Aeromagnetic images; fresh insight to the buried basement, Rolla Quadrangle, Southeast Missouri

Aeromagnetic data from the Rolla 1 degrees X2 degrees quadrangle, Missouri, displayed as a small-scale, grey-tone image, provide a lucid picture of basement features beneath sedimentary-rock cover. Late-stage "tin" granite plutons of the Proterozoic St. Francois magmatic terrane are clearly delineated. The plutons average about 11 km in diameter and occupy a significant fraction (12 percent) of the area surveyed. Gravity data require that certain plutons are at least 3 km thick. Most of the plutons are fat ovals in plan view and show no flattening trend or structural alignment. The ensemble indicates significant extension in an orthorhombic stress field, with the minimum compressional stress being vertical. Exposed St. Francois terrane volcanic rocks show a characteristic intricate pattern in the magnetic signature. This pattern is more widespread than the subsurface distribution of volcanic rocks indicated by drilling and likely also incorporates areas of St. Francois batholith granite. The horseshoe-shaped trend of the southeast Missouri lead deposits may be crudely aligned with the edge of this intricately patterned unit. Small, very high-amplitude magnetic anomalies occur over the well known magnetite-bearing deposits of the region. The magnetic background for all these features is probably provided by extensive areas of the Butler-Hill type biotite granite, characterized by large tracts of low magnetic intensity and a cobblestone-like pattern of ring-shaped, weak magnetic highs.Few of the many regional faults, calderas, ring complexes, and lineaments previously interpreted from geologic, geophysical, and remote sensing data are expressed in the aeromagnetic image.