ReviewSubsurface imaging, TAIGER experiments and tectonic models of Taiwan
Introduction
Taiwan sits on a portion of the convergent boundary between the Eurasian and the Philippine Sea plates, and is seismically very active, with frequent earthquakes under and around the island. Although earthquakes occur much more frequently on the Philippine Sea side, events do occur in the Taiwan Strait, as well. The island is also very young – estimates range from 6 million to perhaps a few millions – but the most active phase may have been in the last million years (Lee et al., 2006). Taiwan is often called a natural laboratory because it is an environment in which most, if not all, of the tectonic processes of mountain building are probably in progress and can therefore be monitored at the surface or imaged at depth. Beginning in the early 1970s when the general geologic framework of Taiwan tectonics became clear (Ho, 1972, Ho, 1986) and when the plate tectonic theory gained momentum (Isacks et al., 1968, Bird and Dewey, 1970), the island of Taiwan was recognized as the product of an arc–continent collision (Biq, 1972, Chai, 1972) (Fig. 1). It soon became the type locality for studies of such collisions (Suppe, 1981, Davis et al., 1983, Moore and Twiss, 1995). A large number of papers either addressing the Taiwan orogen directly, or relevant to it have been published, many in special volumes (e.g., Byrne and Liu, 2002, Brown and Ryan, 2011). Basic surveys on Taiwan geology (Ho, 1986), metamorphic rocks (Liou, 1981, Ernst and Jahn, 1987; Yui, 2000; Beyssac et al., 2007), thermochronology (e.g., Liu et al., 2000, Willett et al., 2003, Lee et al., 2006), tectonics (Biq, 1972, Wu, 1978, Suppe, 1981, Suppe, 1984, Wu et al., 1997) and geodynamics (Willett et al., 2003, Kaus et al., 2008, Lavier et al., 2013) are all very useful introductions. Some of these will be highlighted in the course of our discussion. A simple geologic map with the main Taiwan provinces is shown in Fig. 2. Interpretations of the main geologic features will be discussed in context in the following section.
As is true elsewhere in the world, the acquisition and understanding of geophysical data lagged behind geology in Taiwan. The quality of world wide seismic data has improved markedly since the early 1960s, with modern seismic networks on the island evolving more rapidly since 1973 (Wang and Shin, 1998). A basis for tectonic interpretations in the 1980s and early 1990s was provided from several sources: proprietary use of Chinese Petroleum Company (CPC) seismic profiles in the Foothills (e.g., Suppe, 1976, Chow et al., 1986), better quality seismicity and seismic velocities from modern Taiwan seismic networks (Tsai, 1986) and results from the World Wide Standard Seismograph Network (WWSSN), (Wu, 1970, Wu, 1978, Pezzopane and Wesnousky, 1989). Naturally, some of the earlier conclusions which were based on insufficient data needed to be revised later; for example, the idea of a crustal transform fault across the Ryukyu Trench near 123°E longitude (Wu, 1970) was later thought to be a tear in the PSP (Wu, 1978). Similarly, many conclusions based on early data need to be re-evaluated in light of new observations and ideas.
After 1990 the installment of new seismic networks accelerated. The establishment of the island-wide short period network (Wang and Shin, 1998) improved earthquake reporting and provided the basic data for a series of papers on tomography (Rau and Wu, 1995; Kim et al., 2005; Wu et al., 2007a, Wu et al., 2007b and others) and on precise relocation of seismicity (e.g., Wu et al., 2004, Chou et al., 2006). Later, data from two broadband networks allowed the use of waveforms in routine moment-tensor solutions (e.g., Kao et al., 1998) and S-splitting analysis (Rau et al., 2000, Kuo-Chen et al., 2009). These results contributed in important ways to tectonic studies of Taiwan.
A number of fundamental characteristics of the Taiwan orogen were derived through the previously mentioned works. For example, a thick root of over 50 km under the Central Range was imaged tomographically, while seismicity and moment tensor solutions (from which double-couple focal mechanism can be derived) were applied to understand the mechanics of both shallow- and deep crustal faulting and subduction (Wu et al., 2004, Chen et al., 2002, Kao and Rau, 1998). Additionally, the absence or presence of seismicity noted in different parts of the crust (Wu et al., 1989, Wu et al., 2004) has been interpreted in terms of the rheological properties of the crust. Nevertheless, many outstanding questions remain: for example, what role does the subduction of the EUP play; how much does the PSP deform in the collision; what are the mechanical and petrological processes in the core of the orogen; and what changes occur in crustal and mantle processes in the transition from subduction in southern Taiwan to collision in central Taiwan?
Similar questions have been raised regarding mountain ranges elsewhere in the world, whether they were active a hundred million years ago or only active since the early Tertiary (for example, Dabie, Appalachians, Himalayas and Alps). Studying an orogen as young and active as Taiwan, rare in the world, has distinct advantages. In particular, the mountain building processes we can uncover are most probably directly responsible for the orogeny. A comprehensive understanding can undoubtedly be gained by combining knowledge from Taiwan research with that from old and mature mountains. To advance beyond our present knowledge of Taiwan tectonics it is especially critical to add subsurface and geodetic data, as detailed as possible, to the existing rich archives of geological information upon which many hypotheses are based. New subsurface data can be used to effectively test and improve the models if sufficient spatial resolution and lateral coverage can be achieved. Tomographic Vp imaging has contributed a great deal to tectonic interpretation; with the addition of S-wave velocities, the composition of the crust and, pressure and temperature may now be constrained, as we show later. Furthermore, since offshore eastern Taiwan is a part of the collision system, marine observations are necessary. Resistivity and laboratory rock velocity can, in addition to seismic parameters, provide independent information regarding the state of the rock materials. Finally, now that numerical geodynamics is capable of modeling complex geological processes it would be desirable to explore interpretations that are guaranteed to be mechanically sound and geologically compatible with current knowledge. These are actually the design principles of the TAIGER project (2004–2009). The project has thus far produced a large amount of land and marine geophysical data. Ongoing analyses of this data have produced new images of the lithosphere under Taiwan that can serve as the basis for model building and testing.
In this paper, rather than conducting a comprehensive review of all aspects of works relevant to Taiwan tectonics, we mainly aim to review issues regarding the structures at depth based on fresh results of subsurface imaging from the TAIGER (TAiwan Integrated GEodynamics Research) project. In view of the wide acceptance of Taiwan as an archetype of arc–continent orogeny, identifying all the possible factors that enter into model construction will benefit the study of orogeny as a whole. In the following (Section 2) we shall first discuss the models that have been proposed. In the subsequent section (Section 3) we present a succinct discussion of the geophysical studies of Taiwan; readers familiar with these subjects may elect to skip ahead. After presenting results of new analyses (Sections 4 Imaging the Taiwan orogen – TAIGER tomography and S-splitting, 5 Taiwan tectonics in light of recent observations) we shall discuss their bearing on different aspects of Taiwan tectonics (Section 6). Although our discussion of models in this paper is intimately tied to geodynamic analysis in the TAIGER group, we will not include any details of the analysis at this time, as a separate paper is being prepared (Lavier et al., 2013).
Section snippets
Tectonics of Taiwan, models and issues of model testing
Studies of the geology of a region ultimately aim at learning the time history of the geological processes that have occurred at different locations within the region. Ideally, surface rock samples can yield the age and the environment under which the rocks, or particular components of the rocks, were formed, and the chemical reactions they underwent. But not all rocks of interest are exposed for sampling, nor may they yield the needed information. Difficult terrain and a lack of outcrops would
Evolving facilities and studies of Taiwan subsurface structures
Most of the recent tectonic studies benefit from three seismic “observations”: velocity structures, seismicity and focal mechanisms. But seismic anisotropy also has important implications for regional tectonics and for the depth to which orogenic processes may extend. Magnetotelluric resistivity mapping has also been shown to be an important addition to seismic methods for its ability to indicate fluid and melting conditions. The gravity field in Taiwan has shown that the Bouguer minimum does
Imaging the Taiwan orogen – TAIGER tomography and S-splitting
Kuo-Chen (2011) and Kuo-Chen et al., 2012a, Kuo-Chen et al., 2012b obtained subsurface P and S wave images using first arrival picks from local earthquakes, and from teleseisms and active sources from the TAIGER stations. These were augmented by data from the Taiwan permanent broadband networks described above. The imaging uses a code for combined local earthquake/teleseismic tomography based on spherical geometry by Roecker et al. (2006). Two sets of P-wave tomographic inversions are shown:
Taiwan tectonics in light of recent observations
Taiwan is a relatively compact orogen but the changes in structure and tectonics, both along- and perpendicular to the strike are quite pronounced, especially when subsurface structures are included in the consideration. With its distinct PSP subduction zone offshore of NE Taiwan and the EUP zone in the south, the orogen can conveniently be divided into three subunits, albeit lacking sharp boundaries: (1) southern Taiwan, south of a line linking the cities of Tainan and Taitung (TTL in Fig. 1),
Discussion
The tomographic velocity images, seismicity and focal mechanisms explored in this paper represent a snapshot of cumulative and ongoing deformation resulting from the collision of EUP and PSP, and as such can be used as a “key to the past” for deciphering the processes of mountain building. The TAIGER tomographic and seismicity images of Taiwan in particular highlight the 3-D aspects along-strike of the orogen because of the different tectonic processes in operation, namely, (1) the subduction
Conclusion
The young and active Taiwan orogen provides a unique environment for detailed studies of orogenic processes. Analyses of recently acquired subsurface data have added important constraints to the testing and exploring of models for the collision of plates in Taiwan. The major subsurface structures we have mapped can readily be understood in terms of the EUP/PSP plate geometry and the collision tectonics of Taiwan.
The rapid uplift in the Central Range, the dramatic crustal thickening, the
Acknowledgements
The authors wish to acknowledge the contributions of both US and Taiwan investigators participating in the TAIGER project; specifically Profs. C.S. Lee, C.S. Liu, C.Y. Wang and Dr. B.S. Huang without their collaborations the land and marine operations could not have been carried out. Discussions with US and Taiwan colleagues have helped in clarifying many ideas. IRIS/PASSCAL provided land broadband, active source and sea-land instrumentation and field support. US/OBSIP provided marine passive
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