A broad zone of intense ductile to brittle deformation transects the Paleo-Proterozoic Sudbury Igneous Complex and likely formed during the final stages of the Penokean orogeny on the southern margin of the Superior Province. Herein we present a three-dimensional image of this deformation zone, derived from four seismic reflection profiles. Additional information from rock property measurements, drill results, and published structural mapping suggests that the seismic data define the three-dimensional disposition of listric shear zones. The seismic image of the deformation is best developed within the Onaping Formation. However, the images of the deformation zone envelope are imperfect, being most clear where truncating known contacts within the Sudbury Igneous Complex and its contact with Huronian host rocks. The southern edge of the seismically imaged shear zones does not extend significantly beyond the exposed Sudbury Igneous Complex, suggesting limits to the extent of deformation in the Sudbury Structure. ¿ 2000 American Geophysical Union