MIDDLE PALEOZOIC CLASTIC WEDGE PROVENANCE, DEXTRAL COLLISION, AND THE SOUTHERN APPALACHIAN INNER PIEDMONT: THE SPECHTY KOPF AND ROCKWELL FORMATIONS OF THE CENTRAL APPALACHIANS AS TECTONIC MARKERS

The Inner Piedmont of the Carolinas, Georgia, and Alabama has a tectonothermal history that is different and distinguishable from the adjacent Carolina terrane, and the eastern Blue Ridge. The Inner Piedmont of SC-NC-GA enjoyed peak metamorphic conditions (up to granulite facies) at the Devonian-Mississippian boundary, ca 355-365 Ma. While a Middle Ordovician clastic wedge is well known in the TN-AL-GA Appalachians, the evidence for a Devonian Southern Appalachian clastic wedge is less obvious. However, a great deal of evidence supports very high dextral shear strains in the eastern Blue Ridge and western Inner Piedmont. Nominal shear strains of γ≥20 across a belt not less than 20 km wide restore the Inner Piedmont to the central Appalachians. At these latitudes detritus shed from a collisional orogen is preserved as the Catskill and (younger and to the south) the Pocono-Price clastic wedges. Studies of detrital zircons and sandstone petrology of the Catskill rocks suggest a recycled orogenic provenance, i.e., an uplifted Laurentian margin fold and thrust belt of an earlier orogenic cycle. At the Devonian-Mississippian boundary, between the Catskill and Pocono rocks is preserved an unusual lithology. The Spechty Kopf (PA) and Rockwell (MD-WV) Formations include diamictites and laminites (varved claystones?) with dropstones. In the outcrop belt of Spechty Kopf Fm it is clear that formation thickness is highly variable, and is suggestive of deposition in valleys incised into the underlying Catskill wedge. Additionally, some of the clasts of the Spechty Kopf diamictites are striated, and some represent igneous and metamorphic clasts that are not common in the bounding units. These relations have led to the interpretation that the Spechty Kopf Formation represents a glacial interval that samples exotic Acadian highlands. Thus the hypothesis that the Catskill and Pocono wedges may be derived from higher structural levels of the Inner Piedmont during dextral IP collision with Laurentia may be testable. Such a approach requires indirect methods because U-Pb monazite ages (ca. 359 Ma, 6/8 TIMS) and Ar-Ar mineral ages from the Alto Allochthon indicate the western Inner Piedmont cooled through hornblende blocking temperatures by ca. 350 Ma but did not cool through the blocking temperature for muscovite before ca. 315-316 Ma.