The geometry and magnetic properties of the southeastern margin of the Mississippi Valley graben ate studied by analyzing detailed magnetic-anomaly profiles that are based on measurements made along roads near Memphis, Tennessee. The closely spaced measurements were obtained with a truck-mounted magnetometer system developed by the U.S. Geological Survey. Modeling results suggest that the graben margins represent both structural boundaries and conduits for ascending magma. About 2 km of vertical offset associated with normal faulting occurs within an interpreted 5.5-km-wide zone in which magnetic basement has an average dip of 20° NW into the graben. This zone may contain several large normal faults formed during a stage of rifting in late Precambrian or early Paleozoic time. Subsequent erosion of the scarps probably aided in forming the gentle northwestward slope of the buried Precambrian surface across the fault zone. The high apparent susceptibility (about 3 × 10−3 emu) of magnetic basement associated with this fault zone and with the upblock suggests either that ascending magma intruded the upblock or that the two blocks differed lithologically prior to the formation of the graben.

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