The Keweenaw Fault is a

reverse fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic f ...

that bisects the

Keweenaw Peninsula The Keweenaw Peninsula (, ) is a peninsula of the U.S. state of Michigan. Part of the greater landmass of the Upper Peninsula of Michigan, Upper Peninsula, the Keweenaw Peninsula projects about northeasterly into Lake Superior, forming Keweena ...

of the

Upper Peninsula of Michigan The Upper Peninsula of Michigan—also known as Upper Michigan or colloquially the U.P. or Yoop—is the northern and more elevated of the two major landmasses that make up the U.S. state of Michigan; it is separated from the Lower Peninsula of ...

. The fault thrusts lava flows of the Midcontinent Rift System onto sedimentary rocks of the Jacobsville Sandstone. The fault is part of the inversion of the Midcontinent Rift where a region that had previously undergone extension experienced significant contraction. This contraction occurred during the final stages of the Grenvillian orogeny. The Keweenaw Peninsula, itself, is the southeastern side of a large

syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...

beneath

Lake Superior Lake Superior is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. Lake Michigan–Huron has a larger combined surface area than Superior, but is normally considered tw ...

. The northwestern side forms

Isle Royale Isle Royale (, ) is an Islands of the Great Lakes, island of the Great Lakes located in the northwest of Lake Superior and part of the U.S. state of Michigan. The island and the 450 surrounding smaller islands and waters make up Isle Royale Na ...

. The fault is more than long and extends from the tip of the Keweenaw Peninsula in the northeast to a termination near the Michigan-Wisconsin border in the southeast. The fault is most likely younger than the Jacobsville Formation and the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

Period. A seismic event in 1906 claimed to be an earthquake has been attributed to a rock burst, as the area has been significantly mined.

The Natural Wall

One significant feature along the Keweenaw Fault is known as the ''Natural Wall''. The ''Wall'' is a near-vertical slope of the Jacobsville Sandstone which dips as steeply 85°. The steep dip of the Jacobsville Sandstone is the result of it being folded due to motion on the fault. Interstate State Park at St. Croix Falls, Wisconsin is an example of the Keweenaw Fault. On its trails visitors can find Tholeiitic basalts. File:Tholeiitic basalts (Clam Falls Volcanics, Mesoproterozoic, ~1.1 Ga; Interstate State Park, St. Croix Falls, Wisconsin, USA) 1 (23443260210).jpg, Tholeiitic basalts - Clam Falls Volcanics, Mesoproterozoic File:Tholeiitic basalts (Clam Falls Volcanics, Mesoproterozoic, ~1.1 Ga; Interstate State Park, Taylors Falls, Minnesota, USA) 8 (23707446946).jpg, Tholeiitic basalts - Clam Falls Volcanics, Mesoproterozoic

References

{{coord, 46, N, 87, W, region:US-MI_scale:1250000, display=title Geology of Michigan Seismic faults of the United States