The Goodman swell: a lithospheric flexure caused by crustal loading along the Midcontinent rift system

Rb-Sr biotite ages of Archean and Early to Middle Proterozoic crystalline rocks in northern Wisconsin and adjacent Upper Peninsula of Michigan describe a regionally systematic pattern related to differential uplift. An "age low' occurs in northern Wisconsin where values range from 1070-1172 Ma for rocks with crystallization ages of 1760 to 1865 Ma. These values overlap with the main episode of mafic igneous activity (1090 to 1120 Ma) along the Midcontinent rift system (MRS). We interpret these low biotite ages as registering closure due to cooling below the 300??C isotherm as a consequence of uplift and rapid erosion of an area that we are informally naming the Goodman swell. We interpret the swell to be a forebulge imposed on an elastic crust by loading of mafic igneous rocks along and within the axis of the MRS. -from Authors