15 Structure, Geophysics, Petrology, and Tectonics in the Midcontinent, and Elsewhere

Monday, 18 April 2016
1:30 PM - 5:30 PM
Lincoln Room

CHARACTER OF FAULTING BENEATH THE WESTERN EDGE OF THE LASALLE ANTICLINORIUM, CENTRAL ILLINOIS BASIN: INSIGHT FROM A NEW SEISMIC-REFLECTION PROFILE AND A STRUCTURE-CONTOUR MAP

Paper number: 15-10
Program: Theme Sessions
Time: 4:25 PM – 4:45 PM

Authors

Abstract

The LaSalle Anticlinorium (or LaSalle Deformation Belt), is an overall N-S- to NE-SW-trending, 320 km by 130 km, zone of tectonic deformation along the axis of the Illinois Basin. Where exposed in northern Illinois, the western margin is a west-facing monocline. A new 15-km long E-W-trending seismic-reflection profile, that crosses the county line between Champaign and McLean Counties in east-central Illinois, provides insight into the configuration of underlying faults. This seismic line extends below the Great Unconformity (the Cambrian/Precambrian contact). It reveals a fault that dips steeply to the east, and penetrates Precambrian basement. The Great Unconformity has been uplifted by about 300 m on the hanging wall (eastern) side of the fault. A steeply dipping antithetic fault splays from the main fault just below the Great Unconformity. This splay also displays a reverse-sense component of displacement, but it dies out below the Late Cambrian - Early Ordovician Knox Dolomite. Two new structure-contour maps, one on the base of the Devonian-age New Albany Shale, and one on the top of the Ordovician-age Galena Dolomite, delineates the 3-D shape of the LaSalle Anticlinorium in the central Illinois Basin. Notably, an overall west-dipping slope, trending N25°W, defines the western edge of the La Salle Anticlinorium. Across this boundary, the base of the New Albany drops from an elevation of +120 m (+400') -600 m (-2000') over a horizontal distance of 9 km (30,000'). The uplift appears to consist of several N-S-trending en echelon splays. The study area is underlain by a fault that cuts across and displaces the basement-cover contact, splays up-dip into a positive flower structure in the sedimentary cover, and dies out up-dip into monoclinal folds. It may represent inversion of Proterozoic normal faults. The presence of a flower structure, and of en echelon second-order folds, may indicate a component of strike-slip displacement along the overall belt (i.e., the LaSalle Anticlinorium transpressional). Movement on the faults in the LaSalle Anticlinorium occurred during distinct events in the Paleozoic.

Geological Society of America Abstracts with Programs. Vol. 48, No. 5

doi: 10.1130/abs/2016NC-275297

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