By using a simple lineament analysis technique applied to Landsat imagery (Butler, 1988), it m ay be possible to define the apparent size of the north-northwestern half of the original Sudbury impact structure. Since the impact was pre-Grenvillian in age, apparent ring segments are only visible to the north of the Grenville Front, in areas underlain by Arche an granite-gneiss, greenstone complexes, Lower Proterozoic Huronian Super group rocks, and Nipissing diabase. This study indicates that the tectonically deformed Sudbury igneous complex lies northwest of the original geometric center of a multiring impact basin. One interpreted northwestern ring segment suggests a maximum basin diameter of about 270 km. Middle Proterozoic Penokean orogenic shortening seems to have barely disturbed the original shape of interpreted ring segments in this north-northwest sector. The proposal by Grieve et al. (1981) that the Sudbury Structure should be a multiring basin is supported by this study.

Abstract DNAG Transect E-4. Part of GSA’s DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.

Abstract E-4 is one of eight Geodynamics transects that cross the Atlantic margin of North America between Georgia and Newfoundland. Five of the transects are in the United States and three are in Canada. Transect E-4, which is 110 km wide and more than 1,100 km long, extends from the stable North American craton just west of the Grenville front near Lexington, Kentucky southeastward across Cape Fear, North Carolina, on the Atlantic coast to oceanic crust east of the Blake Spur magnetic anomaly. Like all of the other U.S. Atlantic coast transects, it crosses Cambrian and Jurassic continental margins of North America as well as the Appalachian orogen. The display, based upon published information, portrays the geology, tectonic style and geophysical expression of this segment of the eastern North American continental margin and interprets its Phanerozoic history. The Decade of North American Geology 1983 geologic time scale (Palmer, 1983) is used throughout the display and text.