A dynamic, scaled model of accretion at trenches and its implications for the tectonic evolution of subduction complexes

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Abstract
Popular models of accretion at trenches, based on seismic reflection profiles and geological data from active convergent margins, are that accretionary prisms consist of thrust‐bound wedges of material successively scraped off the descending plate at the base of the inner wall. Geological studies of ancient subduction complexes indicate that deeply buried, metamorphosed rocks, including blueschists, are tectonically elevated at landward margins of accretionary prisms, but such uplift is not easily reconciled with models in which underplated wedges merely rotate arcward as new wedges are added beneath them. We constructed a dynamic, scaled model of a subduction zone to study the large‐scale rheological behavior of accreted materials. Model ratios of λ = 10−5, τ = 2.3 × 10−11, and μ = 1 were applied to a prototype subducting oceanic lithosphere covered with 1 km of sediments at a convergence rate of 1 cm yr−1 for 10 m.y. Water‐based clay of density 2.7 g cm−3 was appropriately scaled to represent materials in the entire prototypal accretionary prism. Successive thrust‐bound lobes formed at the ‘trench,’ but appreciable amounts of ‘sediment’ were carried beneath the lobes to the interior of the developing prism where they rose impressively upward along the landward margin of the entire accretionary prism. This pattern, involving the upward emplacement of deeply buried materials, can be theoretically modeled for the two‐dimensional flow of a viscous fluid through a right‐angle or acute corner in response to sliding a rigid plate beneath another fixed plate. Our kinematic model predicts that subduction‐driven macroscopic flow is a reasonable mechanism for the upward tectonic transport of metamorphic rocks in accretionary complexes.