Recent field studies and petrographic, chemical, and isotopic analyses of rocks from the 1.85-Ga Sudbury Structure lead to two major conclusions: (1) the Sudbury Structure is the erosional remnant of a tectonically deformed and metamorphosed peak ring (or possibly multiring) impact basin with an original rim diameter of approximately 220 km, and (2) the Sudbury Igneous Complex, >2,500 m thick, and the related ore deposits were produced by impact melting of the upper and lower crust and subsequent differentiation of the melt, whose original volume was on the order of 12,500 km 3 . There is clear textural, chemical, and isotopic evidence for the absence of any significant magmatic or volcanic process involved in the formation of the melt complex and the melt-bearing breccias (Onaping Formation). All formations related to the Sudbury Structure, as defined by their structural position or by typical breccia textures and shock metamorphic effects, are compatible with an origin by a single impact event. The impact produced a transient cavity of about 110 km diameter that collapsed to a 220-km-wide shallow peak ring or multiring basin on a time scale of less than 30 min and, subsequently, became filled by pelitic sediments. The depth of the transient cavity, the maximum depth of excavation, and the maximum depth of melting are in the estimated ranges of 28 to 37 km, 15 to 21 km, and 25 to 35 km, respectively. The total impact energy was on the order of 8.6 x 10 30 Present address: Stöffler, Museum für Naturkunde, Invalidenstr. 43, 303 304
D. Stoffler and Others ergs and the size of the projectile near 14 km (for an impact velocity of 20 km/s and a projectile density of 3 g/cm 3 ). On the basis of this impact model, which is supported by the most recent models of impact mechanics, we have reinterpreted the stratigraphy and origin of the individual formation observed at the Sudbury Structure as follows (from bottom to top): (1) Sudbury Breccia, pseudotachylite and clastic matrix breccia dikes of the crater basement; (2) Shocked Footwall rockswith shatter cones and shocked quartz, the upper section of the crater basement below the melt complex; (3) Footwall Breccia, polymict megabreccia of the parautochthonous crater floor, thermally metamorphosed by the overlaying melt complex; (4) Contact Sublayer and Offset Dikes with Ni-Cu ore, clast-bearing basal layer of the coherent impact melt sheet lining the crater floor and forming apophyses of melt into the basement, respectively; (5) Main Mass of the Sudbury Igneous Complex (>2,500 m of norites, quartz gabbro, and granophyre), bulk of the clast-free, internally differentiated melt sheet of the central depression of the peak ring structure; (6) Basal Member of the Onaping Formation (OF), clast-rich impact melt breccia at the top of the melt complex; (7) Gray Member of the OF, polymict allochthonous clastic matrix breccia (suevite) formed by ground surging within the transient cavity; (8) Green Member of the OF, polymict suevitic breccia representing fallback material from the co...
