Plate boundary forces at subduction zones and trench-arc compression

Abstract

Viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone. The subduction fault is included using the dual node technique. It is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench-arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates. This may be masked at the trench by bending stress. It is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces. Slab pull and trench suction were found to be of comparable magnitude within the range 1.0 to 4.0 × 10 12 N/m in models studied with vertical subduction, and there are indications that this may also apply when the slab dips at 45° as a result of viscous flow induced by rollback. When the slab dips beneath the arc-backarc region, it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction. This suggests that backarc tension associated with Marianas type trench-arch systems may be related to the nearly vertical slab whereas backarc compression in the Chilean type may result from the low dip and small downpull of the slab. It is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors.