ARC-TRENCH ROLLBACK AND FOREARC ACCRETION: 1. A COLLISION-INDUCED MANTLE FLOW MODEL FOR TETHYAN OPHIOLITES

Abstract

Tectonically active remnants of Neo-Tethys represented by Mediterranean and western Pacific marginal seas are characterized by rapidly propagating backarc extension episodes. These appear to be triggered by random subduction nucleation events, commonly signalled by the appearance of refractory boninites in volcanic 'proto-arcs'. As backarc basins evolve, active arcs separate from their 'proto-arc' remnants and may split again if more than one basin-opening episode occurs. Accreting arc-forearc terranes are therefore likely to incorporate proto-arc, backarc, and (in some cases) inherited continental fragments, as evidenced by their structural complexity and lithological diversity. Forearc complexes typically show positive Bouguer gravity anomalies and significant age discrepancies within and between their crustal and mantle components. Where exposed, their lower stratigraphic horizons may include boninite-bearing assemblages along with tectonized fragments of mid-ocean ridge basalt (MORB) basement and hydrated refractory peridotite. These are typically intruded by sodic plagiogranite (adakite) and high-temperature Mn-, Fe-rich hydrothermal veins ('epidosites'), further indications of subduction nucleation at, or close to a pre-existing spreading axis. Where the arc-trench rollback process is terminated by collision with an approaching continent, or with another retreating forearc complex, MORB-like backarc lithosphere is rapidly reconsumed, in some cases following a change in subduction polarity. In contrast, given their preponderance of ultra-refractory serpentinized peridotite, forearc complexes are relatively buoyant, resist subduction, and are prone to entrapment during early stages of an orogeny. The associated interplay of extension and compression offers a compelling scenario for resolving the so-called ophiolite 'conundrum' and explaining the near-ubiquity of ophiolites in orogenic belts. We propose that rapid arc-trench rollback pulses are driven largely by collision-induced mantle flow in addition to commonly cited 'slab pull' effects. This is supported by the evidence of isotopic mantle flow tracers, seismic tomography, and the coupled kinematics of marginal basins and continental escape. Model applications to some well-known Tethyan ophiolites are developed in a companion paper.