Taiwan Collision Research Articles

Spatial variation of the crustal stress field along the Ryukyu‐Taiwan‐Luzon convergent boundary

We applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the Philippine Sea plate. Our results indicate that the crustal stress along the Ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction, including the Tokara channel. An extensional stress regime is observed along the entire Ryukyu back arc, implying that back‐arc rifting may have extended northward to Kyushu. A triangular area near the southernmost terminus of the Ryukyu arc is characterized by a unique stress signature. The eastern boundary of this Ryukyu‐Taiwan Stress Transition coincides with the 123°E meridian where the Gagua ridge intercepts the Ryukyu trench; whereas its western boundary agrees remarkably well with the border between the postcollision and waning‐collision domains in northern Taiwan. The Taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the Lukang Magnetization High (LMH), suggesting that the LMH may be critical in controlling the local stress distribution. The stress signature of the Luzon arc–Taiwan collision reaches as far south as 19.5°N. The tectonic stress along the Manila trench–Luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in‐slab membrane stress. Since this extensional regime is observed only south of ∼22°N, it probably marks the northern limit of the contemporary boundary between the subduction along the Manila trench and the collision in Taiwan.

Sedimentology of early Pliocene sandstones in the south-western Taiwan foreland: Implications for basin physiography in the early stages of collision

This work presents sedimentological observations and interpretations on three detailed sections of the Pliocene Yutengping/Ailiaochiao formations, deposited in the early stages of collision in Taiwan. Seven facies associations record paleoenvironments of deposition ranging from nearshore to lower offshore with a strong influence of tidal reworking, even in shelfal sub-tidal environments, and a pro-delta setting characterized by mass-flows. The association of shallow facies of the upper offshore to lower shoreface with pro-delta turbidite facies sourced in the orogen to the east suggests a peculiar setting in which turbidite deposition occurred below wave base but on the shelf, in water depths of probably less than 100 m. This adds to the examples of “shallow turbidites” increasingly commonly found in foreland basins and challenges the classical view of a “deep” early underfilled foreland basin. Time series analysis on tidal rhythmites allow us to identify a yearly signal in the form of periodic changes of sand-supply, energy and bioturbation that suggests a marked seasonality possibly affecting precipitation and sediment delivery as well as temperature. The Taiwan foreland basin may also present a potentially high-resolution record in shallow sediments of the early installation of monsoonal circulation patterns in east Asia. We confirm partly the paleogeography during the early stages of collision in Taiwan: the Chinese margin displayed a pronounced non-cylindrical geometry with a large basement promontory to the west in place of the modern Taiwan mountain range. Collision in Taiwan may have happened at once along the whole length of the modern mountain range, instead of progressively from north to south as classically considered.

Distribution of strain rates in the Taiwan orogenic wedge

To constrain the way Eurasian crust is accreted to the Taiwan orogenic wedge we investigate the present-day 3D seismogenic deformation field using the summation of 1129 seismic moment tensors of events ( M w > 4) covering a period of 11 years (1995 to 2005). Based on the analysis of the principal strain-rate field, including dilatation and maximum shear rates, we distinguish four domains. Domain I comprises the Coastal Plain and the Western Foothills. It is mainly contractional in both the horizontal plane and in cross-section. Domain II comprises the eastern Western Foothills, the Hsuehshan Range and the Backbone Range. It is characterized by the highest contraction rates of 10 − 6 yr − 1 in association with area expansion in cross-section and area contraction in the horizontal plane. Domain III corresponds to the Central Range. It is characterized by area contraction in cross-section and area expansion in the horizontal plane. The maximum contractional axis is typically low and plunges ~ 30°E. Extension is larger, horizontal and strikes parallel to the axis of the mountain range. Domain IV corresponding to the Coastal Range and offshore Luzon Arc shows deformation patterns similar to domain II. This seismogenic strain-rate field, which is found in good agreement with the main features of the geodetic field, supports shortening within a thick wedge whose basal décollement is relatively flat and located in the middle-to-lower crust > 20 km. The east plunges of maximum strain-rate axes below the Central Range argue for the development of top-to-the-east transport of rocks resulting from the extrusion of the whole crust along west-dipping crustal-scale shear zones. The study of seismogenic strain rates argues that the initiation of subduction reversal has already started in the Taiwan collision domain.

Reappraisal of the Arc-Arc Collision in Taiwan

Al though it is evident that Tai wan has been formed by the collision of the west-facing Luzon arc with the Eurasian continental mar gin, there re main a lot of enigmas in this collision. The major ones are: (1) a trans form fault presently connecting the Manila and Ryukyu Trenches in the Philip pine Sea-Eurasia relative motion direction is missing, and in stead, the Ryukyu Trench ex tends near off shore E. Tai wan, (2) the western edge of the intermediate-depth seismicity associated with the Philip pine Sea plate subduction beneath NE Tai wan has a NNW trend, not NW, and (3) a large negative Bouguer gravity anomaly, with undulation of its amplitude in a wave length of 60-80 km, exists along the southernmost Ryukyu forearc. We propose a new model of the collision in Tai wan to re solve these enigmas, assuming that the southern Ryukyu forearc was migrating to the south west with respect to Eurasia for the past several m.y. and the Luzon arc has been colliding with this actively migrating Ryukyu forearc. The northern most Luzon arc is divided into two parts by the NNW line directing along the Philip pine Sea-Ryukyu forearc motion from its initial intersection point with the Ryukyu Trench; the part west of this line has been obducted on the Ryukyu forearc-Eurasian mar gin, producing the collision orogen in Taiwan, and the part east of it has been subducted beneath the Ryukyu forearc. This evolutionary scenario re solves enigmas (1) and (2) kinematically. This model also predicts that the South China Sea slab has to be torn by the west ward component of the motion of the subducting Philip pine Sea slab to Eurasia. This would have brought large lateral compression in the shallow portion of the Philip pine Sea slab at its western border, which might lead to buck ling of the slab causing the ob served undulated gravity anomaly.

Arc-continent collisions, sediment recycling and the maintenance of the continental crust

Abstract Subduction zones are both the source of most new continental crust and the locations where crustal material is returned to the upper mantle. Globally the total amount of continental crust and sediment subducted below forearcs currently lies close to 3.0 Armstrong Units (1 AU=1 km 3 a −1 ), of which 1.65 AU comprises subducted sediments and 1.33 AU tectonically eroded forearc crust, compared with an average of c . 0.4 AU lost during subduction of passive margins during Cenozoic continental collision. Margins may retreat in a wholesale, steady-state mode, or in a slower way involving the trenchward erosion of the forearc coupled with landward underplating, such as seen in the central and northern Andean margins. Tephra records of magmatism evolution from Central America indicate pulses of recycling through the roots of the arc. While this arc is in a state of long-term mass loss this is achieved in a discontinuous fashion via periods of slow tectonic erosion and even sediment accretion interrupted by catastrophic erosion events, probably caused by seamount subduction. Crustal losses into subduction zones must be balanced by arc magmatism and we estimate global average melt production rates to be 96 and 64 km 3 Ma −1 km −1 in oceanic and continental arc, respectively. Critical to maintaining the volume of the continental crust is the accretion of oceanic arcs to continental passive margins. Mass balancing across the Taiwan collision zones suggests that almost 90% of the colliding Luzon Arc crust is accreted to the margin of Asia in that region. Rates of exhumation and sediment recycling indicate that the complete accretion process spans only 6–8 Ma. Subduction of sediment in both erosive and inefficient accretionary margins provides a mechanism for returning continental crust to the upper mantle. Sea level governs rates of continental erosion and thus sediment delivery to trenches, which in turn controls crustal thicknesses over 10 7 –10 9 years. Tectonically thickened crust is reduced to normal values (35–38 km) over time scales of 100–200 Ma.

Taiwan mountain building: insights from 2-D thermomechanical modelling of a rheologically stratified lithosphere

SUMMARY The Taiwan orogen has long been regarded as a case example for studying mountain building in association with subduction processes. In this paper, we present a fully coupled thermomechanical modelling of the Taiwan collision based on a realistic viscous-elastic–plastic rheology. It satisfactorily reproduces available thermochronometric data, long-/short-term deformation patterns, heat flux and erosion/sedimentation distribution across the Taiwan orogeny. We found that a deep seated flux of Asian crustal material into the orogenic wedge should be invoked to counter-balance observed exhumation and erosion in the Central Range. However, in contrast with recent thermokinematic models of exhumation and deformation suggesting that underplating plays a significant role, we show that most constraints on exhumation and deformation can be more straightforwardly interpreted by the frontal accretion of the rheologically layered Asian crust. We finally infer that such a model is in better agreement with the basic expectation that the hot/young and buoyant Chinese continental margin should hardly be subducted beneath the cold/old and dense oceanic plate of the Philippines Sea.

Detrital U–Pb zircon dating of lower Ordovician syn-arc-continent collision conglomerates in the Irish Caledonides

The Early Ordovician Grampian Orogeny in the British Isles represents a classic example of collision between an oceanic island arc and a passive continental margin, starting around 480 Ma. The South Mayo Trough in western Ireland preserves a complete and well-dated sedimentary record of arc collision. We sampled sandstones and conglomerates from the Rosroe, Maumtrasna and Derryveeny Formations in order to assess erosion rates and patterns during and after arc collision. U–Pb dating of zircons reveals a provenance dominated by erosion from the upper levels of the Dalradian Supergroup (Southern Highland and Argyll Groups), with up to 20% influx from the colliding arc into the Rosroe Formation, but only 6% in the Maumtrasna Formation (~ 465 Ma). The dominant source regions lay to the northeast (e.g. in the vicinity of the Ox Mountains, 50 km distant, along strike). The older portions of the North Mayo Dalradian and its depositional basement (the Annagh Gneiss Complex) do not appear to have been important sources, while the Connemara Dalradian only plays a part after 460 Ma, when it supplies the Derryveeny Formation. By this time all erosion from the arc had effectively ceased and exhumation rates had slowed greatly. The Irish Grampian Orogeny parallels the modern Taiwan collision in showing little role for the colliding arc in the production of sediment. Negligible volumes of arc crust are lost because of erosion during accretion to the continental margin.

Does extrusion occur at both tips of the Taiwan collision belt? Insights from active deformation studies in the Ilan Plain and Pingtung Plain regions

We analyse the present-day deformation in two key areas of the Taiwan collision belt, the Ilan Plain to the NW and the Pingtung Plain to the SW. Our approach is mainly based on consideration of horizontal displacement revealed by recent geodetic (GPS) surveys, derived strain rate tensors indicating horizontal deformation and three-dimensional seismotectonic stress regimes issued from inversion of focal mechanisms of earthquakes. We reconstruct a consistent, albeit complex, tectonic pattern involving non-rigid rotations (clockwise in NE Taiwan, anticlockwise in SW Taiwan), transitions from pure compression near the mountains to transtension near the sea and simple shear affecting the deforming domain (NW–SE left-lateral in NE Taiwan, NNE–SSW right-lateral in SW Taiwan). These tectonic patterns reflect lateral extrusion processes towards mechanically weak domains with respect to collision zone, i.e., adjacent subduction zones (Ryukyu to the NE, Manila to the SW). The extrusion of the Ilan Plain area occurs towards the SE, whereas that of the Pingtung Plain area occurs towards the SW. The extrusion in NE Taiwan is facilitated by the opening of the Okinawa Trough, so that velocity and deformation rates are higher than in SW Taiwan despite less active collision in this northern area. The symmetry of the extrusion patterns is altered by differences in relative positions and orientations of collision belt and adjacent subduction zones, which depend on the plate tectonic configuration and tectonic history of the Taiwan region.

The crustal deformation of the Ilan Plain acted as a westernmost extension of the Okinawa Trough

We analyze the current deformation in Ilan Plain of northeastern Taiwan considered as the western extension of Okinawa Trough based on 34 geodetic sites observed between 2002 and 2006. The station velocities are between 1.9 mm/yr and 43.0 mm/yr in a direction between 38 ° and 143 ° relative to stable continent margin of Eurasian plate. This largest velocity is moving southeastward (143 °) at a rate of ~ 43 mm/yr was enhanced by a seismic doublet that occurred on March 6, 2005. The station velocities at the northern margin of Ilan Plain are insignificant with a magnitude of ~ 1.9 mm/yr to ~ 7.5 mm/yr. On the contrary, the station velocities increase approximately southward along the southern margin of Ilan Plain. The stations are moving roughly E–W with a magnitude of ~ 5.8 mm/yr to ~ 21.8 mm/yr. Further southward, the station velocities increase to a value of ~ 43.0 mm/yr in a direction of 143 °. The most prominent feature of the strain distribution patterns in Ilan Plain are the remarkable extensional strain rates observed in central and southern flank of the Ilan Plain. The largest extensional rate is found (2.66 μstrain/yr in 147°) with a shortening rate in the perpendicular direction (1.28 μstrain/yr in 57°), indicating transtensional deformation mode. In the northeastern flank of Ilan Plain, insignificant or minor deformations are observed. Overall the extensional rates in this area clearly increase from north to south. The extension directions in study area mostly trend E–W to NW–SE directions. Using the velocity field of 34 GPS stations and its strain rate filed, we suggest the lateral extrusion process towards mechanically weak domain adjacent Ryukyu subduction zone. The lateral extrusion is facilitated by the opening of the Okinawa Trough. We conclude that the northeast Taiwan is subjected to both the compressional force exerted by the indentation of Philippine Sea plate and the tensile force induced by trench retreat related to the suction force at the Ryukyu trench. Due to the interaction of two forces, the stress regime rapidly changes from pure compression in the Taiwan collision belt to transtension near Ilan Plain and its offshore area.

Crustal structure and deformation at the northern Manila Trench between Taiwan and Luzon islands

The Philippine Sea Plate overrides the Eurasian Plate along the east-dipping Manila Trench between Taiwan and Luzon islands. From south to north, the plate convergence gradually evolves from normal subduction of the South China Sea lithosphere to initial collision of the Taiwan orogen. The subduction-related earthquakes become diffusive close to Taiwan; the accretionary prism is dramatically wider toward Taiwan. To understand the plate convergent features of this subduction-collision transition zone, we have analyzed twelve seismic reflection profiles across the Manila Trench between Luzon and Taiwan. The results show that the basement of northern South China Sea basin generally dips toward east and south. The northern Manila Trench accumulates more trench-fill sediments in the south than in the north. A sequence boundary t 0 is suggested to distinguish the hemipelagic and trench-fill sediments. Probably due to the collision in Taiwan, the sequence boundary t 0 displays more gentle slope or has been uplifted in the north. Structural analysis shows that the subducting crust in the northern Manila Trench area can be characterized by three distinctive zones: a normal fault zone (NFZ), a proto-thrust zone (PTZ) and a thrust zone (TZ). The NFZ is defined by the distribution of numerous normal faults in the top or upper portion of the subducting crust. The normal faults are gradually buried by trench-fill sediments when they are closer to the deep trench. It is suggested that normal fault may take place at the location where the crust starts to bend and induces gravity sliding of the upper sedimentary layers. Some buried normal faults could be reactivated to blind thrust faults because of stronger plate convergence near the accretionary prism. The PTZ is located between the NFZ and the frontal thrust of the accretionary prism; it contains blind thrust faults or folds instead of thrust faults. The successive distribution of the crustal structures of normal faults, blind thrust and thrust faults at the trench area suggests that the blind thrust faults develop along the location of pre-existing normal faults. The brittle deformation occurs at the lower part of the sedimentary layers probably because of stronger compaction and less water content; eventually blind thrust faults may propagate upward and become thrust faults at the seabed.

Morphotectonics and sedimentation in convergent margin basins: An example from juxtaposed marginal sea basin and foreland basin, Northern South China Sea

Using reflection seismic profiles and bathymetric mapping this paper reveals the tectonic–sedimentary characteristics of the convergent margins in the northern South China Sea, where it is strongly related to flexure of Chinese rifted margin and overthrust of Taiwan orogen. Convergent margin tectonics of the South China Sea near southern Taiwan is characterized by a progressively northward transition from oceanic subduction along the Manila Trench to the incipient collision zone offshore southern Taiwan where the continental crust of the Eurasian plate subducts beneath the Philippine Sea plate. North of 21°N, dip angles of the Benioff zone increase up to 80° in the incipient collision zone where the Manila Trench becomes shallower, gradually loses its morphological identity and finally merges into the nearly N–S trending Penghu Canyon. Convergent margin tectonics in the initial collision zone in SW Taiwan is manifested by the beginning of flexure of the Chinese margin under the westward migrating overthrust belt of Taiwan, forming two distinct basins. On the passive Chinese margin, the marginal sea basin becomes smaller and is underlain by the South China Sea Slope, while on the active Taiwan margin, a wedge-top basin has formed above the frontal thrust sheets of the Taiwan orogenic wedge. Sediments derived from the Taiwan orogen progressively overlie the strata of the passive Chinese margin, resulting in sediment thickening and basin shallowing from south to north. Sedimentary facies shows that offshore deep-water mud is gradationally overlain by shallow marine sediments. Sediments of the wedge-top basin are being actively deformed into mud diapiric intrusions and a series of west-vergent thrusts and folds with their associated piggy-back basins, resulting in irregular topography of the sea floor with alternating sea ridges and troughs. Pliocene–Quaternary strata of the passive Chinese margin are a little deformed under the westward compression induced by the arc–continent collision in the southern Taiwan. The slope profile of the passive Chinese margin is characterized by a sigmoidal curvature, representing a typical primary depositional setting of a passive margin. Slope canyons occur mainly on the upper slope and cut the sea floor, resulting in irregular topography and representing effects of erosion. Being under the initial arc–continent condition, the offshore SW Taiwan has experienced the transition from a rifted margin to an overthrust belt and becomes a foreland basin, which is acting as a catch basin for orogenic debris derived from SW Taiwan. Arguments from tectonics and sedimentation suggest that the Koping shelf-slope region is considered to be an immature foreland basin rather than an accretionary wedge. More specifically, the Kaoping shelf-slope can be regarded as a depositional wedge top where exists an overlap area between rapid sedimentation and active deformation of the Kaoping shelf-slope sediments in the external orogenic wedge of southern Taiwan.

Characterization of topographic steady state in Taiwan

The arc-continent collision in Taiwan has resulted in surface uplift, high relief and high rates of erosion. Because the collision is older in the north and has propagated southward, Taiwan offers the opportunity to study the evolution of topography as it evolves towards a large-scale steady state. In this paper, we analyze a 40-m digital elevation model of Taiwan to determine how topographic steady state is expressed in the landscape; particular attention is paid to the eastern Central Range, where major drainage basins are approximately aligned in the direction of the collision propagation. Analyses of orogen size, regional topographic slope, drainage basin hypsometry, and local topographic slope reveal a geomorphological transition 100–125 km north of the southern tip of the island. To the south of this transition, topographic characteristics change with distance along the island; to the north, these characteristics are relatively constant, which we interpret as representing large-scale topographic steady state. Assuming that the collision has propagated southward at a rate of 55 mm/yr, the time to topographic steady state is 1.8–2.3 Myr after emergence above sea level. Within the steady state region, we evaluate regional variability in the topography and find that the most prominent deviations from a steady form are associated with large-scale structures, though our analyses of local topographic slope and river profile form suggest that spatial variations in rock strength and rock uplift rate also contribute to regional topographic variability.

Inversion of the Paleogene Chinese continental margin and thick-skinned deformation in the Western Foreland of Taiwan

New structural data, available seismicity data together with mechanical constraints on the Eurasian continental lithosphere and reconstruction of paleostress trajectories are combined in order to re-assess and discuss the dominant deformation mode (thin-skinned vs thick-skinned) in the Western fold-and-thrust belt of the active Taiwan collision zone. Serial balanced cross-sections and computed paleostress tensors suggest that structural styles and stress trajectories at the front of the Taiwan mountain belt may vary rapidly along-strike depending on the presence of pre-orogenic Paleogene troughs in the Chinese continental margin (Taihsi and Tainan basins) that are favourably oriented to be reactivated and inverted. In localities of the western foreland, where basin inversion and thick-skinned basement-involved shortening predominate, the crustal seismic activity is important and characterized by strike-slip faulting. In these domains of the fold–thrust belt, the stress deviations with respect to the regional transport direction are also important. By contrast, in domains of significant syn-orogenic subsidence, a thin-skinned style of deformation may be prominent due to the lack of available pre-existing features. The seismic activity is limited to few major faulted boundaries such as the active Chelungpu–Sani thrust, and the stress deviations are limited. The timing of deformation at the belt front seems to be independent of the structural styles, so that frontal folds are active since probably 0.5 Ma. However, in inner parts of the Western Foothills of Taiwan, where superimposed thick-skinned and thin-skinned deformation are required, out-of-sequence thrusting occurs to maintain the topographic profile. Finally, we suggest that kinematics and structural styles of the western fold–thrust belt are controlled by the mechanics of the Eurasian continental margin in agreement with a recent study. At the scale of the orogen, the limited shortening amounts across the WF and in the Central Range as well as the absence of continental HP-rocks better support a thick-skinned collision model for Taiwan.

Temporal and spatial records of active arc-continent collision in Taiwan: A synthesis

Well-documented stratigraphy and clearly defined geodynamics in Taiwan, where some of the best records on arc-continent collision have been preserved, offer a unique example for the study of collision belts worldwide. The oblique arc-continent collision in Taiwan caused a simultaneous and sequential migration of four tectonic processes. Beginning from 16 to 15 Ma, subduction of the South China Sea oceanic crust beneath the Philippine Sea plate resulted in volcanism in the Coastal Range and formation of an accretionary prism in the Central Range. Beginning in the latest Miocene–earliest Pliocene, the subduction was followed by initial arc-continent collision, as supported by the following: unroofing and erosion of the deformed accretionary prism, and deposition of sediments thus derived in the adjacent accretionary forearc (5 Ma) and slope basins (4 Ma); waning of volcanism (north, 6–5 Ma; south, 3.3 Ma); buildup of fringing reefs on the gradually quiescent volcanoes (north, 5.2 Ma; south, 2.9 Ma); arc subsidence by strike-slip faulting and the development of pull-apart intra-arc basins (north, 5.2–3.5 Ma; south, 2.9–1.8 Ma); thrusting of forearc sequences to generate a collision complex starting from 3 Ma; and clockwise rotation of the arc-forearc sequences (north, 2.1–1.7 Ma; south, 1.4 Ma). The collision propagated southward and reached southern Taiwan by 5 Ma, as evidenced by the successive deformation of the associated accretionary wedge en route. Afterward, the advanced arc-continent collision stage appeared in the earliest Pleistocene, as marked by the westward thrusting and accretion of the Luzon arc-forearc against the accretionary wedge (north, 1.5 Ma; south, 1.1 Ma) and exhumation of the underthrust Eurasian continent rocks (north, 2.0–1.0 Ma; south, 1.0–0.5 Ma). The final stage of the tectonic process, arc collapse-subduction, began by 1 Ma off the northern Coastal Range. The geologic records compiled and presented in this study strongly support the scenario of a continuous southward migration of tectonic processes and a change in sediment source and structural style. Most importantly, the model has a broad potential for reconstructing and predicting the evolution of arc-continent collision through space and time.

The contribution to tectonic subsidence by groundwater abstraction in the Pingtung area, southwestern Taiwan as determined by GPS measurements

The three sets of Global Positioning System (GPS) data collected from 1996 to 1999 are used to investigate the contribution to tectonic subsidence by groundwater abstraction in the Pingtung plain, southwestern Taiwan. The horizontal station velocities varied from 32 to 54 mm/yr for azimuths ranging from 247.2° to 272.6° with respect to the permanent station S01R located in the Penghu islands. In the central and western part of the study area, GPS stations move generally towards the west, whereas in the Kaohsiung–Pingtung coastal area, the displacement vectors demonstrate a clear counter-clockwise deviation towards the SW. The southern part of the offshore coastal area shows remarkable extension rates of 0.6–2.0 μstrain/yr along azimuth 015–020°. The significant southward increase of extensional strain rates is attributed to the lateral extrusion of blocks bounded by major discontinuities in the study area. For the vertical movement, the station velocities are from ∼13 to −25 mm/yr. Significant subsidence rates from ∼11 to ∼25 mm/yr have been observed. These results clearly demonstrate the existence of transtensional deformation and the southward increase of extensional deformation in the along-strike direction throughout the study area. The comparison with the pattern of Holocene subsidence rates and the isopach of fine-grained sediments suggest that about 75% of this subsidence may result from the decrease in groundwater levels induced by over-pumping. These localized anthropogenic activities contributed to the natural risk that results from tectonic subsidence associated with tectonic extrusion and lateral extrusion at the southern tip of the Taiwan collision belt.

Stress Tensor Inversion for the Chi-Chi Earthquake Sequence and its Implications on Regional Collision

We study the stress regime associated with the 1999 Chi-Chi, Taiwan, earthquake sequence. Broadband waveforms of 115 events recorded by the Broadband Array in Taiwan for Seismology (BATS) are analyzed to determine their focal mechanisms, depths, and seismic moments. These solutions are then used to invert for the reduced stress tensor using the algorithm of Angelier (1998). We perform the stress inversion in three different ways. First, we aim at defining, in the first approximation, a uniform stress field by taking the entire dataset as a whole. The inversion result shows a stress regime with the maximum compression along the direction of N119°E, remarkably consistent with the direction of the relative motion between the Philippine Sea plate and Eurasia. This solution is stable and varies little in terms of the orientations of stress axes (within 8°) and the ratio of stress differences (between 0.26 and 0.44). In the second approximation, we aim at explaining more subtle features of our observations by considering differently built subsets. We divide the dataset according to different seismogenic structures as identified by Kao and Chen (2000). Our results yield rather similar results (axes of maximum compressive stress fall in the range of N99-138°E in terms of trends, and in the range of 1-17° in plunges) for all subsets except a small group in the footwall. Furthermore, we observe a systematic clockwise change in the direction of compression from south to north, representing the more general fan-shaped pattern of stress trajectories that characterizes the whole collision zone. Finally, the inversion is performed for the three main types of faulting, that is, reverse, strike-slip, and normal types. The axis of maximum compression for the reverse mechanisms has the same direction as that for the strike slip, whereas the normal type subset yields a N54°E extension that trends obliquely to the compression. The fact that the stress regime associated with the Chi-Chi aftershock sequence is similar to that derived from geological and seismic data in the region implies that the stress regime corresponding to the regional collision in Taiwan is responsible for the occurrence of the Chi-Chi earthquake. Moreover, systematic, secondary variation in the state of stress is observed for different types of focal mechanisms and seismogenic structures. Such variation may play an important role in producing local stress concentration that leads to the nucleation of disastrous earthquakes in central Taiwan. Manuscript received 12 January 2001.

Nature and Distribution of the Deformation Front in the Luzon Arc-Chinese Continental Margin Collision Zone at Taiwan

Marine seismic reflection profiles from offshore SW Taiwan combined with onland geological data are used to investigate the distribution and nature of the deformation front west of Taiwan. Locations of the frontal structure west of Taiwan are generally connected in a linear fashion, although the alignment of frontal structures is offset by strike-slip faults. The deformation front begins from the northern Manila Trench near 21°N and continues northward along the course of the Penghu Submarine Canyon in a nearly N–S direction north of 21°N until it reaches the upper reaches of Penghu Canyon at about 22°15′N. The deformation front then changes direction sharply to the northeast. It connects to the Chungchou thrust fault or the Tainan anticline in the coastal plain and continues northwards along the outer Western Foothills to the northern coast of Taiwan near 25°N. Characteristics of structural style, strain regime, sedimentation and tectonics vary along the trend of the deformation front. Ramp anticlines, diapiric intrusion and incipient thrust faults are commonly associated with the deformation front. Variations in structural style along strike can be related to different stages of oblique collision in Taiwan. The deformation front (collision front) west of Taiwan can be considered as a boundary between contraction in the Taiwan orogen and extension west of the collision zone. The deformation front east of the Tainan Basin and its northward extension along the outer limit of the Western Foothills is the surface trace separating the foreland thrust belt from the nearby foredeep, not a boundary between the Chinese and Taiwan margins. The submarine deformation front off SW Taiwan is the surface trace separating the submerged Taiwan orogenic wedge from the Chinese passive continental margin, not a surface trace of the plate boundary between the Eurasian and Philippine Sea plates.

Crustal deformation in the central and southern Ryukyu Arc estimated from GPS data

Deformation of the Ryukyu Arc in the period 1999–2001 was investigated on the basis of GPS data for horizontal velocity. The results show that deformation of the Ryukyu Arc is divided into three tectonic blocks, with all counter-clockwise rotation rates of between 18 and 195 nrad/yr. Extensional strain along the Ryukyu Arc is observed in all three blocks, being largest in the southwestern Ryukyu Arc. This suggests that southward bending of the arc occurs in the southwest associated with the arc–continent collision in Taiwan, resulting in clockwise block rotation of the Taiwan–Ryukyu junction. Thus, bending of the Ryukyu Arc appears to be caused by collision of the Luzon Arc with the Eurasian plate, and is likely to contribute to rifting of the Okinawa Trough.

BYRNE, T. B. & LIU, C.-S. (eds) 2002. Geology and Geophysics of an Arc–Continent Collision, Taiwan. Geological Society of America Special Paper no. 358. viii + 211 pp. Boulder: Geological Society of America. Price US $80.00; members' price US $64.00 (paperback). ISBN 0 8137 2358 2

BYRNE, T. B. & LIU, C.-S. (eds) 2002. Geology and Geophysics of an Arc–Continent Collision, Taiwan. Geological Society of America Special Paper no. 358. viii + 211 pp. Boulder: Geological Society of America. Price US 64.00 (paperback). ISBN 0 8137 2358 2 - Volume 140 Issue 6

Rheology and strength of the Eurasian continental lithosphere in the foreland of the Taiwan collision belt: Constraints from seismicity, flexure, and structural styles

Deformation in western Taiwan is characterized by variable depth‐frequency distribution of crustal earthquakes which are closely connected with along‐strike variations of tectonic styles (thin or thick skinned) around the Peikang High, a major inherited feature of the Chinese margin. To fit the calculated high crustal geotherm and the observed distribution of the crustal seismic activity, a Qz‐diorite and granulite composition for the upper and the lower crust is proposed. We then model the plate flexure, through Te estimates, using brittle‐elastic‐ductile plate rheology. Flexure modeling shows that the best fit combination of Te‐boundary condition is for thrust loads acting at the belt front. The calculated Te vary in the range of ∼15–20 km. These values are primarily a reflection of the thermal state of the rifted Chinese margin inherited from the Oligocene spreading in the South China Sea. However, other mechanical properties such as the degree of crust/mantle coupling and the thickness of the mechanically competent crust and mantle are considered. South of the Peikang High, flexure modeling reveals lower Te associated with thinner mechanically strong layers. Variable stress/strain distribution associated with a higher degree of crust/mantle decoupling is examined to explain plate weakening. We first show that plate curvature cannot easily explain strength reduction and observed seismic activity. Additional plate‐boundary forces arising from the strong coupling induced by more frontal subduction of a buoyant crustal asperity, i.e., the Peikang High, with the overriding plate are required. Favorably oriented inherited features in the adjacent Tainan basin produce acceleration of strain rates in the upper crust and hence facilitate the crust/mantle decoupling as attested by high seismic activity and thick‐skinned deformation. The relative weakening of the lower crust and mantle then leads to weaken the lithosphere. By contrast, to the north, more oblique collision and the lack of inherited features keep the lithosphere stronger. This study suggests that when the Eurasian plate enters the Taiwan collision, tectonic inheritance of the continental margin exerts a strong control on the plate deformation by modifying its strength.