The origin of outer topographic rises associated with trenches

The outer topographic rise seaward of many island arcs can be explained in terms of a bending moment applied to the oceanic lithosphere either by the downgoing slab to which it is attached or by modest shear stresses acting along the underthrusting plate boundary. The existence of the outer rise does not require large horizontal compressive stresses to maintain it. In order to explain the outer topographic rises and associated gravity highs seaward of trenches Hanks (1971) and Watts & Talwani (1974) suggested that very large horizontal compressive stresses (5-10 kbar) exist across island arcs. Hanks (1 971) showed that the outer topographic rise south-east of the Kurile trench could be modelled as the response of a thin (- 30 km thick) elastic plate to vertical and horizontal forces at the trench axis. Watts 8c Talwani (1974) demonstrated that such an outer rise existed continuously seaward of the Pacific trench system from the Aleutians to the Marianas arc, and that in many instances, a positive gravity anomaly was associated with the outer rise. This gravity anomaly could be interpreted as that produced by the excess mass at the deformed upper and lower surfaces of the thin elastic plate model. To match the amplitudes of the topographic rises and gravity highs by the model, compressive stresses, up to 12 kbar, acting at the trench were needed. This result presents problems with regard to the material behaviour under such large stresses, and in terms of providing a driving mechanism to maintain these stresses. The purpose of this note is to suggest that such large stresses are not neces-sarily required across the plate boundary, and to give an alternative explanation within the context of the same model. The equation describing the bending of a thin elastic plate embedded in a fluid is where y is the deflection of the plate at a distauce x from the origin (Fig. I), N is the horizontal force per unit length (positive for tension, negative for compression