About This Item
Share This Item
Nine parallel aeromagnetic traverses were flown by the U. S. Geological Survey across the southern Appalachian Mountains and the plateau regions to the west. These traverses are at right angles to the prevailing structural trend and form a strip approximately 20 miles wide and 250 miles long, extending from the Blue Ridge, just north of Asheville, North Carolina, to the Ohio River at Louisville, Kentucky.
There is a pronounced northeast linearity in the trends of the individual magnetic anomalies and the larger magnetic units which parallels the regional tectonic trends of the Appalachian Mountains. The anomaly pattern implies several sharp contrasts in magnetic expression of the crystalline basement rocks. The dominant feature on the profiles is a group of exceptionally large anomalies which delineate a block of strongly magnetic rock, approximately 100 miles in width, underlying the Appalachian Plateau. Available gravity data along this same strip, although sparse, show a marked resemblance to the over-all magnetic pattern. A positive Bouguer anomaly of about 30 milligals coincides with the group of large magnetic anomalies. It is concluded that these magnetic and gravity anomalies a e produced by a large mass of predominantly mafic igneous rock underlying the Paleozoic sedimentary rocks of the Appalachian Plateau.
Estimates of depth to magnetic rock have been made from many individual magnetic anomalies. These magnetic rocks presumably are part of the Precambrian complex underlying the Paleozoic sedimentary rocks. The depths obtained were generally consistent and indicate that in the Appalachian Plateau there are 8,000-10,000 feet of sedimentary rock and the basement surface is 7,000-9,000 feet below sea level. These thicknesses are less than those predicted from stratigraphic considerations but are supported by data from recently drilled wells which reached basement. One well in Leslie County, Kentucky, entered granite 9,412 feet below the surface or 8,233 feet below sea level. The depth analyses of magnetic anomalies in the Valley and Ridge province indicate that the Paleozoic section
thickens to the southeast, although not as much as expected. The average thickness in the Valley and Ridge province is about 17,000 feet and the greatest thicknesses are in the southern part, indicating that the basement surface plunges in that direction.
End_of_Article - Last_Page 1252------------