Taiwan Collision Research Articles

Structural features off southwestern Taiwan

Seismic reflection profiles reveal a complex system of faults, mud diapirs, submarine channels, and sedimentary basins in the offshore area southwest of Taiwan. Structural patterns demonstrate the on-going structural transformation from the passive Chinese continental margin to the Taiwan collision orogen. The boundary between the Chinese continental margin and the Taiwan orogen can be clearly delineated by the deformation front of the contractional structures. West of the deformation front, ENE-WSW trending normal faults prevail in the continental margin, which is characterized by a horst-and-graben structural style. East of the deformation front, west-vergent imbricated folds and thrusts sheets abound in the accretionary wedge. The structural grain of the accretionary wedge trends NNW-SSE in the deep offshore area but bends toward NNE-SSW in the shallow near shore area. Mud diapirs emerge through thick sedimentary layers forming NNE-SSW trending anticlines that extend from near shore areas to onland Taiwan. Mud volcanoes have been observed on 3.5 kHz profiles over several diapirs. Structural styles observed across the frontal portion of the submarine Taiwan accretionary wedge are strongly controlled by the tectonic evolution of the arc-continent collision. The structure and location of the deformation front are affected both by the growth of the accretionary wedge and by the geometry of the Chinese continental margin basement.

Transition between the Okinawa trough backarc extension and the Taiwan collision: New insights on the southernmost Ryukyu subduction zone

Located between the Okinawa trough (OT) backarc basin and the collisional zone in Taiwan, the southernmost Ryukyu subduction zone is investigated. This area, including the southwestern portions of the OT and Ryukyu island arc (RA) and located west of 123.5° E, is named the “Taiwan-Ryukyu fault zone” (TRFZ). West of 123.5° E, the OT displays NNW-SSE structural trends which are different in direction from the ENE-WSW trending pattern of the rest of the OT. Using joint analysis of bathymetric, magnetic, gravity and earthquake data, three major discontinuities, that we interpret as right-lateral strike-slip faults (Faults A, B and C), have been identified. These faults could represent major decouplings in the southern portion of the Ryukyu subduction zone: each decoupling results in a decrease of the horizontal stress on the portion of the RA located on the eastern side of the corresponding fault, which allows the extension of the eastern side of OT to proceed more freely.

Kinematics of convergence, deformation and stress distribution in the Taiwan collision area: 2-D finite-element numerical modelling

Using a 2-D plane stress finite-element model with elastic and elasto-plastic rheologies, appropriate for deformation within the brittle upper crust, we analyse the relationship between kinematics of convergence, deformation and stress distribution in the present Taiwan collision occurring between the Ryukyu and Luzon subduction zones. The distribution of stress trends calculated in our models is compared with a synthetic map of actual stress trajectories based on the most recent data available in the collision zone. These data combine present-day sources (from borehole breakouts and earthquake focal mechanism) with the reconstruction of Quaternary palaeostress (from fault slip data analyses), resulting in a complete map of compressional stress trajectories which is used to constrain our models. We show that the distribution of stress trajectories in the active Taiwan collision is principally controlled by: (1) the geometric configuration of the boundary between Eurasia and the Philippine Sea plate; (2) the shape and rheological properties of major structural units; (3) the direction of convergence of the Philippine Sea plate relative to Eurasia; and (4) the influence of the opening of the Okinawa Trough. The study of a two-dimensional elastic and elasto-plastic finite-element modelling of the subduction-collision in and around Taiwan allows us to estimate the influences of these different parameters in the stress pattern. Taking into account both the simplifying assumptions of the numerical modelling and the angular uncertainties of field determinations, the fit between the calculated stress pattern of the finite-element model and that determined based on the geometrical synthesis of field analyses is rather good in general, indicating that our model is valid to first approximation. Misfits remain minor and can be explained by data uncertainties and simplifying modelling assumptions (for instance, the shape of the corner of the collision zone is critical but is not accurately known; also limited decoupling in the Longitudinal Valley collision zone was not considered in our models although it certainly plays a role). Some interesting features of our model are: (1) the greater influence of the shape of the collision zone in comparison with that of the direction of convergence; (2) the requirement for a trench retreat related to suction force in the Ryukyu Arc; and (3) the crucial role of the interaction between Okinawa Trough opening and collision at the sharp northwestern corner of the Philippine Sea plate including its influence on the geological evolution of northeastern Taiwan.

Miocene basalts in northwestern Taiwan: Evidence for EM-type mantle sources in the continental lithosphere

Cenozoic extension around the Taiwan Strait resulted in intraplate basalt volcanism in the Fujian-Taiwan region of the southeastern China continental margin. In northwestern Taiwan, the basalt volcanism took place in two distinct periods: the early Miocene (23–20 Ma), with the eruption of alkali basalt only, and the late Miocene (13–9 Ma), marked by the emplacement of various basalt types. The early Miocene basalts have uniform SrNdPb isotope compositions comparable with those of the other Fujian-Taiwan basalts, which are believed to have originated from a “plum-pudding” type convecting mantle. These basalts display EM2-type lead isotope signature like that observed in seamount basalts from the South China Sea. By contrast, the late Miocene basalts show distinctive isotope characteristics indicating additional involvement of an EM1-type mantle source that has never been identified before for any volcanics in southern China. We propose that the “enriched mantle” components (EMI and EM2) reside in different levels of the continental lithospheric mantle. Reactivation of the unique EM1 source may be ascribed to the arc-continent collision in Taiwan during the last 12 million years which terminated the intraplate volcanism around this region.

Kinematics of arc‐continent collision in the eastern Central Range of Taiwan inferred from geomorphic analysis

Active arc‐continent collision in Taiwan is an example of a typical collisional orogen, with a deformed continental margin (Foothills Belt, Hsueshan Range, and Central Range) separated from an accreted island arc (Coastal Range) by a collapsed forearc basin (Longitudinal Valley). The geomorphology of the eastern Central Range reflects spatial and temporal variations in uplift rate and pattern along the orogen during late Quaternary time. Digitized drainage basin perimeters, stream channels, and the mountain‐piedmont junction of a 200‐km‐long segment of the eastern Central Range provide data for calculating quantitative geomorphic parameters. Statistical analyses of these parameters define four domains of distinct landscape character. The northern domain extends northward along the coast from the northern end of the Longitudinal Valley (LV) and has landscape characteristics suggesting uplift near the mountain front. The central two domains are adjacent to the LV and show evidence of uplift concentrated in the interior of the range, away from the front. The boundary between the two central domains occurs near the middle of the LV. The southern domain extends southward along the coast from the southern end of the LV and has characteristics suggesting a transient response to increased uplift rate. Analysis of fluvial terrace profiles in the Central Range adjacent to the southern half of the LV indicates a change in uplift pattern through time, with migration of maximum uplift rate from the mountain front to the interior of the range. This change probably occurred just prior to late Pleistocene time.

Oblique convergence, indentation and rotation tectonics in the Taiwan Mountain Belt: Insights from experimental modelling

Experimental modelling allows one to describe the development and kinematics of structures in mountain belts formed during oblique convergence. In the collision geometry of the Taiwan mountain belt, the Chinese continental margin is oriented about N60°E, whereas the N16°E Philippine Sea plate backstop is moving toward the Eurasian plate in the N55°W direction. Sand☐ experiments have been conducted to simulate the Cenozoic arc-continent collision in Taiwan with a foreland-dipping backstop. The kinematics of deformation comprises a combination of compression and rotation which locally result in a partitioning between thrusting and wrenching. The experimental modelling shows that the faults or shear zones are rotated around the indentation point of the backstop by transcurrent and bookshelf faulting. An overall clockwise rotation (or right-lateral movements) occurs to the north of the indentation point and anticlockwise rotation occurs to the south of the indentation point. Oblique indentation and rotation structures can be demonstrated by (1) the crescent shape of the thrust wedge, (2) pull-apart structures induced by strike-slip tectonics, and (3) bookshelf-type strike-slip faulting resulting from the block rotation around the indentation point.

Late Cenozoic basaltic volcanism around the Taiwan Strait, SE China: Product of lithosphere-asthenosphere interaction during continental extension

Late Cenozoic intraplate basaltic volcanism in SE China and Taiwan occurred as a result of lithospheric extension related to the opening of the South China Sea. It was gradually terminated by the compression which propagated westwards from the arc-continent collision in Taiwan since ∼ 12 Ma ago. The basalts show a spatial variation in chemical composition. Their alkalinity, degree of silica undersaturation and abundance of incompatible elements increase whilst Pb isotopic ratios decrease ( 206 Pb 204 Pb from 19.0 to 18.2) progressively away from a NE-trending extension axis in the western Taiwan Strait. Abundant tholeiites were emplaced near the axial zone whereas alkali basalts, basanites and nephelinites took place further away from this axis. Overall, the basalts have Sr-Nd-Pb isotopic systematics similar to those of seamount basalts in the South China Sea which are characterized by a Dupal-type Pb isotopic anomaly. Based on geochemical constraints from the basalts, depth estimates for mantle xenoliths and regional geophysical data, a passive extension model is proposed for the geodynamic evolution of this region. It suggests greatest lithosphere thinning during the Miocene beneath the axial zone. The lithospheric mantle was thermo-mechanically eroded by convective upwelling of the asthenosphere, a process that raised the lithospheric geotherm and resulted in a plum-pudding-type convecting mantle. The spatial chemical and isotopic variation in the basalts can be explained by different degrees of decompression melting of this convecting mantle, corresponding to a change in lithosphere thickness, compounded by various contributions from the continental lithospheric mantle (CLM) -derived plum component. This model, which involves continental extension followed by magma generation via lithosphere-asthenosphere interaction, should be applicable to other areas of Cenozoic rift magmatism around the South China Sea and in the eastern Eurasian continental margin.

Morphological and seismic characteristics of the Kaoping Submarine Canyon

SeaMARC II side-scan sonar images and swath bathymetry plus 6-channel seismic reflection profiles reveal details of major canyons in the submarine portion of the Taiwan collision belt. The Kaoping Submarine Canyon is the largest of these, extending over 240 km from the mouth of the Kaoping River across the accretionary wedge to the Manila Trench. Morphological features and structural settings vary along the course of the Kaoping Submarine Canyon, defining three sections. The first section extends southwest from the mouth of the Kaoping River, cutting across the shelf and upper slope roughly perpendicular to local bathymetric contours, to about 22°03'N, where the canyon turns sharply southeast. From this point, the second section of the canyon follows the trace of a major thrust fault, paralleling local structure to 21°35'N, at which point the canyon turns southwest again. Over the third section the canyon meanders, cutting through low-relief fault-bend anticlines and distributing orogenic sediments in intervening slope basins, to where it joins the Manila Trench. Seismic profiles reveal that this canyon, which forms a major sediment conduit between the Taiwan mountain belt and the Manila Trench, has a complicated evolutionary history, and that its development has been controlled strongly by accretionary structural processes.

Collapse of the Luzon Volcanic Arc during onset of arc‐continent collision: Evidence from a Miocene‐Pliocene unconformity, eastern Taiwan

A regional Miocene‐Pliocene unconformity in the Coastal Range of eastern Taiwan provides information about vertical motions that affected the colliding volcanic arc during initiation of arc‐continent collision. The unconformity separates precollisional volcanic and volcaniclastic rocks of the Miocene Luzon arc and forearc basin from an overlying thick sequence of syncollisional, Pliocene‐Pleistocene sedimentary rocks derived from the metasedimentary accretionary wedge of Taiwan. The duration of time represented by the unconformity averages 1–2 m.y. and locally ranges up to 4 m.y. The basal unit of the younger syncollisional sequence consists of bathyal, mega‐slump‐folded mudstone, pebbly mudstone, and thin‐bedded turbidites. Sharp stratigraphic juxtaposition of these deep‐marine deposits directly above shallow‐marine limestone and epiclastic rocks (upper part of the older arc sequence) requires extremely rapid subsidence during initiation of the collisional basin. The Coastal Range unconformity is interpreted to represent a period of latest Miocene nondeposition and/or erosion in the Luzon arc that was related to initial subduction of the Eurasian continental margin along the Manila Trench. This period was followed in Pliocene time by very rapid subsidence, or collapse, of the arc platform to 1–2 km water depth. There are two dissimilar mechanisms of basin subsidence that may have controlled initiation of the collisional basin: (1) extension, rifting, and crustal thinning in the Luzon arc massif to form a series of small rift or pull‐apart subbasins or (2) flexural subsidence of oceanic lithosphere in a foreland‐style basin due to crustal loading in the expanding accretionary wedge of proto‐Taiwan. Although data are insufficient to provide a unique solution, the flexural subsidence model is supported by systematic younging of the initial subsidence event from west to east, which may record eastward migration of an outer flexural bulge. Notable differences between the unconformity and paleogeography of the early collisional basin and analogous present‐day basins in the offshore region to the south suggest that arc‐continent collision and basin development in eastern Taiwan have experienced secular change since late Pliocene time.

Stress tensor analysis in the Taiwan area from focal mechanisms of earthquakes

We produce a map of the stress pattern in and around Taiwan based on 200 earthquake focal mechanism solutions. These solutions were determined by using data from Taiwan Telemetered Seismographic Network, microearthquake surveys and WWSSN. The stresses are derived through a minimization of angles between the slip vector and the shear stress on each nodal plane considered as a fault, employing appropriate weighting factors. The whole set of focal mechanisms is divided into several groups, mainly according to apparent clustering of the event locations. The results show that the direction of maximum principal stress in Taiwan area is nearly horizontal and SE-NW on average. This is in good agreement with the direction of relative motion between the Philippine Sea plate and the Eurasian plate. In western Taiwan, the fan-shaped distribution of the maximum principal stress is consistent with the direction of Philippine Sea-Eurasian plate convergence through a simple model of viscous material indented by a rigid wedge. In the northeastern part of Taiwan, a nearly horizontal minimum principal stress oriented N–S is found for shallow depths; it occurs in a region of low seismic velocities, probably related to the back-arc activity of the Okinawa Trough. Down-dip compressional and down-dip extensional stresses have been identified in different depth ranges within the subducting slab of the Philippine Sea plate in the northern Taiwan; this may reflect the slab characteristics in this area. A complex stress pattern prevails in the Hualien area, at the junction between the Ryukyu subduction system and the Taiwan collision zone.

Geotectonic evolution of late Cenozoic arc-continent collision in Taiwan

The active collision between the Luzon arc and the Asian continent in the Taiwan area is investigated in terms of plate kinematics and geological records. Regarding plate kinematics, the tectonic evolution of the collision can be reconstructed by superimposing the paleopositions of Luzon arc on the pre-collisional Asian continental margin. Regarding geological records, the collisional history can be interpreted from the stratigraphy of the Coastal Range and the Western Foothills and from the diastrophism of the Central Range of Taiwan. By incorporating geological information into plate kinematics, it appears that the Luzon arc could have begun overriding the Asian continental margin in the late Middle Miocene (about 12 Ma). In the Late Miocene, the impingement of the arc deformed part of the continental margin and might have caused metamorphism of part of the Central Range, but no distinct effects were produced in the sedimentary record. In Mio-Pliocene times (about 5 Ma), the arc changed its direction of motion from north-northwesterly to west-northwesterly and began to override the continental margin rapidly. The accretionary wedge grew increasingly to emerge above sea level and feed continental detritus to the Luzon forearc basin and to induce foreland subsidence on the continental margin. In the early Late Pliocene (about 3 Ma), the collision drastically uplifted the mountain ranges in northern Taiwan, which shed voluminous orogenic sediments into the forearc and foreland basins. As the collision propogated toward the west and the south, the forearc and foreland basins were progressively accreted to the collisional orogen which eventually grew up to its present configuration.

Tectonic analysis and the evolution of a curved collision belt: The Hsüehshan Range, Northern Taiwan

Detailed studies of fracture patterns and paleostress reconstructions based on analyses of fault-slip data sets allow identification of four main patterns of compressional paleostress orientations in the Cenozoic rocks of northern Taiwan. These patterns correspond to five successive tectonic events dominated by (1) N-S, (2) WSW-ENE, (3) WNW-ESE, (4) NW-SE and (5) WNW-ESE directions of compression. The first two events are characterized by relatively minor structures in the foreland fold-and-thrust belt; the first event probably reflects major left-lateral shear along NNE-SSW trends which occurred in the adjacent eastern metamorphic belt during the middle-late Miocene. These first two events do not affect the Plio-Pleistocene in the localities studied. In contrast, the last three events created the major compressional structures of the foreland belt during the Plio-Pleistocene collision in Taiwan. Preliminary studies suggest that the magnetic fabric of the rocks has been significantly influenced by compressional deformation along WNW-ESE and NW-SE trends, especially in the Slate Belt, thus confirming the importance of these last three events. On the basis of paleomagnetic work in progress the northernmost segment of the Taiwan Belt has been affected by a clockwise rotation of about 20°. Taking into account the distribution of successive compressional paleostress patterns, we conclude that of the 55° angle of the sharply curved belt of northern Taiwan, about 20° probably correspond to actual clockwise rotation during the Plio-Pleistocene whereas the remaining 35° reflect the initial shape of the curved Plio-Quaternary belt. The paleostress history of northern Taiwan is examined in terms of geodynamic evolution and kinematic reconstructions. The Plio-Quaternary development of the collision between the Luzon Arc and Eurasia followed a late Miocene predominantly transform motion from south to north of the Philippine Sea Plate relative to Eurasia. The succession of Plio-Pleistocene paleostress patterns in northern Taiwan is found to be consistent with the longitudinal shift (from northeast to southwest) and the transverse migration (from the inner to the outer zones) of compressional phenomena related to the evolution of oblique collision. Some major changes in compressional trends are also accounted for by variations in the kinematics of plate convergence: the most recent charge comprises a counterclockwise change in the direction of Philippine Sea Plate-Eurasia convergence.

Paleostress analysis as a key to margin extension: The Penghu Islands, South China Sea

The Penghu Islands belong to the Eurasian passive margin, west of the collision zone of Taiwan. From south to north, the South China Sea basins become narrower and disappear in the Taiwan Strait area. The presence of the Penghu Islands allow description of the tectonic evolution of the margin through the reconstruction of tectonic paleostress orientation based on analysis of fault slip data collected in the field. Brittle structures have been studied on four islands in Neogene basalt flows with thin interbedded sediments. Sets of minor striated faults, tension gashes, dikes and joints enabled us to reconstruct the orientation of late Cenozoic stresses. Two main extensional tectonic events were thus identified: a N-S extension dominated during the Middle-Late Miocene (especially in the northern Penghu Islands), and a more recent NW-SE extension prevailed during the Late Miocene (especially in the southernmost island). The latter extension induced complicated patterns of perpendicular normal fault systems and strike-slip conjugate and transfer fault systems, through permutations between principal stress axes ( σ 1/ σ 2 and σ 2/ σ 3 modes). The ages of these tectonic events are ascertained by the existence of syndepositional normal faults, dike injections and superposed brittle structures. A third, less important compressional tectonic event, is correlated with the Plio-Quaternary collision in adjacent Taiwan (ENE-WSW compression). The comparison between the results of marine studies in the South China Sea basins and our paleostress reconstruction in the Penghu Islands suggests that repeated changes between N-S and NW-SE directions of extension occurred in the past. The counterclockwise change from N-S to NW-SE recorded in the Penghu Islands took place between about 12 and 8 Ma ago, indicating that significant extensional phenomena along the South China margin may postdate the seafloor spreading activity in the central ridge.

Tectonic Evolution of Late Cenozoic Arc-Continent Collision in Taiwan: ABSTRACT

Tectonic Evolution of Late Cenozoic Arc-Continent Collision in Taiwan: ABSTRACT

Tectonic record of convergence changes in a collision area: the Boso and Miura peninsulas, Central Japan

Late Cenozoic formations in Boso and Miura have been affected by several tectonic events. Tectonic analysis enables us to reconstruct six different paleostress types: (1) early extension affecting the Oligocene/early Miocene Mineoka Group, (2) and (3), relatively minor compressional and extensional events probably early Pliocene in age, (4) major NNE-SSW compression dominating prior to 2 or 3 Ma ago, and (5) and (6) more recent major NNW-SSE compression to the west and WNW-ESE extension to the east, both types affecting the Pleistocene and prevailing since 1–2 Ma ago. The counterclockwise change from NNE-SSW to NNW-SSE compression is not accurately dated, but very likely occurred between 2 and 3 Ma ago; it is compared to similar evolutions in other areas of the Izu collision zone. We conclude that it corresponds to a major counterclockwise change in the direction of plate convergence (from SSE-NNW to SE-NW). The relationships between the directions of convergence and the distributions of Plio-Quaternary compressional paleostresses in and around the collision zone are described through a simple analogy, for the two stages of Plio-Quaternary collision. This counterclockwise change in stress fields and relative motions, also described in the Taiwan collision zone along the same Philippine Sea plate-Eurasia boundary, is interpreted as a major event at the scale of the plate. The possible significances of the other paleostress types identified in Boso are discussed. We conclude that tectonic analysis in and along collision boundaries provides a key for understanding kinematic evolution.

Opening of the Okinawa basin and collision in Taiwan: a retreating trench model with lateral anchoring

Using a two-dimensional finite element model with an elasto-plastic behavior, we show that the opening of the Okinawa basin behind the Ryukyu trench since about 6 Ma can be explained by a retreating trench model with lateral anchoring due to the collision in Taiwan. We assume that a suction force is applied to the edge of the overriding plate. This force corresponds to the difference between the mean lithospheric pressure in the overriding lithosphere and the normal pressure applied to its edge. It results in an outward motion of the edge of the continental margin (retreating trench) and in the opening of the Okinawa basin. If, on the contrary, no suction force is applied, the collision in Taiwan does not result in any extension in the Okinawa area. On both extremities of the Ryukyu trench, this outward motion is locked by the collision in Taiwan and the buoyant subduction of the Palau-Kyushu ridge. It results in the arcuate shape of the Ryukyu arc. In order to explain the actual amount of extension in the Okinawa basin, it is however necessary to take into account the presence of the Miocene volcanic arc which results in a weakened lithosphere. In our model, this weak zone is simulated by a lower yield condition. Finally, the dissymmetry of the Ryukyu arc and Okinawa basin can be explained by a lateral variation of the suction force related to the variation of the length of the subducting slab, thus of the slab pull force.

Collision and stress trajectories in Taiwan: a finite element model

Field analyses of Plio-Quaternary compressional deformations in Taiwan have enabled us to reconstruct the paleostress trajectories resulting from the collision of the Luzon arc (Philippine Sea plate) with the Chinese continental margin (Eurasian plate). The direction of the maximum compressional stress σ 1 shows a fan-shaped pattern that we interpret as resulting from the collision of a rigid body (the Luzon arc) indenting a more deformable material (the thick sediments of the Chinese continental margin). Simple analytical models qualitatively explain the fan-shaped pattern, but the influence of various parameters such as boundary conditions and rheology cannot be quantitatively accounted for by this approach. Consequently, we have used a finite element technique to compute the stresses and strains induced by the push of a rigid body against a two-dimensional, viscous material. The boundary conditions are the velocities based on plate kinematics. A motion in the N300°E direction best explains the stress trajectories observed in central Taiwan. Viscosity contrasts as well as small changes in the shape of the northern edge of the indenter have little influence on the computed stress pattern. The most important parameter is the direction of convergence. Our model quantitatively explains the general pattern of the stress trajectories observed in the collision zone of Taiwan, between the Philippine Sea plate and Eurasia.

Active collision in eastern Taiwan: The Coastal Range

A detailed analysis of Late Cenozoic faulting has been carried out in the Coastal Range of eastern Taiwan. 4000 measurements have been collected in 60 sites where faults affect formations that range from upper Miocene to Pleistocene and scattered Quaternary deposits. This large data set enables us to accurately determine paleostress axes in the area. During the major tectonic event (the Pleistocene Coastal Range orogeny), the direction of the maximum compressional stress σ 1 has remained constant. 25% of reconstructed σ 1 axes trend approximately 120; 90% trend between 090 and 140. Taking into account the internal deformation in the Coastal Range during folding and thrusting, these reconstructed directions of compression are remarkably homogeneous. Present faulting in the Longitudinal Valley is consistent with the paleostress pattern recorded in the adjacent Coastal Range formations. The Longitudinal Valley fault is a thrust, with a minor left-lateral strike-slip component (20% of the total movement). This quantitative estimate of motion is compatible with reconstructed Pleistocene paleostress and more general reconstructions of recent plate motion in the vicinity of Taiwan. The analysis of the focal mechanisms of earthquakes provides similar results. Finally, the analysis of the Coastal Range faults systems related to the collision of the volcanic Luzon arc (including the Luzon trough) with the Central Range of Taiwan provides a consistent picture of the distribution of stress patterns and deformations. The strikes of folds and thrusts in the Coastal Range are mainly controlled by the direction of convergent plate motion (125), whereas the overall trend of the Coastal Range is similar to that of the Central Range (020). The obliquity of convergent motion has resulted in the complex en échelon structure of the Coastal Range. The relative plate motion is not the sole factor controlling the development of a collision chain such as the Coastal Range; the existence of major pre-existing structures also plays an important role.

The Okinawa Trough: Genesis of a back-arc basin developing along a continental margin

The Okinawa Trough is the only example of a marginal back-arc basin type at a young stage of evolution that developed along a continental margin. The first phase of crustal stretching and subsidence occurred during late Miocene-early Pliocene times. A second major phase occurred after the latest Pliocene or early Pleistocene, with en échelon rift axes along the axes of the Okinawa Trough, oblique to the general trend of the arc and trench. Crustal separation and active spreading has occurred in the central and southern Okinawa Trough, but the north Okinawa Trough is only in a rifting stage. The development of the basin has several correlations with the general tectonism of the arc system: 1. (1) There was a synchronism between opening and subsidence of the Okinawa Trough and tilting and subsidence of the fore-arc terrace. 2. (2) Back-arc rifting was initiated in the volcanic arc and propagated along it during the Neogene. Remnants of a volcanic arc are found on the continental side of the basin. The beginning of the back-arc spreading observed in the central and southern Okinawa Trough correspond to a relative minimum in the activity of the volcanic arc. 3. (3) The timing and kinematic evolution of the Taiwan collision and the south Okinawa Trough opening suggest a connection between these two events. Such a collision-lateral back-arc opening model could explain the initiation of the opening of back-arc basins, but additional phenomena such as thermal convection, retreating trench model or anchored-slab model are indispensable to maintain extension in the back-arc basin.

Plate collision and paleostress trajectories in a fold-thrust belt: The foothills of Taiwan

Field analyses of compressional faulting and folding in the Foothills of western Taiwan enable us to reconstruct paleostress trajectories over a large area and to establish the relative chronology of tectonic events. Two main compressional events have contributed to the present structure of the fold-thrust belt. Stratigraphic data show that these events are Plio-Pliocene in age. Older normal faulting indicates NNW-SSE extension across the Chinesse passive continental margin during the Neogene. The two main compressional events of the Taiwan collision correspond to similar fan-shaped distributions of maximum compressive stress trajectories, with a counterclockwise shift of 30°–50° between the two events. Using the relationship between recent stress trajectories and the direction of recent plate motion as a guide, we reconstruct the direction of plate convergence for the older event. We suspect that the relative motion Philippine Sea plate-Eurasia has rotated counterclockwise of at least 35°–45° in Taiwan during collision. This conclusion is in agreement with independent plate tectonic reconstructions. Several problems provide objectives to further tectonic and paleomagnetic studies, including the duration and diachronism of compressional events as well as possible clockwise rotation of northernmost Taiwan.