--> ABSTRACT: Structure of Mid-Continent Rift Beneath Lake Superior, by William F. Cannon; #91022 (1989)

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Structure of Mid-Continent Rift Beneath Lake Superior

William F. Cannon

The 1.1 Ga Mid-Continent rift system extends from Kansas through the Lake Superior region and into southern Michigan. The rift is filled with thick sequences of basalt and clastic sedimentary rocks, which are now mostly buried beneath Paleozoic rocks. Rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan supergroup. Seismic reflection surveys by GLIMPCE in 1986 imaged much of the deep structure of the rift beneath the lake in detail.

Reflection profiles reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented. Volcanic and sedimentary rocks, in places greater than 30 km thick, fill the central graben, which is bounded by normal growth faults. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the rift axis, the pre-rift crust is thinned to about one-fourth of its original thickness, apparently by low-angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferre from abrupt disruptions in the Bouguer gravity anomaly associated with the rift.

Late uplift of the central graben transposed graben-bounding normal faults into high-angle reverse faults with throws of 5 km or more.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.