Sudbury Structure as an Astrobleme

Abstract

The Sudbury structure is interpreted as a 1.7-billion-year-old asteroid impact structure or "astrobleme" created by a $$3 X 10^{29}$$ erg event. If traveling at 15 km/sec, the asteroid was about 4 km. in diameter. A crater was formed 30 miles across and 2 miles deep. Along with melted country rock, the bolide, possibly a copper-rich nickel-iron meteorite, is still partially preserved, although converted to sulfides, as a marginal sheet along the crater wall and as injections into radial tension cracks. Shock brecciation and rock were heaved up, forming the crater wall. The collar is still easily recognized along the southern periphery of the Sudbury structure, but it can only be permissibly assumed in the massive granitic rocks around the northern periphery. Because of its great magnitude, the Sudbury event triggered magmatism by offloading the lower crust and mantle and by adding shock heat. Partial fusion of already critically warm rock resulted. A saucer-shaped pool of magma, an extrusive lopolith, formed and differentiated in the crater bottom, also laying down a thick capping of welded tuffs (Onaping tuffs). A body of water subsequently occupied the basin, in which the Whitewater sediments were deposited. Rebound, isostasy, tectonism (especially the Grenville thrust from the southeast making the lopolith oval), and finally erosion have modified the structure. The writer considers that lunar maria may also be created by impact-triggered magmatism to form extrusive lopoliths. Hence, while other astroblemes so far known may be the terrestrial equivalents of lunar craters, the Sudbury structure may be the equivalent of a small lunar mare.