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In September 1980, the U.S. Geological Survey conducted a seismic refraction investigation of the northern Mississippi Embayment. During the investigation, 34 shots from nine shotpoints were recorded along a series of profiles. The profiles were parallel to and across an inferred Precambrian rift zone which is outlined by a series of magnetic anomalies and covers an area at least 200 km (125 mi) long and 70 km (45 mi) wide.
Along the northeast-southwest trending axis of the rift, a 0.7 to 1.1-km (2,297 to 3,609-ft) thick 1.8-km/sec layer representing the unconsolidated Tertiary and Cretaceous sediments overlies a 2-km (6,562 ft) thick 5.95-km/sec layer representing a Paleozoic carbonate sequence. Beneath the carbonates is a 3-km (9,843-ft) thick low-velocity layer which probably consists of late Precambrian clastics. The presence of this low-velocity layer is indicated on the seismic profiles by an abrupt cutoff of the 5.95-km/sec arrivals at about 20 km (12 mi) from the shotpoints. This cutoff is followed by a 30 to 40-km (20 to 25-mi) long zone of no arrivals beyond which are seen delayed arrivals from a 6.2-km/sec crystalline basement. The 11-km (6.8-mi) thick 6.2-km/sec basement overlies a lower crus composed of a 6.6-km/sec layer whose thickness decreases to the northeast owing to its replacement by an anomalous high-velocity 7.3-km/sec basal layer. The base of the crust ranges from a 39 km (24 mi) depth at the southern end of the profile to a 46 km (29 mi) depth at the northern end where the 7.3-km/sec layer is 20 km (12 mi) thick. Below the base of the crust (Moho), the upper mantle has a velocity of 8.0 km/sec.
The velocity structure beneath the west flank of the embayment is simpler than that of the axis: 5.95, 6.2, 6.6, 7.3, and 8.0 km/sec. The low-velocity layer is absent and the anomalous 7.3-km/sec layer is thinner than along the axial profile. The profiles perpendicular to the rift show that the low-velocity layer is restricted to the axial zone and that the anomalous 7.3-km/sec basal crustal layer reaches maximum thickness there.
The tectonic model proposed to explain the origin of this embayment velocity structure includes a late Precambrian mantle plume that intruded the lower crust of the northern embayment, causing uplift (and/or crustal stretching) and subsequent rifting of the axial area. This was followed by erosion, subsidence, and subsequent deposition of sediments in the resulting trough.
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