Controls of subduction geometry, location of magmatic arcs, and tectonics of arc and back-arc regions

Abstract

Most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors. Combinations of (1) rapid absolute upper-plate motion toward the trench and active overriding of the subducted plate, (2) rapid relative plate convergence, and (3) subduction of intraplate island-seamount chains, aseismic ridges, and oceanic plateaus (anomalously low-density oceanic lithosphere) cause low-angle subduction. Under conditions of low-angle subduction, the upper surface of the subducted plate is in contact with the base of the overlying plate, the wedge of low-density asthenosphere is replaced by subducted lithosphere, and the width of the arc-trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate. The fourth factor is age of the subducting lithosphere. Subduction of young lithosphere produces two opposing tendencies: (1) low-angle subduction and increased arc-trench distance, owing to its low density; and (2) decreased arc-trench distance, owing to its higher temperature. Two factors of secondary importance contribute to variation in subduction-zone geometry and arc-trench distance. Accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward. Prolonged subduction thickens the upper plate, depresses the isotherms in the subducted plate, and may create a broader arc. Both factors increase the arc-trench gap. The four primary factors also control development of other tectonic elements, such as regional subsidence (for example, the Amazon basin and a portion of the Cretaceous Interior Seaway of western United States), intra-arc extension (for example, the Basin and Range province), foreland fold and thrust belts, and Laramide-style tectonics.