A possible mechanism for spontaneous cyclic back-arc spreading

Abstract

Back-arc spreading is a non-steady-state process exemplified by the repeated cycles of spreading of the South Fiji and the Lau Basins behind the Tonga arc, and the Parece Vela Basin and the Mariana Trough behind the Mariana arc. Spreading in these regions starts with rifting within the volcanic arc before shifting to the back-arc region where it develops into a phase of well-defined spreading. 2D thermo-mechanical subduction modeling incorporating phase transitions at depths of 410 km and 660 km suggests the presence of a low-viscosity and low-density mantle wedge is an important condition for arc rifting to occur. Back-arc spreading starts when a nearly vertical slab impinges upon the 660 km discontinuity causing downdip compressive stress that is transmitted up the slab resulting in extensional within-arc stress. Trench retreat during a phase of back-arc spreading causes a decrease in slab dip angle and buckling of the slab. Back-arc spreading ceases during this buckling phase. Rifting starts once more when the nearly vertically dipping ‘heel’ of the buckled slab again impinges upon the 660-km boundary. The second phase of rifting initially focuses within the arc but subsequently shifts to the back-arc region leading to renewed back-arc spreading. Our modeling predicts that subduction of thick (old age) and weak (small yield stress) slabs, which have intermediate resistance to slab bending, leads to cyclic back-arc spreading. In contrast, continuous back-arc spreading is predicted for thick and strong slabs with a large resistance to bending, and no back-arc spreading is predicted for slabs with a small resistance to bending (thin slabs). Geological processes such as toroidal mantle flow around the lateral edges of a slab, collisions with buoyant lithosphere and interactions with third plates may have important roles in the development of cyclic back-arc spreading in specific cases. However, the presence of a common timescale of ~ 20 Myr suggests there a general underlying control on back-arc basin formation that is common to many if not all subduction zones. The new model presented here can account for the main features of cyclic back-arc spreading seen in the Tonga-Kermadec and the Calabrian arcs.