TECTONICS OF SHORT-LIVED INTRAARC BASINS IN THE ARC-CONTINENT COLLISION TERRANE OF THE COASTAL RANGE, EASTERN TAIWAN

The Coastal Range in eastern Taiwan was originated from an oblique col lision between the Luzon volcanic are and Asian continent since the la te Neogene. In this collision terrane, two intra-arc basins, the Plioc ene Chingpu and Pleistocene Chengkung basins, were developed on the ea stern part of the Neogene Chimei and Chengkuangao volcanic islands, re spectively, prior to their accretion to eastern Taiwan. The tectonic e volution of these Neogene volcanic islands and associated intraarc bas ins is reconstructed by stratigraphic and sedimentological analysis, i gneous rock geochemistry, and comparison with observations in modem co llision zone in the regions off southeastern Taiwan. In the Coastal Ra nge, the intra-arc basin sequences are 1.5-10 km wide and 40 km long, comparable in size to their modern analogues in the active collision z one. The basin axis trends subparallel to the volcanic ridge. In both basins, deepwater flysch overlies shallow marine reef carbonates, whic h in turn rest on volcanic basement, indicating rapid are collapse (mi nimum rate of 1 km/m.y.) soon after the are-continent collision. The a re collapse occurred earlier in the north (Chimei, between 5.1 and 3.5 Ma) and later in the south (Chengkuangao, between 2.9 and 1.8 Ma), in concert with a southward propagation of the oblique collision. Sedime ntation ended about 2 Ma and 1 Ma in the Chingpu and Chengkung basins, respectively, coeval with rotation of the Neogene volcanic islands. T his suggests that the rotation inverted the intra-arc basin into thrus ting, uplifting, and final emergence. Thus the duration of sedimentati on for the intra-are basins spanned only about 0.8-3.1 m.y. On the bas is of land geology, offshore observations, and a clay model experiment simulating oblique are-continent collision, a model for the intra-are basin evolution in eastern Taiwan is proposed. During the collision, strike-slip faults would have been developed in the eastern part of vo lcanic islands to induce transtension movements, thus forming pull-apa rt, intra-are basins on the collapsed volcanic island. This mechanism is believed to be responsible for the formation of the Pliocene Chingp u and Pleistocene Chengkung basins as well as the present-day offshore intra-are basins found on the Lutao and Lanhsu volcanic islands. The two intra-arc basins on Lutao and Lanhsu are predicted to be short liv ed. As collision continues, these two basins, together with their unde rlying northern part of the Luzon are, will be rotated, thrust, and up lifted in the next 1 m.y. and, finally, will become part of the southe rn extension of the Coastal Range.