Philippine Trench Research Articles

Lebensspuren and müllspuren: Drifting plastic bags alter microtopography of seafloor at full ocean depth (10,000 m, Philippine Trench)

Tracks, trails, burrows, faecal castings and other sedimentary structures made by organisms on the deep-seafloor, known as “Lebensspuren”, form a natural and significant component of the microtopography at the sediment-water interface. These structures are often more conspicuous than the errant organisms responsible for them, particularly at hadal depths (>6000 m). Recent explorations of the deepest places in the oceans have revealed the occurrence of long, often parallel tracks running in unnaturally straight lines along the trench floors, erasing lebensspuren. During a crewed submersible dive to 10,000 m deep in the Philippine Trench (west Pacific Ocean), the source of these features was confirmed to be the result of plastic bags drifting in the near-bottom currents. During the 90 min transect of the trench floor, nine individual bags were observed compared to only one benthic holothurian. Assuming the bags are drifting at similar speeds to the current, these tracks, documented here for the first time, that we have coined “müllspuren” are forming up to 12 times faster than hadal holothurians create lebensspuren. The area affected is also much larger as the tracks laid down by plastic bags often consist of multiple, parallel gouges with smoothed intermediate sediment. The ecological effects of particle perturbation and the erasing of lebensspuren is unknown but is becoming ubiquitous at the deepest points in our oceans.

“Double Door” Opening of the Japan Sea Inferred by Pn Attenuation Tomography

AbstractThe extension of back‐arc basins and formation of marginal seas following the subduction of oceanic lithosphere are usually attributed to the rollback of subducting slabs and distorted mantle convection. However, for the Japan Sea, the largest marginal sea in the northwestern Pacific, its opening is unlikely only resulted from the subduction of the Pacific plate because of the coeval Philippine plate subduction and the arcuate arc volcanic zone. Therefore, the present‐day thermal structure in the uppermost mantle, which can be directly constrained by strong Pn‐wave attenuation, plays a vital role in understanding the Japan Sea opening. Here, we observe two belts of strong Pn attenuation beneath the Japan Sea; their strikes are generally consistent with local Pn anisotropy and the retreat directions of the Pacific and Philippine trenches. Hence, there seem to be two divergent mantle flows in the uppermost mantle, pushing a “double door” for the Japan Sea opening.

Mapping Submarine Geomorphology of the Philippine and Mariana Trenches By an Automated Approach Using GMT Scripts

Abstract This paper presents a geospatial analysis of two oceanic trenches using a GMT (Generic Mapping Tools) cartographic method that exploits the scripting approach to visualisation of their geometric shapes. To this end, the research applies the high-resolution datasets GEBCO and ETOPO1 and ETOPO5 for modelling of the submarine relief. This allows taking into account the 2D and 3D shape deviations in the geomorphology of the two selected segments of the trenches by transecting a series of the cross-section profiles. A scripting algorithm of spatial data processing based on the GMT techniques visualised the topography of the submarine objects in 2D and 3D forms and extracted the topographic data from raster grids for statistical analysis of depth using the cross-section transect profiles of both trenches. The bathymetry of the Mariana Trench was evaluated in the southern segment located near the Challenger Deep area, southwest of Guam Island, in comparison with the segment of the Philippine Trench, which was transected in the surroundings of Mindanao Island. The study presented a comparative submarine geomorphic modelling and spatial analysis of the Philippine Sea basin area. The bathymetric analysis of the relief in the Mariana and Philippine trenches showed effective performance of the GMT scripting toolset in advanced cartographic data analysis and visualisation.

Rejuvenated extension of the Philippine Sea plate and its effect on subduction dynamics in the Nankai Trough

AbstractThe Nankai Trough, Japan, is a subduction zone characterized by the recurrence of disastrous earthquakes and tsunamis. Slow earthquakes and associated tremor also occur intermittently and locally in the Nankai Trough and the causal relationship between slow earthquakes and large earthquakes is important to understanding subduction zone dynamics. The Nankai Trough off Muroto, Shikoku Island, near the southeast margin of the rupture segment of the 1946 Nankai earthquake, is one of three regions where slow earthquakes and tremor cluster in the Nankai Trough. On the Philippine Sea plate, the rifting of the central domain of the Shikoku Basin was aborted at ~15 Ma and underthrust the Nankai forearc off Muroto. Here, the Tosa‐Bae seamount and other high‐relief features, which are northern extension of the Kinan Seamount chain, have collided with and indented the forearc wedge. In this study, we analyzed seismic reflection profiles around the deformation front of accretionary wedge and stratigraphically correlated them to drilling sites off Muroto. Our results show that the previously aborted horst‐and‐graben structures, which were formed around the spreading center of the Shikoku Basin at ~15 Ma, were rejuvenated locally at ~6 Ma and more regionally at ~3.3 Ma and have remained active since. The reactivated normal faulting has enhanced seafloor roughness and appears to affect the locations of slow earthquakes and tremors. Rejuvenated normal faulting is not limited to areas near the Nankai Trough, and extends more than 200 km into the Shikoku Basin to the south. This extension might be due to extensional forces applied to the Philippine Sea plate, which appear to be driven by slab‐pull in the Ryukyu and Philippine trenches along the western margin of the Philippine Sea plate.

Intra-trench variations in flexural bending of the subducting Pacific Plate along the Tonga-Kermadec Trench

We conducted a detailed analysis of along-trench variations in the flexural bending of the subducting Pacific Plate at the Tonga-Kermadec Trench. Inversions were conducted to obtain best-fitting solutions of trench-axis loadings and variations in the effective elastic plate thickness for the analyzed flexural bending profiles. Results of the analyses revealed significant along-trench variations in plate flexural bending: the trench relief (W0) of 1.9 to 5.1 km; trench-axis vertical loading (V0) of −0.5×1012 to 2.2×1012 N/m; axial bending moment (M0) of 0.1×1017 to 2.2×1017 N; effective elastic plate thickness seaward of the outer-rise region (\(T_{\rm{e}}^{\rm{M}}\)) of 20 to 65 km, trench-ward of the outer-rise (\(T_{\rm{e}}^{\rm{m}}\)) of 11 to 33 km, and the transition distance of 20 to 95 km. The Horizon Deep, the second greatest trench depth in the world, has the greatest trench relief (w0 of 5.1 km) and trench-axis loading (V0 of 2.2×1012 N/m); these values are only slightly smaller than that of the Challenger Deep (W0 of 5.7 km and V0 of 2.9×1012 N/m) and similar to that of the Sirena Deep (W0 of 5.2 km and V0 of 2.0×1012 N/m) of the Mariana Trench, suggesting that these deeps are linked to great flexural bending of the subducting plates. Analyses using three independent methods, i.e., the \(T_{\rm{e}}^{\rm{M}}\)/\(T_{\rm{e}}^{\rm{m}}\) inversion, the flexural curvature/yield strength envelope analysis, and the elasto-plastic bending model with normal faults, all yielded similar average Te reduction of 28%–36% and average Te reduction area \({S_{\Delta {T_{\rm{e}}}}}\) of 1 195–1 402 km2 near the trench axis. The calculated brittle yield zone depth from the flexural curvature/yield strength envelope analysis is also consistent with the distribution of the observed normal faulting earthquakes. Comparisons of the Manila, Philippine, Tonga-Kermadec, Japan, and Mariana Trenches revealed that the average values of \(T_{\rm{e}}^{\rm{M}}\) and \(T_{\rm{e}}^{\rm{m}}\) both in general increase with the subducting plate age.

Punctuated growth of an accretionary prism and the onset of a seismogenic megathrust in the Nankai Trough

Ocean drilling in the Nankai Trough forearc suggests a new scenario for the evolution of the Nankai subduction zone. Continuous subduction since the Late Cretaceous has been a common tectonic scenario, although the plate subduction was transferred from the Pacific Plate to the Philippine Sea Plate during the Miocene. Seismic reflection studies coupled with drilling have demonstrated that two episodes have controlled the recent evolution of the Nankai forearc: a resurgence of subduction at ~ 6 Ma after cessation since ~ 12 Ma and rapid growth of the accretionary prism since ~ 2 Ma because of the influx of large amounts of terrigenous sediments from the Japan Alps in central Japan. Both episodes were synchronous with large-scale plate reorganizations. The westward subduction of the Philippine Sea Plate initiated both in the Ryukyu and the Philippine trenches at ~ 6 Ma. Rifting in the Okinawa and Mariana troughs started at ~ 6 Ma. Compressive tectonics in northeast Japan started at ~ 3–2 Ma, and resultant mountain building with active surface erosion commenced in central Japan at ~ 2 Ma. This recent compressive tectonic phase might be due to the initiation of convergence of the Amurian Plate with the Okhotsk or North American Plate along the eastern margin of the Japan Sea. In addition to this event, the strong collision and indentation of the Izu-Bonin Arc since ~ 2.5 Ma was also enhanced in central Japan.

Plate Convergence and Block Motions in Mindanao Island, Philippine as Derived from Campaign GPS Observations

We conducted yearlyGlobal Positioning System(GPS) campaigns in the eastern part of Mindanao from March 2010 to March 2014. The obtained station velocities with respect to the Sunda plate (SU) show that WNW motions are dominant due to the convergence of the Philippine Sea plate (PHS). However, it was found that elastic deformations caused by a full coupling of the plate interface down to 80 km could explain a maximum of only 29% of the observed station velocities. In order to interpret the displacement pattern, we applied a rigid block rotation model and determined the Euler vector. As a result, we determined that Mindanao Island could be divided into at least three blocks and that the Philippine fault is one of the block boundaries. Although it was not possible to determine the coupling ratio at the Philippine trench, the dislocation pattern of the Philippine fault showed along-strike variation in Mindanao Island.

Uplifted marine terraces in Davao Oriental Province, Mindanao Island, Philippines and their implications for large prehistoric offshore earthquakes along the Philippine trench

We conducted systematic mapping of Holocene marine terraces in eastern Mindanao Island, Philippines for the first time. Raised marine platforms along the 80-km-long coastline of eastern Davao Oriental Province are geomorphic evidence of tectonic deformation resulting from the westward subduction of the Philippine Sea plate along the Philippine trench. Holocene coral platforms consist of up to four terrace steps: T1: 1–5 m, T2: 3–6 m, T3: 6–10 m, and T4: 8–12 m amsl, from the lowest to highest, respectively. Terraces are subhorizontal, exposing cemented coral shingle and eroded coral heads, while terrace risers are 1–3 m high. Radiocarbon ages, 8080–4140 cal yr BP, reveal that erosional surfaces were carved onto the Holocene transgressive reef complex which grew upward until ∼8000 years ago. The maximum uplift rate is ∼1.5 mm/yr based on the highest Holocene terrace at <11.4 m amsl. The staircase topography and meter-scale terrace risers infer that at least four large earthquakes have uplifted the coast in the past ∼8000 years. The deformation pattern of the terraces further suggests that seismic sources are probably located offshore. However, historical earthquakes as large as M W 7.5 along the Philippine trench were not large enough to produce meter-scale coastal uplift, suggesting that much larger earthquakes occurred in the past. A long-term tectonic uplift rate of ∼1.3 mm/yr was also estimated based on Late Pleistocene terraces.

隆起サンゴ礁が示すマニラ海溝・フィリピン海溝の 巨大地震発生履歴

隆起サンゴ礁が示すマニラ海溝・フィリピン海溝の 巨大地震発生履歴

Kinematics of convergence and deformation in Luzon Island and adjacent sea areas: 2-D finite-element simulation

The Luzon Island is a volcanic arc sandwiched by the eastward subducting South China Sea and the northwestward subducting Philippine Sea plate. Through experiments of plane-stress, elastic, and 2-dimensional finite-element modeling, we evaluated the relationship between plate kinematics and present-day deformation of Luzon Island and adjacent sea areas. The concept of coupling rate was applied to define the boundary velocities along the subduction zones. The distribution of velocity fields calculated in our models was compared with the velocity field revealed by recent geodetic (GPS) observations. The best model was obtained that accounts for the observed velocity field within the limits of acceptable mechanical parameters and reasonable boundary conditions. Sensitivity of the selection of parameters and boundary conditions were evaluated. The model is sensitive to the direction of convergence between the South China Sea and the Philippine Sea plates, and to different coupling rates in the Manila trench, Philippine trench and eastern Luzon trough. We suggest that a change of ±15° of the direction of motion of the Philippine Sea plate can induce important changes in the distribution of the computed displacement trajectories, and the movement of the Philippine Sea plate toward azimuth 330° best explains the velocity pattern observed in Luzon Island. In addition, through sensitivity analysis we conclude that the coupling rate in the Manila trench is much smaller compared with the rates in the eastern Luzon trough and the Philippine trench. This indicates that a significant part of momentum of the Philippine Sea plate motion has been absorbed by the Manila trench; whereas, a part of the momentum has been transmitted into Luzon Island through the eastern Luzon trough and the Philippine trench.

Analysis of crustal deformation in Luzon, Philippines using geodetic observations and earthquake focal mechanisms

We utilize regional GPS velocities from Luzon, Philippines, with focal mechanism data from the Harvard Centroid Moment Tensor (CMT) Catalog, to constrain tectonic deformation in the complex plate boundary zone between the Philippine Sea Plate and Eurasia (the Sundaland block). Processed satellite imagery and digital elevation models are used with existing gravity anomaly, seismicity, and geologic maps to define a suite of six elastic blocks. Geodetic and focal mechanism data are inverted simultaneously to estimate plate rotations and fault-locking parameters for each of the tectonic blocks and faults comprising Luzon. Major tectonic structures that were found to absorb the plate convergence include the Manila Trench (20–100 mm yr − 1 ) and East Luzon Trough (∼ 9–15 mm yr − 1 )/Philippine Trench (∼ 29–34 mm yr − 1 ), which accommodate eastward and westward subduction beneath Luzon, respectively; the left-lateral strike-slip Philippine Fault (∼ 20–40 mm yr − 1 ), and its northward extensions, the Northern Cordillera Fault (∼ 17–37 mm yr − 1 transtension), and the Digdig Fault (∼ 17–27 mm yr − 1 transpression). The Macolod Corridor, a zone of active volcanism, crustal thinning, extension, and extensive normal and strike-slip faulting in southwestern Luzon, is associated with left-lateral, transtensional slip of ∼ 5–10 mm yr − 1 . The Marikina Fault, which separates the Central Luzon block from the Southwestern Luzon block, reveals ∼ 10–12 mm yr − 1 of left-lateral transpression. Our analysis suggests that much of the Philippine Fault and associated splays are locked to partly coupled, while the Manila and Philippine trenches appear to be poorly coupled. Luzon is best characterized as a tectonically active plate boundary zone, comprising six mobile elastic tectonic blocks between two active subduction zones. The Philippine Fault and associated intra-arc faults accommodate much of the trench-parallel component of relative plate motion.

Determination of geoid over South China Sea and Philippine Sea from multi-satellite altimetry data

A new computational procedure for derivation of marine geoid on a 2.5′ × 2.5′ grid in a non-tidal system over the South China Sea and the Philippine Sea from multi-satellite altimeter sea surface heights is discussed. Single-and dual-satellite crossovers were performed, and components of deflections of the vertical were determined at the crossover positions using Sandwell’s computational theory, and gridded onto a 2.5′ × 2.5′ resolution grid by employing the Shepard’s interpolation procedure. 2.5′ × 2.5′ grid of EGM96-derived components of deflections of the vertical and geoid heights were then used as reference global geopotential model quantities in a remove-restore procedure to implement the Molodensky-like formula via 1D-FFT technique to predict the geoid heights over the South China Sea and the Philippine Sea from the gridded altimeter-derived components of deflections of the vertical. Statistical comparisons between the altimeter-and the EGM96-derived geoid heights showed that there was a root-mean-square agreement of ±0.35 m between them in a region of less tectonically active geological structures. However, over areas of tectonically active structures such as the Philippine trench, differences of about −19.9 m were obtained.

The May 17, 1992 earthquakes in southeastern Philippines

Two large earthquakes (Q1 and Q2) occurred off the eastern coast of Mindanao along the Philippine Trench on May 1992. During these events, the southeastern coasts of the island experienced one strong ground shaking and suffered from one set of tsunamis waves. To investigate this phenomenon, teleseismic body wave inversions and tsunami waveform analysis were undertaken for the quakes. Correlation of simulation results and field observations show that Q1 was the most probable source of the damaging tsunami wave. Results also imply that the relation between the Q1 and Q2 that may have ruptured the southern and northern portion of the one‐day aftershock area, respectively. Furthermore, closer examination of the Philippine trench indicates the presence of a bend in between the events that could have prevented the Q1 to rupture in just one big event.

GPS determined eastward Sundaland motion with respect to Eurasia confirmed by earthquakes slip vectors at Sunda and Philippine trenches

GPS measurements acquired over Southeast Asia in 1994 and 1996 in the framework of the GEODYSSEA program revealed that a large piece of continental lithosphere comprising the Indochina Peninsula, Sunda shelf and part of Indonesia behaves as a rigid `Sundaland' platelet. A direct adjustment of velocity vectors obtained in a Eurasian frame of reference shows that Sundaland block is rotating clockwise with respect to Eurasia around a pole of rotation located south of Australia. We present here an additional check of Sundaland motion that uses earthquakes slip vectors at Sunda and Philippine trenches. Seven sites of the GEODYSSEA network are close to the trenches and not separated from them by large active faults (two at Sumatra Trench, three at Java Trench and two at the Philippine Trench). The difference between the vector at the station and the adjacent subducting plate vector defines the relative subduction motion and should thus be aligned with the subduction earthquake slip vectors. We first derive a frame-free solution that minimizes the upper plate (or Sundaland) motion. When corrected for Australia–Eurasia and Philippines–Eurasia NUVEL1-A motion, the misfit between GPS and slip vectors azimuths is significant at 95% confidence, indicating that the upper plate does not belong to Eurasia. We then examine the range of solutions compatible with the slip vectors azimuths and conclude that the minimum velocity of Sundaland is a uniform 7–10 mm/a eastward velocity. However, introducing the additional constraint of the fit of the GEODYSSEA sites with the Australian IGS reference ones, or tie with the NTUS Singapore station, leads to a much narrower range of solutions. We conclude that Sundaland has an eastward velocity of about 10 mm/a on its southern boundary increasing to 16–18 mm/a on its northern boundary.

Regional variation in surface wave group velocities in the Philippine Sea

Group velocities of Rayleigh and Love waves have been measured within the period range 30–80 s to study the regional variation of seismic wave velocity structures of the crust and upper mantle beneath the Philippine Sea. These group velocity measurements were made by applying the moving window method to long-period seismograms recorded at six WWSSN stations, which are located around the Philippine Sea plate. We divided the Philippine Sea region into seven small areas on the basis of tectonic features, age and topography of ocean floor, and determined the group velocities of the Rayleigh and Love waves in each region as a function of period. The surface wave group velocities in the Philippine Sea plate are higher than those in tectonic regions such as the Mariana arc, the Ryukyu-Japan arc and the Philippine trench. The low group velocities in tectonically active regions are attributable to the presence of an extremely low velocity layer in the upper mantle. In the Philippine Sea plate, the group velocities in the western region (the west Philippine basin) are higher than those in the eastern region (the Shikoku-Parece Vela basin). This result is interpreted as being due to the difference in the lithospheric thicknesses of the two regions. The lithospheric thickness estimated from the group velocity data of the Rayleigh wave is about 55 km in the western region and 40 km in the eastern region. The northern part of the west Philippine basin, covering the Amami plateau, the Daito ridge and the Oki-Daito ridge, has a thick crust, which is needed to explain the fact that the short-period group velocities are lower than those in the western and eastern regions of the Philippine Sea plate. These features are commonly found in the seismic velocity structures inferred from the group velocity data of Rayleigh and Love waves.

Oblique plate convergence, slip vectors, and forearc deformation

Slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis, suggesting that oblique convergence is taken up by partial decoupling. Decoupling means that a component of arc‐parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench. Partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike‐slip faults that are parallel to the trench and to each other and, starting with a force equilibrium condition, a relationship between the obliquity and the earthquake slip vector orientation is derived. Assuming that either fault slips when shear stress on it reaches a yield stress, oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle. For obliquity at or greater than this angle the stress on the strike‐slip fault is large enough to start it slipping, and when both faults are active, the arc‐parallel motion of the forearc deflects the slip vector back toward the trench‐normal. If we assume that continued slip on either fault occurs at constant stress (but the two faults can be at different stresses), the slip vector will maintain a constant angle relative to the trench‐normal even when obliquity is larger than the critical angle. This limiting angle of the slip vector, called ψmax (measured relative to the trench‐normal), is simply the arcsine of the ratio of the shear forces resisting slip on the strike‐slip and thrust faults. A consequence is that when the obliquity exceeds ψmax the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates. Slip vectors at the Java trench southwest of Sumatra show the relationship clearly with ψmax =20°±5°, while slip vectors at the Aleutian trench show the relationship less clearly with ψmax=25° to 45°. The greater angle at the Aleutian trench suggests that the upper plate is stronger in the Aleutian arc (relative to the thrust fault) than in the Sumatran arc, consistent with the Sumatran arc being continental and having a well‐developed strike‐slip fault while the Aleutian arc is oceanic and without a clear transcurrent fault. Slip vectors at the Philippine trench which, like Sumatra, has a large strike‐slip fault inboard of it, tend to stay within 25° of the trench‐normal when obliquity is as large as 50°. If obliquity exceeds ψmax and continues to increase along a subduction zone, the rate of motion of the forearc relative to the upper plate will vary with obliquity, in which case the forearc sliver should extend or contract parallel to the arc. From the geometry of modern island arcs, arc‐parallel extension should be the more common and has been hypothesized for both Sumatra and the Aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations. From estimates of ψmax and the arc‐parallel gradients in obliquity, arc‐parallel strain rates are estimated to be 1 to 3×10−8/yr for the Sumatran forearc, 2 to 6×10−8/yr for the Aleutian forearc, and 0.3 to 3×10−8/yr for the Philippine forearc. Oblique convergence and subsequent arc‐parallel extension, if accompanied by crustal thinning, may provide an important yet little appreciated mechanism for bringing high‐grade metamorphic rocks to the surface of subduction complexes.

Seismicity and stress state in the South China Sea, Indochina and their vicinity

The distribution of earthquakes from 1973 to 1982 in the South China Sea, Indochina and their vicinity was studied using the data from I. S. C. It was found that the earthquakes are mainly concentrated along the boundaries of plates. Beside, some of shallow eartqhuakes are distributed in west part of Burma and the boundary between Burma and China, a few of earthquakes occurred in South China Sea. The features of Benioff zone along the boundaries between India plate, Philippine Sea plate and Eurasia plate were studied. The plate do not coupled well under the Java trench and the Philippine trench. The subducted India plate under Burmese range, Andaman—Nicobar arc moves NNE. The fault plane solutions of earthquakes were studied using the first motions of P wave. The stress state on subduction zones and within the area were deduced from the fault plane solutions and the fault movement. It was found that the direction of principal compression axis of stress is in the NNE in west part of Burma, in S—N in south and middle part of Bruma and Thailand, and in NNE or S—N in the South China Sea. It was also found that the stress state has close relation with the interaction of plates.

The Manokwari Trough and the western end of the New Guinea Trench

The New Guinea Trench is a sparsely surveyed seafloor depression which runs parallel to the north coast of New Guinea for a distance of some 700 km. Its western end lies about 600 km east of the Philippine Trench; the intervening region is occupied by a complex series of north trending ridges and troughs, of which the Ayu Trough and the Tobi and Mapia ridges are the most prominent. It has been suggested and is now widely accepted that the trench marks a site of subduction, but the degree of present‐day activity is disputed. The western termination has recently been defined by combined single‐channel seismic and long range sidescan sonar surveys as lying at the ridge system which culminates in Mapia Island. The trench contains as much as 1 km of flat‐lying undisturbed sediments within the area surveyed, and the southern slopes are extensively channeled, suggesting a lack of recent deformation. The same survey has also shown that for a distance of some 400 km west of the western end of the trench, the north coast of New Guinea is flanked, at a distance of only a few tens of kilometers, by a deep trough. The sonar imagery of this “Manokwari Trough” suggests recent convergence as well as transcurrent movement. Both the existence of this trough and the abrupt termination of the New Guinea Trench are consequences of the seafloor spreading which took place in the Ayu Trough after subduction had ceased at the trench.

The western Central Luzon volcanic arc, the Philippines: two arcs divided by rifting?

The western Central Luzon arc is apparently a complex zone of volcanism and tectonism. The volcanoes of the northern segment of the arc (the Bataan arc, BA) fall along two semi-parallel lineaments, the Western Bataan Lineament (WBL) and the Eastern Bataan Lineament (EBL). This northern segment is cut off from the southern extension of the arc by a NE-SW “cross arc” zone of volcanism (the Macolod Corridor, MC). The volcanoes of the southern segment of the arc (Mindoro arc, MA) fall along two parallel lineaments, the Western Mindoro Lineament (WML) and the Eastern Mindoro Lineament (EML). Samples collected from the MC, EBL, and MA are alkali (and LIL) enriched compared with the WBL. The enrichment of the EBL is believed to be the result of the release of smaller amounts of hydrous-rich fluids within the mantle below the EBL than below the WBL. This process generated smaller degrees of partial melting below the EBL (as compared to the WBL) which resulted in the higher concentrations of LIL elements in the EBL magmas as compared to the WBL magmas. The alkali and LIL enriched nature of the MC is believed to be the result of small degrees of partial melting during rifting. The enriched nature of the MA is thought to be due to slivers of crustal material from the Palawan-Mindoro Block being incorporated into the mantle during subduction. This region has the highest radiogenic Sr values found in the Philippines. The MC may represent a pull-apart rift zone between the Philippine and West Luzon shear zones and the transform between the Manila and Philippine Trenches.

ルソン島中部におけるフィリピン断層の第四紀後期の断層運動

The Philippine fault zone is one of the most remarkable active faults in the world. It streches on the Philippine archipelago and runs parallel with the Philippine trench. It can be traced for more than 1, 200 km from Luzon Island to the southwest, through Masbate Island, Leyte Island and Mindanao Island (Fig. 15). In central part of the Luzon Island ma ny geomorphic features of late Quaternary displacement are recognized along the fault zone.The aim of our study is to clarify the nature of the fault zone in central Luzon Island on the basis of the interpretation of topographic maps, vertical aerial photographs and detailed field investigations.The Philippine fault zone extends with high certainty in the direction of SW-NE more than 100 km long from Dingalan Bay to Lingayen Gulf. It runs across the Shiera Madre Mountains in the southern part. In the northern part of the fault zone, it separates the Cordillera Central Range on the north-eastern side from the Central Valley with the elevation less than 100 m on the southwestern side.The Philippine fault zone is divided into three fault systems based on the continuity of the fault traces and the strike of the faults. The first system extends from Dingalan Bay to Digdig (fault length ca. 90 km), the second is northeast of Lupao and Umingan (ca. 20km), and the third is San Manuel and its surrounding (ca. 30km) (Figs. 1 and 2).The main results obtaind follow ;1) In the study area, various fault topographic features develope along the Philippine fault zone. The southern end of it, from Dingalan Bay to Gabaldon, some lineaments and a number of fault sag develope along it (Fig. 3). The features in the southern part between Gabaldon and Rizal are characterized by displacements of alluvial fans, river terraces and a few offset streams along or near the foot of the Sierra Madre Mountains (Figs, 4, 7 and 8). In the northern part, between Lupao and San Manuel, fault displacements appear sharply just along the foot on the Cordillera Central Range. The fault features are offset streams and fault scarplets on alluvial fans and river terraces (Figs. 9, 10 and 12).2) The dip of fault plain is vertical (Fig. 6) or high as 60°E on fault outcrops.3) The sence of horizontal displacement is sinistral in the study area. Vertical one is variable in the southern part, but in the northern part it is downthrown to southwest side (Fig. 13).4) The ratio of horizontal and vertical displacements id deduced from clear breaks on geomorphic surfaces. While it is 1 : 1 to 3 : 2 in the southern part, but it becomes 8 : 1 in the northern part (Table 1).5) The average rate of horizontal displacements along the fault is estimated to be 1.5 5mm/year based on the relation between values of horizontal offsets and upstream lengths from the fault. The vertical one which is estimated from the proportion of the vertical displacement to horizontal displacement is calculated to be 0.190.63 mm/year (northern part), and 1.03. 3 to 1.5 5.0mm/year (southern part).6) It seems that the last earthquake along the fault in the study area took place in 1645 A.D. based on the 14C age data of plant fossil samples which were collected from terrace deposits in some places and the catalogue of historical earthquakes (REPETTI 1946). The magnitude of the earthquake is estimated to be a 8 class and the fault dislocation should appear along the fault.7) The recurrence interval of earthquakes is calculated as 1, 6005, 300 years from the average slip rate and the displacement of the last earthquake.8) The fault features and activity of the Philippine fault zone are remarkable, and its nature is similar to that of the faults which are located on other island arcs in the world.