Abstract

Aeromagnetic and gravity surveys of the northern Mississippi embayment have delineated a northeast-trending basement depression about 70 km wide and more than 300 km long. The feature, the Mississippi Valley graben, is remarkably linear over its known length, has nearly parallel sides, and has estimated depths relative to surrounding basement of more than 2 km. Both gravity and magnetic anomalies show an alignment of dense magnetic plutons along the boundaries of the graben. Estimated magnetization directions for the plutons indicate a Mesozoic age. The graben, of probable earliest Paleozoic age, underlies the northwest side of the embayment and contains the area of major seismicity. In particular, much of the instrumentally located contemporary seismicity and the estimated locations of the large-magnitude 1811–1812 earthquakes are in the central part of the graben. We interpret the early Paleozoic graben, the Mesozoic plutonism, and the sedimentation cycles as evidence of stretching events. Seismicity is attributed to strain in and below the central graben area, which would appear to be a focal point of deformation in the stretching model. We suggest that the sequence of events can be explained by the action of a rotating horizontal stress field on a crustal flaw that persisted well back in Pre-cambrian time. The postulated rotating stress field is presumably controlled by the system that drives the global plates.

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