Formation of high‐pressure metabasites in the southern Appalachian Blue Ridge via Taconic continental subduction beneath the Laurentian margin

Abstract

Tectonic models that account for the occurrence of eclogite, retrograde eclogite, peridotite, and migmatitic basement gneisses in the southern Blue Ridge invoke closure of a small ocean basin between the eastern Blue Ridge (EBR) and western Blue Ridge (WBR). In this “obducted ophiolite” model, Franciscan‐type, eastward subduction of oceanic crust beneath the Piedmont arc occurred prior to obduction of the arc onto the Laurentian margin. The discovery of kilometer‐scale eclogite bodies near the EBR‐WBR boundary are cited as evidence for obducted ophiolite/accreted island arc models. However, thermobarometric estimates based on mineral chemistry from HP metabasites of the western, central, and eastern Blue Ridge indicate that eclogites and granulites are medium‐temperature type (550–900°C), inconsistent with the obducted ophiolite model. Mid‐ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550°C. Thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the Blue Ridge allow for a model of westward subduction of continental material beneath Laurentia during the Taconic orogeny that produced a megamélange now preserved in the central and eastern Blue Ridge tectonic provinces. P‐T paths determined from Blue Ridge HP metabasites are similar to P‐T paths determined previously from Caledonian and Variscan eclogites. Thermobarometric similarities imply a common tectonic history of continental collision, subduction of continental crust to depths of 50–65 km, followed by rapid eduction.

Crustal evolution across the southern Appalachians: Initial results from the SESAME broadband array

“…The global implication is that crustal refining during convergence may be an effective means of recycling mafic rocks into the mantle and enriching the crust in felsic components [Ernst, 2010;Hacker et al, 2011]. This hypothesis is consistent with the felsic crustal structure in other orogenic domains such as the Variscides [Villaseca et al, 1999] and Irish Caledonides [van den Berg et al, 2005;Hauser et al, 2008].…”

“…In the southern Appalachians, the low Vp/Vs ratios and low-to-moderate average Vp values (6.2-6.6 km/s) may be explained by a combination of quartzofeldspathic gneisses, metasedimentary rocks, and felsic granulites in the lower crust [e.g., Kern and Schenk, 1988]. The increase in Vp to~7.0 km/s in the lowermost crust [Hawman et al, 2012] may be indicative of metasedimentary rocks containing garnet or sillimanite and possibly significant amounts of quartz [Fountain, 1976;van den Berg et al, 2005], rather than mafic granulites or arc rocks.…”

“…The low Vp/Vs values indicate that the crust is more silica rich than intact arc crust of andesitic‐to‐mafic composition [ Jagoutz and Schmidt , ], which implies that volcanic and plutonic arc rocks exposed at the surface are not representative of lithologies at depth. Instead, arc rocks likely accreted farther outboard during the Neo‐Acadian (380–355 Ma) were later transported northwestward during Alleghanian thin‐skinned thrusting over either North American or Inner Piedmont basement [ Anderson and Moecher , ]. The results are consistent with models that show that the Alleghanian detachment extends southeastward beneath the Carolina terrane and possibly the Coastal Plain [ Hatcher et al ., ].…”

Crustal structure of the eastern Piedmont and Atlantic coastal plain in North Carolina and Virginia, eastern North American margin

“…A period of rifting occurred in the early Paleozoic, leading to the breakup of Rodinia and the opening of the Iapetus Ocean (Hartz and Torsvik 2002). Within the Iapetus Ocean, a volcanic island arc formed, later being accreted onto Laurentia (Anderson and Moecher 2009). As part of the Taconic orogeny in the Ordovician, this volcanic island arc accretion was due to the closing of the Iapetus Ocean (Anderson and Moecher 2009).…”

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

“…In the model presented here, arc rocks sensu stricto would have developed outboard and southeast of the proposed back-arc terrane and may have been rifted away from the Laurentian margin during extensional accretionary orogenesis, similar to the separation of Japan from the Asian mainland during formation of the Sea of Japan (Kim et al, 2007). This model for the initial phases of the Taconic orogeny would explain the palinspastic position of sequences interpreted to contain fragments of an arc/accretionary prism in North and South Carolina and north Georgia, including parts of the western Tugaloo, Cartoogechaye, and Cowrock terranes (Raymond et al, 1989;Hatcher et al, 2007;Anderson and Moecher, 2009) and arc plutons sensu stricto (Persimmon Creek Gneiss-Cowrock terrane; Whiteside pluton-western Tugaloo terrane). Although these terranes have been interpreted to belong to a volcanic arc system, they are overwhelming dominated by sedimentary rocks (Coweeta Group, Tallulah Falls Formation) and do not appear to constitute any signifi cant plutonic/volcanic edifi ce of a volcanic arc.…”