Identification of a Precambrian Rift through Missouri by digital image processing of geophysical and geological data

Abstract

Digital free air and Bouguer gravity anomaly images have been constructed from approximately 287,000 station readings for the region bordered by 25°–49°N latitude and 80°–110°W longitude. The technique used to interpolate between station locations was based on a two‐dimensional spatial filter, where the average of the anomaly values located within the filter area was computed. The images contain as many as 256 contours (values in byte variable), so that subtle anomaly patterns can be identified and traced with much greater certainty than on most contour maps. A newly discovered feature in the midcontinent is a gravity low that begins at a break in the midcontinent gravity high in SE Nebraska, extends across Missouri in a NW‐SE direction, and intersects the Mississippi Valley graben to form the Pascola arch. The anomaly varies from 120 to 160 km in width, extends about 700 km, and is best expressed in southern Missouri, where it has a Bouguer amplitude of approximately −34 mGal. The magnitude of the anomaly cannot be explained on the basis of a thickened section of Paleozoic sedimentary rock. The gravity data and the sparse seismic refraction data for the region are consistent with an increased crustal thickness beneath the gravity low. Some of the discrete positive magnetic anomalies in Missouri are located along the borders of the gravity low. Digitally enhanced thermal infrared images from the Heat Capacity Mapping Mission show a distinct alignment of linear structures with the gravity feature. The linear features in some cases correspond to mapped high‐angle normal faults, to drape folds over relief within the Precambrian basement, and in some cases to extensions of mapped structures. The gravity anomaly also cuts across the major Precambrian boundary in SE Missouri, marking the change from older, sheared granites and metasedimentary rocks to younger granites and rhyolites. Given the cumulative evidence, the gravity anomaly is probably the present expression of a failed arm of a rifting event, perhaps one associated with the spreading that led to or preceded formation of the granite and rhyolite terrain of southern Missouri.