1977 Volume 30 Issue 3 Pages 359-368
Lithosphere anomalies beneath seamounts and trenches are derived from differece between observed gravity and expected gravity from subterranean structure, and the following results have been obtaind;
(1) Negative free air gravity anomaly surrounding seamount in the Mid-Pacific is interpreted by thinner lithosphere under the seamount. This anomalous lithosphere is 20-30km thinner in thickness and its horizontal extent is once or twice as large as horizontal scale of the seamount. The seamount is expected to be supported with buoyancy of asthenosphere, the density of which is smaller than that of lithosphere.
(2) Outer gravity high seaward of trench is interpreted by thicker and heavier lithosphere near the trench. Lithosphere will sink downwards due to its excess weight at the trench. According to the sinking of lithosphere, the trench axis will move seawards.
(3) If a trench axis approaches seamounts, moving rate of the axis near the seamounts will be slown down. Because, thinner lithosphere under the seamounts cannot sink downwards until its thickness become larger than normal. This mechanism can explain the origin of the large negative free air gravity anomalies landward of trench junctions as well as the origin of nodes of trenches.
