The Mid Continental Rift

The Midcontinental Rift System began to split apart the stable continental crust of the Great Lakes region a little over a billion years ago. The sequence of mixed sediments and extrusive basaltic rocks in the lake Superior region associated with rifting are called the Keweenawan and crop out around the rim of western Lake Superior itself.

Geophysical studies and drill holes have shown Keweenawan rocks and the rift structure to underlie the lake, and have traced it in the sub surface through the western tip of Lake Superior down into Kansas, and to the east as well down through the center of the upper and lower peninsulas of Michigan.
A minor third limb shoots of to the north at the top of the crescent shape of Lake Superior. This third limb completes a three way 120 degree rifting arrangement that is a characteristic feature of a continental break up. This type of feature can be shown to have broken up the Pangean supercontinent at the end of the Paleozoic.

Rifting began as early as 1.11 bya and opened to its full extent within 15 million years. This rifting episode lacks evidence of early doming or elevation of the area by mantle currents. The crust is believed to have been under tension which would not require doming uplift, in other words it was pulled apart, rather than simply pushed apart by doming. A mantle plume was associated with the volumes of basaltic volcanism seen in the Keweenawan rocks.

It is probable that mantle convection currents began beneath the continent, rather than the continent drifting over an existing current cell. This model does not require early uplift of the crust. The theory is that as a cell gets going, the front of the new plume rises and trys to work its way through the upper mantle which acts to fan out the front. As the plume rises, it undergoes adiabatic decompression and melts and causes unusually high volumes of volcanism for a short period until the currents establish themselves in a deeper circulation pattern. This decompression and mixing with the upper mantle acts to spread out the forces on the overlying plate which might account for the apparent tension with out doming. The volumes of basalts associated with rifting that filled in, or created gaps in the crust, and surficial low lying areas. Huge piles of sediments helped to fill in the low lying areas as they piled in from the higher portions of the continent onto the quickly subsiding rifting region. These volcanic and sedimentary rocks of the Keweenawan can be seen in thick beds around the perimeter of Lake Superior as they dip slowly toward the rift axis beneath the lake.

The first rocks deposited in the western Lake Superior area are some quartz sandstones and gravels from streams and rivers that total a relatively thin less than 100 meters. These underlie the first basalts which flowed over them around 1.109 bya. The sandstones were probably deposited in a basin developed by the onset of rifting.

The basalts total some 20 km deep in places and may have originally totaled two million cubic kilometers overall. This enormous volume erupted in a 23 million year time span. The peak volcanism was from 1.109 to 1.096 bya when the main block faulting associated with rifting took place. After this period was dominated by sedimentation which eventually filled in the basins, leaving quiet the basement rocks of the Great Lakes region.