Gravity Anomalies Seaward of Deep‐Sea Trenches and their Tectonic Implications *

Summary Studies of free‐air gravity anomaly profiles across island arcs show an important belt of positive anomalies seaward of deep‐sea trenches. This belt of positive anomalies is called the Outer Gravity High. The Outer Gravity High is well developed seaward of the central and eastern Aleutian, Kuril, Japan, northern Bonin and Philippine Trenches where it correlates with a regional rise in topography of a few hundred metres. The Outer Gravity High can be most satisfactorily explained by a stress system associated with the convergence of lithospheric plates at island arcs. The computed gravity effect of simple models of flexure of an oceanic plate approaching an island arc generally explain both the amplitude and wavelength of the Outer Gravity High. The Outer Gravity High seaward of the central and eastern Aleutian, Kuril, Japan, northern Bonin and Philippine Trenches can be explained by a horizontal compressive stress of the order of a few kilobars acting on the oceanic plate. The Outer Gravity High seaward of the southern Bonin and Mariana Trenches, however, can be explained in the absence of horizontal compressive stresses. These conclusions are consistent with differences in the stress field of island arcs as indicated by the seismicity and regional tectonics of the north‐western Pacific. The Outer Gravity High is considered an important part of the regional gravity field of island arcs and the field derived from satellite observations. The close correlation of the Outer Gravity High with regional topography seaward of trenches suggest that the gravity effect of a dense downgoing slab beneath island arcs may be small and confined in lateral extent to the region of the island arc and trench. The prominent positive anomalies in island arc areas derived from satellite observations therefore owe a major part of their existence to causes other than the gravity effect of a dense downgoing slab.