THREE PHASES OF TACONIC ACCRETIONARY OROGENESIS IN THE SOUTHERN APPALACHIANS

A growing body of evidence indicates that the Taconic orogeny in the southern Appalachians formed during three short-lived (10-20 my) accretionary orogenic phases associated with alternating back-arc extension and contraction along the southern Laurentian plate margin (LPM). This tectonic ‘switching’ is similar to the development of other accretionary orogens (e.g. Tasmanides), but unlike the classic collisional model proposed for initial phases of the Taconic in the northern Appalachians. Subduction was initiated outboard of Laurentia during the latest Cambrian-earliest Ordovician, followed by extension at the edge of the LPM and resulted in formation of an extensive (>400km long) back-arc system (Hillabee Greenstone-Emuckfaw Group-Pumpkinvine Creek Formation-Dahlonega Gold Belt) between ~480-460 Ma. Distal to the orogenic front, destabilization along the outer parts of the passive margin resulted in formation of the post-Knox unconformity and initial intrabasinal deposition of the Blount clastic wedge, which contains Middle Ordovician k-bentonites presumably sourced from this back-arc system. Following initial extension, ~460 Ma inversion of the back-arc region resulted in cessation of magmatism, subduction erosion of the overriding LPM and formation of high pressure metamorphic rocks of the central Blue Ridge and Lick Ridge eclogite, as well as a thermal pulse and possibly mild deformation within Laurentian margin rocks of the western Blue Ridge-Talladega belt. Some silicic magmatism (e.g. ~458 Ma Villa Rica Gneiss) may have occurred during this contractional phase, similar to magmatism associated with tectonic switching events in other accretionary orogens. Continued deposition in the Blount basin may have resulted from a topographically subdued thin-skinned thrust belt or inversion of basement faults in the new compressional regime. By ~450 Ma, a return to extensional orogenesis outboard of the central Blue Ridge-eastern Blue Ridge terranes resulted in suprasubduction igneous activity in the Tugaloo terrane (e.g. ~448 Ma Whiteside and ~445 Ma Poor Mountain) and extensive ash beds (e.g. ~450 Ma Deicke and 448 Ma Millbrig k-bentonites) in the Appalachian foreland. Recently recognized ~440-430 Ma plutons (e.g. Mulberry Rock Gneiss) suggest this final phase continued for at least 20 m.y.