THE SUDBURY STRUCTURE - CONTROVERSIAL OR MISUNDERSTOOD

The origins of the Sudbury Structure and associated Igneous Complex have been controversial. Most models call for a major impact event followed by impact-induced igneous activity, although totally igneous models are still being proposed. Much of the controversy is due, in our opinion, to a misunderstanding of the size of the original Sudbury Structure. By analogy with other terrestrial impact structures, the spatial distribution of shock features and Huronian cover rocks at the Sudbury Structure suggest that the transient cavity was approximately 100 km in diameter, which places the original final structural rim diameter in the range of 150-200 km. Theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in approximately 10(4) km3 of impact melt, more than sufficient to produce a melt body the size of the Igneous Complex (present volume 4-8 x 10(3) km3). For the Igneous Complex to be an impact melt sheet it must have a composition similar to that of the target rocks. Evidence for this has been presented previously for Sr and Nd isotopic data, which suggest a crustal origin. Here, we also present new evidence from least squares mixing models that the average composition of the Igneous Complex corresponds to a mix of Archean granite-greenstone terrain, with possibly a small component of Huronian cover rocks. This is a geologically reasonable mix, based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size. The Igneous Complex is differentiated, which is not a characteristic of previously studied terrestrial impact melt sheets. This can be ascribed, however, to its great thickness and slower cooling. That large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted. If this working hypothesis is accepted, namely, that both the Sudbury Structure and the Igneous Complex are impact in origin, then previous hybrid impact-igneous hypotheses can be discarded and the Sudbury Structure can be studied specifically for the constraints it provides to large-scale cratering and the formation of basin-sized (multiring?) impact structures.