EVIDENCE FOR POST-ALLEGHANIAN BASIN AND RANGE STYLE EXTENSION IN THE SOUTHERN APPALACHIAN OROGEN

The southern Appalachian orogen (SAO) records the nexus of the Laurentia-Gondwana collision that formed Pangea during the culminating Alleghanian orogeny. Lithosphere thickened through preceding Paleozoic orogenic cycles reached a maximum thickness in the late Mississippian to Pennsylvanian, resulting in widespread metamorphism and granitic plutonism. An ensuing 100 million years of extension and erosion uniformly thinned the SAO lithosphere and led to development of the nascent Atlantic basin, placing the SAO again at a nexus for the breakup of Pangea. Although there is overlap, suites of late to post-orogenic granitoids in the SAO generally yield U/Pb ages from ca. 335-325 Ma in the Inner Piedmont, and ca. 315-290 Ma eastward to the accreted Gondwanan terranes. Peak metamorphic conditions broadly accompanied magmatism and ⁴⁰Ar/³⁹Ar ages generally record younger deformation and cooling, from ca. 335 Ma in the western Blue Ridge eastward to as young as ca. 260 Ma near the Coastal Plain unconformity. ⁴⁰Ar/³⁹Ar cooling age contours generally strike parallel to the Appalachian orogen and appear to extend from the subsurface of northern Florida to Virginia. The progressive eastward cooling and exhumation can be interpreted in the context of a Basin and Range style of extension, with the Blue Ridge and Piedmont terranes in the footwalls of a series of large-scale detachment faults, and an upper plate with a horst-and-graben system (e.g., South Georgia basin). Maximum average extension rates along the detachments are estimated to be ca. 10 km/m.y. during the interval from 300-280 Ma. The nature and fate of the terranes that once buried the SAO lithosphere above the present erosion surface are unclear, as are the variations in lithospheric thickness that once existed. Erosion accompanying extension provided sediment to the Appalachian foreland basins, and later to the evolving Atlantic margin, but the balance of those crustal thinning processes and locations of the sinks for sediment are also unclear. These uncertainties will be addressed through future detailed field mapping of extensional structures, targeted strategies for dating plutons and increasing the thermochronologic/PTt database, and tying the record of the crystalline basement to the late Paleozoic and Mesozoic sedimentary record.