Lithospheric Structure of Philippine Sea Plate near the Western End of Ryukyu Subduction Zone and Some of its Tectonic Effects

The Ryukyu subduction zone, comprised of the Ryukyu trench, the Ryukyu arc and the Okinawa trough, extends between southern Japan and northern Taiwan. This subduction zone is produced by the northwestward subduction of the Philippine Sea plate beneath the Eurasian plate along the Ryukyu trench. The motion of the Philippine Sea plate is, however, im­ peded at its northwestern corner by the island of Taiwan, which is reflected in the complicated structure of the Philippine Sea plate at the western end of the Ryukyu subduction zone. Using seismicity, as well as other geophysi­ cal and geological data in the Taiwan region, we have investigated the litho­ spheric structure of the Philippine Sea plate beneath the westhern end of the Ryukyu subduction zone and its tectonic effects. Although the Philippine Sea plate moves northwestward in general, at its northwestern corner the direction of subduction changes from the north­ west to the north. The lithosphere of this northwestern corner of the plate underthrusts and extends westward beneath the island of Taiwan. A west­ ern boundary of the lithosphere exists beneath Taiwan. The boundary lies almost along the 121. 0E meridian line. The boundary is migrating west­ ward at a rate of about 6 cm/y along with the movement of the Philippine Sea plate. The westward migration of the lithospheric boundary of the plate causes the westward extension of the Okinawa trough, which lags behind the former by about 600,000 years (-35 km). The initial stage of the forma­ tion of the Okinawa trough, in turn, put the whole northern Taiwan area under tensional stress. (


INTRODUCTION
The Philippine Sea plate moves northwestward and subducts beneath the Ryukyu arc and the Okinawa trough on the Eurasian plate along the Ryukyu trench (Fig. 1).The Ryukyu subduction zone, including the Ryukyu trench, the Ryukyu arc and the Okinawa trough, ex tends from southern Japan to northern Taiwan.It runs northeast-southwest in general.However, near Taiwan, it bends northward and runs east-west.Also, it seems to terminate on the eastern side of the Taiwan island, which is built from the intensive interaction between the Eurasian and Philippine Sea plates.This complicated tectonic structure is mainly due to the fact that the motion of the northwesternmost portion of the Philippine Sea plate is impeded by the island.
Therefore, in order to understand the tectonic structure of the northern Taiwan area and its behavior, it is desirable to depict the lithospheric structure of the Philippine Sea plate near the western end of the Ryukyu subduction zone.In this study, using seismological as well as geophysical and geological data, we delineate the lithospheric structure of the subducting Phil ippine Sea plate near its northwestern corner and examine the interaction between the Eur asian and Philippine Sea plates for the complicated tectonic structure in the northern Taiwan area.

FORMATION OF THE TAIWAN ISLAND •
The Tai wan island has been formed since 4 ma (e.g., Chi et al. 1981; Teng 1990; Lee and Lawver 1994) on the eastern edge of the Chinese continent by the process of arc-continent collision (e.g., Suppe 1984; Teng 1990) or arc-arc collision (Hsu and Sibuet 1995).
Figure 2 shows the structural provinces of Taiwan and its surrounding offshore region.In Fig. 2, the Coastal Range (province v) in eastern Taiwan plays an important role in the arc continent collision model (e.g., Barrier and Angelier 1983).It is believed that Coastal Range is a part of both the Luzon arc, and the uplifted part of the forearc basin (e.g., Cheng et al. 1996Cheng et al. , 1998)), which has formed from the eastward subduction of the South China Sea lithosphere beneath the Philippine Sea plate along the Manila trench.In the arc-continent collision model, the Luzon arc has been colliding with the Eurasian plate, resulting in the growth of the western Fig. 2. Structural provinces on the Taiwan island (Ho 1986(Ho , 1988) ) and its sur rounding offshore region.The two belts on the map are Ryukyu arc and Luzon arc newly defined by Cheng et al. (1996) and Shyu et al. (1996).
The bathymetric data are from Liu et al. (1996Liu et al. ( , 1998)).1995), the Luzon arc has been colliding with the Ryukyu arc, lying on the edge of the Eurasian plate.Shyu et al. (1996) and Cheng et al. (1996Cheng et al. ( , 1998) )    According to Sena (1977) and Sena et al. (1993), the speed of the northwestward move ment of the Philippine Sea plate is about 7 cm/y toward Taiwan.The velocities of Lanhsu and Lutao GPS stations on the Luzon arc relative to the Paisha GPS station on one of the Penghu islands (Fig. 7) in the western offshore area of Taiwan (Yu et al. 1997) shows that the Philip pine Sea plate is moving N50°W at a speed of about 8 cm/y with respect to Eurasia.Calculat ing from the velocity vector of the plate movement of Yu et al. (1997), we obtain western and northern components for the velocity of the Philippine Sea plate in the Taiwan region.These are about 6 cm/y and 5 cm/y, respectively.Therefore, the western boundary of the subducting lithosphere beneath the Taiwan island along the meridian line of 121.5°E is migrating west ward at a speed of about 6 cm/y.

DOUBLE PLUNGING OF THE LITHOSPHERE
The geomagnetic study by Shyu et al. (1996) showed that the Luzon arc subsides toward the north along with the Philippine Sea plate and meets the eastern coastline of Taiwan at 23.5°N.Geodetic data from Yu et al. (1997) show that most of the crustal shortening on Tai wan occurs to the south of 23.6°N, i.e., in the southern half of the island (Fig. 7); north of .: • : .= .
,-... .100 ;:.��: .\.. � . . .From the E-W directional hypocentral profiles shown in Figs. 5 and 8, it is clear that, near Taiwan, the northward-subducting lithosphere of the Philippine Sea plate also underthrusts and extends westward beneath eastern offshore area of Taiwan and northern Taiwan.This lithospheric underthrusting may be caused by the mountain building of Taiwan island, be cause the island rides on the meeting zone of the Euarsian and Philippine Sea plates along the meridian line of 121.5°E (Fig. 6).
The forearc basin (Fig. 2) of the Ryukyu subduction zone east of the Gagua ridge along the meridian line of 123°E runs northwest-southeast, and the westward underthrusting of the Philippine Sea plate starts from this forearc basin.The westward lithospheric underthrusting is most obvious near the latitude of 24°N, near which the Chi-Chi earthquake of Richter magni tude ML 7.3 occurred on September 21, 1999, which is a episode of the vivid mountain build ing of the Taiwan island due to the pushing of the Philippine Sea plate on the Chinese conti-12fe 12f o-1---"-� �+ +� ��-:rt� ��., ,,.,.;:.:� o--1-->-... .,._-o-.k� �� �� �am�" ""'"'"f: .� nental shelf edge (e.g., Wang et al. 2000).This phenomenon of the westward lithospheric underthrusting disappears when the forearc basin terminates against the eastern coastline of Taiwan at about 24.4 °N.The relationship between the phenomenon of the lithospheric un derthrusting and the existence of the forearc basin of the Ryukyu subduction zone is intriguing.

WESTWARD EXTENTION OF OKINAWA TROUGH
The Okinawa trough is a part of the Ryukyu subduction zone, which is running between southern Japan and Taiwan and is convex toward the southeast (Fig. 1).The western edge of the trough lies at the northeastern coastline of Taiwan.It has been found that at its western end near northeastern Taiwan, the formation of the Okinawa trough has been propagating west-.ward along the whole width of about 100 km (e.g., Wang et al. 2000).In other words, the southwestern part of the Okinawa trough is not formed simultaneously, but has been growing from the east to west.The northward subduction of the plate causes the opening of the Okinawa trough in the northern Taiwan area.Since the Philippine Sea plate moves northwestward, the migration of the western boundary of the subducting lithosphere, along with the movement of the plate, initiates the westward propagation of the opening of the Okinawa trough towards northern Taiwan.The rate of the westward extension of the Okinawa trough should be the same as that of the migration of the western lithospheric boundary.The opening of the western part of Okinawa trough started about 2Ma (e.g., Lee et al. 1980;Sibuet et al. 1998).A volcanic sea mount situated at 123.1°E, 25.2°N (designated "x" in Fig. 9), near the northern edge of the Okinawa trough, is believed to have been created about 2Ma, at the beginning of the formation of the Okinawa trough; it was created in the center of the Okinawa trough, and then migrated to its present position due to the spreading of the Okinawa trough (Sibuet et al. 1998).Kueishan Tao (designated "y" in Fig. 9), a 7000-year-old volcanic island (Chen et al. 1998) situated at 121.95°E, 24.8°N off the Ilan plain, represents the western end of the inner volcanic arc of the Ryukyu subduction zone.Using the distance between the volcanic seamount and island, and the difference in age, we calculate an approximate value of 6.4 cm/y fo r the average extension rate of the Okinawa trough.This value confirms the extension rate, estimated from GPS data, of about 6 cm/y.
The westward migration of the opening and formation of the Okinawa trough causes the slumping and collapsing of material on the continental shelf-slope area toward the center of the trough.Apart from the morphological phenomena in the northeastern offshore area of Taiwan described by Wang et al. (2000), Figure 10 also shows the collapsing and slumping phenomena in the northern offshore area of Taiwan.The material in this area has been trans ported toward the Okinawa trough mainly through the Chilung Valley and Mien-hua Canyon (Fig. 9).Material on the sides of these two channels slumps or collapses toward the channels.In northern Taiwan there are several faults striking NE.Traces of these faults extend north eastward in the northern offshore area of Taiwan morphologically (Fig. 10).At present, nor mal faulting appears prevalent along these fault.The material seems to move toward the center of the Okinawa trough northeast of Taiwan.Also, some small lots are sinking directly (deeper than the surrounding area), such as the small area denoted by "a" in Fig. 10.These indicate that the northern offshore area of Taiwan is under tensional stress.
From both the focal mechanisms in northern Taiwan, the slumping and collapsing phe nomena in the northeastern offshore area of Taiwan (Wang et al. 2000) and the above-dis cussed morphology in the northern offshore area of Taiwan, we can conclude that the whole northern Taiwan area is presently under tensional stress.This is most probably caused by the initial formation of the Okinawa trough due to the westward lithosphere migration of the Philip pine Sea plate beneath the western end of the Ryukyu subduction zone.

DISCUSSION
Several points concerning the Okinawa trough, the western boundary of the subducting Philippine Sea plate and the stress status are worth noting.
(1) The morphology (Fig. 9), sug gests that the traditionally recognized Okinawa trough reaches, at most, the eastern coastline of the Ilan plain in eastern Taiwan.
(2) Judging from the Wadati-BenioffZone (Figs.In order to provide information on the strength of material beneath the Taiwan area, Cheng (2000) derived a three-dimensional seismic attenuation structure from the seismic intensity data on Taiwan.The attenuation of the S wave can be presented by a parameter Q., defined in terms of the fractional loss of energy per cycle, -LIB/E: 2JC/ Q =-Lill /E.In this way, 1/ Q can be s s called the attenuation factor of the S wave; its value represents the degree that the material is softened or weakened in the Earth, mainly due to the presence of heat or water.Figure 11 shows the distribution of the attenuation factor 1/ Q, of a layer 49-74 km deep in the Taiwan area.We can see that the attenuation factor of the material beneath the whole Okinawa trough is higher than the rest of the Taiwan area, indicating that seismic waves will experience more serious attenuation when they travel through the space beneath the Okinawa trough.In the northern Taiwan area, the higher value of the attenuation factor of this layer may indicate the location of material that has been softened, weaken or become half-molten due to the subduc tion of the Philippine Sea plate.It is very interesting to notice that the zone of high attenuation factor extends westward at least to the 121.5°E meridian line, i.e., the western lithospheric

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Hua-ping Yu (volcanic islet) (1) The Philippine Sea plate moves generally northwestward.However, it subducts northward at its northwestern corner due to the impediment of Taiwan island, with its western bound ary almost being along the 12L5°E meridian line, beneath northern Taiwan.
foothills and Central Range of Taiwan through deformation of strata and basement rocks of the Asian continental margin.The suture zone of the collision (the Longitudinal Valley of eastern Taiwan; province iv in Fig. 2) lies along the length of Taiwan.Based on the P-wave velocity structure derived from seismic refraction experiments in the northeastern Taiwan area, Cheng et al. (1996) found that the eastern flank of the Central Range in central Taiwan (designated by EFCR in Fig. 2) and the western end of the Ryukyu arc have similar characteristics in seismic velocity and speculated that they are parts of the same tectonic unit.Delineating the Luzon arc from the magnetic anomaly, Shyu et al. (1996) found that the Luzon arc extends northward on the eastern side of the Coastal Range and that northern Coastal Range is now a part of the Luzon arc.From a 3-D P-wave velocity study based on signals from earthquakes and air gun shots in the southeastern area of Taiwan, Cheng et al. (1998) showed a zone ofrelatively lower P-wave velocity near the surface, between the Luzon arc and the eastern coastline of Taiwan.In the arc-arc collision model (Hsu and Sibuet Fig. I. Bathymetry and tectonic units in the Ryukyu-Taiwan-Philippine region.TW: Taiwan; 1.Continental slope of East China Sea; 2. Okinawa trou g h; 3. Ryukyu island arc; 4. Ryukyu trench; 5. Gagua ridge; 6. Luzon island arc; 7. Central Basin ridge of West Phili p pine Basin; 8. Philippine trench; 9. Manila trench.The arrow to the southeast of Taiwan shows the gen eral direction of movement of the Philippine Sea plate.The box in the northern Taiwan area is the study area of this paper.The contour lines of water depth are in 1,000 meters. Fig.2.Structural provinces on the Taiwan island(Ho 1986(Ho , 1988) ) and its sur rounding offshore region.The two belts on the map are Ryukyu arc and Luzon arc newly defined byCheng et al. (1996) andShyu et al. (1996).The bathymetric data are fromLiu et al. (1996Liu et al. ( , 1998)).A: continent shelf of East China Sea; B: Okinawa trough (backarc basin of Ryukyu arc); C: Ryukyu arc; D: Nan-ao Basin (forearc basin of Ryukyu arc); E: Ryukyu trench; F; Philippine Sea plate; G: Gagua ridge; H: Luzon arc; I: Ilan plain; L: Manila trench; j: Lutao (island); k: Lanhsu (island); m: Tatun vocano; i: Western Coastal Plan; ii: West Foothills; iii: Central Range; iv: Longitudinal Valley; v: Coastal Range; EFCR: Eastern flank of Cen tral Range.The numbers on the bathymetric contour lines are the depth in 1,000 meters.

Figure 3
Figure3shows the distribution of earthquake epicenters in the Taiwan region and zones of earthquakes used for plotting the hypocentral profiles.Figures4 and 5show hypocentral profiles in the northern Taiwan area in the N-S and E-W directions, respectively.By compari son of these profiles with the seismicity maps in Fig.3, it can be seen that most earthquakes occur at depths of less than 40 km, including those in the seismic belts.Earthquakes in the northern Taiwan area which are deeper than 40 km (Figs.4 and 5) mostly fall into the Wadati-Benioff Zone associated with the subduction of the Philippine Sea plate.In Fig.4, the hypocentral profiles in the N-S direction show that to the east of 121.5°E (the right column) the Wadati-Benioff Zone is clear.To the west of 121.5°E (the left column), the earthquakes deeper than 40 km are much fewer and scattered.In Fig.5, the hypocentral profiles show that most earthquakes fall into two groups.The top group includes those earth quakes shallower than 40 km.The bottom group is the Wadati-Benioff Zone which is situated deeper and deeper when the hypocentral profiles move to the north.From the hypocentral profiles in Figs.4 and 5, it can be seen clearly that, in general, the Wadati-Benioff Zone exists to the east of 121.5°E, though there are still some fine structures in the zone.From the location and attitude of the Wadati-Benioff Zone shown in Figs.4 and 5, we interpret that the Philippine Sea plate subducts northward in the Taiwan region along the Ryukyu trench, with the meridian line of 12L5°E as the present western boundary of the active sub ducting lithosphere (Fig.6).Tsai et al. (1977) used this meridian line to separate two seismic areas of different tectonic significance in the Taiwan region: the Northeastern Seismic Area and the Western Seismic Area.This western boundary line of the northward-subducting plate in the Taiwan region intersects the eastern coastline of Taiwan at 23.5°N, 121.5°E, whereShyu et al. ( 1996) separated the direct collision and the indirect collision of the Luzon arc against the Ryukyu arc (Fig.2).

Fig. 4 .
Fig. 4. The N-S directional hypocentral profiles in the northern Taiwan area.See Feg. 3 for the locations of the profilers.

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Fig. 5.The E-W directional hy p ocentral p rofiles in the northern Taiwan area.See Fe g .3 for the locations of the p rofilers.

Fig. 6 .
Fig. 6.Model of double lithospheric plunging of the Philippine Sea plate.The shaded zones in the north-south profile is the Wadati-Benioff Zone rep resenting the upper part of the subducting lithosphere.The east-west tec tonic profile is revised from Wang et al. (2000).This profile shows the interaction between the Eurasian and the Philippine Sea plates, including locations of the seismic wedge (Wang et al. 2000), the sedimentary wedge above the Eurasian plate, and a frictional zone between the two plates.

From
the Wadati-Benioff Zone shown in Fig.4, it is obvious that near the eastern coast line of Taiwan, this seismic zone (the lowest zone) is not bending smoothly downward to the north; instead, it has intrinsic structure and seems to be composed of several segments with different slopes.It may be associated with the NW striking left-lateral faultings in the lithos phere(Wang 1988;Wang and Wu 1991), near the western lithospheric boundary, caused by the horizontal lithospheric bending of the Philippine Sea plate due to the impediment of the northwestward motion of the plate by the Eurasian plate (Wang 1982; Wang 1988; Wang and Wu 1991; also Fig.6).It may also be associated with the formation process of the Ryukyu subduction zone.The cause and effect of this complicated structure of the lithosphere in the eastern Taiwan area still need further study.

Fig. 7 .
Fig. 7. Velocities of OPS stations in the Taiwan region relative to Paisha station (designated by A) west of Taiwan (Yu et al. 1997).The 95% confidence ellipse is shown at the tip of each vector.In the bsckground, the contours of water depth, the tectonic units and the two subduction systems (solid lines with solid trianges) are shown.The motion speed of the Philippine Sea plate near Taiwan with respect to Eurasial is estimated from velocity vectors of OPS stations on two islands southeast of Taiwan, Lutao and Lanhsu (designated by B and C, respectively).Numbers on contour lines of water depth are in 1000 meters.The dashed line along the median line of 121.S°E is the surface projection of the western lithospheric boundary of the Philippine Sea plate.ECS: East China Sea; OT: Okinawa trough; RA: Ryukyu arc; PSP: Philippine Sea plate.

Fig. 8 .
Fig. 8.The east-west hypocentral profiles across the Taiwan island near the lati tude of 24°N, near which the Chi-Chi earthquake with Richter magni tude ML 7.3 occured in central-western Taiwan on September 21, 1999.These profiles clearly show the westward lithospheric underthrusting of the Philippine Sea plate, exhibited by the attitude of the Wadati-Benioff Zone (the lowest seismic zone).

Fig. 9 .
Fig. 9. Morphology of the western end of the Ryukyu subduction zone.The three small arrows, a, b and c, on the continental shelf/slope show the locations of the Chilung Valley, the Mien-hua Canyon and the North Mien-hua Canyon.See Wang et al. (2000) for detailed description.ECS: East China Sea; OT: Okinawa trough; RA: Ryukyu arc; I: Ilan plain; N: Northern central graben; S: Southern central graben; x,y: volcanic sea mount and islet (the Kueishan islet) used for calculating the rate of west ward extension of the Okinawa trough.
4 and 5), the active subducting lithosphere of the Philippine Sea plate extends westward to the meridian line of 121.5°E.(3) The morphology of the continental shelf-slope area of the East China Sea suggests the slumping and collapsing of material, and the sinking of the sea floor, reach 121.5°E in the northern offshore area of Taiwan.(4) The focal mechanisms in northern Taiwan indicate tensional stress to the north of about 24.5°N (Wang et al. 2000).(5) There are scat tered hypocenters of earthquakes deeper than 40 km in northern Taiwan west of the 121.5°E meridian line (the western boundary of the subducting lithosphere of the Philippine Sea plate) (Figs. 4 and 5).From (1) and (2) above, we can conclude that the subducting lithosphere of the Philippine Sea plate advances westward about 35 km before the opening of the traditionally-recognized Okinawa trough.From (2) and (3), we know that the slumping, collapsing and sinking of the sea floor are associated with the presence of the subducting lithosphere.From (2), (4) and (5),we know that the tensional stress and the scattered deep hypocenters of earthquakes advance well before the western lithospheric boundary of the Philippine S ea plate.The physical-chemical interpretation of these points is desired.

Fig. JO .
Fig. JO.Shaded topography of the northern offshore area of Taiwan.This figure show some morphological evidence of tensional stress.

Fig. 11 .
Fig. 11.Distribution of the values of the attenuation factor 1/Q of a la yer 49-74 ' km in depth beneath the Taiwan area.A zone of consistent high value of the attenuation factor can be clearly seen in the whole Okinawa trough area.(2)Moving at a rate about 6 cm/y along with the movement of the Philippine Sea plate, the western boundary results in the westward extension of the Ryukyu subduction zone, most apparently in the Okinawa trough.The westward extension of the Okinawa trough causes the slumping and collapsing of material in the continental shelf-slope area in the northern and northeastern offshore areas of Taiwan, and puts the whole northern area of Tai wan under tensional stress.(3)Comparing the location of the western end of the traditionally-recognized Okinawa trough, to the east of the Ilan plain in northeastern Taiwan, with that of the western lithospheric interpreted that the Coastal Range extends northward to 24.2°N and that the arc-arc collision is direct north of 23.5°N, and is indirect ( with the deformed and uplifted sediment layers of the southern Coastal Range and its southern extension between the Luzon arc and the eastern coastline of Taiwan as buffer) south