Skip to main content
U.S. flag

An official website of the United States government

Integration of geophysical evidence suggests that anorthosite composes a significant portion of Grand Marais ridge, an inferred basement high in western Lake Superior

May 31, 2021

The Midcontinent Rift System (MRS) is expressed geophysically by a semi-linear, regional gravity high that trends across the Midcontinent and Great Lakes region of North America. The gravity high is interrupted by two prominent, semi-circular gravity lows, which have been interpreted from modeling and seismic-reflection sections as basement highs of Archean granite (Allen et al., 1997). One is centered southwest of Isle Royale in western Lake Superior (Grand Marais ridge) and the other over Bayfield Peninsula (White’s Ridge). Allen et al. (1997) suggest that the Archean granite highs were pre-rift features that remained high while lava basins of the MRS subsided adjacent to them. Hart et al. (1994) questioned the presence of granitic rocks underlying Grand Marais ridge (GMR) because heat flow measurements there are much lower than is typical for Archean granitic upper crust. They argued that the region must instead be underlain by rocks of low radiogenic heat production, such as gabbro, extending to at least 15 km depth. However, gabbro has high densities and would not produce the observed gravity low. Thus, the geophysical observations appear contradictory. 

Citation Information

Publication Year 2021
Title Integration of geophysical evidence suggests that anorthosite composes a significant portion of Grand Marais ridge, an inferred basement high in western Lake Superior
DOI
Authors V. J. Grauch, Samuel J. Heller
Publication Type Conference Paper
Publication Subtype Conference Paper
Series Title
Series Number
Index ID 70222361
Record Source USGS Publications Warehouse
USGS Organization Northern Rocky Mountain Science Center