SIGNIFICANCE OF BACK-TO-BACK FACING DIRECTIONS ALONG THE SOUTH RANGE OF THE SUDBURY IGNEOUS COMPLEX, ONTARIO

The 1850 Ma Sudbury Igneous Complex has been variously interpreted as an intrusive and most recently as a differentiated impact melt sheet. The complex is differentiated, with norite along its outer (lower) circumference overlain by transitional rocks and finally granophyre in the inner (upper) portion. Along its southern margin, the complex is north-facing, but is in intrusive contact with south-facing metavolcanic and metasedimentary rocks of the early Proterozoic Huronian Supergroup. Both the igneous complex and the adjoining Proterozoic rocks are nearly vertical. This back-to-back facing relationship requires that the Huronian rocks have been overturned before the emplacement of the igneous complex. The melt sheet model requires that the Huronian rocks were overturned through nearly 180 degrees. Recent reinterpretations of the Sudbury Basin have suggested that major breccia zones are actually superfaults created during terrace collapse following the impact. Terrace collapse along listric normal faults would not supply sufficient rotation, and is hard to reconcile with the stratigraphically lowest Huronian rocks being exposed adjacent to the igneous complex. A modified superfault model involving collapse of a peak-ring fits the observed structure much better. In a peak-ring crater, rebound of the crater floor results in a very high central uplift that collapses and spreads outward by overturning and thrust-faulting the uplifted rocks. In such a collapsed central uplift, the rocks would be overturned, then overlain by an upright melt sheet. Later Penokean tectonism (2000-1800 Ma ago) tilted the rocks to their present vertical orientation. In this model the breccia zones are still superfaults, but some of the inner superfaults are thrust faults rather than normal faults. This model also would thrust lowermost Huronian rocks upward and outward, accounting for their presence immediately adjacent to the igneous complex. A thrust superfault would override fallback breccia and probably impact melt, resulting in a thicker fault breccia zone and incorporation of impact melt in the breccia. In addition, overthrusting of impact melt could provide a mechanism for injecting impact melt into dikes and sills, accounting for the so-called "offset" intrusions at Sudbury.