Central Appalachian Tectonics and the Deep Basin

Abstract

Much of the central Appalachian region fits a geometric pattern that is bilaterally symmetrical to an axis passing N. 40° W. from the Baltimore dome through the high point of the Nittany arch. Many elements are likewise concentric to a focus situated on that axis near Baltimore, Maryland, and are symmetrically tangent to a base-line that crosses the axis at right angles. There is possible distortion of this symmetry along a conjectured slip or wrench fault at about N. Lat. 40°, which may involve a dextral offset amounting to 80 or more miles along a trace now concealed by younger sediments or the Atlantic Ocean from the Susquehanna River eastward to the Kelvin Seamount Group, 400 miles offshore at N. Lat. 40°. It is suggested that these symmetrical features and the fault result from (a) primary uplift of the Baltimore dome with outward gravitational sliding in the overlying skin of sediments, (b) a secondary forward movement along the aforementioned axis of a crustal block containing the Baltimore dome at its outer corner, and (c) dextral displacement along the wrench fault. It is possible that c is the cause of b. The deep part of the central Appalachian basin is reviewed in the light of a general theory of Appalachian tectonic deformation which accepts the foregoing hypotheses as valid. This reappraisal supports the no concept of deformation wherein structures of the sedimentary cover are independent of those in the basement. It suggests that depths of the basement may be considerably less than those projected downward by customary calculations; that the true configuration of the top of the basement may not be calculated implicitly from the assumed thickness of overlying sediments; and that structural trend-lines in the basement may not be those of the sediments above them. The location of the deep Appalachian moat is placed well east of the Allegheny Front, and evidence is presented that it strikes toward or under the present Blue Ridge in northern Virginia. Similar evidence suggests that the grain of basement structures and the lines of original sedimentation both strike into the Blue Ridge overthrust in southwest Virginia. These conclusions are supported by magnetic data, and an original domal source for Siluro-Devonian sediments is confirmed by a palinspastic map. Some argument is offered for the development of a general tectonic theory of Appalachian deformation involving a sedimentary carpet distorted by differential movements of a fragmented crust. The process is called Pattern Tectonics.