Abstract

A preliminary interpretation of about 135 km of seismic-reflection data provides new information on the structural relations between the the Crittenden County fault zone and the subjacent rift-bounding faults along the southeastern margin of the Reelfoot rift in the New Madrid seismic zone. On the reflection data, the rift boundary is marked by a 4- to 8-km-wide zone of incoherent reflected energy and disrupted reflectors in the lower part of the well-stratified, lower Paleozoic sedimentary rocks and in the underlying Precambrian crystalline basement. In places, the zone of disrupted reflectors extends into the upper part of the Paleozoic rocks, and, on some lines, disrupted reflectors and distinct faults are present in the Upper Cretaceous and Tertiary rocks of the Mississippi Embayment. The Crittenden County fault zone is interpreted as a northwest-dipping, high-angle reverse fault with an up-to-the-northwest throw, which is opposite to the net structural relief in the subjacent graben. The fault zone is at least 32 km long and coincides with the rift margin in southwestern Crittenden County, but to the northeast, it diverges away from the aeromagnetically defined margin of the rift by almost 4 km. Most faults in the Crittenden County fault zone are apparently ancient rift-bounding normal faults that were reactivated with a significant amount of reverse slip during the Mesozoic and Cenozoic. On the basis of its apparent connection with the rift-bounding faults, the evidence of its long history of recurrent movement, and its orientation with respect to the modern stress field, the Crittenden County fault zone might be considered to potentially generate major earthquakes. In contrast, the possibility that the Crittenden County fault zone could be a bending-moment fault argues against it being extremely hazardous. Precambrian crystalline basement interpreted on the profiles is commonly deeper than magnetic basement by as much as 2.5 km. This discrepancy between shallow magnetic basement and deeper crystalline basement could be explained by the presence of igneous intrusions in the Paleozoic strata immediately above Precambrian basement.

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