ALBERT

Description: During arc-continent collision, buoyant sections of sediments and rifted continental crust from a subducting plate will accrete to the forearc of the upper plate as long as this backstop remains intact. Deformation of the oceanic arc and forearc block may ultimately lead to accretion of these mafic rock units to the new orogen. The Taiwan mountain belt, which formed at ~6.5 Ma by oblique convergence between the Eurasian passive margin and the overriding Luzon arc in northern Taiwan, offers important insight in this process, since the collision is more advanced in the north than in the south. The incipient stage of arc-collision can be studied in southern Taiwan, while the northern portion of the orogen is presently undergoing collapse due to a flip in the subduction polarity between the Eurasian Plate and the Philippine Sea Plate. In this study, we seismically image the structure of the northern section of the mountain belt with a tomographic inversion. We present marine and land-based seismic refraction data, as well as local earthquake data, from transect T6 of the Taiwan Integrated Geodynamic Research (TAIGER) program across the Taiwan mountain belt and the adjacent Ryukyu arc. Our 2-D compressional seismic velocity model for this transect, which is based on a tomographic inversion of 10 213 P -wave arrival times, shows that the Eurasian crystalline continental crust thickens from ~24 km in the Taiwan Strait to ~40 km beneath the eastern Central Range of Taiwan. The detailed seismic velocity structure of the Taiwan mountain belt shows vertical continuity in the upper 15 km, which suggests that rocks are exhumed to the surface here from the middle crust in a near-vertical path. The continental crust of the westernmost Ryukyu arc is almost as thick (~40 km) as in the adjacent northern Central Range of Taiwan, and it appears to override the leading edge of the Philippine Sea Plate offshore northeastern Taiwan. If we assume that the western Ryukyu arc crust also thickened in the collision, then the mountain belt is wider and less thick in northern Taiwan than in central Taiwan (~50 km), which may be the result of post-collisional extension in the north.

Description: In this paper, we combine a novel finite difference method (FDM) with a manifold-based exponentially convergent algorithm (MBECA) to solve a nonlinear elliptic boundary-value problem defined in an arbitrary plane domain. It is very difficult to solve nonlinear and geometric complexity problems by conventional FDM. To overcome these problems, the concepts of internal residual and boundary residual in a fictitious rectangular domain are introduced. Besides, by adding a fictitious time coordinate, we avoid the need to treat complex boundary conditions but only use shape functions. Also, it is not necessary to solve an inverse matrix of algebraic equations when by using the MBECA. Moreover, in order to increase the numerical stability of the MBECA, we introduce a group-preserving scheme (GPS) to address fictitious time integration. Given the cone structure of the GPS and MBECA and their manifold properties, we can preserve the manifold path on the cone structure by a weighting factor such that the MBECA must also exhibit a cone construction, Lie algebra and group properties at each fictitious time step. Finally, the accuracy and convergence behaviour of this present method are demonstrated in several examples.