Floor‐fractured crater models of the Sudbury Structure, Canada: Implications for initial crater size and crater modification

Abstract

The pattern of radial and concentric offset dikes at Sudbury strongly resembles fracture patterns in certain volcanically modified craters on the Moon. Since the Sudbury dikes apparently formed shortly after the impact event, this resemblance suggests that early endogenic modification at Sudbury was comparable to deformation in lunar floor‐fractured craters. Although regional deformation has obscured many details of the Sudbury Structure, such a comparison of Sudbury with lunar floor‐fractured craters provides two alternative models for the original size and surface structures of the Sudbury basin. First, the Sudbury date pattern can be correlated with fractures in the central peak crater Haldane (36 km in diameter). This comparison indicates an initial Sudbury diameter of between 100 and 140 km but requires loss of a central peak complex for which there is little evidence. Alternatively, comparison of the Sudbury dikes with fractures in the two‐ring basin Schrödinger indicates an initial Sudbury diameter of at least ∼ 180 km, which is in agreement with other recent estimates for the size of the Sudbury Structure. In addition to constraining the size and structure of the original Sudbury crater, these comparisons also suggest that crater modification may reflect different deformation mechanisms at different sizes. Most lunar floor‐fractured craters are attributed to deformation over a shallow, crater‐centered intrusion; however, there is no evidence for such an intrusion at Sudbury. Instead, melts from the evolving impact melt sheet probably entered fractures formed by isostatically‐induced flexure of the crater floor. Since most of the lunar floor‐fractured craters are too small (<100‐km diameter) to induce significant isostatic adjustment, crater modification by isostatic uplift apparently is limited to only the largest of craters, whereas deformation over igneous intrusions dominates the modification of smaller craters.

Impact melt dikes in the Sudbury multi‐ring basin (Canada): Implications from uranium‐lead geochronology on the Foy Offset Dike

“…This implies age identity between the SIC and the offset material; complex contact relationships are thought to be due to late adjustments of the final crater (e.g., Deutsch et al, 1995, and references therein). Based on mechanical arguments and seemingly supported by comparison with analogous impact melt dikes at Manicouagan, however, Wichman and Schultz (1993) questioned Offset Dike emplacement during the impact event and interpreted these features as a "post-impact intrusion sequence beneath, and probably derived from, the evolving impact melt sheet. "…”

Extracts of impact breccia samples from Sudbury, Gardnos, and Ries impact craters and the effects of aggregation on C60 detection

“…The Sudbury impact structure, in southern Ontario, Canada, is ϳ1.85 billion years old and has been deformed and metamorphosed since formation, making it difficult to know its original size (Wichman and Schultz, 1993). The structure contains several distinct impact-produced geologic units, including the carbon-bearing "Black Member" of the Onaping Formation, a 1600-m-thick unit of heterolithic breccias formed by deposition in the crater immediately after impact (for descriptions, see French, 1968;Muir and Peredery, 1984;Avermann, 1994;Heymann et al, 1999).…”

Origin and emplacement of Offset Dykes in the Sudbury impact structure: Constraints from Hess

“…In this model the proximal Foy penetrates the horst via a fault/fracture breach and feeds the Hess Offset through the horst's northern fault system. Large, rapid displacements would be required on the horst to generate pseudotachylyte, but this would be compatible with rebound and various isostatic adjustments following hypervelocity impact (e.g., Wichman and Schultz, 1993).…”