Study On Present-day Crustal Deformation In The Arc-Continent Collision Zone Of Taiwan And Adjacent Areas

The arc-continent collision zone between the Philippine Sea Plate and Eurasian Plate is a convergent plate boundary zone. It is one of the most seismically active regions in the western portion of the Pacific Rim seismic belt.The interaction between the Philippine Sea Plate and Eurasian Plate is given priority to with subduction and collision, which affects the dynamics process and tectonic setting of southeastern Eurasian Plate. The crustal deformation and seismicity of this arc-continent collision zone has always been a hot topic in geological study.The present-day crustal deformation, seismicity and geodynamic mechanism of the arccontinent collision zone is studied in this paper.Based on the analysis of the previous studies, this paper makes a comprehensive utilization on gravity, earthquakes and GPS observations and puts deep analysis on the distribution, geometrical morphology and regional seismicity of the research area. Based on the fact that spatial distributions of GPS observations are not even over a wide range, a series formulas that calculating GPS strain field is derived in this paper. Combine with the horizontal GPS velocity field, this paper makes a deep analysis on the crustal deformation of reasearch area and each of its main segments such as taiwan orogenic belt, Manila subduction zone, west coast of taiwan straits. This paper also analysises the crutal deformation of the SW taiwan orogenic belt during its tectonic escape process and proposes a new dynamical mechanism combine with seismic wave observations. The main innovation points are as follows:1. The inversion results of principal stress distribution in the regional crust shows a fan structure from north to south and counterclockwise rotation from east to west within taiwan orogenic belt. The E-W transformation of principal stress directions might be the modulation effect of strike slips on aseismic strike-slip boundaries among the N-S geological units.2. The subduction of the Philippine Sea Plate along the Ryukyu Trench significantly enhanced after Chi-Chi earthquake, which forms an obvious gradient zone of hypocenters’ distribution. The subduction of Eurasian plate towards the Philippine Sea plate is also enhanced and its hypocenters’ distribution forms an eastern trending gradient zone. Besides, the seismicity in the taiwan straight and fujian area are also enhanced.3. Effections on the princple stress field of Chi-Chi earthquake has not reached the depth over 17 km. The Chi-Chi earthquake has enhanced adjustment function on the stress field on the aseismic strike-slip boundaries among the N-S geological units.4. Derived three GPS strain rate calculating models: spherical-coordinate MQ model, spherical-coordinate least squares collocation model and ellipsoidal-coordinate least squares collocation model. Propose a method of area computation of any simply connected region on spherical surface using Delaunay triangulation, which achived practical application of regional interpolation using spherical MQ model.5. The trend of GPS velocity field has a reverse transformation near 24°N, its shifting mechanism might be change of interactions between the Philippine Sea plate and the Eurasian plate. South of 24°N, the interactions between the two plates are oblique collision; In the northern part, the Philippine Sea plate mainly subducted below the Eurasian plate along the Ryukyu Trench, thus the regional crustal deformation mainly affected by the motion of Eurasian Plate. Besides, the shifting mechanism between the two plate may also affect by the strikeslip of longitudinal valley fault.6. The trend of GPS strain field is polarized by the boundary of the Gagua Ridge, which illustrates the disunity of rigid rotations between the west Philippine sea basin and the east China sea basin.7. This paper propose a decoupling mechanism model about upper and lower crust of the escape structure in SW taiwan orogenic belt. A 10km-depth relatively narrow unclosed boundary in SW taiwan alows the plastic lower crust material to pass through and resist south motion of rigid upper crust.