KINEMATICS OF CONVERGENCE, DEFORMATION AND STRESS-DISTRIBUTION IN THETAIWAN COLLISION AREA - 2-D FINITE-ELEMENT NUMERICAL MODELING

Using a 2-D plane stress finite-element model with elastic and elasto- plastic rheologies, appropriate for deformation within the brittle upp er crust, we analyse the relationship between kinematics of convergenc e, deformation and stress distribution in the present Taiwan collision occurring between the Ryukyu and Luzon subduction zones. The distribu tion of stress trends calculated in our models is compared with a synt hetic map of actual stress trajectories based on the most recent data available in the collision zone. These data combine present-day source s (from borehole breakouts and earthquake focal mechanism) with the re construction of Quaternary palaeostress (from fault slip data analyses ), resulting in a complete map of compressional stress trajectories wh ich is used to constrain our models.We show that the distribution of s tress trajectories in the active Taiwan collision is principally contr olled by: (1) the geometric configuration of the boundary between Eura sia and the Philippine Sea plate; (2) the shape and rheological proper ties of major structural units; (3) the direction of convergence of th e Philippine Sea plate relative to Eurasia; and (4) the influence of t he opening of the Okinaiva Trough. The study of a two-dimensional elas tic and elasto-plastic finite-element modelling of the subduction-coll ision in and around Taiwan allows us to estimate the influences of the se different parameters in the stress pattern. Taking into account bot h the simplifying assumptions of the numerical modelling and the angul ar uncertainties of field determinations, the fit between the calculat ed stress pattern of the finite-element model and that determined base d on the geometrical synthesis of field analyses is rather good in gen eral, indicating that our model is valid to first approximation. Misfi ts remain minor and can be explained by data uncertainties and simplif ying modelling assumptions (for instance, the shape of the corner of t he collision zone is critical but is not accurately known; also limite d decoupling in the Longitudinal Valley collision zone was not conside red in our models although it certainly plays a role). Some interestin g features of our model are: (1) the greater influence of the shape of the collision zone in comparison with that of the direction of conver gence; (2) the requirement for a trench retreat related to suction for ce in the Ryukyu Are; and (3) the crucial role of the interaction betw een Okinawa Trough opening and collision at the sharp northwestern cor ner of the Philippine Sea plate including its influence on the geologi cal evolution of northeastern Taiwan.