COLLISION-INDUCED MANTLE FLOW AND THE GENESIS OF OPHIOLITES: A TECTONIC FACIES APPROACH

The facies concept has been useful in characterizing igneous, metamorphic, and sedimentary environments in space and time. Recently, a ‘tectonic facies’ model was invoked to address the paradox of continuously solid deformation during continental evolution and rigid-body plate tectonics (Hsü, 1997). This notion ascribes orogenic lithologies to discrete oceanic margin settings and provides a basis for exploring a further paradox, the spatial-temporal coexistence of extension and compression, implied by the ubiquitous presence of ophiolites in orogens. Ophiolite studies raise fundamental questions about the relationship between lithospheric plates and asthenosphere flow. For example, the record of Neo- and Paleo-Tethyan ophiolites may be interpreted to reflect repeated episodes of subduction rollback and backarc basin opening prior to the final closure of Tethyan oceans. These subduction-accretion cycles appear to typify impending collision scenarios and, we suggest, may be attributable to pre- and syn-collision mantle perturbations. The continued eastward propagation of subduction-accretion cycles in the western Pacific provides a basis for actualistic models. Relict arcs and backarc basins in the eastern Philippine Sea plate are probably the best documented examples, subduction initiation at the Palau-Kyushu Ridge being signalled by the appearance at c. 45 Ma of ultramafic (hence thermally-anomalous) boninite magmas. As they evolve, accreting forearcs increasingly resemble lithologic 'high tide marks' (HTMs) as they progressively incorporate relict arc, backarc, and (in some cases) continental lithosphere, typically fragmented with large age discrepancies within and between their crust and mantle components. Typically, the termination of rollback by collision with a continent is followed by basin ‘collapse’ resulting from the subduction of recently-formed backarc lithosphere as extension tectonics give way to compression. Accreted HTM lithosphere, in contrast, resists subduction and becomes entrapped as ‘ophiolite’. Several lines of evidence in the western Pacific suggest subduction-accretion cycles leading to HTM formation occur in response to collision-induced mantle extrusion whereby lateral squeezing of ductile asthenosphere leads directly to arc rollback and backarc basin formation.