THE SUDBURY STRUCTURE AS A TERRESTRIAL MARE BASIN

This paper reviews the origin, development, and Present status Of the hypothesis that the Sudbury Structure of Ontario, Canada, is a deformed Precambrian impact crater analogous to the circular lunar maria. The petrologic similarity of the maria and terrestrial lopoliths, such as the Bush-veld, Duluth, and Sudbury igneous complexes, was suggested in 1961, assuming these to be extrusive features roofed by their own silicic differentiates. This led to the proposal by R. S. Dietz in 1962 that the Sudbury Structure was a tectonically deformed impact basin, occupied by an extrusive lopolith, and analogous to the small lunar maria. He then searched for and found shatter cones at Sudbury. Discovery of shock metamorphic features in the Onaping Formation by B. M. French in 1967 showed that this formation was a fallback breccia. Detailed studies since the 1970s have further strengthened the impact theory. Isotopic and impact modeling investigations have shown that the Sudbury Igneous Complex may be largely impact melt rather than internally generated magma. The shape and size of the original impact crater are not well known. The absence of compressional deformation of the North Range supports an originally elliptical shape, which has been shown to be similar to those of impact craters on the Moon and Mars. However, field mapping and reflection profiling suggest that much of the present ellipticity may have been produced by northwest directed ductile thrusting concentrated on the South Range and interior of the complex. The size of the original structure is also uncertain. Recent investigations indicate that the present igneous complex is only the central part of a much larger and perhaps multiring structure. The analogy between the Sudbury Structure and lunar maria, though petrologically wrong, is valid to the extent that both classes of structure were initially formed by impact. However, the size and shape of the original crater and the degree to which the Sudbury Intrusive Complex and its ore deposits are impact-generated are still open to debate.