Crustal Thinning of the Northern Continental Margin of the South China Sea

Magnetic data suggest that the distribution of the oceanic crust in the northern South China Sea (SCS) may extend to about 21 °N and 118.5 °E. To examine the crustal features of the corresponding continent–ocean transition zone, we have studied the crustal structures of the northern continental margin of the SCS. We have also performed gravity modeling by using a simple four-layer crustal model to understand the geometry of the Moho surface and the crustal thicknesses beneath this transition zone. In general, we can distinguish the crustal structures of the study area into the continental crust, the thinned continental crust, and the oceanic crust. However, some volcanic intrusions or extrusions exist. Our results indicate the existence of oceanic crust in the northernmost SCS as observed by magnetic data. Accordingly, we have moved the continent–ocean boundary (COB) in the northeastern SCS from about 19 °N and 119.5 °E to 21 °N and 118.5 °E. Morphologically, the new COB is located along the base of the continental slope. The southeastward thinning of the continental crust in the study area is prominent. The average value of crustal thinning factor of the thinned continental crust zone is about 1.3–1.5. In the study region, the Moho depths generally vary from ca. 28 km to ca. 12 km and the crustal thicknesses vary from ca. 24 km to ca. 6 km; a regional maximum exists around the Dongsha Island. Our gravity modeling has shown that the oceanic crust in the northern SCS is slightly thicker than normal oceanic crust. This situation could be ascribed to the post-spreading volcanism or underplating in this region.     

Crustal Structures of the Northernmost South China Sea: Seismic Reflection and Gravity Modeling

The South China Sea (SCS) is a marginal sea off shore Southeast Asia. Based on magnetic study, oceanic crust has been suggested in the northernmost SCS. However, the crustal structure of the northernmost SCS was poorly known. To elaborate the crustal structures in the northernmost SCS and off southwest Taiwan, we have analyzed 20 multi-channel seismic profiles of the region. We have also performed gravity modeling to understand the Moho depth variation. The volcanic basement deepens southeastwards while the Moho depth shoals southeastwards. Except for the continental margin, the northernmost SCS can be divided into three tectonic regions: the disturbed and undisturbed oceanic crust (8–12 km thick) in the southwest, a trapped oceanic crust (8 km thick) between the Luzon-Ryukyu Transform Plate Boundary (LRTPB) and Formosa Canyon, and the area to the north of the Formosa Canyon which has the thickest sediments. Instead of faulting, the sediments across the LRTPB have only displayed differential subsidence offset of about 0.5–1 s in the northeast side, indicating that the LRTPB is no longer active. The gravity modeling has shown a relatively thin crust beneath the LRTPB, demonstrating the sheared zone character along the LRTPB. However, probably because of post-spreading volcanism, only the transtension-shearing phenomenon of volcanic basement in the northwest and southeast ends of the LRTPB can be observed. These two basement-fractured sites coincide with low gravity anomalies. Intensive erosion has prevailed over the whole channel of the Formosa Canyon.     

Tectonic Significance of the Taitung Canyon, Huatung Basin, East of Taiwan

Since the beginning of formation of Proto-Taiwan, the subducting Philippine (PH) Sea plate has moved continuously through time in the N307° direction with respect to Eurasia (EU), tearing the EU plate. The subducting EU plate includes a continental part in the north and an oceanic part in the south. The boundary B between these two domains corresponds to the eastern prolongation of the northeastern South China Sea ocean-continent transition zone. In the Huatung Basin (east of Taiwan), the Taitung Canyon is N065° oriented and is close and parallel to B. Seismic profiles show that the southern flank of the canyon corresponds to a fault with a normal component of a few tens of meters in the sediments and possible dextral shearing. Several crustal earthquakes of magnitude >%6 are located beneath the trend of the Taitung Canyon and focal mechanisms suggest that the motion is right-lateral. Thus, faulting within the sedimentary sequence beneath the Taitung Canyon is a consequence of underlying dextral strike-slip crustal motions. As the continental part of the EU slab located north of B has been recently detached, some subsequent dextral strike-slip motion might be expected within the EU slab, along the ocean-continent transition zone, which is a potential zone of weakness. We suggest that the dextral strike-slip motion along the ocean-continent boundary of the EU slab might trigger the observed dextral strike-slip motion within the overlying PH Sea crust and the associated faulting within the sediments of the Huatung Basin, beneath the Taitung Canyon. An erratum to this article is available at .     

Change of crustal gravitational potential energy in the Taiwan orogen by the Chi-Chi earthquake sequence

The active convergence between the northwest corner of the Philippine Sea Plate and the southeast margin of the Eurasian Plate has given rise to the Taiwan mountain-building and produced numerous earthquakes. Among the earthquakes, the 1999 Chi-Chi earthquake is the largest one recorded in the century. In this study, we examine the crustal gravitational potential energy (GPE) change in the Taiwan orogen caused by the Chi-Chi earthquake sequence, which was catalogued by the regional broadband seismometer array for a whole year. As a result, we find that the crust was going up and down randomly during the earthquake sequence, but an overall cumulative gain of the crustal GPE, +1.82×10¹⁶ J, was rapidly achieved in 1 month after the main shock. The crustal GPE was nearly still afterwards and reached +1.90×10¹⁶ J in 1 year. Spatially, although the main surface faulting has occurred in western Taiwan, the crustal GPE gain is mainly distributed in central Taiwan at the area where the existing crustal GPE is high and the existing lithospheric GPE is relatively low. The crustal GPE loss by the Chi-Chi earthquake sequence can also be observed and is generally distributed at both sides of the crustal GPE gain area. The crustal GPE gain mainly found in central Taiwan corroborates that the uplift of the Taiwan orogen is principally taking place in central Taiwan, rather than in the more hazardous western Taiwan.     

A derivative-based interpretation approach to estimating source parameters of simple 2D magnetic sources from Euler deconvolution, the analytic-signal method and analytical expressions of the anomalies

The major advantage of using either the analytic‐signal or the Euler‐deconvolution technique is that we can determine magnetic‐source locations and depths independently of the ambient earth magnetic parameters. In this study, we propose adopting a joint analysis of the analytic signal and Euler deconvolution to estimate the parameters of 2D magnetic sources. The results can avoid solution bias from an inappropriate magnetic datum level and can determine the horizontal locations, depths, structural types (indices), magnetization contrasts and/or structural dips. We have demonstrated the feasibility of the proposed method on 2D synthetic models, such as magnetic contacts (faults), thin dikes and cylinders. However, the method fails to solve the parameters of magnetic sources if there is severe interference between the anomalies of two adjacent magnetic sources.     

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.     

Improvement of earthquake locations with the Marine Cable Hosted Observatory (MACHO) offshore NE Taiwan

In order to improve the locating capability for offshore earthquakes and tsunamis monitored off northeastern Taiwan, a cable-based ocean bottom seismographic observatory named “Marine Cable Hosted Observatory” (MACHO) was constructed and began operation at the end of 2011. The installed instruments of the observatory include a broadband seismometer, a strong-motion seismometer and a pressure gauge. In addition, various scientific instruments could be deployed for other purposes as well. At present, the seismic data are transmitted in real-time via a fiber cable, and integrated into the current inland seismographic network in Taiwan. The ocean bottom station has contributed to provide high quality seismic data already. According to observations from January 2012 to June 2013, there were a total of 15,168 earthquakes recorded by the system. By using the data from the ocean bottom station, the number of relocated earthquakes with an azimuth gap less than 180 degrees substantially increase about 34 %. Meanwhile, the root–mean–square of the time residual, the error in epicenter, and the error in depth of the earthquake locations decrease. Therefore, the implementation of MACHO has the advantage of extending the coverage of existing the Taiwan seismic network to the offshore, providing more accurate and real-time seismic data for offshore earthquakes monitoring. The results show that MACHO is crucial and necessary for monitoring seismic activities in northeastern Taiwan.     

Crustal features of the northeastern South China Sea: insights from seismic and magnetic interpretations

We interpret seven two-dimensional deep-penetration and long-offset multi-channel seismic profiles in the northernmost South China Sea area, which were collected by R/V Marcus G. Langseth during the TAIwan GEodynamics Research (TAIGER) project in 2009. To constrain the crustal characteristics, magnetic inversion and forward magnetic modeling were also performed. The seismic results clearly show tilted faulting blocks in the upper crust and most of the fault plane connects downward to a quasi-horizontal detachment as its bottom in the south of the Luzon-Ryukyu transform plate boundary. North of the plate boundary, a small-scale failed rifted basin (minimum 5 km in crustal thickness) with negative magnetization probably indicates an extended continental origin. Significant lower crustal material (LCM) was imaged under a crustal fracture area which indicated a continent and ocean transition origin. The thickest LCM (up to 6.5 km) is located at magnetic isochron C15 that is probably caused by the magma supply composite of a Miocene syn-rift volcanic event and Pliocene Dongsha volcanic activity for submarine volcanoes and sills in the surrounding area. The LCM also caused Miocene crustal blocks to be uplifted reversely as 17 km crustal thickness especially in the area of magnetic isochron C15 and C16. In addition, the wide fault blocks and LCM co-existed on the magnetic striped area (i.e. C15–C17) in the south of the Luzon-Ryukyu transform plate boundary. Magnetic forward modeling suggests that the whole thick crustal thickness (>12 km thick) needs to be magnetized in striped way as oceanic crust. However, the result also shows that the misfit between observed and synthetic magnetic anomaly is about 40 nT, north of isochron C16. The interval velocity derived from pre-stack time migration suggests that the crust is composed of basaltic intrusive upper crust and lower crustal material. The crustal nature should refer to a transition between continent and ocean. Thus, the magnetic reversals may be produced in two possible ways: basaltic magma injected along the crustal weak zone across magnetic reversal epoch and because some undiscovered ancient piece of oceanic crust existed. The crustal structure discrimination still needs to be confirmed by future studies.     

Possible northward extension of the Philippine Fault Zone offshore Luzon Island (Philippines)

The Philippine Fault Zone, a system of left-lateral strike-slip faults traversing the length of the Philippine Islands, is associated with the oblique convergence between the Philippine Sea Plate (PSP) and the Eurasian Plate (EP). Although it is a major deformational structure within the diffuse PSP–EP convergent boundary, some of its segments, particularly its marine extensions, are not well studied. To investigate the crustal deformation in the marine prolongation of the Philippine Fault Zone offshore Luzon Island, multi-channel seismic (MCS) data, gravity data and centroid moment tensor solutions were used in this study. Focal mechanism solutions from the Global CMT catalog were inverted to determine the average principal stress directions and consequently understand the prevailing stress regime in the study area. The stress inversion results indicate that the direction of maximum compression (σ1) is 321°N, which coincides with the PSP–EP convergence direction. From the MCS profiles, the study area was subdivided into deformation zone and a relatively stable zone. Thrust faulting, folding and general uplift are observed in the deformation zone. This zone is further subdivided into the active and inactive segments. In the active segment, uplift is occurring in the submarine ridge. This deformation pattern can be related to the ongoing uplift in some regions bisected by the PFZ. The inactive segment, characterized by intense folding of the sequences and faulting of the basement and overlying sequences, is suggested as the precursor of the Philippine Fault Zone. Deformation appears to be recently shifted to the east as delineated by an uplifted N-NW trending submarine ridge offshore NW Luzon Island.