Geoid anomalies over deep sea trenches

Abstract

Summary. Utilizing radar altimeter data from the GEOS‐3 satellite, it has been possible to identify a characteristic geoid anomaly which occurs over major deep‐sea trenches in the world's oceans. The geoid increases in height as it approaches the trench axis from the seaward side. This increase starts in the range of 1000–3000km seaward of the trenches for which we have examined the profiles. We observed a maximum average slope at 37m/1000 km (7.6arcsec) for the geoid anomaly. In shape the geoid anomaly in this region is in some cases concave downward (Aleutian Trench) concave upward (Java Trench); or nearly linear (Tonga Trench). Directly over the trench axis there is a narrow minimum in the geoid. This low is from 100 to 200km in width and from 3 to 15m in depth. From the base at the trench axis low there is a steep increase of up to 30m to a region landward of the trench axis. In this region, which includes island arcs, the geoid anomaly is variable although it generally remains high. In some cases there is a distinct geoid high above the island arc. This is most obvious with the area associated with the Bonin and the Aleutian Trench. Behind the Aleutian Arc there is a gradual lowering of geoid height. However, landward of the Java Trench the geoid continues to rise. In other regions the geoid remains fairly level at a high value landward of the trench. By using a combination of radar altimeter and marine gravimeter measurements, it has been possible to construct a density model of the crust and upper mantle in the Tonga—Kermadec trench region. This model is used to calculate geoid and gravity on a spherical earth in agreement with the observed data. It is more constrained than a model based on either geoid or gravity data alone. Landward of the trench, in this model, the ocean depths are shallower than in the deep ocean basin. Thus for these depths average crustal densities (including the water column as a part of the crust) are higher than those for the Southwest Pacific Basin. On the other hand, lower relative densities occur deeper within the mantle under the area landward of the trench. These lesser densities partially compensate the higher values at shallow depth. Thus a dipolar layered mass distribution landward of the trench is the fundamental cause of the gradual slope of the geoid. This slope extends across the trench from the landward to the seaward side. The effect of the descending lithosphere is only of secondary importance in explaining this geoid anomaly. These results are dependent on the assumptions that density variations occur only within the crust and uppermost mantle and that there are no changes in density along strike. Copyright © 1982, Wiley Blackwell. All rights reserved