The Keweenaw peninsula is bisected by the Keweenaw fault, which is the boundarybetween the mid continent rift and other Precambrian terrains. Lake Superior isthe result of a combination of a geological syncline which helped form astructural basin that has been subsequently enhanced by the actions of glaciers whichexcavated the softer rocks which previously occupied the middle of the syncline.

The regional distribution of various rock types and structural geology of theLake Superior region. From USGS Bulletin 1309 figure 39.

    The midcontinent rift (Precambrian in age) starts in northern Kansas and runs under Lake Superior andthen turns south to run through lower Michigan. It was similar to the East Africa rift as parts of the continent tried toseparate, but were ultimately unsuccessful in the attempt to break into separateland masses. As the rift developed, there weretwo bounding graben faults at either side of the rift, the Keweenaw fault on theeastern side of the rift and the Isle Royale fault on the western edge of therift. As the rocks were beingdown-warped, igneous extrusions filled the rift valley. These eruptions werealmost exclusively on dry land ( there is one location on the Keweenaw peninsulaand several small areas on Isle Royale that were erupted under water - as seenin the development of pillow lavas). During periods of volcanic quiescence, thedown-warping continued and sedimentary rocks were formed from the erosion of thesurrounding higher land areas.

The sequence of volcanic eruptions, development of sedimentary rocks from rockspreviously erupted, and the continuation of the cycle. This sequence of eventshappened numerous times during the deposition of the volcanic and sedimentaryrocks of the Keweenaw peninsula and Isle Royale. From USGS Bulletin 1309 figure43.

    TheKeweenaw is the eastern flank of a large syncline under Lake Superior, while therock outcrops on Isle Royale represent the western flank of the syncline. Mostof the rocks generally dip about 20 degrees to the west on the Keweenawpeninsula. There are minor gentle folds, synclines and anticlines that haveamplitudes extending over 10 km distances in the Keweenaw thatalong with cross faults, tend to provide pathways for the copper ore solutionsand localize the deposition of native copper. The Allouez gap fault provided the mostproductive pathway through the volcanics. More rock formations were mineralized next tothis fault than any other structure in the Keweenaw.

View to the north from Bumbleton hill. The low lying area to the north is thesurface expression of the Allouez gap fault and subsidiary fractures. Thesefractures created an area of relatively soft rock that was eroded more easily bythe glaciers during the Pleistocene. These faults also played a key role in thechanneling of the ore solutions.

While the Keweenaw fault is a major structural element in the area, it is notwell exposed either in outcrop or in any roadcuts (although it's presence ismade known in features such as Douglas Houghton falls and the Hungarian falls).Minor faulting is preserved in the mines in samples such as this example ofslickensides on the dump of the Iroquois mine. This specimen of basalt shows thepolishing of the fault trace (with the creation of minerals such as epidote).

    About 30 million years after the rift formed, the region was placed undercompression by the northwest movement of the plate along the Grenville front (this is located several hundred miles to the southeast). This compressionresulted in the normal graben faults becoming reverse faults. Although theKeweenaw fault is present throughout the length of the peninsula, it is poorlyexposed along it's entire length.