Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: Characteristics, extent, and tectonic significance

Abstract

Fault-dismembered segments of a distinctive, extensive, highly allochthonous, and tectonically signifi cant Ordovician (ca. 480-460 Ma) basin, which contains suites of bimodal metavolcanic rocks, associated base metal deposits, and thick immature deepwater (turbiditic) metasediments, occur in parts of the southern Appalachian Talladega belt, eastern Blue Ridge, and Inner Piedmont of Alabama, Georgia, and North and South Carolina. The basin's predominantly metasedi mentary strata display geochemical and isotopic evidence of a mixed provenance, including an adjacent active volcanic arc and a provenance of mica (clay)-rich sedimentary and felsic plutonic rocks consistent with Laurentian (Grenvillian) upper-crustal continental rocks and their passive-margin cover sequences. Geochemical characteristics of the subordinate intercalated bimodal meta volcanic rocks indicate formation in a supra subduction environment, most likely a back-arc basin, whereas characteristics of metasedimentary units suggest deposition above Neoproterozoic rift and outer-margin lower Paleozoic slope and rise sediments within a marginal basin along Ordovician Laurentia's Iapetus margin. This tectonic setting indicates that southernmost Appalachian Ordovician orogenesis (Taconic orogeny) began as an extensional accretionary orogen along the outer margin of Laurentia, rather than in an exotic (non-Laurentian) arc collisional setting. B-type subduction polarity requires that the associated arc-trench system formed southeast of the palinspastic position of the backarc basin. This scenario can explain several unique features of the southern Appalachian Taconic orogen, including: the palinspastic geographic ordering of key tectonic elements (i.e., back-arc, arc, etc.), and a lack of (1) an obducted arc sensu stricto on the Laurentian margin, (2) widespread Ordovician regional metamorphism, and (3) Taconic klippen to supply detritus to the Taconic foreland basin.

Linking metamorphism, magma generation, and synorogenic sedimentation to crustal thickening during Southern Appalachian mountain building, USA

“…Previous thermobarometic studies in the eastern Blue Ridge estimated regional metamorphic conditions of 580 °C ± 65 °C to 665 °C ± 50 °C and of 4.7 ± 1 to 8.5 ± 0.8 kbar from garnet-muscovite-biotite-plagioclase equilibrium (Gibson and Speer, 1986;Drummond et al, 1988;Stowell et al, 1996). Wedowee Group metasediments contain detrital zircon with a significant Grenville component and include mid-ocean-ridge basalt (MORB)-like metabasalts (Tull et al, 2014;Barineau et al, 2015). Tull et al (2014) used these data to interpret the eastern Blue Ridge rocks as an Ordovician back-arc basin.…”

“…Wedowee Group metasediments contain detrital zircon with a significant Grenville component and include mid-ocean-ridge basalt (MORB)-like metabasalts (Tull et al, 2014;Barineau et al, 2015). Tull et al (2014) used these data to interpret the eastern Blue Ridge rocks as an Ordovician back-arc basin. The Emuckfaw Group, southeast of the Wedowee Group exposures ( Fig.…”

“…Previous publications have proposed that the Inner Piedmont includes the core of a Taconic magmatic arc that was constructed above a NWdipping subduction zone (e.g., Hatcher, 2010). The eastern Blue Ridge rocks have been interpreted as an Ordovician back-arc basin built upon the Laurentian margin (Tull et al, 2014;Barineau et al, 2015). NWdipping subduction beneath an outboard island arc most likely began during the Late Ordovician (e.g., Hatcher, 2010).…”

“…440 Ma Farmville Metagranite (Hawkins et al, 2013), and the 450-440 Ma Dadeville Complex (Tull et al, 2018). The Late Ordovician magmatic arc may not have developed on the Laurentian margin and instead may have been offshore or separated from the continent by a back-arc basin (Tull et al, 2014;Barineau et al, 2015). The ages and metamorphic data presented here indicate that the Ordovician arc magmatism likely did not extend northwest into what is currently the eastern Blue Ridge, and that the metasedimentary rocks in the Blue Ridge were not heated (and loaded) until near the end of the Neoacadian orogeny.…”

Detrital zircons and sediment dispersal in the Appalachian foreland

“…From the New York promontory northward to Newfoundland, late Paleozoic clastic sedimentation along the Appalachian orogen filled fault-bounded pull-apart basins along a regional system of dextral strike-slip faults (Fig. 1) (e.g., Thomas and Schenk, 1988;van de Poll, 1995). From the Alabama promontory westward along the Ouachita and Marathon embayments, late Paleozoic synorogenic clastic wedges GEOSPHERE GEOSPHERE;v.…”