Benioff Zone Research Articles

On the clustering of seismicity in the Southern Tyrrhenian area

High-quality shallow, intermediate and deep shocks from the Southern Tyrrhenian area (interval: 1985-1995) are analysed to draw inferences on the complex shape of the underlying Benioff zone. The hypothesis of an active NW-oriented subduction of the lithosphere, generated and stressed by the opening of the Tyrrhenian basin, is confirmed. The fractal behaviour of seismic data in its five-dimensional set (magnitude, time and space) is significantly identified, even if within specific ranges. The light but significant variations of the different fractal dimensions measured during more recent years is explained in terms of a reduction in the fracturing degree of the Southern Tyrrhenian crust.

Microseismicity evidence for subduction of the Caribbean plate beneath the South American Plate in northwestern Venezuela

Over 1100 microearthquakes with body wave magnitude mb<4 have been located in western Venezuela and the southwestern Caribbean region since the installation in 1980 of the Venezuelan Seismological Array, together with 120 events of mb≥4, one of them with surface wave magnitude Ms∼6. This tectonically complex region is part of the boundary between the Caribbean and the South American plates. The main seismically active feature inland in western Venezuela is the northeast striking, 600‐km long, 100‐km wide, right‐lateral strike‐slip Boconó fault zone along the Venezuelan Andes. About 80% of the earthquakes located in the entire region in the period 1980‐mid‐1995 have occurred on this fault zone, at focal depths <20 km. Microearthquake activity at lower rates also occurs northwest of the Venezuelan Andes, both in the continental and Caribbean sea regions. Part of this activity takes place at depths down to ∼150 km. Northwest oriented seismicity depth profiles show the existence of a Benioff zone dipping to the southeast beneath northwestern Venezuela and northern Colombia. This indicates the presence of a northeast striking, southeast dipping subducted slab of the Caribbean plate beneath the South American plate. Hypocentral locations show that the northeastern end of this subduction occurs northwest of the Curaçao‐Aruba region, in the vicinity of a northwest trending, right‐lateral strike‐slip fault zone that joins up with the northeastern end of the Boconó fault zone. This latter place turns out to be the western end of the east‐west striking San Sebastián fault along the Venezuelan coast.

A reassessment of the deep Fiji earthquake of 26 May 1932

The 1932 deep Fiji earthquake is studied from the point of view of comparing it to the recent large events in the transition zone. Spectral analysis and body-wave modeling suggest a moment of 3.4 × 10 27 dyn cm (within a multiplicative or divisive factor of 1.5), which makes it comparable to, but not significantly larger than, the great 1994 earthquake, 770 km to the north. Its seismic moment remains well below that of the largest deep shocks (1994 in Bolivia or 1970 in Colombia). Relocation efforts show that the earthquake took place on the front edge of the Wadati-Benioff Zone, in an area where the latter features a complex geometry involving significant warping. The mechanism of the earthquake involves down-dip compression and can be readily explained in terms of the prevailing large-scale stresses in the slabs.

Relocation of earthquakes in the Cook Inlet area, south central Alaska, using the joint hypocenter determination method

AbstractThe Joint Hypocenter Determination (JHD) method was used to relocate 1604 earthquakes that occurred during 1988 to 1993 in the Wadati-Benioff zone (WBZ) beneath the Cook Inlet in south central Alaska. The study utilizes both P- and S-wave arrivals recorded by a recently improved regional seismic network, and it provides an improvement over previous studies. The JHD locations of the events allowed improved delineation of the geometry of the subducted slab and the detection of a double seismic zone within the WBZ in the 40- to 100-km-depth range. The JHD station corrections obtained correlate with the major surface geological features in the study area (slow-velocity sedimentary basins) as well as with crustal and mantle structure (high-velocity subducting slab and low-velocity anomalies beneath the active volcanoes). To investigate the dependence of the JHD relocation procedure on factors such as the hypocentral depths of the events used, the velocity model, the νp/νs ratio, and random and systematic errors in travel-time readings, synthetic travel times were calculated using an exact raytracing technique, a three-dimensional tomographic P-wave velocity model, the present network configuration, and the actual seismicity distribution. Then, the synthetic events were located with the JHD algorithm. The synthetic experiments showed that the JHD procedure is rather insensitive to the choice of plane-layered velocity model and νp/νs ratio, preserves relative hypocenter locations well, and can handle systematic and large random reading errors effectively.

Shiveluch volcano: seismicity, deep structure and forecasting eruptions (kamchatka)

The deep structure, Wadati-Benioff zone (focal zone) geometry and the magma feeding system of Shiveluch volcano are investigated based on 1962–1994 detailed seismic surveillance. A focal zone beneath Shiveluch is dipping at an angle of 70° at depths of 100–200 km. Based on the revealed interrelations between seismicity at depths of 105–120 km and an extrusive phase of its eruptions in 1980 through 1994, it is inferred that primary magmas, periodically feeding the crustal chamber, are melted at depths of at least 100 km. An upsurge of extrusive-explosive activity at the volcano is preceded and accompanied by the increasing number and energy of both volcanic earthquakes beneath the dome and tectonic or volcano-tectonic earthquakes in the zones of NW-striking crustal faults near the volcano.The eruption of April 1993 has been the most powerful since 1964. It was successfully predicted based on interactive use of all seismic data. At the same time the influence of seismicity at depths of 105–120 km under the volcano on the style (and consequently on prediction) of its activity is decisive.

Tectonic control of the subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile

Near the latitude of Valparaiso, Chile, a fundamental change in configuration of the Benioff Zone, volcanic arc activity, and the structure of the continental margin occurs opposite the subducting Juan Fernández Ridge. Upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two‐degree segment offshore Valparaiso, extending from the shelf edge seaward across the eastern end of Juan Fernández Ridge. Near the O'Higgins group of seamounts, the Juan Fernández Ridge strikes northeast rather than continuing its east‐west trend across the Pacific Basin. The ridge uplifts the upper plate, and sediments of the Valparaiso Basin are deformed against its southern flank. This deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench‐normal vector of plate convergence. In the trench axis, the ridge forms a basement barrier behind which sediments 2.5 km deep have ponded. The lower slope over the ridge appears eroded, whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide. Nazca Plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust; its relation to deeper processes segmenting the Andean Orogen appears to involve prior tectonic events.

Temporal variations of the coda decay rate on Kamchatka: Are they real and precursory?

Temporal variations of the decay rate of the coda of local earthquakes are studied for four Kamchatka stations. To suppress any bias caused by the coda Q versus lapse time dependence, I use the deviation of an individual coda decay function from the empirical reference decay function, and the analysis is done over a fixed lapse time window. For each individual coda record, a decay rate parameter “α” is determined by the following procedure. First, the reference log coda decay function is subtracted from an individual one giving a residual function; second, the slope of this residual versus lapse time is determined over the fixed lapse time window giving an α estimate. The data used are records of 150–250 Kamchatka earthquakes of 1967–1990 (ML=3.5–5) obtained by four stations employing three‐component 1‐s seismographs. The processed data show apparent temporal variation of α with high statistical significance. Several possibilities are then investigated of mimicking the genuine temporal variation of α by systematic variation of other parameters, first of all epicenter wandering and nodal plane rotation. This analysis does not reveal any strong bias and thus suggests that the observed variation is genuine. Then I show the significance of five apparently precursory anomalies identified retroactively. Also, I describe briefly the real‐time prediction experiment conducted on Kamchatka for 1982–1990 using coda decay, aimed at the intermediate‐term forecasting of large Benioff zone earthquakes. This experiment resulted in the successful forecast of the August 17, 1983, Mw=7.0 event with an accuracy of 2 months in time and 100–200 km in location; magnitude was overestimated by 0.5. Later, a false alarm was also issued. Thus the experiment confirms the reality of the coda decay rate precursor but also shows that it needs further improvement to become reliable. Physically, the variations of the coda decay rate are associated with time‐dependent local variations of scatterer density in the lithosphere.

P and S deep velocity structure of the Hellenic area obtained by robust nonlinear inversion of travel times

We derived a new model for the P and S velocity structure of the lithosphere in the subduction area of the Hellenic arc and the Aegean Sea from the inversion of travel times of local events. The inversion technique applied is nonlinear, since three‐dimensional ray tracing is incorporated. At the same time, an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results. The P‐S coherency is controlled by an additional damping of the VP/VS ratio. The study focuses mainly on the structure of the Hellenic subduction in the southern Aegean region. Interesting features and details of the subducted slab can be recognized in the final tomographic images. On the western part, the shallower section of the slab is dipping at a very small angle (≃10°). This changes to approximately 25° for the deeper part of the subduction, resulting in a prominent kink at a depth of about 70 km, which is in accordance with the general characteristics of the associated Benioff zone. The smaller eastern part of the slab is not resolved with the same detail, but a clearly steeper subduction zone can be recognized. Moreover, detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments. Finally, the results confirm the recent suggestion concerning the existence of a low‐velocity crustal layer at shallow depths (≃10–15 km) in the accretionary prism of the Alpine belt.

Mapping the continuity of the Nazca Plate through its aseismic part in the Arica Elbow (Central Andes)

Along the Peru-Chile trench, the subduction of the Nazca Plate under South America is well underlined by the Wadati Benioff zone. In the Central Andes, the seismicity defines a moderately deepening slab down to about 325 km. The occurrence of a scarce seismicity in the 550–650 km depth range, after a complete seismic quiescence, raises the question of the continuity of the slab. To answer this question we performed a 3-D teleseismic tomography of the mantle at 20°S. On the tomographic image, a higher velocity zone is associated with the subducted plate where it is defined by the Wadati Benioff zone. At greater depth, this higher velocity body can be followed through the model down to the lower mantle. Our results provide clear evidence of the continuity of the slab throughout its seismically quiescent part. Moreover, they allow the imaging of the bending of the Nazca Plate in the Arica Elbow, where the general strike of the Andean chain changes and no deep seismicity has never been recorded. Finally, our results confirm the increase of the dip of the slab between 350 and 550 km in depth.

Details of stress directions in the Alaska subduction zone from fault plane solutions

We used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions. We propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses. Using a synthetic data set of 50 fault plane solutions, composed of two halves corresponding to two incompatible stress tensors, we tested the validity and efficacy of the cumulative misfit method. Our acceptance criteria for results of stress tensor inversions are (1) the directions must be well constrained, that is, the 95% confidence regions of the greatest and least principal stresses do not overlap and (2) there must be evidence for homogeneity in the sample, that is, the average misfit of the inversion is less than 6°. We estimated stress directions in the Alaska Wadati‐Benioff Zone (WBZ) using 470 fault plane solutions, which we determined for earthquakes with ML∼3, and 67 published fault plane solutions for earthquakes with Ms∼5. We succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets, with the average misfit ranging from 3.2° to 5.5°. The average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set (F∼10°) by factors of 2 to 3. We estimated the stress fields at two scales along the Alaska WBZ. The stress directions measured by the large earthquakes (Ms∼5) were homogeneous over large volumes, with extension downdip and the direction of greatest compression along strike. This unusual orientation of the greatest principal stress is attributed to the bend of the slab under central Alaska, which generates compressive stresses along strike. The stress orientations revealed by small earthquakes, in contrast, exhibited a great deal of heterogeneity as a function of space, although their general trend confirms the overall stress directions obtained from the large events. We propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50. The estimated stress directions of the crustal earthquakes corresponded to the following mechanisms: (1) strike‐slip faulting with the greatest principal stress oriented N‐S near Fairbanks and (2) thrusting with the greatest principal stress oriented NW‐SE near Mount McKinley.

Aligned buoyant highs, across-trench deformation, clustered volcanoes, and deep earthquakes are not aligned with plate-tectonic theory

Bathymetry shows the regional interaction of aseismic, buoyant highs in northern Pacific subduction zones. Seamounts, ridges, and fractures on the seaward side of the trench are associated with events that do not support the accepted plate-tectonics paradigm, including an altered slab dip angle (Benioff zone) and the clustered volcanoes and earthquakes within the convergent margin. Most of the examples in this study show a reduction in the number of total earthquakes but an increase in the deeper earthquakes, an abnormal amount of across-trench deformation, and a larger amount of volcanism on the active arc than if no bouyant highs existed in the subduction zone. The connections between the seaward highs and the landward clustered highs are the transverse faults, which widen by turbidite scour as they age. Forearc canyons are the modern-day bathymetdc expression of these faults. All of the parameters introduced disagree with the plate-tectonic hypothesis, making an alternate explanation for the genesis necessary. That explanation falls into the realm of the surge-tectonic hypothesis, which can explain by fluid mechanics and eastward flow each of the introduced parameters.

Relationships among the tectonic components of the island arcs and their plate tectonic implications

Relationships among the tectonic components of the simple (non-circum Philippine plate) east-facing island arcs of the western Pacific are as follows: positive relationship between strike (strike of the subduction zone) and latitude (Euler and geographic), radius of curvature and latitude, slab dip and latitude, radius of curvature and slab dip, strike and radius of curvature, dip and strike of the slab, and negative relationship between the dip of the slab and convergence rate. In the east-facing island arcs, the slab dip is inversely related to the convergence rate, whereas in subduction zones which are from various orientations, a similar relationship has not been recognized. Plots of the latitudinal variation of tectonic components of the island arcs as proposed in this study can be used to forecast the geographic position of the possible arc-trench systems in the tectonically unknown regions of the western Pacific. These plots can also be used to forecast the geometrical and kinematic characteristics of the newly explored subduction zones in this region. In regions where the slope of Benioff-Wadati zone is known, plot of slab dip vs convergence rate proposed here, can be used to estimate the rate of convergence. Variations in radius of curvature with convergence rate indicate that convergence rate is another controlling factor which determines the radius of curvature, and provides a means for estimating the former convergence rate of extinct arcs.

Cocos-Nazca slab window beneath Central America

Integration of petrologic and tectonic data favours a model of slab window formation beneath Central America in the Pliocene-Pleistocene. Central America has been the site of voluminous Cenozoic arc volcanism. The Cocos and Nazca plates, which are subducting beneath Central America, are diverging along the east-trending Cocos-Nazca spreading ridge. Since 25 Ma the Americas have advanced about 1800 km west over the ridge-transform system. Since at least 8 Ma, plate integrity and the ridge-transform configuration have been preserved during convergence, resulting in subduction of the spreading ridge and development of a slab window. The Panama fracture zone, an active transform fault, is the part of the ridge-transform system currently being subducted. The ridge-transform system formerly adjoining the northern end of the Panama fracture zone is likely to have been left-stepping. We use present-day plate motions to design a slab window to fit known variations in igneous composition, hypocentre distribution, and mantle anisotropy. The modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 Ma. Cessation of ridge subduction occurred between 1 and 3 Ma, when subduction of the Panama fracture zone is considered to have begun. The slab window is continuing to expand and migrate northeastward below the Central American volcanic arc. The absence of a Wadati-Benioff zone from southeastern Costa Rica through Panama corresponds to the position of the slab window. Within this region, dacitic and rhyolitic volcanic rocks have “adakitic” compositions, and are thought to result from anatexis of the young, buoyant crust which forms the trailing edges of the slabs bounding the window. Basalts in this area were derived from an enriched ocean-island type mantle source, whereas basalts from the rest of the arc, in Nicaragua, El Salvador and Guatemala, are mainly derived from slab-modified depleted mantle, characteristic of volcanic arcs. The presence of ocean-island type mantle beneath southern Costa Rica and Panama is explained by eastward flow of enriched asthenosphere from the Galapagos plume-head through the slab window and into the volcano source region. Eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the Nazca and Caribbean plates. The flow of peridotite is a consequence of progressive shrinkage of the Pacific mantle reservoir and concurrent expansion of the Atlantic mantle reservoir.

On the interpretation of large-scale seismic tomography images in the Aegean sea area

A review of the interpretations given to large-scale seismic tomography anomalies in the Aegean area indicates a strong contradiction between these interpretations and the traditional ones based on seismic and tectonic data: 1) the tomographic lithospheric slab penetrates down to at least 400 km which doubles the maximum depth of the Benioff zone seismicity; 2) the resulting minimum time estimates for the subduction process duration at least doubles the seismotectonically calculated maximum time of 13 Ma for the subduction initiation. An alternative interpretation is proposed: the tomography detects not only the South Aegean active Benioff zone but also large remnants of lithospheric material subducted in two phases beneath the Aegean during the Miocene. Given that the geometric features of the paleosubductions are compatible with the features of the tomographic anomalies, it is suggested that the latter represent cool, non-assimilated lithospheric remnants. Then, no contradiction any longer exists and both seismotectonic and tomographic data are well explained.

Morphology of the subducted Indian plate in the Indo-Burmese convergence zone

Earthquake hypocenters and travel time residuals have been analysed to constrain the geometry and physical state of the subducted Indian plate in the Indo-Burmese convergence zone. A critical analysis of earthquake hypocenters reveals the existence of a non-uniform Benioff zone, progressively shortening from north to south. The deepest level of seismicity is observed beneath the Naga hills (160 km) followed by that under the Chin hills (120 km) and Arakan-Yoma ranges (80 km). The region seems to be devoid of moderate sized shallow (< 40 km) earthquakes. Differential travel time residuals from pairs of shallow and intermediate depth earthquakes recorded at teleseismic distances show significantly faster travel time (up to l.2s) in the north-northeast and south-southwest azimuths, whilst slower arrivals (1.2 to 1.5 s) are recorded in the transverse direction. This observation points to the presence of a high velocity slab possibly linked to the subduction of the Indian oceanic lithosphere.

Large seismic faults in the Hellenic arc

Using information concerning reliable fault plane solutions, spatial distribution of strong earthquakes (Ms³ 6.0) as well as sea bottom and coastal topography, properties of the seismic faults (orientation, dimension, type of faulting) were determined in seven shallow (h &lt; 40 km) seismogenic regions along the convex part of thc Hellenic arc (Hellenic trench) and in four seismogenic regions of intermediate depth earthquakes (h = 40-100 km) along the concave part of this arc. Except for the northwesternmost part of the Hellenic trench, where the strike-slip Cephalonia transform fault dominates, all other faults along this trench are low angle thrust faults. III thc western part of the trench (Zante-west Crete) faults strike NW-SE and dip NE, while in its eastern part (east Crete-Rhodos) faults strike WNW-ESE and dip NNE. Such system of faulting can be attributed to an overthrust of the Aegean lithosphere on the eastern Mediterranean lithosphere. The longest of these faults (L = 300 km) is that which produced the largest known shallow earthquake in the Mediterranean area (21 July 365, Ms = 8.3) which is located near the southwestern coast of Crete. The second longest such fault (L = l 70 km) is that which produced a large earthquake (December 1303, Ms = 8.0) in the easternmost part of the trench (east of Rhodos island). Both earthquakes were associated with gigantic tsunamis which caused extensive damage in the coast of many Eastern Mediterranean countries. Seismic faults of the intermediate depth earthquakes in the shallow part of the Benioff zone (h = 40- 100 km) are of strike-slip type, with a thrust component. The orientations of these faults vary along the concave part of the arc in accordance with a subduction of remnants of all old lithospheric slab from the convex side (Mediterranean) to the concave side (Aegean) of thc Hellenic arc. The longest of these faults (L = 220 km) is that which produced the largest known intermediate depth earthquake in the whole Mediterranean area (12 October 1856, M = 8.2) north of Crete. The second longest such fault (L = 160 km) produced a large earthquake (26 June 1926, M = 8.0) in the easternmost part of the concave part of the arc (near Rhodos). Both earthquakes caused very serious damage in several Eastern Mediterranean countries but were not associated with tsunamis.

The Andean subduction zone between 22 and 25°S (northern Chile): precise geometry and state of stress

One year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the Andean subduction between 22 and 25°S in the northern Chile seismic gap. A sharp image of the Wadati-Benioff Zone (WBZ) is obtained down to 270 km in depth. A seismically quasi-quiescent zone is observed in the WBZ below the volcanic arc, between 150 and 210 km in depth. Hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the WBZ image at depth greater than 100–150 km in depth. No shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the Atacama Fault. The stress field and the characteristics of faulting along the subducted slab are investigated. Underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth (Locked zone). Downdip, intra-slab normal faulting prevails (Tensile zone), but some strike-slip faulting is observed. A transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous NNW- to NW-oriented fault plane is found at about 80 km in depth. It is found that the stress axes σ 1 and σ 3 in the Locked zone are oriented in the convergence direction (75–80°E). Downdip, in the tensile zone, σ 3 has a mean azimuth 60–65°E. There, the slab is hence submitted to a tensional force (slab pull) oblique relatively to the convergence. The transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental Moho encounters the Wadati-Benioff Zone, suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface. The pre-seismic state of this segment of the Andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the Wadati-Benioff Zone.

Estimates of stress directions by inversion of earthquake fault-plane solutions in sicily

SUMMARY The tectonic stress orientation is estimated in the lithosphere of northern Sicily, the southern Tyrrhenian sea and southern Calabria, and in the Wadati-Benioff zone below the Tyrrhenian, by inversion of fault-plane solutions of earthquakes covering a magnitude range from 2.5 to 7.1. Focal mechanisms of 97 earthquakes are taken from the literature, after a critical evaluation of their data quality. An average misfit of F= 13 indicates that the set of all shallow (<50 km) earthquakes is generated by a heterogeneous stress field. For three subsets, based on regional and magnitude separation, F was small enough (2.8lFs5.9) to support the assumption of a homogeneous stress direction; for an additional subset, with F= 7.4, such a condition is close to being fulfilled even though some heterogeneity appears to be present. The number of earthquakes in these subsets ranged from nine to 22, and the uncertainties of the principal stress directions were generally of the order of 20 at the 90 per cent confidence level. The earthquakes with A425 define a regional stress field with the greatest principal stress, ul, dipping at a shallow angle to the south. In north-eastern Sicily and south-western Calabria the stress field estimated by earthquakes is extensional, with o3 in a direction of WNW, and a near-vertical ol, in agreement with the graben tectonics mapped geologically in this area. In western Sicily the c1 direction is oriented WNW, but this result is judged less reliable than the others, based on the broader confidence limits of the solution and the average misfit of 7.4. The earthquakes in the Wadati-Benioff zone define ol dipping at about 70 to the NW, subparallel to the zone, with o2 horizontal and striking parallel to the zone.

New data on the geodynamics of southern Peru from computerized analysis of SPOT and SAR ERS-1 images

Several geological structures are extracted from SPOT-Panchromatic and ERS-1 radar images of the Central Andes (Arequipa area, Southern Peru). Our new methodology of numerical image processing is an aid for identification of the structures. This numerical approach helps to locate, characterize and determine the amplitude of the vertical offset of structures. The results show that the NW-SE-trending major faults are principally normal with a small sinistral strike-slip component. The open NE-SW faults are tension gashes. Extension in the studied area trends NE-SW and the major NW-SE-striking faults form an extensional “en échelon” pattern including an active volcanic zone. These new kinematic results lead to a regional geodynamic model which takes into account oblique convergence between Pacific and South America Plates within the Wadati-Benioff zone of the Peru trench.

A double island arc between Taiwan and Luzon: consequence of ridge subduction

Analysis of geomorphological, geochronological, geochemical and geophysical features in the segment of the Taiwan-Luzon Arc between Taiwan and Luzon (the Bashi Segment) allows the recognition of a double arc structure. The two volcanic chains are separated by 50 km just north of Luzon (18°N), and converge near 20°N. Islets in the western chain are older and largely composed of volcanic rocks of Miocene to Pliocene age. They all show low relief, lateritic platforms and wave-cut terraces, and are covered by massive recrystallized limestone. In contrast, all active volcanoes in this segment of the Taiwan-Luzon Arc belong to the eastern chain, where most islets are Quaternary in age. The volcanoes have well-developed cone shapes, and well-preserved deposits of near-vent facies. Magmas of the eastern chain have higher K Si, (La) n,(La/Yb) n, and lowerε Nd than their counterparts at the same latitude in the western chain. Therefore, the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain. Moreover, the available seismological data seem to suggest an abrupt increase of the dip angle from 30° at 18°N to 80° at 20°N. Thus, the double arc structure is located in the region where the Benioff zone suddenly changes. In analogy with the Lesser Antilles Arc, we propose a geodynamic model in which the double arc in the Bashi Strait is the tectonic manifestation of the subduction of the aseismic Scarborough Seamount Chain, the extinct mid-ocean ridge of the South China Sea. Before that ridge reached the Manila Trench, the western chain was the volcanic front. When the ridge reached the subduction zone at 5–4 Ma, its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity. Furthermore, this ridge-arc collision was probably also responsible for regional uplift causing extensive sub-aerial weathering and erosion as well as massive reef formation in the western chain. When subduction started again, the dip angle became shallower in response to the extra buoyancy of the downgoing ridge. If the depth of magma generation remained constant, the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain. Moreover, we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest, thus imprinting a geochemically more enriched signature on the magmas of the eastern chain.