Deep‐seated flow as a mechanism for the uplift of broad forearc ridges and its role in the exposure of high P/T metamorphic terranes

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Abstract
Forearc systems evolve through the interplay of numerous processes, but aside from lateral movements much of the observed variability in morphology can be related to evolution of ductility in the deeper levels of the subduction complex. In the eastern Aleutian, Lesser Antilles, Makran, and Cascade arc trench systems a 50‐ to 200‐km‐wide forearc ridge is developed along the inboard edge of the subduction complex. These forearc ridges did not form until subduction accretion had expanded the arc trench gap to 275–300 km and the near vertical rise of the ridge occurred 50 km or more from the trench slope break; the forearc high apparently maintained by accretion at the trench. Thus these broad forearc ridges are not emergent trench slope breaks; the process responsible for uplift is occurring within the subduction complex, and the size of the accreted mass is apparently a major factor. Rheological models assuming frictional sliding laws at shallow levels and power law rheologies at depth predict different behaviors for various parts of a “typical” large subduction complex. The older forearc basement beneath which the inboard part of the subduction complex is emplaced represents a rigid, brittle massif that extends 100–150 km from the volcanic arc, a prediction supported by seismicity studies in modern convergent margins. Because of the wet, quartz‐rich lithologies of the subduction complexes, however, the brittle‐to‐ductile transition apparently occurs at depths of 10–20 km within the accreted mass. Thus in large subduction complexes, where the downgoing slab is 40–50 km beneath the inboard edge, a significant region at the deeper levels of the subduction complex is apparently capable of flow. The predicted zone of ductile deformation is directly beneath the area in which a forearc ridge is generally developed; hence we conclude that the broad structural highs of large subduction complexes are dynamically maintained by deep‐seated flow. The character of this flow is uncertain, yet it is likely that the stripping of low‐viscosity, partially subducted sediments plays a role, and the volume of sediments added by underplating may be the controlling factor in the dynamic uplift of broad forearc ridges. Kinematically, the flow could be a pure shear deformation induced by subhorizontal loading, but we prefer the familiar “corner flow“ mechanism because it provides the simplest explanation for the observations. The interaction between offscraping at the trench and underplating affects the distribution of mass on the downgoing slab and probably controls uplift rates in broad forearc ridges. Because of the complexity of the interaction and the observation that modern forearc ridges appear to be rising slowly, we conclude that high‐pressure rocks are only exposed in unusual forearc systems, e.g., systems with either a prolonged period of uplift or large influxes of deep underplated sediment. The evolution of ductility plays a major role in the development of forearc morphologies. When the subduction complex is small, brittle deformation and processes related to accretion at the trench dominate and the “classic” growth models are applicable. However, as continued growth of the subduction complex depresses the downgoing lithosphere and increases the arc trench gap, the subduction complex becomes ductile at depth, and ultimately, deep‐seated flow leads to abrupt uplift along the inboard edge of the subduction complex.