TERTIARY AND QUATERNARY TECTONIC FAULTING IN SOUTHERNMOST ILLINOIS

Tertiary and/or Quaternary tectonic faulting is documented in three ar eas of southernmost Illinois: the Fluorspar Area Fault Complex (FAFC) in Pope and Massac Counties, the Ste. Genevieve Fault Zone (SGFZ) in A lexander and Union Counties, and the Commerce Fault Zone (CFZ) in Alex ander County. In the FAFC, faults that strike NE and NNE displace Moun ds Gravel (late Miocene to early Pleistocene) and, locally, the Metrop olis terrace gravel (Pleistocene; pre-Woodfordian). No Woodfordian or younger deposits are deformed. Faults typically outline narrow, linear grabens that formed under tension with a component of strike slip. No rth-south to NW-trending vertical faults near the southeast end of the SGFZ displace Eocene sediments. Again, faults outline narrow grabens and show indications of strike slip. Deformed Quaternary sediments hav e not been observed. The CFZ, which trends northeast, displaces Mounds Gravel in Illinois and units as young as Peoria Silt (Woodfordian) in Missouri. Quaternary movement has been interpreted as right-lateral s trike-slip. The CFZ coincides with a subtle gravity and magnetic linea ment and seems to reflect a major feature in the basement. Surface exp ression in Illinois is subtle, but mafic and ultramafic intrusions, hy drothermal alteration and small faults align with the Commerce geophys ical lineament. Earthquake foci in Missouri and Illinois lie on or clo se to the CFZ; some focal mechanisms fit the fault trend. Among these structures, only the CFZ exhibits slip that conforms to the current st ress field (principal compressive stress axis E-W to ENE-WSW). Possibl y, the stress field changed during Neogene time. Alternatively, high f luid pressures or local stress concentrations may have induced slip on less favorably oriented fractures. Tighter constraints are needed on timing, magnitude, and direction of Neogene displacement. (C) 1997 Els evier Science B.V.