Late Proterozoic tectonostratigraphic evolution of the Avalonian and Cadomian terranes

Abstract

The Avalonian and Cadomian terranes of the northern Appalachians and western Europe together record a ∼ 700-540 Ma orogenic event characterized by; (1) widespread, late Precambrian arc volcanism and calc-alkaline plutonism; (2) broadly coeval volcanogenic turbidite deposition often accompanied by mafic volcanism; (3) predominantly sinistral motions on major shear zones locally associated with latest Precambrian terrestrial sedimentation and bimodal volcanism; and (4) deposition of shallow-marine Cambro-Ordovician overstep sequences containing Acado-Baltic faunas. The record is interpreted to trace a tectonostratigraphic evolution from late Precambrian extensional magmatic arc, through latest Precambrian transform margin, to early Paleozoic shallow-marine platform. Arc-related activity is attributed to oblique subduction and the development of predominantly ensialic volcanic arc basins. The diachronous cessation of subduction was not the result of continental collision and is associated with the development of major ductile shear zones, the closure of some volcanic arc basins, and the opening of pull-apart rifts. This is attributed to transform activity of predominantly sinistral sense that yielded to shallow-marine platformal conditions with rising Cambrian sea levels. Paleomagnetic reconstructions for the late Precambrian place the Avalonian and Cadomian terranes along the northern African margin of a proto-Gondwanan supercontinent. The proposed timing of the accretion and breakup of this supercontinent coincides with the onset and cessation of orogenic activity within the Avalonian and Cadomian terranes and provides a potential tectonic framework in which to evaluate their tectonostratigraphic evolution.