A plume-generated segment of the rifted margin of Laurentia, Southern Canadian Appalachians, seen through a completed Wilson Cycle

Abstract

The southern part of the Canadian Appalachians and the adjacent parts of the craton show a range of distinctive features pertaining to the evolution of the rift and rift-drift transition stages of a segment of the continental margin of Laurentia, despite the effects of a completed Wilson Cycle. Several lines of evidence point to an initial lithospheric rupture, in the form of a three-pronged rift, possibly induced by a rising mantle plume. The earliest synrift igneous products to penetrate the crust in the area are represented by a swarm of tholeiitic diabase dykes intruded at ca. 590 Ma. The swarm is more than 700 km long and radiates from the inferred location of the rrr triple junction. Dyke emplacement and the initiation of rifting were probably synchronous. Approximately 35 m.y. later, a rapid outburst of volcanism occurred at the triple junction, probably as a harbinger of sea-floor spreading. The volcanic products are predominantly basalts of mildly alkaline to transitional character but with a minor comenditic component. The voluminous nature of the volcanic outburst and the compositional characteristics of its products indicate higher than normal asthenospheric temperatures, coupled with thinning of the lithosphere suggesting continued mantle plume activity. The volcanic sequence is overlain by rift-facies elastics that are locally interrupted and augmented by a deltaic deposit, representing the delta of a river that drained the failed arm into the nascent lapetus Ocean. The rift-facies elastics and the associated deltaic deposits are overlain by rocks of marine origin, signalling the beginning of the drifting stage, possibly at ca. 550 Ma.