Ridge Subduction and Slab Windows

Abstract

Ridge subduction occurs where a mid-ocean spreading ridge intersects a subduction zone. As one or both of the plates flanking the spreading ridge subduct into the mantle, the spreading ridge becomes “unzipped” and widens into a slab window, a slab-free region beneath the overriding plate. The transition from a normal subduction zone to a slab window leads to changes in the mantle which, in turn, affect the overriding plate. As the slab window opens, hot, dry mantle from beneath the slab wells up and displaces cool, hydrated and metasomatized mantle. The upwelling mantle also adds heat to the forearc region in what is termed the “blow-torch effect.” The margins of the slab window, i.e., the edges of the subducting slabs, typically become thermally eroded and physically degraded, leading to microplate formation near the trench and plate disintegration at depth. The thermal erosion involves partial melting of the slab edges and the genesis of felsic melts, some of which have adakitic compositions. These processes lead to changes in igneous activity within the overriding plate that may include forearc magmatism, extinguishment of the volcanic arc, and establishment of a broad volcanic field characterized by alkaline compositions similar to those found in ocean islands. Slab window formation is typically coupled with changes in the stress field within the overriding plate, with consequences that may include crustal uplift, extension and strike-slip faulting. Slab windows have affected the Earth's crust and mantle since the onset of plate tectonics in the Precambrian.