QUANTITATIVE INVESTIGATIONS OF THE MISSOURI GRAVITY LOW - A POSSIBLE EXPRESSION OF A LARGE, LATE PRECAMBRIAN BATHOLITH INTERSECTING THE NEWMADRID SEISMIC ZONE

Analysis of gravity and magnetic anomaly data helps characterize the g eometry and physical properties of the source of the Missouri gravity low, an important cratonic feature of substantial width (about 125 km) and length (> 600 km). Filtered anomaly maps show that this prominent feature extends NW from the Reelfoot rift to the Midcontinent Rift Sy stem. Geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal, Il-km-thick batholith with either near vert ical or outward dipping boundaries. Considering the modeled characteri stics of the batholith, structural fabric of Missouri, and relations o f the batholith with plutons and regions of alteration, a tectonic mod el for the formation of the batholith is proposed. The model includes a mantle plume that heated the crust during Late Precambrian and melte d portions of lower and middle crust, from which the low-density grani tic rocks forming the batholith were partly derived. The batholith, ca lled the Missouri batholith, may be currently related to the release o f seismic energy in the New Madrid seismic zone (earthquake concentrat ions occur at the intersection of the Missouri batholith and the New M adrid seismic zone). Three qualitative mechanical models are suggested to explain this relationship with seismicity.