Plate-boundary strain partitioning along the sinistral collision suture ofthe Philippine and Eurasian plates: Analysis of geodetic data and geological observation in southeastern Taiwan

Crustal deformation and strain partitioning of oblique convergence between the Philippine Sea plate and the Eurasian plate in the southern Longitudina l Valley of eastern Taiwan were characterized, based on geodetic analysis o f trilateration network and geological field investigation. The Longitudina l Valley fault, one of the most active faults on Taiwan, branches into two individual faults in the southern Longitudinal Valley. These two active fau lts bound the Plio-Pleistocene Pinanshan Conglomerate massif between the Co astal Range (the Luzon island are belonging to the Philippine Sea plate) an d the Central Range (the metamorphic basement of the Eurasian plate). A geo detic trilateration network near the southern end of the valley shows a sta ble rate of the annual length changes during 1983-1990. The strain tensors for polygonal regions (including triangular regions) of the Taitung trilate ration network reveal that there are two distinct zones of deformation: a z one of shortening (thrusting) between the Pinanshan massif and the Central Range on the west and a strike-slip movement between the Pinanshan massif a nd the Coastal Range on the east. The analysis of a discontinuity model con sisting of three rigid blocks separated by two discontinuities has been car ried out. The results show that the deformation in this region can be chara cterized by two major faults. A reverse fault is located between the Plio-P leistocene Pinanshan massif and the metamorphic basement of the Central Ran ge, with a shortening rate of about 12 mm/yr in the direction N280 degrees E. A strike-slip fault is located principally along the river between the P inanshan massif and island are system of the Coastal Range with a purely st rike-slip component of about 22 mm/yr in the direction N353 degrees E. The analysis of the geodetic data further suggests that substantial deformation (probably strike slip in type) occurs within the Pinanshan massif. Geologi cal evidence of deformation in the Plio-Pliestocene Pinanshan Conglomerate includes regional folding, a conjugate set of strike-slip fractures at the outcrop scale, and morphological lineaments related to fracturing, all indi cating that the Pinanshan massif is being deformed within a transpressive s tress regime. Regional kinematic data indicate that a significant portion o f the 82 mm/yr of motion between the Eurasian plate and the Philippine Sea plate is absorbed in the southern Longitudinal Valley by the decoupling of two distinct major faults. The geometry of the oblique convergence and the rheology of the rock units (the well-consolidated Plio-Pleistocene conglome rate and the sheared melange formation) play the two important roles in the partitioning of crust deformation.