Reappraisal of the Arc-Arc Collision in Taiwan

Abstract

Al though it is evident that Tai wan has been formed by the collision of the west-facing Luzon arc with the Eurasian continental mar gin, there re main a lot of enigmas in this collision. The major ones are: (1) a trans form fault presently connecting the Manila and Ryukyu Trenches in the Philip pine Sea-Eurasia relative motion direction is missing, and in stead, the Ryukyu Trench ex tends near off shore E. Tai wan, (2) the western edge of the intermediate-depth seismicity associated with the Philip pine Sea plate subduction beneath NE Tai wan has a NNW trend, not NW, and (3) a large negative Bouguer gravity anomaly, with undulation of its amplitude in a wave length of 60-80 km, exists along the southernmost Ryukyu forearc. We propose a new model of the collision in Tai wan to re solve these enigmas, assuming that the southern Ryukyu forearc was migrating to the south west with respect to Eurasia for the past several m.y. and the Luzon arc has been colliding with this actively migrating Ryukyu forearc. The northern most Luzon arc is divided into two parts by the NNW line directing along the Philip pine Sea-Ryukyu forearc motion from its initial intersection point with the Ryukyu Trench; the part west of this line has been obducted on the Ryukyu forearc-Eurasian mar gin, producing the collision orogen in Taiwan, and the part east of it has been subducted beneath the Ryukyu forearc. This evolutionary scenario re solves enigmas (1) and (2) kinematically. This model also predicts that the South China Sea slab has to be torn by the west ward component of the motion of the subducting Philip pine Sea slab to Eurasia. This would have brought large lateral compression in the shallow portion of the Philip pine Sea slab at its western border, which might lead to buck ling of the slab causing the ob served undulated gravity anomaly.