APPALACHIAN GREAT GRAVITY GRADIENT: GEOLOGICAL EVIDENCE FOR ITS AGE AND ORIGIN

The Pennsylvania Piedmont’s GGG lies just SE of the allochthonous Reading Prong slab of Grenville basement, continuous with New England geology. A 70-km gap separates the prong from Blue Ridge and other basement sheets that continues through the S. Appl. The Alleghanian Orogeny detached these slabs as megathrusts to form this region’s Piedmont Superthrust by their juxtaposition at depth and displacement 100-150 km to the NW. The Cambrian surface on both sheets now lies ~ 0.5 km above s.l. and 12-15 km below s.l. on the underlying craton. In this region, the GGG must be a deeper cratonic and/or lithospheric feature localized just outboard and below the slabs’ far-travelled Cambrian cratonic edges. Similar geometry along the chain suggests this association is not a local tectonic accident.

The Martic Thrust/Mine Ridge Region, at the SE edge of the Reading Prong slab, exposes the Cambrian cratonic edge imbricated by the Late Ordovician Taconian orogeny. It is a greatly displaced Early Paleozoic cratonic margin and its presumed gravity gradient cannot be the source of this gravity gradient 100+ km inland. Conceivably the local gradient could result from large displacement of the Moho by the Mesozoic Newark Basin’s border fault but the basin’s tilting and listric fault requirements preclude such interpretations. Absence of regional metamorphic effects excludes a related massive Mesozoic gabbroic intrusion. The GGG in this area is a Mid-Paleozoic feature. Locally, a Mid-Paleozoic very large dextral fault just SE of the GGG places a terrane of NY City Piedmont geology next to this region. This and many other Mid-Paleozoic circum-Rhreic Ocean processes are candidates for this density change to the Appl. Piedmont edge.

The Later-Paleozoic continental collision loaded and tilted that modified cratonic margin, detached its upper surface along the brittle-ductile transition, transported that slab up the ramp, and stranded it on the undisturbed craton just inboard of the GGG