Aeromagnetic images: Fresh insight to the buried basement, Rolla quadrangle, southeast Missouri

Abstract

Aeromagnetic data from the Rolla 1° × 2° 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.

The decompensative gravity anomaly and deep structure of the region of the Rio Grande Rift

“…added. Because of the large area covered and the high wave number content of the data, the decompensative anomaly is best shown as a grey scale image(Figure 4)to preserve high wave number content[Cordell and Knepper, 1987]. The Rio Grande rift is clearly delineated in the lower central third ofFigure 4by gravity lows over lowthe point of intersection of this feature with the rift the rift shifts abruptly in a right echelon sense, described as an accommodation zone byChapin [1989].Steep gravity gradients associated with gravity lows over low-density graben fills delineate the master graben faults by which the rift is defined(Figure 4).…”

9. Magnetic Exploration Methods

“…Automated color-filled contour maps made their debut in the early 1980s, which facilitated the assessment of regional trends and magnitude variations (Paterson and Reeves, 1985). By the late 1980s, magnetic interpreters were borrowing from remote-sensing imaging technologies in the form of gray and color gradational images and shaded-relief images (Cordell and Knepper, 1987). By the late 1980s, magnetic interpreters were borrowing from remote-sensing imaging technologies in the form of gray and color gradational images and shaded-relief images (Cordell and Knepper, 1987).…”

Application of aeromagnetic data to mineral potential evaluation in Minnesota

“…The interpreter outlines magnetic/lithologic units and interprets structure using the magnetic contacts map that is overlain on either a magnetic intensity contour map and/or any one of several computer-enhanced data maps such as shadow, grey-tone (see Cordell and Knepper, 1987), vertical derivative maps, etc. In this study, the magnetics contact overlay was used in conjunction with the total field contour map, the pole reduced contour map, and the second vertical derivative map.…”