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Much of the central Appalachian region fits a single geometric pattern that is bilaterally symmetrical to an axis or radius 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 and (or) are symmetrically tangential to a baseline that crosses the above axis at right angles in the vicinity of Baltimore. It is suggested that all of these symmetrical features 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 axis of a crustal block containing the Baltimore dome; or (c) some combination of these two factors.
There is possible distortion of this symmetry along a conjectured slip- or wrench-fault at about Lat. 40° N., 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 Lat. 40° N.
The nature of 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 strongly supports the "no-basement" concept of deformation wherein structures of the sedimentary cover are independent of those in the basement. It suggests that depths to the basement may be considerably less than those predicted 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.
The loose-ended crystalline Reading prong (of the Hudson Highlands) and South Mountain prong (of the Blue Ridge) are both believed to have slid westward away from the Baltimore dome. Both are regarded as having over-ridden their own original roots and now occupy an "overthrust" position. The unusual deformation of the anthracite region of eastern Pennsylvania is attributed to this shift of the Reading prong. The Burning Springs anticline is thought to be a late effect, also crescentic or arcuate with respect to the Baltimore dome.
Most deep Appalachian wells are drilled on surface-visible anticlines. Probably the synclines show strata least removed from their pre-deformational position. An isopach map utilizing only synclinal data seems to show sedimentary trends extending toward, rather than converging along, the site of the present Blue Ridge. A palinspastic map is presented on which the arcuate folds of the central Appalachians are eliminated in line with the thesis here explained. When delineated on this restored base map, isopachs for several mid-Paleozoic series still show curvature concentric with the Baltimore dome. This is believed to demonstrate an original domal, rather than lineal, source for these clastic sediments.
Finally, the lineation or extension of certain undescribed "non-fold" trends--perhaps joints facilitating the migration of oil or gas--seems also to follow the geometric pattern controlled by the N. 40° W. axis and its N. 50° E. baseline. It is thus suggested that the regionwide Appalachian occurrence of oil and gas is a definite function of the pattern being described.
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