanics, and kinematics. Neither, however, has addressed at depth fault inversion, kinematics, and geometry. The Flynn Creek impact structure is a complex impact structure formed in a 1.5 km-thick carbonate section in east Central Tennessee. It has been remapped in detail to reveal a circular shape, frequent buckle folds, and an inward-verging thrust fault replacing a bounding normal fault in the eastern modified rim. This segment of the fault did not propagate to the surface during impact and therefore did not become reactivated. Further evidence of negative inversion during the modification stage is breached concentric buckle folds associated with the bounding faults, indicating the normal faults developed in earlier zones of compression during the excavation stage. During the modification stage, downward movement along the inverted bounding faults caused the modified crater rim to collapse and the fault blocks to rotate away from the impact. As the normal faults propagated inward along a master décollement, fault tips passed into compression, and folds or thrust faults developed as noted in other media. Compression could therefore account for some crater floor uplift, but has not been demonstrated. Seismic reflection data from other impact structures indicate that impact deformation and modification are relatively shallow as deep reflectors and rock units usually appear undisturbed.