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The Cottage Grove fault system (Illinois Basin): Late Paleozoic transpression along a Precambrian crustal boundary

Amanda B. Duchek¹, John H. McBride^,2, W. John Nelson^,3 and Hannes E. Leetaru^,3

¹ Department of Geology and Illinois State Geological Survey, University of Illinois at Urbana— Champaign, 1301 West Green Street, Urbana, Illinois 61801, USA
² Department of Geology, Brigham Young University, P.O. Box 24606, Provo, Utah 84602, USA
³ Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820, USA

The Cottage Grove fault system in southern Illinois has long been interpreted as an intracratonic dextral strike-slip fault system. We investigated its structural geometry and kinematics in detail using (1) outcrop data, (2) extensive exposures in underground coal mines, (3) abundant borehole data, and (4) a network of industry seismic reflection profiles, including data reprocessed by us. Structural contour mapping delineates distinct monoclines, broad anticlines, and synclines that express Paleozoic-age deformation associated with strike slip along the fault system. As shown on seismic reflection profiles, prominent near-vertical faults that cut the entire Paleozoic section and basement-cover contact branch upward into outward-splaying, high-angle reverse faults. The master fault, sinuous along strike, is characterized along its length by an elongate anticline, 3 km wide, that parallels the southern side of the master fault. These features signify that the overall kinematic regime was transpressional. Due to the absence of suitable piercing points, the amount of slip cannot be measured, but is constrained at less than 300 m near the ground surface. The Cottage Grove fault system apparently follows a Precambrian terrane boundary, as suggested by magnetic intensity data, the distribution of ultramafic igneous intrusions, and patterns of earthquake activity. The fault system was primarily active during the Alleghanian orogeny of Late Pennsylvanian and Early Permian time, when ultramafic igneous magma intruded along en echelon tensional fractures.

Key Words: seismic reflection • strike slip • Illinois basin • folding • Precambrian • transpression

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