Seismic reflection data show the deep geometry of the Sudbury structure to be markedly asymmetric. The Sudbury North Range comprises shallowly south dipping sedimentary strata overlying a thick unit of heterolithic breccia and melt bodies, layered rocks of the Sudbury igneous complex, and footwall gneiss. Reflections from the upper layers are interrupted by faults near the center of the Sudbury basin, whereas the basal mafic units and footwall gneiss continue dipping southward and appear to be tightly folded or truncated near the Creighton fault. In contrast, the seismic image of the South Range is dominated by a distinctive series of moderately south dipping reflections interpreted as thrust faults or shear zones on which severe telescoping and imbrication of lithological units and considerable northwest-southeast shortening of the Sudbury structure have occurred. Such shortening explains the noncircular shape of the structure, and thus removes a critical objection to the Sudbury impact crater hypothesis. The new structural model is shown to be consistent with regional magnetic and gravity fields; a large hidden ultramfic-mafic mass is no longer required to explain the potential field data.

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