Subduction - Subduction Angle

Subduction Angle

Subduction typically occurs at a moderately steep angle right at the point of the convergent plate boundary. However, anomalous shallower angles of subduction are known to exist as well some extremely steep.

• Flat-slab subduction (<30°): occurs when subducting lithosphere, called a slab, subducts horizontally or nearly-horizontally. The flat slab can extend for hundreds of to over a thousand kilometers. This is abnormal, as the dense slab typically sinks at a much steeper angle directly at the subduction zone. Because subduction of slabs to depth is necessary to drive subduction zone volcanism (through the destabilization and dewatering of minerals and the resultant flux melting of the mantle wedge), flat-slab subduction can be invoked to explain volcanic gaps. Flat-slab subduction is ongoing beneath part of the Andes causing segmentation of the Andean Volcanic Belt into four zones. The flat-slab subduction in northern Peru and Norte Chico region of Chile is believed to be the result of the subduction of two buoyant aseismic ridges, the Nazca Ridge and the Juan Fernández Ridge respectively. Around Taitao Peninsula flat-slab subduction is attributed to the subduction of the Chile Rise, a spreading ridge. The Laramide Orogeny in the Rocky Mountains of United States is attributed to flat-slab subduction. During this time, a broad volcanic gap appeared at the southwestern margin of North America, and deformation occurred much farther inland; it was during this time that the basement-cored mountain ranges of Colorado, Utah, Wyoming, South Dakota, and New Mexico came into being.

• Steep-angle subduction (>70°): occurs in subduction zones where earth's oceanic crust and lithosphere are old and thick and have therefore lost buoyancy. At present the steepest dipping subduction zone lies in the Mariana Trench, which is also where the Earth's oldest oceanic crust lies (Jurassic age), if exempting ophiolites. Steep-angle subduction is in contrast to flat-slab subduction associated with back-arc extension of crust making volcanic arcs and fragments of continental crust wander away from continents over geological times leaving behind a marginal sea.