We have compiled new free-air gravity anomaly (FAA) and magnetic anomaly maps, shedding light on the tectonics in the Taiwan-Luzon region.To have a suitable datum level for both the available gravity and magnetic anomaly data, the set of data from an ACT cruise, conducted during May 27 to June 21, 1996, was chosen as a reference. Based on the cross-over error analysis, all the other data were adjusted accordingly. Some satellite derived, airborne or land data were also added to the compilation to obtain better coverage.Several major new insights into the Taiwan-Luzon region are revealed by the new maps. (1) A prominent NE-SW trending belt of gravity and magnetic anomalies is present in the onshore and offshore areas of south western Taiwan. The Peikang High is located on this belt. (2) Located in the offshore region west of Taiwan and to the north of the belt described above, the Taishi Basin, in contrast, occupies a relatively low FAA area. It could be regarded as a flexural basin on account of the loading of a thrust-and fold belt in western Taiwan. (3) A probable NW-SE trending old transform fault is well imaged off southwestern Taiwan, which separates the lithos phere (plate) of the South China Sea from a trapped piece of the Philippine Sea plate. (4) Located east of the Luzon Arc, the Huatung Basin contains several E-W trending magnetic reversals and two N-S trending old frac ture zones. The Huatung Basin is separated from the West Philippine Ba sin by the "123E Fracture Zone". Accordingly, the Gagua Ridge corre sponds to a transverse ridge bounding the 123E Fracture Zone. (5) The Luzon Arc is abnormally concave toward the Manila Trench and becomes

The Coastal Range in eastern Taiwan was originated from an oblique collision between the Luzon volcanic arc and Asian continent since the late Neogene. In this collision terrane, two intra-arc basins, the Pliocene Chingpu and Pleistocene Chengkung basins, were developed on the eastern part of the Neogene Chimei and Chengkuangao volcanic islands, respectively, prior to their accretion to eastern Taiwan. The tectonic evolution of these Neogene volcanic islands and associated intraarc basins is reconstructed by stratigraphic and sedimentological analysis, igneous rock geochemistry, and comparison with observations in modem collision zone in the regions off southeastern Taiwan. In the Coastal Range, the intra-arc basin sequences are 1.5-10 km wide and 40 km long, comparable in size to their modem analogues in the active collision zone. The basin axis trends subparallel to the volcanic ridge. In both basins, deepwater flysch overlies shallow marine reef carbonates, which in turn rest on volcanic basement, indicating rapid arc collapse (minimum rate of 1 krrdm.y.) soon after the arc-continent collision. The arc 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. Sedimentation ended about 2 Ma and 1 Ma in the Chingpu and Chengkung basins, respectively, coeval with rotation of the Neogene volcanic islands. This suggests that the rotation inverted the intra-arc basin into thrusting, uplifting, and final emergence. Thus the duration of sedimentation for the intra-arc basins spanned only about 0.8-3.1 m.y. On the basis of land geology, offshore observations, and a clay model experiment simulating oblique arc-continent collision, a model for the intra-arc basin evolution in eastern Taiwan is proposed. During the collision, strike-slip faults would have been developed in the eastern part of volcanic islands to induce transtension movements, thus forming pull-apart, intra-arc basins on the collapsed volcanic island. This mechanism is believed to be responsible for the for-1Institute of Geology, National Taiwan University, Taipei. 2Institute

The three-dimensional P-wave velocity structure of the obliquely convergent zone in the eastern Taiwan area has been determined by using traveltimes of seismic waves from 1826 local earthquakes and air-gun shots recorded by the Central Weather Bureau Seismographic Network, and 8334 earthquakes have been relocated for better understanding of the current tectonics. The possible location of the plate boundary between the Eurasian and Philippine Sea plates, characterized by a sharp gradient in the velocity structure, is found beneath the eastern flank of the Central Range to the north of 23.5ЊN, eastern Taiwan. To the south of 23.5ЊN, this boundary is generally along the Longitudinal Valley fault and its southern projection. The distribution of the relocated earthquakes also shows a spatial pattern closely related to the boundary and the state of plate collision. To the east of the boundary, a prominent high-velocity anomaly in the middle to lower crust is found beneath the Longitudinal Valley and the Coastal Range in eastern Taiwan; this anomaly could be interpreted as the oceanic crust of the Luzon forearc. To the west of the boundary, the Central Range has a relatively low velocity at the same depth. The velocity structure and relocated seismicity have led to the recognition of interaction between the materials on opposite sides of the boundary. The relatively high P-wave velocity of the Luzon forearc suggests that it can accumulate strain energy and then release it as brittle failure. However, the relatively low P-wave velocity of the Central Range implies that it responds to the convergence by silent or ductile deformation.

Using travel time data from local earthquakes and air-gun shots re corded by the Central Weather Bureau Seismographic Network, the tran sition from a typical subduction to a collision suture in the southeastern Taiwan area is imaged in terms of a three-dimensional Vp structure. The southern prolongation of the Longitudinal Valley Fault (PL VF), which is characterized by a sharp contrast in velocity on either side, is the primary feature in the velocity structure. West of the PLVF, a high velocity volume exists from the surface to about 9-km in depth, which can be interpreted as being related to the Central Range. The Central Range structure seems to end near 22.2° N beneath the Hengchun Peninsula. East of the PLVF, a major high velocity anomaly in the middle-to lower-crust beneath the South ern Longitudinal Trough and Huatung Ridge is observed. According to the velocity structure and the estimated composition, the high velocity body could be the forearc oceanic crust, which might have been torn off and separated from the Philippine Sea plate after the Luzon arc was formed, and has been shortened during the collision of the Eurasian and Philippine Sea plates. The other conspicuous feature of the Vp model is a clearly lat eral velocity variation across the Taitung Canyon from the surface to about 25-km in depth, which might be associated with the segmentation of the Luzon arc. Using the three-dimensional Vp model, earthquake events that occurred from 1990 to 1997 were relocated. Most of the relocated hypo centers in the study area tend to lie on the locations where there is a greater gradient in the Vp model.

Seismic refraction data from onshore and offshore experiments in the eastern-northeastern Taiwan region \\'ere used to study the velocity struc ture by the two-dimensional ray-t . racing method. In the \1elocity model, a structural fault boundary located beneath the Longitudinal Valley "ras used to separate the northern Coastal Range (CR) on the eastern side from the eastern flank of the Central Range (EFCR) on the western side. The P 'vave 'relocities from the surface to the dept .h of 12-15 km varied from 3.9 to 5.8 km/s beneath the CR and from 4.8 to 6.1 km/s beneath the EFCR. Com paring the velocity structures along various latitudes, it \\'as found that the CR extends northward to 24.2 <> N. The velocity structures of the CR, the Hsinchen Ridge (HR) and the Yaeyama Ridge (YR) indicate that the HR and the YR both belong to t . he same type of tectonic unit as the CR. To the north of 24.2 <• N, the velocit)' structure of the Ilan Ridge (IR), located be t,veen the EFCR and the southwest . ern end of the Ryukyu arc, is similar to that . of the EFCR; hence, probably indicating it is the northeastern exten sion of the EFCR. This suggests that the EFCR bends eastward and be longs to the same tectonic unit as the southwestern Ryukyu arc. From a comparison of the velocity structures of the CR, EFCR and of other typical continental arcs, orogens and oceanic arcs in the literature, it can be con cluded that the northern CR b�longs to an oceanic arc and that the EFCR is a continental arc. Further more, from the analysis of the velocity struc tures beneath the CR and EFCR, it is believed that the upper crust of the CR is weaker in strength than the EFCR, which means that the arc-conti nent collision is not an appropriate model for the formation of Tai\\ran is land.