North Luzon Trough Research Articles

Bottom simulating reflectors in the Manila Trench forearc and its implications on the occurrence of gas hydrates in the region

Gas hydrate occurrence and stability in active plate margins have been reported in various localities, with investigations focusing on its potential as an alternative energy resource or the threat it poses due to dissociation and methane release into the atmosphere. The Manila Trench forearc, proximal to active margins and probable methane-rich sediments, can be an analog for gas hydrate formation and geologic preconditions constrained by tectonics and sedimentation processes. The possible occurrence of gas hydrates in offshore western Luzon Island is reported for the first time based on bottom simulating reflectors (BSRs) in multi-channel seismic reflection datasets. Results indicate a prevalence of BSRs on the frontal wedge seaward of the North Luzon Trough (NLT) and in the West Luzon Trough (WLT) basin fill. Continuous, discontinuous, double, and pluming BSRs in the forearc encompass a total area of approximately 15,400 km2 at depths of 223–553 mbsf in the frontal wedge and 486 to 1428 mbsf within the basins. Enhanced amplitude reflectors (EARs) above and below the BSRs indicate possible gas hydrate and free gas accumulations, respectively. Gas chimneys associated with the upward migration of methane-rich fluids appear to be controlled by deep structural features. The accretion of continent-derived sediments along the northern segment of the frontal wedge promotes the formation of fluid migration structures, enhancing fluid migration and gas hydrate formation at shallower depths. Estimated geothermal gradient values for the frontal wedge range from 28 to 92 °C/km, consistent with previously reported in-situ measurements from offshore SW Taiwan. In contrast, arc-derived sediments within the NLT and west-ward fluid migration along the landward-tilted sequences limit gas hydrate accumulation within the basin. To the south, the preferential formation of gas hydrates within the WLT basin points to deep gas sources within the forearc basin sediments, with upward movement of methane-rich fluids along normal faults. Estimated geothermal gradient values in the basin are lower, ranging from 12 to 45 °C/km, reflecting lower thermal conditions. The absence of BSRs seaward of the WLT signifies widespread gas hydrate dissociation in the frontal wedge slope.

A forearc pull-apart basin under oblique arc-continent collision: Insights from the North Luzon Trough

The North Luzon Trough, on the East Asian continental margin, is under transpression due to the oblique convergence between the Eurasian and Philippine Sea plates. Based on a seismic profile across the Manila subduction system and other geological data, we propose a strike-slip pull-apart basin model to account for the formation of the North Luzon Forearc Basin. The subduction system experienced four tectonic evolutionary stages: (1) initial subduction (~22–20 Ma): the Eurasian Plate began to subduct beneath the Philippine Sea Plate, and the incipient Manila Trench-North Luzon Volcanic Arc system started to form; (2) upper crustal deformation (~20–15 Ma): the overriding Philippine Sea Plate was strongly deformed by continuous subduction, and thrust faults, volcanic activity, and possibly strike-slip faulting weakened the forearc areas; (3) initiation of strike-slip faults (~15–6 Ma): continuous rotation of the Philippine Sea Plate initiated strike-slip faults in the weak zones of the forearc areas; (4) pull-apart basin formation (~6–0 Ma): the pull-apart basin formed in the forearc due to sinistral movement of two sets of strike-slip faults. We propose an indentation-escape process involving coupling between the sinistral strike-slip faults in the Taiwan-Luzon area and dextral strike-slip faults in the East Asia continental margin, for the evolution of the eastern South China Sea Basin margin.

Forearc structures and deformation along the Manila Trench

The Manila Trench subduction zone is characterized by varying tectonic structures from north to south. Analyses of new seismic reflection data and bathymetric data indicate distinct morphological and deformational patterns in the forearc region. Differences in the nature of the subducting oceanic lithosphere (i.e. seafloor relief related to seamounts and ridges, sediment supply, reactivated features and faults associated with the South China Sea opening) cause along-strike heterogeneity in the Manila Trench and the Luzon forearc region. The northern segment is classified as an accretionary margin while the southern segment is mainly an erosive margin (with a narrow, steep, and often eroded frontal wedge). This sharp contrast is attributed to abundant sediment supply to the trench in the north and to the highly eroded frontal wedge in the south due to scarce sediment supply. The southern trench segment is prone to submarine slope failures and mass wasting processes. The 17°N latitude boundary also separates the forearc basin into the North Luzon Trough and the West Luzon Trough. Associated with this is the initiation of the Scarborough Seamount Chain (South China Sea extinct spreading ridge) subduction at 16°N latitude. A combination of forearc uplift and submarine mass movements attributed to subduction of bathymetric highs near and south of 17°N latitude produced the Stewart Bank. Seamount and other seafloor spreading features induced complex responses arising to diverse forearc architectures in the southern segment. Seamounts and other seafloor spreading-related features in the subducting slab induce slope steepening and significant vertical movement in the frontal wedge and the forearc region, respectively. Deformation associated with the subduction is overprinted by shearing related to the Philippine Fault Zone splay faults with the frontal wedge shortening associated with the ongoing subduction, further complicating the forearc development in the trench and marine forearc region.

Origin of the serpentinites in the Lichi mélange, eastern Taiwan, China: implication from petrology and geochronology

ABSTRACT Lichi melange, located in the southern coastal range, eastern Taiwan, China, is a typical tectonic melange of the plate’s boundary zone between the Eurasian Plate and the Philippine Sea Plate. It formed during the collision of the Luzon arc with the Eurasian Continent (arc-continent collision). It is composed of sandstone and/or mudstone matrix and many kinds and sizes of rock fragments, including some sedimentary rocks, volcanic rocks and a few metamorphic rocks. The serpentinite is one of the common fragments in the Lichi melange. By the petrographic characteristics and the zircon U-Pb chronology analyses, protolith of the serpentinite is peridotite, the age is 17.7 ± 0.5 Ma. Taking the tectonic background into account, it is inferred that the serpentinite (serpentinised peridotite) come from the forearc basin (the North Luzon Trough) and was taken into the melange by a second thrust westwards. The origin of the serpentinite in Lichi melange is helpful to understand the formation of the Lichi melange and can provide reliable detailed information for the study of the arc-continent collision orogenic activity in and offshore Taiwan.

Stratigraphy and provenance of forearc sequences in the Lichi Mélange, Coastal Range: Geological records of the active Taiwan arc‐continent collision

AbstractThe Lichi Mélange in the Coastal Range, eastern Taiwan, is considered as sheared forearc sequences. However, the age of these sequences is still uncertain, and their significance for the arc‐continent collision has been overlooked. Based on field surveys and micropaleontological analysis, four independent biostratigraphic units ranging from <8.5 to 3.0 Ma are discerned in the Lichi Mélange. These units are obviously older than the remnant forearc sequences in the east (3.4–1.2 Ma), supporting the interpretation that the Lichi Mélange arose from the shearing of the lower forearc basin sequences. The stratigraphy also suggests that the older forearc strata (<8.5–3.4 Ma) were deformed and uplifted as a bathymetric high similar to the Huatung Ridge in the western North Luzon Trough by arcward back thrusting before 3.4 Ma, while sedimentation continued in the remnant forearc basin in the east during 3.4–1.2 Ma. The older forearc strata possess vitrinite reflectance values even lower than those of the remnant forearc sequences, which also supports that they were uplifted by back thrusting and therefore did not experience significant burial. Neodymium isotope analysis shows that the <8.5–6.4 Ma forearc sediments were sourced from both the volcanic arc and the accretionary prism, indicating the emergence of the accretionary prism within <8.5–6.4 Ma. It might have resulted from the underplating of thinned continental crust and provides a good time constraint for the onset of arc‐continent collision. The post 6.4 Ma forearc sediments were mainly derived from the accretionary prism, reflecting its constant uplift and large‐scale exposure.

Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system

AbstractWe examine interseismic coupling of the Manila subduction zone and fault activity in the Luzon area using a block model constrained by GPS data collected from 1998 to 2015. Estimated long‐term slip rates along the Manila subduction zone show a gradual southward decrease from 90–100 mm/yr at the northwest tip of Luzon to 65–80 mm/yr at the southern portion of the Manila Trench. We provide two block models (models A and B) to illustrate possible realizations of coupling along the Manila Trench, which may be used to infer future earthquake rupture scenarios. Model A shows a low coupling ratio of 0.34 offshore western Luzon and continuous creeping on the plate interface at latitudes 18–19°N. Model B includes the North Luzon Trough Fault and shows prevalent coupling on the plate interface with a coupling ratio of 0.48. Both models fit GPS velocities well, although they have significantly different tectonic implications. The accumulated strain along the Manila subduction zone at latitudes 15–19°N could be balanced by earthquakes with composite magnitudes of Mw 8.8–9.2, assuming recurrence intervals of 500–1000 years. GPS observations are consistent with full locking of the majority of active faults in Luzon to a depth of 20 km. Inferred moments of large inland earthquakes in Luzon fall in the range of Mw 6.9–7.6 assuming a recurrence interval of 100 years.

Depleted deep South China Sea δ13C paleoceanographic events in response to tectonic evolution in Taiwan–Luzon Strait since Middle Miocene

The most distinctive feature of the deep South China Sea (SCS) paleoceanography is the occurrence of long-term depleted deep-sea benthic foraminiferal δ13C values. They are lower than the global and the Pacific composite records in the last 16Ma, especially at 13.2, 10.5, 6.5, 3.0 and 1.2–0.4Ma. This distinct deep SCS paleoceanograhic history coincides with the subduction–collision history in the Taiwan region where waters of the West Pacific (WP) and the SCS exchange. The depleted deep-sea benthic foraminiferal δ13C events indicate that the SCS deep basin became progressively a stagnant environment in the last 16Ma due to either closure of the connection with the WP bottom water or temporary reduction of the WP deep water flowing into the deep SCS. Both the Taiwan accretionary prism and the Luzon arc became the main tectono-morphological barriers for the WP bottom water flowing into the SCS deep basin when eastward subduction of the SCS oceanic lithosphere beneath the Philippine Sea Plate started from the Middle Miocene (18–16Ma). This began a long-term trend of depleted SCS deep-sea benthic δ13C values in the last 16Ma. The oblique arc–continent collision since ~6.5Ma uplifted the Taiwan accretionary prism rapidly above sea level and further isolated the SCS from the open Pacific. The collision simultaneously causes backthrusting deformations in the North Luzon Trough forearc basin and sequentially closes interarc water gates between volcanic islands from north to south. The Loho and the Taitung interarc water gates in the advanced collision zone were closed at ~3.0Ma and ~1.2Ma, coinciding with the very low SCS deep-sea benthic δ13C events at 3.0 and 1.2–0.4Ma, respectively. The Taitung Canyon between the Lutao and Lanyu volcanic islands in the incipient collision zone is semi-closed presently. These closure events also lead to the result that the WP deep water intrudes westward into the SCS principally through the Bashi Channel between the Lanyu and Batan volcanic islands in the subduction zone.

Plate coupling along the Manila subduction zone between Taiwan and northern Luzon

We use GPS data, trench parallel gravity anomaly (TPGA), and bathymetry to infer plate coupling patterns along the Manila subduction zone. Using a block model and a fault geometry constrained by seismicity, we simultaneously solve for the location of Euler pole and angular velocity between the Sunda and Luzon blocks as well as the slip-deficit rate on plate interface. Our estimates show that the Euler pole between the Sunda and Luzon blocks is situated at southern Palawan near 8.3°N and 119.4°E with the angular velocity of 4.6Myr−1. The estimated convergence rate along the Manila Trench continuously decreases southward from 91mm/yr at the northern tip of Luzon to 55mm/yr north of Mindoro. The inversion of GPS data reveals partially locked fault patches extending from the West Luzon Trough to the east of Scarborough Seamount chain. The slip-deficit rate in this region is in the range of 20–30mm/yr corresponding to a coupling ratio of 0.4. However, the fault slip behavior is not well resolved near the North Luzon Trough. Based on a good correlation between locations of large subduction zone earthquakes and areas possessing gravity low, we investigate a variety of TPGA-based plate coupling models assuming different scaling between TPGA values and plate coupling ratios. The TPGA-based plate coupling models offer plausible rupture scenarios which are not constrained by current GPS data. The partially locked fault zone near 15–16.5°N may be associated with the subducted Scarborough Seamount wherein oceanic floor is highly fractured. The great subduction zone earthquake propagates beneath the Scarborough Seamount seems to be unlikely. The densification of GPS network in central Luzon and seafloor geodetic observations close to trench axis are crucial to distinguish the detailed fault coupling patterns and the role of subducted seamounts.

Sediment compositions in offshore southern Taiwan and their relations to the source rocks in modern arc-continent collision zone

Modal analyses of 31 sand samples collected by piston coring document variations in sediment composition along and across the developing collision zone off southern Taiwan and help constrain sediment transport paths and, by inference, of sediment sources. Overall, sand composition from this region is dominated by lithic-fragment populations, with a QtFL average of Qt 27F 21L 52. Three geographic domains are based on morphotectonics and variations in sand composition: (1) South China Sea and the adjacent slope of the accretionary prism; (2) suture zone south of southern Taiwan and north of the Luzon forearc; and (3) the Luzon forearc basin (North Luzon Trough) and immediate adjacent slopes. Sands from the accretionary prism and the suture zone contain subequal amounts of sedimentary and metamorphic lithic fragments (Ls 51Lv 5Lm 45 and Ls 47Lv 5Lm 48), respectively, whereas sands from the forearc basin are dominated by volcanic lithic fragments (Ls 29Lv 49Lm 22). In addition, compositions of individual sand beds vary dramatically in the forearc basin. This heterogeneity indicates that sediments from different sources have been deposited sequentially, but do not typically mix during transport and deposition. Similar, but less dramatic, within-core variations occur over the accretionary prism. The major sediment input for the accretionary prism is from western Taiwan and is dominated by sedimentary and low-grade metamorphic lithic fragments. There appears to be a second sediment source, however, from southeastern China. This implies that a significant amount of sediment is the input from China to this region. The major component of sands in the suture zone is derived from Taiwan, but medium-grade metamorphic lithic fragments are rather sparse in these sands, considering the extensive and high-relief exposures of metamorphic rocks on the island of Taiwan. The major source of sands in the forearc basin is the active volcanoes of the Batan islands. Episodically, minor components appear to be fed to the forearc basin from the arcward slope of the accretionary prism, presumably by submarine mass wasting. This observation provides support for the hypothesized olistostromal origin for the Lichi Mélange of eastern Taiwan.

Structural evolution from Paleogene extension to Latest Miocene-Recent arc-continent collision offshore Taiwan: comparison with on land geology

Marine structures observed in seismic profiles across the NE South China Sea–Taiwan Strait–Luzon Arc region show that they are comparable to those observed onshore Taiwan. The Hanjiang Depression and Dongsha Uplift off the SE China coast extend to the Penghu Basin and Penghu Platform, respectively, in the Taiwan Strait, while the Tainan Basin in the south of Penghu Platform is connected to the Chaoshan Depression south of the Dongsha Uplift. The Hanjiang Depression and the Penghu Basin are composed of Paleogene syn-rift sequences on the Mesozoic basement and are unconformably overlain by Neogene post-rift strata. Similar successions are found underneath the coastal plain or have been thrust up as a fold-and-thrust belt bordering the Peikang Basement High onshore central Taiwan.East of the Manila Trench, off southwestern Taiwan, parts of the Tainan Basin are thrust into the submarine syn-collision accretionary wedge (the fold-and-thrust Kaoping Slope) in the initial arc-continent collision zone. They have also been exposed in the fold-and-thrust Western Foothills south of the Peikang Basement High in southern Taiwan during an advanced stage of arc-continent collision since the Late Pliocene. Further east, the pre-collision accretionary wedge (the Hengchun Ridge) shoals northward to the Hengchun Peninsula, with Late Miocene turbidites unconformably overlain by Plio–Pleistocene shallow-marine slope basin sequences. In the arc domain, east of the arc-prism boundary fault, the western forearc sequences in the North Luzon Trough are deformed. Two normal-faulted intra-arc basins occur on the eastern part of the Lanhsu Island in the Luzon Arc. The scenario of the deformed forearc sequences and the development of the two intra-arc basins in the initial arc-continent collision zone are analogous to the Lichi Mélange–Taiyuan Basin and the intra-arc basins in the southern Coastal Range, eastern Taiwan.Additionally, the lateral change in the marine structure from the passive Asian continental margin to the Manila subduction system is comparable to the temporal evolution from Paleogene normal faulting, through Neogene subduction, to Latest Miocene-Recent arc-continent collision observed onshore Taiwan.

Forearc-basin closure and arc accretion in the submarine suture zone south of Taiwan

The obliquely propagating Taiwan collision provides an active example of an intraoceanic arc being accreted to a young rifted continental margin. The actual accretion of the exotic arc is taking place immediately south of Taiwan, in a complex area of rapid uplift and shortening between the emerging crest of the submarine accretionary prism and the extinct northernmost segment of the Luzon volcanic arc. The northern part of this region accommodates over half of the convergence between the Philippine Sea and Eurasian plates, based on recent results of triangulation and Global Positioning System studies. Assuming that nearly all the plate convergence to the south, in the region of normal intraoceanic subduction, is concentrated near the active trench, as is true in most subduction zones, this region of arc accretion is a zone across which approximately 60% of the total plate convergence, amounting to about 50 mm/yr, is being actively transferred. This transfer of slip is presumably caused by the buoyant nature of continental crust that has been subducting beneath the Taiwan orogen. This arcward transfer of plate convergence has strongly affected development of the suture between the Luzon arc and the continental margin, represented by the Taiwan mountain belt. Backthrusting of the accretionary prism in this region is accommodated on east-vergent thrust faults, which locally reach the surface and deform the entire forearc sequences, thereby building the Huatung Ridge, a distinct structural and bathymetric ridge east of the main accretionary prism (the Hengchun Ridge). The Huatung Ridge dams orogenic sediment from the emergent collision in the Southern Longitudinal Trough, a suture basin that projects directly northward to the Longitudinal Valley of eastern Taiwan. Growth strata in the Southern Longitudinal Trough document progressive uplift of the Huatung Ridge to the east, apparently along east-verging thrust systems. Seismic reflection and sidescan sonar data south of about 23°N provide no evidence of back-arc thrusting along the eastern margin of the Luzon arc, as has been hypothesized in order to transfer shortening to the Philippine Sea plate. Neither do these data show clear evidence of west-vergent thrusting of the Luzon arc over adjacent elements of the forearc, in contrast to the very active thrusting documented onland to the north, along the Longitudinal Valley of eastern Taiwan. The arcward-vergent structures in the region of arc accretion have closed the North Luzon Trough, the major forearc basin. These structures have also built the Huatung Ridge as a compressional ridge of orogenic strata that serves to broaden the accretionary prism toward the arc (eastward) and, in so doing, have formed small collisional or suture basins and redirected orogenic sedimentation patterns throughout this key area. Thus the arcward flank of the collision has evolved in a much more complicated fashion than the relatively smooth progression followed by the western, frontal slope of the submarine accretionary prism as it evolves northward to the fold-and-thrust belt exposed along strike in western Taiwan. This complexity on the arcward flank of the collision zone is likely a response of the collision complex to continued plate convergence in the face of increasing resistance, to the north, to the subduction of continental crust.

A Regional Gravity Map for the Subduction-Collision Zone Near Taiwan

To delineate the tectonic character of the subduction-collision zone in the marine area near Taiwan, we have constructed a regional gravity map by reconciling the shipboard data of 15 cruises that have surveyed the region of 20-26°N and 119-124°E over the past 20 years. Among the 15 cruises, three have known, reliable base-tie information. The absolute level for the other 12's cruises are adjusted in such a way that their crossover errors with respect to the other, three are minimized. The root-mean-square difference of the totally 865 crossovers in the final data set is 6.0mgal, representing an acceptable quality considering the multi-agency nature of the source and the rugged seafloor. The distribution of the ship tracks is uneven, with the better resolution around Taiwan but a progressive undersampling toward the east and northwest. To make a regional map, we interpolate both the marine data and the 640 free-air anomaly measurements on Taiwan and its offshore islands into 5 minute spaced grids using a minimum curvature technique. The SEASAT altimetry derived gravity values are placed on part of the map's boundary as an additional constraint in the minimum curvature calculation. On the map, the free-air anomaly ranges from -250 to 350mgal, primarily reflecting the topography, with the maximum and minimum at the high mountains of Taiwan and the Ryukyu Trench-Taiwan intersection, respectively. The Lutao-Lanyu volcanic arc is manifested as a prominent, continuous high from the southern border of the map, merging northerly with the Coastal Range in eastern Taiwan. The Hengchun Ridge shows a much more subdued gravity anomaly than that of the volcanic arc at a similar elevation. The map also delineates the North Luzon Trough as a forearc basin to the west of the arc, the Nanao Basin at the Ryukyu Trench, and to a less extent the Gagua Ridge at 123°E. To illustrate how the marine gravity data constrain crustal structure, a 2-D gravity analysis is performed with a free-air anomaly profile across the Ryukyu Trench taken from this map.

Marine geology in the arc-continent collision zone off southeastern Taiwan: Implications for late neogene evolution of the coastal range

Marine geology in the region between Taiwan and Luzon is characterized by a convergent tectonics in the south and an oblique arc-continent collision in the north. In the collision zone six geologic provinces are recognized. They are the Manila Trench, the accretionary prism of the Hengchun Ridge, the sutural basin of the Southern Longitudinal Trough, the subduction mélange of the Huatung Ridge, the forearc basins of the Taitung and the North Luzon Troughs, and the volcanic arc of the North Luzon Ridge. The oceanic crust of the South China Sea is subducting eastward beneath the Philippine Sea plate along the Manila Trench south of 20°N. Further to the north, the Trench has changed progressively its axis toward the west from the subduction mélanges off southeastern Taiwan at 3 Ma to the Hengchun peninsula of southern Taiwan at 2-1 Ma, and then to the present-day location off southwestern Taiwan. Because of the continuing collision and the westward shifting of the Manila Trench, sediments deposited in the region are passively incorporated into the accretionary prism and deformed to the Hengchun Ridge. After the collision the Southern Longitudinal Trough, consisting of a 2 km thick of orogenic sediments, represents a sutural basin superposed on the plate boundary between the accretionary prism of the Hengchun Ridge (Asian continent) and the subduction mélange of the Huatung Ridge (Philippine Sea plate). The Huatung Ridge is a bathymetric high connected to the southern Coastal Range and is characteristic of low, negative free-air gravity and magnetic anomalies. The Ridge contains deeply weathered sedimentary blocks with abraded deep-water late Pliocene foraminifera and identical clay mineral compositions as those in the onshore Lichi Mélange. This implies that at least a portion of the Huatung Ridge is composed of mélange materials and represents a former subduction zone extending from the Lichi Mélange of the southern Coastal Range. The North Luzon Trough and the Taitung Trough are the constructed foreac basins with 2 km of orogenic sediments unconformably overlying the volcanic islands and the subduction melange. The Taitung Trough is southward connected to the North Luzon Trough through a bathymetric passage, which was formed by narrowing the North Luzon Trough as a consequence of clockwise rotation of the Lutao volcanic island during the late Pleistocene. A detailed study of the four Neogene tectonostratigraphic units in the Coastal Range shows that the overall history of the formation of the Range can be related to the present-day geologic provinces and the on-going collision processes in the region off southeastern Taiwan. The Range had evolved from a southward accretion of the Shuilien-Loho remnant forearc basin and the Chimei volcanic pair at 2 Ma, followed by the Chengkung-Taiyuan remnant forearc basin and the Chengkuangao volcanic island pair at 1 Ma, and the Taitung Trough and the Lutao volcanic island pair presently in process. By analogy, the northern part of the North Luzon Trough and the Lanhsu volcanic island will be accreted to the Coastal Range in the next million years. The paired forearc basin-volcanic island accretion model proposed in this study may thus have implications for mountain ranges that evolved from similar tectonics of oblique arc-continent collision.

FOCAL MECHANISMS AND TECTONICS IN THE TAIWAN-PHILIPPINE REGION

Seismic activity and focal mechanisms in the vicinity of Taiwan and the Philippines are studied to elucidate the tectonics in this complex region. Epicentral distribution and vertical profiles of earthquake foci support the idea that the entire Philippines is not a part of the Eurasian plate, but another block of lithosphere and the relative motion between the Philippine Sea and the Eurasian plates is shared by the subduction along the two boundaries, west and east of the Philippines. Thrust type of mechanisms are the dominant mode of deformation along the eastern margin of the Philippines; in contrast, no thrust type of solutions are obtained along the western margin of this islands. Between Taiwan and Luzon, mode of plate consumption is most complex. Seismic activity is mostly shallow and diffuse in a 200km wide zone. Reverse faultings along the eastern margin of Taiwan, strike-slip faultings off the southeastern coast of Taiwan, and normal faultings between the Manila trench and the North-Luzon trough are the major mode of deformation. We believe that the region between Taiwan and Luzon constitutes a left-lateral shear zone due to the gradual transition of site of plate consumption from eastern margin of Taiwan to east Luzon. Subduction under the west-facing Luzon arc in this region is likely to be ending now; this might be causing the opening of the sedimentary wedge behind the Manila trench. Normal faultings in this study probably indicate the initial phase or prespreading phase of opening of the area behind the Manila trench.