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The eastern flank of the Appalachian orogen is composed of extensive Neoproterozoic–early Paleozoic crustal blocks that originated in a peri-Gondwanan setting. Three of these blocks record the evolution of Neoproterozoic magmatic-arc systems, including Carolinia in the southern Appalachians and Ganderia and Avalonia in the northern Appalachians. Relationships among these three crustal blocks are important for understanding both the accretionary history of the orogen and the evolution of the Iapetus and Rheic Oceans, first-order geographic features of the Paleozoic globe.

Traditionally, Carolinia and Avalonia have been considered to represent a single microcontinental magmatic arc that accreted to Laurentia in the middle to late Paleozoic. The early lithotectonic history (ca. 680–570 Ma) of the two blocks is obscure; however, their latest Neoproterozoic-Paleozoic histories are distinct. This disparity is manifest in the first-order features of (1) timing and style of magmatic-arc cessation and (2) the nature of their Paleozoic lithotectonic records. Magmatic arc activity ceased in Avalonia in the late Neoproterozoic (ca. 570 Ma), succeeded by extension-related magmatism and sedimentation that was transitional into a robust latest Neoproterozoic–Silurian platformal clastic sedimentary sequence. This platform was tectonically unperturbed until the Late Silurian–Early Devonian. In contrast, Carolinia records late Neo-proterozoic tectonothermal events coeval with arc magmatism, which extended into the Cambrian; a relatively thin Middle Cambrian shallow-marine clastic sequence is preserved unconformably atop the Carolinia arc sequences. Subsequently, Carolinia experienced widespread Late Ordovician–Silurian deformation and metamorphism.

However, we note striking similarities between Carolinia and Ganderia; specifically, in Ganderia, like Carolinia, late Neoproterozoic tectonism was accompanied by arc magmatism that extended into the Cambrian. Ganderian arc rocks are capped unconformably by a Middle Cambrian to Early Ordovician clastic sequence, and they were tectonized in the Late Ordovician–Silurian, similar to relations in Carolinia. Independent studies indicate that the Late Ordovician–Silurian tectonism in both blocks was related to their accretion to Laurentia. Thus, Carolinia and Ganderia show parallel development of first-order lithotectonic characteristics for two endpoints in their global strain path, i.e., their Gondwanan source region and their accretion to Laurentia.

Consequently, we posit that Carolinia appears to be more closely affiliated with Ganderia than with Avalonia. The recognition of this linkage between Appalachian peri-Gondwanan realm crustal blocks in light of paleomagnetic and isotopic data leads to a unified model for the accretion of these blocks to the eastern margin of Laurentia.

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