NATURE AND TIMING OF MULTIPLE(?) TERRANE ACCRETION ALONG THE EASTERN/WESTERN BLUE RIDGE (TALLADEGA BELT) BOUNDARY, ALABAMA

The Eastern-Western Blue Ridge boundary of Alabama and western Georgia is a major thrust, the Hollins Line fault(HLF), emplacing middle to upper amphibolite facies rocks of the hanging wall Ashland-Wedowee belt(AWB) over lower greenschist facies rocks of the footwall Talladega belt(TB). Lithologies of the upper TB, Talladega Group(TG), represent a Devonian to earliest Mississippian(?) successor basin which developed on the Laurentian shelf, whereas lithologies of the suspect AWB, Ashland-Wedowee Group(AWG), are of unknown affinity, but were most likely emplaced against Laurentia by the Devonian. At the structural top of the TB is the Hillabee Greenstone(HG), recently interpreted to be Ordovician in age(~470Ma) based on U/Pb zircon ion probe dating. The HLF is a footwall duplex imbricating the HG and upper TG lithologies into a series of right-stepping horses, with significant displacement along the roof thrust(>40km horizontal and >9km vertical). Dextral transpression along the HLF initiated no earlier than late Mississippian and had subsequently ceased no later than late Carboniferous. An Ordovician age for the HG would indicate an earlier thrusting event juxtaposing an exotic volcanic terrane against the TG shelf sediments prior to metamorphism, becoming subsequently metamorphosed with the TB, and then being overprinted by the middle to late Carboniferous HLF and the final suturing of the Pangean supercontinent. Tight age constraints dictate that an exotic HG terrane be emplaced no earlier than the late Devonian to earliest Mississippian(Tournasian) age of the upper TG metasediments and prior to the late Mississippian metamorphic age(~328Ma). The existence of an exotic HG volcanic terrane necessitates that a significant strike-slip or transpressional boundary exist between the HG and TG with an order of displacement many 10's km. Additionally, an exotic origin for the HG would require a shallow thrust boundary between it and the TG which is concordant to TG stratigraphy along a footwall-hanging wall flat >100km in length and >10km in width. Other than the isotopic age for the HG, no structural evidence of a pre-metamorphic terrane boundary between the HG and TG has been recognized to date.