MESOSCOPIC ANALYSIS OF BRITTLE DAMAGE ZONES, STE. GENEVIEVE FAULT ZONE, OZARK PLATEAU, MISSOURI

The Ste. Genevieve Fault forms the border between the Ozark Plateau, structurally high and sharp on its NE side, and the Illinois Basin. The 200 km long WNW-trending Ste. Genevieve Fault Zone extends from southeastern Missouri, across the Mississippi River into southern Illinois. Net slip across the fault zone is ~7 km. Two main Paleozoic periods of faulting have been suggested, but new Earthscope data suggest that small earthquakes may still occur in this zone. This mid-continental fault zone is generally deeply weather and poorly exposed and has been difficult to study at the sub-macroscopic scale. We study the relative timing and kinematics of a beautifully exposed suite of mesoscopic structures (cataclastic deformation bands, fault cores) in a well-developed, intense brittle damage zone related to the Ste. Genevieve Fault Zone. The brittle structures are developed in the basal Cambrian LaMotte Sandstone at Pickle Creek Recreation Area in the uplifted and exhumed SE Ozark block of the Ste. Genevieve Fault. Our descriptions, and provisional chronology and kinematics of the main structures are as follows: 1) an early set of subhorizontal deformation bands (kinematics to be determined; possibly related to early layer-parallel shortening) are cut by 2) N60W-striking, NE- and SW-(steeply) dipping, nearly pure dip-slip, normal-sense shear deformation bands and normal faults, and 3) NE-striking, subvertical, dilational deformation bands and joints. We are currently developing and testing two working models: 1) The Ozarks moved structurally up and over the Illinois Basin along a giant Laramide-style NE-verging (SW-dipping) basement thrust whose surface expression today is the steeply dipping Ste. Genevieve Fault. Accordingly, the Ozark Dome is a stretched fault-propagation fold at surface associated with this deep thrust. 2) The Ozarks are the uplifted and exhumed footwall block of a NE-dipping normal fault whose surface expression today is the steeply dipping Ste. Genevieve Fault. To test the two models we plan to refine our chronological/kinematic analysis and to to collect additional outcrop data at more localities.