OPHIOLITES, HISTORICAL CONTINGENCY, AND THE WILSON CYCLE

Abstract

Lithologic assemblages termed "ophiolites" were at the heart of the mid-twentieth century "plate tectonics" paradigm shift when they were widely accepted as representing sections of the oceanic crust created during the early stages of "Wilson cycles." However, the past four decades have seen lengthy, if acrimonious, debates about this enigmatic phenomenon, fueled by a growing body of evidence for their association with processes of lithosphere subduction. Today, the core of the debate continues to be the coexistence of structural evidence for seafloor spreading and the chemical and petrographic signatures of subduction. More recently, the discussions have acquired a philosophical flavor, following the suggestion that a "historically contingent" model could allow subduction-related geochemical indications to be reconciled with processes at active mid-ocean ridges. Offering powerful, if not controversial insights into the nature of biological and cultural change, historical contingency is an appropriate context for resolving dilemmas in interpreting earth history, including the subject of this paper, the so-called ophiolite conundrum. Here, an actualistic model for Tethyan ophiolites is offered for testing against the geological record and contingent factors absent or underrepresented today. The model is based on processes observed in the Mediterranean and western Pacific regions and is developed on the premise that "proto-ophiolites" form as accreting forearc complexes during sub-cycles of basin opening and arc-trench rollback in closing, pre-collision stages of Wilson cycles. Rollback appears to be triggered by spontaneous subduction nucleation and may be driven by lateral mantle flow. If a rollback episode successfully evades the "jaws" of a plate collision (as observed in the western Pacific), forearc accretion may proceed indefinitely or until mantle flow dissipates. On the other hand, entrapped by a continental plate collision, forearcs are preferentially preserved in an ensuing orogeny, given their relative buoyancy relative to conjugate backarc basin lithosphere. The case for studying the ophiolite conundrum from a historically contingent perspective is supported in principle, but remains accountable to petrologic constraints and evidence from the geological record.