LEADING CONTINENTAL EDGES, ACCRETIONARY OROGENS AND RECONSTRUCTION OF ANCIENT SUPERCONTINENTS

Repeated amalgamation and subsequent breakup of continental lithosphere have profoundly affected Earth’s evolution since the Archean. Following breakup, distinctive rift-drift sequences along trailing edges of dispersing continents identify such margins in the geologic past. Identification of leading continental edges has not been as widely applied to previous episodes of supercontinent dispersal. However, following the accretion of terranes, magmas produced along the outermost margin of western North America show distinctive Sm-Nd isotopic compositions which can be used to determine the characteristics of the underlying mantle lithosphere. As Sm and Nd behave compatibly during most intra-crustal processes, this approach requires only the availability of Nd isotopic data from igneous complexes that post-date terrane accretion. These signatures should also be recognizable along leading continental edges during previous episodes of supercontinent dispersal.

Using western North America as an analogue, we show that the leading edges of dispersing continents have isotopic characteristics that can be used to identify these margins. For example, the Sm-Nd isotopic signatures of Late Neoproterozoic and Early Paleozoic igneous rocks along the northern margin of Gondwana indicate derivation from 0.7 to 1.1 Ga mantle lithosphere. This lithosphere originated in the Mirovoi Ocean surrounding Rodinia. It subsequently accreted to northern Gondwana in response to Rodinia breakup, and provided a source for subsequent magmatism. Accretion and subsequent recycling of oceanic mantle lithosphere should be common along the leading edges of dispersing continents following supercontinent breakup, providing an additional aid in paleocontinental reconstructions.

We show that a Sm-Nd isotopic record like that of the Cordillera occurs in Late Neoproterozoic-Early Paleozoic igneous rocks along the northern margin of Gondwana. The record identifies this margin as a leading continental edge following the 0.8–0.75 Ga breakup of Rodinia, a supercontinent that formed at ca. 1.1–1.0 Ga. This approach may help identify the leading edges of continental margins and provide another constraint for palaeocontinental reconstructions.