HETEROGENEOUS WRENCH-DOMINATED TRANSPRESSION IN THE DEEP CRUST RECORDED BY THE BURNSVILLE FAULT AND RELATED STRUCTURES, BLUE RIDGE, NORTH CAROLINA: IMPLICATIONS FOR THE ACADIAN OROGENY IN THE SOUTHERN APPALACHIANS

The Burnsville fault juxtaposes Precambrian Laurentian crust and the Ashe Metamorphic Suite (AMS) within the Fries thrust sheet of the Blue Ridge thrust complex of western North Carolina. The Burnsville Fault and adjacent AMS accommodated high strain at amphibolite facies (~700°C and ~9 kb) during the Acadian orogeny and then were tilted southeast during Alleghanian thrusting. Deformation resulted in three kilometer-scale structural domains: the Burnsville Fault dextral strike-slip domain, the “transitional” domain, and the Otter Knobs domain. Structures recording the orientation of the finite flattening plane in the three domains are subparallel; those southeast of the Burnsville Fault shear zone are rotated counterclockwise by ~10-15° consistent with dextral shear. Across strike from the Burnsville Fault shear zone into the transitional domain, shear sense indicators become scarce, fabric grades to S>L, and lineations change from sub-horizontal to down-dip. Across strike into the Otter Knobs domain, lineations grade to moderately southwest-plunging and the orientation distribution of poles to foliation indicate moderately southwest-plunging folding. The macroscale Otter Knobs fold, a tight-to-isoclinal synform whose hinge line, associated lineations, and minor fold hinges plunge moderately southwest, is interpreted to represent this structural element. No evidence of oblique or reverse shear is observed in these domains. The across-strike changes between these domains are consistent with heterogeneous wrench-dominated (10-20° from the plate boundary) transpression. Changes across strike from the Otter Knobs domain into the transitional domain record part of the deformation path for a zone with an “effective” convergence angle of 14-18°, including the rotation of structures recording the maximum incremental stretch from subhorizontal towards down-dip.