Numerical modelling of arc-continent collision: application to Taiwan

Two-dimensional finite element numerical modelling is applied to study the deformation and failure of the overriding plate during are-continent collis ion (continental margin subduction). This plate has elasto-plastic rheology with strain weakening and contains a 'volcanic arc' with thinned and weake ned lithosphere. The overriding plate deforms due to the normal and tangent ial stresses applied along the interplate surface. These boundary condition s represent the friction and pressure between the plates. The latter is due to the flexural rigidity of the subducting lithosphere and the buoyancy fo rce generated by the subducting continental crust. The modelling shows that continental margin subduction results in increasing compression and failur e of the overriding plate, which occurs along the surface dipping under the are in either of two possible directions. The failure mode is largely cont rolled by the two competitive factors: the rigidity of the subducting plate and thickness gradient of the subducted continental crust. A high rigidity favors failure along an ocean-vergent fault, which is followed by a subduc tion reversal, while a high thickness gradient favors failure in the opposi te direction, which is followed by a fore-are block underthrusting beneath the are. Both scenarios seem to have natural analogs. We consider one of th em, the ongoing are-continent collision in Taiwan, and argue that this proc ess occurs according to the second scenario corresponding to the fore are u nderthrusting. Wavelet statistical analysis of the seismicity distribution to the south of Taiwan has clearly displayed a shallow (0-40 km) zone of hi gh density, coherently distributed seismicity beneath the Luzon Are. This z one, interpreted as a lithospheric-scale fault, dips from the forearc basin to the east and corresponds to the initiation of the forearc block subduct ion. A self-consistent, combined mechanical-gravity-topography model is use d to see whether failure of the overriding plate to the south of Taiwan can be 'captured' by this model. By 'tuning' different controlling parameters, we did not succeed in obtaining realistic topography and gravity held in a model where failure of the overriding plate was not allowed. Introduction of this failure and underthrusting of the forearc block under the Luzon Are allowed us to fit both topography and gravity data. (C) 2000 Elsevier Scie nce B.V. All rights reserved.