COMMERCE GEOPHYSICAL LINEAMENT - ITS SOURCE, GEOMETRY, AND RELATION TO THE REELFOOT RIFT AND NEW MADRID SEISMIC ZONE

The Commerce geophysical lineament is a northeast-trending magnetic an d gravity feature that extends from central Arkansas to southern Illin ois over a distance of approximate to 400 km. It is parallel to the tr end of the Reelfoot graben, but offset approximate to 40 km to the nor thwest of the western margin of the rift floor. Modeling indicates tha t the source of the aeromagnetic and gravity anomalies is probably a m afic dike swarm, The age of the source of the Commerce geophysical lin eament is not known, but the linearity and trend of the anomalies sugg est a relationship with the Reelfoot rift, which has undergone episodi c igneous activity. The Commerce geophysical lineament coincides with several topographic lineaments, movement on associated faults at least as young as Quaternary, and intrusions of various ages. Several earth quakes (M-b > 3) coincide with the Commerce geophysical lineament, but the diversity of associated focal mechanisms and the variety of surfa ce structural features along the length of the Commerce geophysical li neament obscure its relation to the release of present-day strain. Wit h the available seismicity data, it is difficult to attribute individu al earthquakes a specific structural lineament such as the Commerce ge ophysical lineament, However, the close correspondence between Quatern ary faulting and present-day seismicity along the Commerce geophysical lineament is intriguing and warrants further study.