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Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, USA
A corollary of plate tectonics is that spreading ridges ultimately interact with trenches; this is a consequence of the closure phase of the Wilson cycle that eliminates ocean basins. Ridge-trench interactions generate distinctive igneous, metamorphic, structural and sedimentation effects, which commonly are diachronous parallel to the trench; effects vary with plate boundary geometry and rate of migration is controlled by relative motion vectors. Identification of such interactions in the geological record is uncommon, which suggests that geological events triggered by ridge-trench interactions are attributed to other phenomena, such as changes in plate motion or a singularity at the trench. The estimated greater length of plate boundaries in the past means that subduction of a spreading ridge system could have been more common earlier in Earth history.
High-T-low-P metamorphism requires a tectonic setting that allows enhanced heat flux, transient advection of heat due to magma ascent, unusually large internally generated heat, or some combination of these features. Subduction of a spreading ridge system produces anomalously high temperatures at shallow crustal depth through the development of a slab window, which allows transfer of sub-slab asthenospheric mantle across the slab to create an enhanced heat flux beneath the overriding plate. This process is important along convergent margins that develop a subduction-accretion complex, and drives low P/T ratio metamorphism and anatexis of the complex. Heat transport occurs by rising magma near the trench, which leads to formation of plutons with characteristic contaminated MORB geochemistry. Near-trench igneous activity and low P/T ratio metamorphism are the most striking features attributable to spreading ridge subduction and slab window formation, and together are likely to be diagnostic of ridge-trench interactions. Geological features of the Cretaceous Ryoke and Abukuma high-T-low-P metamorphic belts in Japan are consistent with the expectations of ridge subduction and slab window formation. Based on modelled plate interactions in the North Pacific Ocean basin during the Cretaceous, a tectonic scenario is presented for the evolution of the Japanese Islands that involves interactions between the Farallon-Izanagi ridge and a trench along the Asian continental margin. Other low P/T ratio metamorphic belts that owe their high-T lineage to interactions between spreading ridges and trenches are the Shimanto and Hidaka belts of Japan, and the Chugach belt of Alaska, USA. The complex tectonic evolution of Palaeozoic orogenic belts such as the Appalachian orogen of eastern Canada and USA, implies that ridge-trench interactions were involved in their evolution, although terminal continental collision makes the effects of these interactions difficult to identify in the geological record. There are many older belts for which ridge subduction and slab window formation should be evaluated, including the Palaeoproterozoic evolution of the Arunta Inlier, Australia, and the late Archean evolution of the Slave Province, northern Canada.
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