Geoid lows over deep‐sea trenches

Abstract

In many instances, negative gravity anomalies over trenches are slightly displaced toward the inner slope of the subduction complex indicating an accumulation of low density material. This effect, known for many years in gravity data, is also observed in geoid data. We have analyzed regularly spaced geoid height profiles from the Seasat satellite across the Aleutian, Kuril, Mariana and Tonga‐Kermadec trenches. Regional variations of the offset between topography and geoid minima have been recorded. Along the Aleutian trench, the offset increases from a few kilometers in the eastern part to a few tens of kilometers (up to 40 km) in the central part (between 170° W and 180° W), and then decreases again to less than 10 km in the western Aleutians. Along the Kuril trench, the offset is more or less constant (∼20 km) except for a larger offset (up to ∼30 km) at the southern end of the arc. Along the Mariana and Tonga trenches, the geoid low is in near coincidence with the trench axis, while in the southern end of the Kermadec trench, a small offset is detected. The displacement of the geoid low away from the trench axis is related to the width of accreted material beneath the inner trench slope: no offset is observed in regions of very narrow accretionary prism whereas a ∼30‐km offset may result from an accretionary prism of ∼60 km in width supported without isostatic compensation by the subducting elastic lithosphere; geoid lows centered over the axis of trenches with narrow accretionary prism (e.g., Mariana and Tonga) are almost completely explained by the seawater trench topography alone, unlike displaced geoid lows which require an additional negative density contrast. On the other hand in areas of very extensive accretionary prism (as in the eastern Aleutians) the situation is complicated by sediment loading and additional flexure of the descending plate which insures partial compensation of the prism.