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 within the Fries thrust sheet of the Blue Ridge thrust complex of western North Carolina. The Burnsville Fault and adjacent Ashe metamorphic suite accommodated high strain at amphibolite facies (approximately 700 degrees C and approximately 9 kbar) 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 finite flattening plane are subparallel; those southeast of the Burnsville Fault shear zone are rotated counterclockwise by approximately 10 degrees -15 degrees , consistent with a component of dextral shear. Across strike into the transitional domain, shear sense indicators become scarce, fabric grades to S > L, and lineations change from subhorizontal to downdip. Across strike into the Otter Knobs domain, lineations grade to moderately southwest-plunging, and the orientation distribution of poles to foliation indicates moderately southwest-plunging folding. The macroscale Otter Knobs fold, a tight-to-isoclinal synform in which the 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. The across-strike changes between these coeval domains are consistent with heterogeneous wrench-dominated (10 degrees -20 degrees 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 degrees -18 degrees , including the rotation of structures recording the maximum incremental stretch.