PALEOMAGNETISM AND K-AR AGES OF NEOGENE ROCKS OF NORTHERN TAIWAN - TECTONICS OF THE ARC JUNCTION OF RYUKYU AND LUZON ARCS

More than 100 block samples were collected for palaeomagnetic and geoc hronological studies from 14 sites in Miocene and Plio-Pleistocene vol canic rocks in the western foothills province of northern Taiwan, in a n attempt to understand the tectonic evolution of the junction of the Ryukyu and the Luzon arcs. Potassium-argon dating yields ages of 7.9 /- 0.5 Ma and 10.1 +/- 0.6 Ma for two samples from the Miocene Chiopan shan volcanic series, and 1.2 +/- 0.1 Ma for a sample from the Plio-Pl eistocene Chilung volcanic group. The palaeomagnetic direction from se ven sites of the Miocene volcanics shows no deflection of declination value (D = 5.9-degrees, I = 43.8-degrees, alpha95, = 14.0-degrees). Th e direction from the volcanic rock of 1.2 Ma is D = 2.5-degrees and I = 53.3-degrees. The reliability of the direction is ascertained throug h thermal demagnetization, positive fold test and the presence of norm al and reversed polarities. Comparison of Neogene palaeopoles between northern Taiwan and China indicates that little relative tectonic move ment has occurred in northern Taiwan since 20 Ma. These results establ ish that the arcuate shape of geological belts in northern Taiwan is n ot due to tectonic bending but to an inherited geometry. Reconstructio n of the 10 Ma palaeoposition of Taiwan and the south Ryukyu arc impli es that the cusp shape at the arc junction has already existed prior t o 10 Ma.