ABSTRACT

The Coastal Plain province of Virginia is characterized by a coincidental gravity and magnetic high parallel with the northeast-southwest Appalachian trend in the north changing to a north-south trend in the south. Both ends of the anomaly steeply grade into the regional field in Maryland and North Carolina.

Interpretation of 2 cross-strike geophysical profiles indicates that the anomaly is due to a high-density mafic unit flanked by low-density granitic(?) units. Geophysical signatures to the east of the anomaly are not parallel with Appalachian regional trends that characterize areas to the west.

Gravity models extending to 3 km depths on the southern profile (parallel with 1-64 USGS line) and 4 km on the northern profile were chosen experimentally for optimal estimation of density contrasts. First approximations of the profile using vertical blocks to obtain density contrasts showed that east-dipping (≈60°) crustal blocks better represent the data. Well logs show that the anomaly is characterized by metagabbro, metabasalt, and amphibolite flanked by coherent lower-density granitic crustal blocks. Triassic-Jurassic clastic basins on the flanks of the anomaly to the west are fault-bound, but occur as vast basin infills to the east.

A conceptual model suggests that the anomaly may represent a suture zone between the North American crustal block to the west and possibly a remnant Avalonian(?) microplate to the east. Marked lineaments observable on computer-enhanced Landsat images closely parallel the subsurface trend of the anomaly.

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