Implications for cratering mechanics from a study of the Footwall Breccia of the Sudbury impact structure,Canada

Abstract— The Footwall Breccia layer in the North Range of the Sudbury impact structure is up to 150 m thick. It has been analyzed for several aspects: shock metamorphism of clasts, matrix texture, mineralogy, and geochemistry with respect to major and trace element compositions. The matrix of this heterolithic breccia contains mineral and lithic fragments, which have suffered shock pressures exceeding 10 GPa, along with clasts of breccia dikes originating from the crater basement. The matrix in a zone near the upper contact of the breccia layer is dominated by a dioritic composition with intersertal textures, whereas beneath this zone the matrix is characterized by poikilitic to granular textures and a tonalitic to granitic composition. Major and trace element analyses of adjacent slices of a thin-slab profile from the breccia show that the matrix is chemically inhomogeneous within a range of 3 mm. The breccia layer has been thermally annealed by the overlying Sudbury Igneous Complex, which is interpreted as a coherent impact melt sheet. The Rb-Sr isochron age of 1.825 ± 0.021 Ga for the matrix is a cooling age after partial melting of fine grained clastic material by the melt system. Two-pyroxene thermometry calculations give temperatures in excess of 1000 °C for this thermal overprinting. Clasts were affected by recrystallization, melting, and reactions with the surrounding matrix at that time. The crystallization of the molten matrix resulted in the observed variety of igneous textures. Results of clast population statistics for the Footwall Breccia along with both geochemical considerations and the Sr-Nd isotopic signature of the matrix indicate that the breccia constituents exclusively derived from the Levack gneiss complex, which forms the local country rock to the breccia layer in the Levack area. K-feldspar-rich domains, which tend to replace parts of matrix and felsic gneiss fragments have been formed due to metasomatic activities during the Penokean orogeny, ～ 1.7 Ga ago. The available observations suggest that the Sudbury structure represents the remnant of a multi-ring basin with an apparent diameter between 180 and 200 km and a diameter of the transient cavity of about 100 km. For a crater of the size of the Sudbury basin a maximum depth of excavation of ～21 km and a depth of shock-melted target rocks of ～27 km are obtained. In the Sudbury crater, the Footwall Breccia layer represents a part of the uplifted crater floor directly underlying the thick coherent impact melt sheet.