INTEGRATED GEOLOGICAL, SEISMIC REFLECTION, POTENTIAL FIELD, AND SEISMICITY STUDY OF A CLASSIC INTRA-PLATE STRIKE-SLIP ZONE: COTTAGE GROVE FAULT SYSTEM, ILLINOIS BASIN

Mapping and reprocessing of industry reflection data provide new deep subsurface images of deformation along the Cottage Grove fault system (CGFS), a major dextral strike-slip zone in the southern Illinois basin. Well-log data were used to map subsurface structure, and to confirm previous observations from coal-mine mapping. Isotraveltime structural contour mapping delineates distinct monoclines, broad anticlines, and synclines that express Late Paleozoic strike-slip deformation. Prominent near-vertical faults that cut through the entire Paleozoic section and basement-cover contact branch upward into positive flower structures, which are characteristic of strike-slip systems. 3-D contour maps based on well-log data reveal a previously undetected ~3-km wide elongate ridge that follows the southern margin of the master fault. The prevalence of compressional features, including this ridge and positive structural features (flower structures and fault-propagation folds) at bends in the fault, is consistent either with local irregularities that "locked up" the fault and intensified compression, or with the CGFS being overall a transpressive system (compression dominated). The CGFS marks the boundary between a region of numerous small to moderate magnitude earthquakes, on the north, and a region of practically no earthquakes on the south. Also, a belt of strong magnetic anomalies (using the second vertical derivative) closely follows the trend of the master fault. In places, these anomalies appear to be offset across the fault (in a dextral sense). These observations imply that the CGFS originated as a major Precambrian crustal boundary, separating rocks of markedly different physical properties. This boundary was reactivated in strike-slip during the Alleghanian orogeny.