Gravity Anomalies Seaward of Deep-Sea Trenches and their Tectonic Implications

Abstract

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 cffect of a dense downgoing slab. Gravity anomalies seaward of deep-sea trenches 59

Seismic and aseismic slip along subduction zones and their tectonic implications

“…The above model is very consistent with the topographic and gravity highs seaward of trenches [Walcott, 1970;Hanks, 1971;Watts and Talwani, 1974]. A recent extensive study of Watts and Talwani [1974] is particularly intriguing in this context.…”

“…A recent extensive study of Watts and Talwani [1974] is particularly intriguing in this context. Figure 9 shows some of the representative profiles perpendicular to the Aleutians, Kuriles, Bonin and Marianas.…”

“…The topographic high and the gravity high are most conspicuous for the Aleutians and the Kuriles, but are not very obvious for the southern Bonin and Marianas. Watts and Talwani [1974] explained these features in terms of a flexural bending of the oceanic lithosphere due to compression and vertical loading at the trench. They found that a large compressive stress is required to explain the topographic and gravity highs for the Aleutians and Kuriles, but for the southern Bonin and Marianas, they can be Watts and Talwani, 1975].…”

Gravity in the junction between the Japanese and the Izu-Bonin Islands.

“…A broad area of high free air anomaly (>100 mgal) can be recognized around the Hachijo-shima Island this region may be due to the lack of the topographic low observed generally in the arc-trench gap. The belt of positive free air anomalies east of the IzuBonin Trench is named Outer Gravity High (WATTS and TALWANI, 1974) and correlated with the Outer Topographic Rise. Other notable negative free air anomalies are observed from the Bay of Tokyo eastwards to the continental shelf across the neck of the Boso Peninsula, and in the northern Shikoku Basin.…”

“…14) Graf-Askania Gss2 meter measurements of the Lamont-Doherty Geological Observatory (Only plotted sheets of free air anomalies (WATTS and TALWANI, 1974) are available to the authors).…”