Geophysical evidence for an allochthonous Alleghanian(?) granitoid beneath the basement surface of the Coastal Plain near Lumberton, North Carolina

Abstract

Geophysical data obtained over the Atlantic Coastal Plain near Lumberton, North Carolina, indicate the presence of a granitic pluton buried beneath basement volcanic rocks similar to those of the Carolina slate belt. The volcanic rocks were cored southeast of the city of Lumberton. They consist of interlayered felsic and mafic rocks metamorphosed to lower amphibolite facies. Excellent reflections from a nearby VIBROSEIS line originate from this volcanic sequence, which is interpreted as being >3.5 km in thickness. The volcanic rocks are underlain by an acoustically transparent interval interpreted as being associated with a granitic pluton. This hypothesis is supported by a −35 mgal Bou-guer gravity anomaly and by a relatively high heat flow of 63.5 ± 5.4 mW/m 2 obtained in the drill hole. These are both characteristics of late Paleozoic Alleghanian granitoids in the southeastern United States. Results of gravity modeling suggest that the body is nearly circular in shape (∼45 km diam) and 13 km in thickness. Subhorizontal reflections at 5 to 7 sec on the seismic data are interpreted as being at and below the base of the granitoid at a depth of ∼ 17 km. The deep layering may be stratigraphy or mylonites, and, in either case, the granitoid is allochthonous and did not intrude the layering. High heat flow suggests, from our experience in the Piedmont, that the body is unmetamorphosed and of late Paleozoic age; the thrusting may postdate, or be coeval with, the age of intrusion and suggests that late Paleozoic thrusting in this part of the Coastal Plain is confined to depths below 17 km.