Chao-Shing Lee

Inversion of a hyper-extended rifted margin in the southern Central Range of Taiwan

Seismic reflection and wide-angle data acquired across, south, and west of Taiwan show that extended to hyper-extended continental crust of the Chinese continental margin is present more than 200 km south of the shelf and is subducting at the Manila Trench. Furthermore, crustal-scale tomographic velocity models show that this crust is underthrusted to ∼15 km depth below the accretionary prism, where it then is structurally underplated to the base of the prism. We document an increasing volume of accreted crust from south to north, and in our northern transect high-velocity material of the accretionary prism can be directly linked to outcrops of Central Range basement rocks. In map view the Central Range of Taiwan is clearly contiguous with the Hengchun Peninsula and Hengchun submarine ridge to the south. Accordingly, we propose a new model in which the Central Range forms directly from the accretionary prism, including the basement core, which originates from subducted, and then accreted, extended to hyper-extended continental crust.

Deep crustal structure of an arc‐continent collision: Constraints from seismic traveltimes in central Taiwan and the Philippine Sea

The collision of continental crust of the Eurasian Plate with the overriding Luzon Arc in central Taiwan has led to compression, uplift, and exhumation of rocks that were originally part of the Chinese rifted margin. Though the kinematics of the fold-thrust belt on the west side of the orogen has been described in detail, the style of deformation in the lower crust beneath Taiwan is still not well understood. In addition, the fate of the Luzon Arc and Forearc in the collision is also not clear. Compressional wave arrival times from active-source and earthquake seismic data from the Taiwan Integrated Geodynamic Research program constrain the seismic velocity structure of the lithosphere along transect T5, an east-west corridor in central Taiwan. The results of our analysis indicate that the continental crust of the Eurasian margin forms a broad crustal root beneath central Taiwan, possibly with a thickness of 55 km. Compressional seismic velocities beneath the Central Range of Taiwan are as low as 5.5 km/s at 25 km depth, whereas P wave seismic velocities in the middle crust on the eastern flank of the Taiwan mountain belt average 6.5–7.0 km/s. This suggests that the incoming sediments and upper crust of the Eurasian Plate are buried to midcrustal depth in the western flank of the orogen before they are exhumed in the Central Range. To the east, the Luzon Arc and Forearc are deformed beneath the Coastal Range of central Taiwan. Fragments of the rifted margin of the South China Sea that were accreted in the early stages of the collision form a new backstop that controls the exhumation of Eurasian strata to the west in this evolving mountain belt.

Marine Cable Hosted Observatory (MACHO) Project in Taiwan

Taiwan is located in a junction corner between the Philippine sea plate and Eurasian plate. Because of active convergence, numerous earthquakes have taken place in and around Taiwan. On average, there are about two earthquakes greater than magnitude 6 each year and over 70% of earthquakes occurred in the offshore area. Because of the subduction of Philippine Sea Plate beneath the western end of the Ryukyu Arc and northern Taiwan, both the tectonics and seismic activity are intensive. The 2004 Sumatra earthquake has induced giant tsunami attacking coastal countries of South Asia. In a similar geodynamic context, the Sumatra event has aroused the attention of Taiwan government. Specialists from Taiwan earth scientists and ocean engineers have quickly teamed up to discuss the potential and mitigation of natural hazards from the western end of the Ryukyu subduction zone. To construct a submarine cable observatory off eastern Taiwan (MACHO project) was proposed. MACHO means a sea goddess who protects people at sea. The purpose of MACHO project has several folds. Firstly, the extension of seismic stations on land to offshore area can increase the resolution of earthquake relocating. Secondly, the extension of seismic stations may obtain tens of second before the destructing seismic waves arrive on land or tens of minute before the arrival of giant tsunami, which is helpful for earthquake or tsunami warning. Thirdly, the seafloor scientific station can monitor the active volcanoes in the Okinawa Trough, which is directly adjacent to the Ilan plain in northeastern Taiwan. Fourthly, the seafloor observatory can be used to continuously study the Kurosho current, off eastern Taiwan. The MACHO project has been granted for the fiscal year of 2007. The MACHO project is expected to be fulfilled in 2009.

Spatial variations in the frequency-magnitude distribution of earthquakes in the southwestern Okinawa Trough

The relations between the frequency of occurrence and the magnitude of earthquakes are established in the southern Okinawa Trough for 2823 relocated earthquakes recorded during a passive ocean bottom seismometer experiment. Three high b-values areas are identified: (1) for an area offshore of the Ilan Plain, south of the andesitic Kueishantao Island from a depth of 50 km to the surface, thereby confirming the subduction component of the island andesites; (2) for a body lying along the 123.3°E meridian at depths ranging from 0 to 50 km that may reflect the high temperature inflow rising up from a slab tear; (3) for a third cylindrical body about 15 km in diameter beneath the Cross Backarc Volcanic Trail, at depths ranging from 0 to 15 km. This anomaly might be related to the presence of a magma chamber at the base of the crust already evidenced by tomographic and geochemical results. The high b-values are generally linked to magmatic and geothermal activities, although most of the seismicity is linked to normal faulting processes in the southern Okinawa Trough.

Introduction to the TAIGER special issue of Marine Geophysical Research

With the advent of the plate tectonics paradigm in the 1960s and 1970s, Earth scientists began to understand mountain-building, basin formation, earthquakes and a myriad other processes that shape our world in terms of moving plates and deformation focused along their boundaries. In this period, early adopters such as Chai (1972), Biq (1972), and Wu (1978) recognized that Taiwan was formed by a special process: the collision of an island arc with a continental margin. This is the special or terminal case of subduction where the ocean basin floored by the subducting plate is completely consumed and the advancing arc and forearc make contact with the continental margin. While this general idea to understand Taiwan has stood the test of time and remains valid, the quest to better understand how arc-continent collision works has continued. As one of the few sites of active arccontinent collision on the Earth today, Taiwan has continued to be one of the ‘‘type’’ study areas for this process. The TAIGER (TAiwan Integrate GEodynamic Research) project was developed to again use the Taiwan area as a natural laboratory to reveal the evolution and lithospheric structure of an ongoing collision. While previous work in the Taiwan area had been fruitful, improved instrumentation and analysis techniques available for TAIGER and the lessons learned elsewhere in the world suggested that significant progress could be made. The concept behind TAIGER was to use a dense passive seismic network onshore in Taiwan to determine full crustal and lithospheric structure, and to use active-source seismology, onshore with explosive sources, offshore with airgun sources, plus offshore to onshore recording, to develop more detailed models of crustal structure and images of crustal deformation. With the generous support of the United States National Science Foundation Continental Dynamics program, Taiwan’s National Science Council, Ministry of the Interior, and Central Geological Survey of the Ministry of Economic Affairs, the TAIGER project became the largest coordinated geophysical effort to study the tectonics of Taiwan and evolution of arc-continent collision in this vicinity. Figure 1 shows most instrument locations for passive and active seismic recording as well as seismic acquisition ship tracks followed by the R/V Marcus Langseth during the TAIGER project. The marine, active-source data acquisition took place during three R/V Langseth cruises, MGL0905, MGL0906, and MGL0908, starting in 1 April 2009 and ending in 25 July 2009. A shorter, 8-day cruise, MGL0907, occurred during 7–14 June 2009 to recover 20 broad band ocean bottom seismographs (OBS) that had been deployed a year earlier from the R/V Melville. Determining crustal structure across Taiwan and in a wide area surrounding was a prime objective for TAIGER. Thus *269 OBS stations and 243 land stations were deployed and more than 10,000 km of multichannel seismic reflection (MCS) data were recorded. In addition, there were 10 other ships working together with the R/V Langseth at various times during the TAIGER marine surveys for the OBS deployments and recoveries, including Taiwanese research vessels, fishing boats, tourist ships, and others. As seen in Fig. 1, the TAIGER project targeted all aspects of the arc-continent collision. The particular advantages of K. D. McIntosh (&) Institute for Geophysics, University of Texas at Austin, Austin, TX 78758, USA e-mail: kirk@ig.utexas.edu

Imaging of Gas Hydrates in the Northernmost South China Sea from Multi-component OBS Data

More than 100 Ocean-bottom seismometers (OBS), with a relatively dense spacing of about 500-1000 m, were deployed and recovered for exploring gas hydrates in the northernmost South China Sea. P-wave velocity models have been determined from travel-time inversion of hydrophone and vertical components of OBS data whereas the Poisson’s ratio models are determined from shear waves, converted at the sea floor and the bottom-simulating reflector (BSR), selected from two horizontal components of OBS data. A layer-stripping and Monte-Carlo inversion of the blocky model of the Poisson’s ratio is applied. Furthermore, based on P-wave velocity, Poisson’s ratio and rock physics of the three-phase effective medium, saturation of gas hydrates is estimated. The results show that hydrates are imaged by a relatively low Poisson’s ratio (0.44-0.46) below anticlines and free gas is characterized by a relatively high Poisson’s ratio (0.485-0.5) beneath most of the BSR. We also observe that saturation of hydrates (30%) in the passive continental margin of the South China Sea is greater than that (15-25%) in the active accretionary wedge off SW Taiwan.

Origin of Gas Hydrates Imaged from the Acoustic Basement in the Northernmost South China Sea by Using OBS

During the survey of TAiwan Integrated GEodynamics Research (TAIGER) in 2009, we recovered 36 ocean-bottom seismometers (OBS) along 3 multi-channel seismic (MCS) profiles for investigating the origin of gas hydrates in the northernmost South China Sea off SW Taiwan. In the accretionary prism of the Manila subduction zone, we observed gas hydrates accumulated in the closed areas where were formed by folding imaged from a strong variation of the lateral velocity (3-4.5 km/s) in the acoustic basement. In the continental slope, gas was migrated from the rifting basement to the anticline. The rifting basement with a relatively low velocity of 3-4.5 km/s in the extended continent of the northernmost South China Sea resulted from magma intrusion along normal faults since the paleo-seafloor spreading. We suggest that gas generated in the deeper structures may be migrated along faults to accumulate hydrates in the sedimentary layers.