Shu-Kun Hsu

East Asia plate tectonics since 15 Ma: constraints from the Taiwan region

Abstract 15 Ma ago, a major plate reorganization occurred in East Asia. Seafloor spreading ceased in the South China Sea, Japan Sea, Taiwan Sea, Sulu Sea, and Shikoku and Parece Vela basins. Simultaneously, shear motions also ceased along the Taiwan–Sinzi zone, the Gagua ridge and the Luzon–Ryukyu transform plate boundary. The complex system of thirteen plates suddenly evolved in a simple three-plate system (EU, PH and PA). Beneath the Manila accretionary prism and in the Huatung basin, we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations. These two patterns are rotated by 15°. They were formed by seafloor spreading before 15 Ma and belonged to the same ocean named the Taiwan Sea. Half-spreading rate in the Taiwan Sea was 2 cm/year from chron 23 to 20 (51 to 43 Ma) and 1 cm/year from chron 20 (43 Ma) to 5b (15 Ma). Five-plate kinematic reconstructions spanning from 15 Ma to Present show implications concerning the geodynamic evolution of East Asia. Amongst them, the 1000-km-long linear Gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the PH Sea plate; the formation of Taiwan was driven by two simple lithospheric motions: (i) the subduction of the PH Sea plate beneath Eurasia with a relative westward motion of the western end (A) of the Ryukyu subduction zone; (ii) the subduction of Eurasia beneath the Philippine Sea plate with a relative southwestward motion of the northern end (B) of the Manila subduction zone. The Luzon arc only formed south of B. The collision of the Luzon arc with Eurasia occurred between A and B. East of A, the Luzon arc probably accreted against the Ryukyu forearc.

Geodynamics of the Taiwan arc-arc collision

Abstract Hsu and Sibuet (1995), on the basis of an overview of the satellite-derived marine gravity anomalies, postulated that the Ryukyu subduction zone extended before the formation of Taiwan a few hundreds kilometres south of its present-day termination, and that Taiwan resulted from an arc-arc collision rather than from an arc-continent collision. An analysis of the structure and timing of rifting in the basins of the Southeast Asia continental shelf offshore and onshore Taiwan shows that they are located within four belts parallel to the main China shoreline. Rifting occurred at the same time within basins belonging to each of these four belts and becomes younger oceanward for each belt. As a first approximation, the four rifting phases occurred during Paleocene, Eocene, Oligocene to Early Miocene and early Middle Miocene times to Present. Ridges with volcanic products are present between these belts. They seem to be the same age as basins located immediately northwest. We interpret these basins and associated ridges as relict backarc basins and arcs of the Ryukyu subduction system which were successively active since the early Tertiary. The geographic distribution of basins and ridges suggests that the Ryukyu subduction zone extended from Japan to southwest Taiwan from early Tertiary to Early Miocene times. During the early Middle Miocene, the southeast portion of the subduction zone facing the Tainan basin and the future island Taiwan became inactive. Southwest of the Tainan basin, the Pearl River basins are tensional basins formed during the rifting of the northern South China Sea margin. Consequently, the geology of the Southeast Asia continental shelf supports the existence of a former subduction zone with which the Luzon arc entered into collision in the Late Miocene. The kinematic evolution of the Southeast Asia region is compatible with such constraints. Such a detailed kinematic evolution of the collision between the Luzon arc and the former Ryukyu subduction zone is proposed both in plan views and in cross-sections. Collision started with the compression and uplift of the Hsuehshan trough backarc basins, where the continental crust and lithosphere were thin and weak, followed by the compression and uplift of the Luzon and Ryukyu arcs. The Lichi and Kenting melanges are explained in the framework of the arc-arc collision model.

Depth to magnetic source using the generalized analytical signal

The advantage of using analytic signal techniques to determine magnetic parameters from magnetic anomalies is the independence of magnetization direction. In addition, using the amplitude ratio method to determine depths to magnetic sources can avoid tedious operations with several characteristic points and reduce interference effects. A drawback of this method, however, is the assumption that near-surface structures can be characterized adequately by step models. To improve mapping resolution, we have extended the amplitude ratio method to allow both step-like and dike-like structures. A criterion is constructed that discriminates between maxima from dike-like or step-like structures and significantly improves near-surface structural mapping. This avoids a bias in geological interpretation caused by the original assumption that all structures can be characterized by step models. Synthetic data and real data acquired offshore northern Taiwan demonstrate the effectiveness of this method.

Evidence for Early Cretaceous oceanic crust trapped in the Philippine Sea Plate

Abstract The Huatung Basin is a small oceanic basin located east of Taiwan. Previous age estimates from magnetic lineation studies indicated an Eocene age for the basin, and formation from the Central Basin Spreading Center of the West Philippine Basin during the last phase of spreading in Middle Eocene. New Ar/Ar ages obtained on gabbros dredged on oceanic basement highs of the Huatung Basin are Early Cretaceous. These old ages are consistent with Early Cretaceous ages determined on radiolarian assemblages from Lanyu Island (Luzon Arc). We have performed magnetic anomalies modeling for an Early Cretaceous oceanic crust. Our results are in good agreement with new Ar/Ar ages determinations. The best fit is indeed obtained with an opening of the Huatung Basin during the Early Cretaceous from 131 to 119 Ma, with a half spreading rate varying between 25 and 30 mm/yr. The spreading center appears to be located south of the actual basin. The abnormal depth (5500 m instead of 5900 m) and thickness (∼12 km instead of 6 km) of the crust beneath the basin indicate that there was probably an excess supply of magma during its formation. We propose that the basin is a fragment of the former ‘proto-south China Sea’ or possibly the ‘New Guinea Basin’ that has been trapped by the Philippine Sea Plate.

Is Taiwan the result of arc-continent or arc-arc collision?

Abstract Conventionally, it has been accepted that the formation of Taiwan results from the collision of the Luzon arc with the Eurasian continental margin. We suggest that Taiwan results from the collision of the Luzon arc with the former Ryukyu subduction zone. Before the collision, the latter extended a few hundred kilometres southwest of its present-day termination. In the early to middle Miocene, the subduction became inactive in its southwest portion and the Manila trench migrated northeastward, giving rise to the formation of the Luzon arc. The collision started in the late Miocene by closing the oceanic domain located between the Luzon arc and the former Ryukyu arc. Because the Luzon arc moved northwestward with respect to the former NE-SW trending Ryukyu arc, the oblique collision first resulted in the indentation on the south Ryukyu arc, west of 123.5°E, and the narrowing of the corresponding portion of the Okinawa trough prior to the uplift of Taiwan. Today, the Hsuehshan and Backbone Ranges represent the uplifted portion of the backarc basin and the Longitudinal valley corresponds to the suture zone between the former Ryukyu and the Luzon arcs. In this model, we propose to link, although they are not of the same age, two truncated backarc basins: the active Okinawa trough to the northeast and the former active Tainan backarc basin to the southwest. The former backarc basin extended from southwest of Taiwan to southern Japan and could have been initiated near the paleo-location of Taiwan as early as the Eocene-Oligocene. The result of the collision is the truncation of the ancient backarc basin associated with the shortening, deformation and uplift of the ancient outer arc and related backarc region, which provides the fundamental mechanism of the Taiwan mountain building. This model is in agreement with a counterclockwise rotation of the portion of Taiwan located west of the Coastal Range (belonging to the Luzon arc) and a clockwise rotation of the south Ryukyu arc. Earthquake focal mechanisms, geodetic observations and the distribution of the petrologic and structural features are also consistent with the proposed arc-arc collision model.

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.We demonstrate that the 30° clockwise bending of the southwestern RA and the consecutive faulting in the TRFZ are mainly due to the collision of the Luzon arc with the former RA. After the formation of Fault C, the counterclockwise rotated portion of the ancient RA located west of the Luzon arc was more parallel to the Luzon arc. This configuration should have increased the contact surface and friction between the Luzon arc and the ancient RA, which could have reduced the northward subduction of the Luzon are. Thus, the westward component of the compressive stress from the collision of the Luzon arc should become predominant in the collisional system resulting in the uplift of Taiwan. Presently, because the most active collision of the Luzon arc has migrated to the central Taiwan (at about 23° N; 121.2° E), the southwestern OT has resumed its extension. In addition, the later resistent subduction of the Gagua ridge could have reactivated the pre-existing faults A and B at 1 M.y. ago and present, respectively. From 9 to 4 M.y., a large portion of the Gagua ridge probably collided with the southwestern RA. Because of its large buoyancy, this portion of the ridge resisted to subduct beneath the Okinawa platelet. As a result, we suggest that a large exotic terrane, named the Gagua terrane, was emplaced on the inner side of the present Ryukyu trench. Since that period, the southwestern portion of the Ryukyu trench was segmented into two parallel branches separated by the Gagua ridge: the eastern segment propagated westward along the trench axis while the western segment of the trench retreated along the trench axis.

Geodynamic Context of the Taiwan Orogen

Four independent arguments suggest that the Ryukyu subduction zone extended from Japan to southwest Taiwan (118°E) from the late Cretaceous to early Miocene (17-18 Ma): i) An analysis of the structure and timing of rifting in the basins of the East Asia continental shelf and west of Taiwan shows that they are located within four belts parallel to the mainland Chinese shoreline, which becomes younger oceanward since early Tertiary. Ridges with volcanic products are present between these belts. We interpret these basins and associated ridges as relict backarc basins and arcs of the Ryukyu subduction system. ii) Subsidence curves across west Taiwan Basins show that rifting ceased 17-18 Ma. iii) A new shear wave velocity model suggests that the Ryukyu slab extended in the past southwest of Taiwan, beneath the northern China Sea margin. iv) A deep seismic line shot across the north-eastern South China Sea margin also suggests that this margin was active in the past. We conclude that about 15-20 Ma, the southwestern extremity of the Ryukyu subduction zone jumped from 118°E (southwest of the Tainan Basin) to 126°E (where the present-day trend of the Ryukyu subduction zone changes direction). Since that time, the southwestern extremity of the Ryukyu subduction zone continuously moved westwards to its present-day location at 122°E. Since the beginning of formation of proto-Taiwan during late Miocene (9 Ma), the subducting PH Sea plate moved continuously through time in a N307° direction at 5.6 cm/yr with respect to EU, tearing the EU plate.

Distribution of the East China Sea continental shelf basins and depths of magnetic sources

The acoustic basement map of the East China Sea, established by the Shanghai Offshore Petroleum Bureau with all available industry seismic data, shows the existence of a 30-km-wide, 10-km-deep basin, that we named the Ho Basin. The Ho Basin belongs to a series of elongated deep basins extending over 600 km east of the Taiwan-Sinzi Ridge and flanked to the East by a ridge named the Longwan Ridge in its northern part. This new system of basin and ridge was probably formed during middle Miocene, sometimes in between rifting episodes occurring in the Taipei Basin and Okinawa Trough. It complements the already defined system of five belts of backarc basins and associated arc volcanic ridges in the East China Sea, which are progressively younger from the Mainland China shoreline (late Cretaceous/early Tertiary) to the Okinawa Trough (Present). In order to determine the crustal thickness beneath the East China Sea continental shelf, we used a power spectrum method to calculate the depth of the top (Zt) and the centroid (Zo) of the magnetic basement by fitting a straight line through the high- and low-wave number portions of the power spectrum, respectively. Then, the depth of the base (Zb) is estimated from Zt and Zo. After optimizing the size of the data squares, we demonstrate that, except for basins more than 10 km deep, Zt corresponds to the basement depths and Zb, the depth of the Curie point, to the Moho depth. As wide-angle reflection and refraction data are scarce in the East China Sea, this method provides a way to characterize the crustal thickness of the East China Sea and to compute the theoretical heat flow values.

Lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone: an example of decreasing plate coupling

Abstract The Okinawa Trough is a backarc basin located behind the Ryukyu arc–trench system. The southernmost part of the Okinawa Trough (SPOT) displays different tectonic features from the rest of the Okinawa Trough. The SPOT area includes abundant seamounts with active hydrothermal venting and high heat-flow values. To understand better the rifting and magmatism context of the SPOT area, we examine the lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone. The results show that beneath the SPOT area the continental crust and mantle lithosphere thickness of ∼25–30 and 120 km, respectively, are thick with little continental thinning. The analysis of mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates. However, the two plates are actually decoupled as indicated by present-day interface earthquakes. This situation indicates that the southernmost Ryukyu subduction zone displays a transitory case of a changing plate coupling, from a strong to a weak plate coupling. Such a coupling/decoupling transition is probably associated with the collision of the Luzon arc with the Asian continental margin. Additionally, the curve of the mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone indicates that the submarine magmatism in the SPOT area is located within the volcanic arc area, suggesting early arc magmatism in the SPOT area.

Could a Sumatra-like megathrust earthquake occur in the south Ryukyu subduction zone?

A comparison of the geological and geophysical environments between the Himalaya-Sumatra and Taiwan-Ryukyu collision-subduction systems revealed close tectonic similarities. Both regions are characterized by strongly oblique convergent processes and dominated by similar tectonic stress regimes. In the two areas, the intersections of the oceanic fracture zones with the subduction systems are characterized by trench-parallel high free-air gravity anomaly features in the fore-arcs and the epicenters of large earthquakes were located at the boundary between the positive and negative gravity anomalies. These event distributions and high-gravity anomalies indicate a strong coupling degree of the intersection area, which was probably induced by a strong resistance of the fracture features during the subduction. Moreover, the seismicity distribution in the Ryukyu area was very similar to the pre-seismic activity pattern of the 2004 Sumatra event. That is, thrust-type earthquakes with a trench-normal P-axis occurred frequently along the oceanward side of the mainshock, whereas only a few thrust earthquakes occurred along the continentward side. Therefore, the aseismic area located west of 128°E in the western Ryukyu subduction zone could have resulted from the strong plate locking effect beneath the high gravity anomaly zone. By analogy with the tectonic environment of the Sumatra subduction zone, the occurrence of a potential Sumatra-like earthquake in the south Ryukyu arc is highly likely and the rupture will mainly propagate continentward to fulfill the region of low seismicity (approximately 125° E to 129° E; 23° N to 26.5° N), which may generate a hazardous tsunami.