A geophysical investigation of a concealed granitoid beneath Lumberton, North Carolina

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Virginia Polytechnic Institute and State University


Interpretation of geophysical data obtained near Lumberton, North Carolina suggests the presence of a granitic pluton buried beneath Slate Belt-like rocks. A 239 m drill core retrieved from a hole 7.5 km southeast of the city of Lumberton consists of interlayered felsic and mafic volcanics of lower amphibolite grade metamorphism. Dipping reflections in a nearby 16. 5 km long seismic reflection line are believed to be from these volcanics, which are interpreted to be 3.5 km or more in thickness. Below these volcanics is an acoustically transparent zone which is interpreted to be caused by a granitic pluton. This hypothesis is supported by gravity data, which show a -35 mgal Bouguer gravity anomaly, and the relatively high heat flow of 63.4±5 mW/m² obtained in the drill hole, both of which are characteristic of Hercynian granitic plutons in the southeastern United States. Gravity modeling suggests that the body is nearly circular in shape, about 45 km in diameter, and nearly 14 km in thickness.

Deep, nearly horizontal reflections in the 5 to 7 sec time range are interpreted to be from the base of the granitoid at a depth of about 17 km. One possible explanation for these reflections is that the granitoid is allochthonous; emplaced elsewhere and then transported to its present position along a sole thrust. The high heat flow suggests that the body is unmetamorphosed and the thrusting, which may post-date or be coeval with the intrusion, would thus be late Paleozoic in age.