Journal of Asian Earth Sciences 90 (2014) 173–208

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Journal of Asian Earth Sciences

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Review Subsurface imaging, TAIGER experiments and tectonic models of ⇑ Francis T. Wu a,b,c, , H. Kuo-Chen c,a, K.D. McIntosh d a Department of Geological Sciences, SUNY, Binghamton, NY, USA b Department of Earth Sciences, USC, Los Angeles, CA, USA c Department of Earth Sciences, National Central University, Taiwan d Institute for Geophysics, University of Texas at Austin, TX, USA article info abstract

Article history: The seismicity, deformation rates and associated erosion in the Taiwan region clearly demonstrate that Received 19 April 2013 plate tectonic and orogenic activities are at a high level. Major geologic units can be neatly placed in Received in revised form 25 March 2014 the plate tectonic context, albeit critical mapping in specific areas is still needed, but the key processes Accepted 31 March 2014 involved in the building of the island remain under discussion. Of the two plates in the vicinity of Taiwan, Available online 19 April 2014 the (PSP) is oceanic in its origin while the (EUP) is comprised partly of the Asian continental lithosphere and partly of the transitional lithosphere of the South China Sea basin. It Keywords: is unanimously agreed that the collision of PSP and EU is the cause of the Taiwan orogeny, but several Tectonics models of the underlying geological processes have been proposed, each with its own evolutionary his- Orogeny Tomography tory and implied subsurface tectonics. Seismicity TAIGER (TAiwan Integrated GEodynamics Research) crustal- and mantle-imaging experiments recently Focal mechanisms made possible a new round of testing and elucidation. The new seismic tomography resolved structures Collision under and offshore of Taiwan to a depth of about 200 km. In the , the steeply east-dipping high velocity anomalies from southern to central Taiwan are clear, but only the extreme southern part is Plate boundary associated with seismicity; toward the north the seismicity disappears. The crustal root under the Central Range is strongly asymmetrical; using 7.5 km/s as a guide, the steep west-dipping face on the east stands in sharp contrast to a gradual east-dipping face on the west. A smaller root exists under the Coastal Range or slightly to the east of it. Between these two roots lies a well delineated high velocity rise spanning the length from Hualien to Taitung. The 3-D variations in crustal and mantle structures parallel to the trend of the island are closely correlated with the plate tectonic framework of Taiwan. The crust is thickest in the central Taiwan collision zone, and although it thins toward the south, the crust is over 30 km thick over the subduction in the south; in northern Taiwan, the northward subducting PSP collides with Tai- wan and the crust thins under northern Taiwan where the subducting indenter reaches 50 km in depth. The low Vp/Vs ratio of around 1.6 at a mid-crustal depth of 25 km in the Central Range indicates that cur- rent temperatures could exceed 700 °C. The remarkable thickening of the crust under the Central Range, its rapid uplift without significant seismicity, its deep exhumation and its thermal state contribute to make it the core of orogenic activities on Taiwan Island. The expanded network during the TAIGER deployment captured broadband seismic data yielding enhanced S-splitting results with mainly SKS/SKKS data. The polarization directions of the fast S-waves follow very closely the structural trends of the island, supporting the concept of a vertically coherent Tai- wan orogeny in the outer few hundred kilometers of the Earth. Ó 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 174 2. Tectonics of Taiwan, models and issues of model testing ...... 176 3. Evolving facilities and studies of Taiwan subsurface structures ...... 180

⇑ Corresponding author at: Department of Geological Sciences, SUNY, Binghamton, NY, USA. Tel.: +1 607 727 1933. E-mail address: [email protected] (F.T. Wu). http://dx.doi.org/10.1016/j.jseaes.2014.03.024 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved. 174 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

3.1. Development of local/worldwide seismic networks and Taiwan research ...... 180 3.2. Local, teleseismic and global tomography ...... 180 3.3. Active source crustal studies, joint active/passive onshore/offshore profiles and receiver functions ...... 181 3.4. Surface wave studies...... 181 3.5. Magnetotellurics, gravity and magnetics ...... 182 3.6. S-splitting ...... 182 3.7. GPS and leveling in Taiwan ...... 182 3.8. TAIGER project...... 182 4. Imaging the Taiwan orogen – TAIGER tomography and S-splitting ...... 183 5. Taiwan tectonics in light of recent observations ...... 189 5.1. Southern Taiwan subduction and the transition to collision ...... 189 5.1.1. Benioff seismicity and subduction under southern Taiwan ...... 189 5.1.2. Deformation of the crust from the southern Coastal Range to Hengchun ...... 190 5.1.3. Central Range, Foothills and the Coastal Plain – from central to southern Taiwan ...... 191 5.1.4. Plate motion-related deformation of southern Taiwan ...... 192 5.2. Central Taiwan – mature collision ...... 193 5.2.1. Deformation, vertical and horizontal, in central Taiwan ...... 194 5.2.2. Seismicity in the higher ranges, under the Western Foothills and under the Coastal Plain ...... 194 5.2.3. Crustal seismicity and rheology...... 195 5.2.4. High angle reverse structure under the Lishan fault ...... 196 5.2.5. Seismicity associated with the Longitudinal Valley and faulting ...... 197 5.2.6. High velocity rise under eastern Taiwan...... 198 5.2.7. Resistivity profile ...... 199 5.3. Northern Taiwan ...... 199 6. Discussion...... 200 6.1. Regarding the detachment, the deformation front and the Manila Trench ...... 201 6.2. Trench, forearc block and collision...... 202 6.3. Mountain building and crust/mantle processes in Taiwan ...... 203 6.4. Vertical coherent deformation vs. EUP subduction under Taiwan ...... 204 7. Conclusion ...... 204 Acknowledgements ...... 205 References ...... 205

1. Introduction As is true elsewhere in the world, the acquisition and under- standing of geophysical data lagged behind geology in Taiwan. Taiwan sits on a portion of the between The quality of world wide seismic data has improved markedly the Eurasian and the Philippine Sea plates, and is seismically very since the early 1960s, with modern seismic networks on the island active, with frequent earthquakes under and around the island. evolving more rapidly since 1973 (Wang and Shin, 1998). A basis Although earthquakes occur much more frequently on the Philip- for tectonic interpretations in the 1980s and early 1990s was pro- pine Sea side, events do occur in the , as well. The vided from several sources: proprietary use of Chinese Petroleum island is also very young – estimates range from 6 million to per- Company (CPC) seismic profiles in the Foothills (e.g., Suppe, haps a few millions – but the most active phase may have been 1976; Chow et al., 1986), better quality seismicity and seismic in the last million years (Lee et al., 2006). Taiwan is often called velocities from modern Taiwan seismic networks (Tsai, 1986) a natural laboratory because it is an environment in which most, and results from the World Wide Standard Seismograph Network if not all, of the tectonic processes of mountain building are prob- (WWSSN), (Wu, 1970, 1978; Pezzopane and Wesnousky, 1989). ably in progress and can therefore be monitored at the surface or Naturally, some of the earlier conclusions which were based on imaged at depth. Beginning in the early 1970s when the general insufficient data needed to be revised later; for example, the idea geologic framework of Taiwan tectonics became clear (Ho, 1972, of a crustal transform fault across the near 123°E 1986) and when the plate tectonic theory gained momentum longitude (Wu, 1970) was later thought to be a tear in the PSP (Isacks et al., 1968; Bird and Dewey, 1970), the island of Taiwan (Wu, 1978). Similarly, many conclusions based on early data need was recognized as the product of an arc–continent collision (Biq, to be re-evaluated in light of new observations and ideas. 1972; Chai, 1972)(Fig. 1). It soon became the type locality for stud- After 1990 the installment of new seismic networks acceler- ies of such collisions (Suppe, 1981; Davis et al., 1983; Moore and ated. The establishment of the island-wide short period network Twiss, 1995). A large number of papers either addressing the Tai- (Wang and Shin, 1998) improved earthquake reporting and pro- wan orogen directly, or relevant to it have been published, many vided the basic data for a series of papers on tomography (Rau in special volumes (e.g., Byrne and Liu, 2002; Brown and Ryan, and Wu, 1995; Kim et al., 2005; Wu et al., 2007a,b and others) 2011). Basic surveys on Taiwan geology (Ho, 1986), metamorphic and on precise relocation of seismicity (e.g., Wu et al., 2004; rocks (Liou, 1981; Ernst and Jahn, 1987; Yui, 2000; Beyssac et al., Chou et al., 2006). Later, data from two broadband networks 2007), thermochronology (e.g., Liu et al., 2000; Willett et al., allowed the use of waveforms in routine moment-tensor solutions 2003; Lee et al., 2006), tectonics (Biq, 1972; Wu, 1978; Suppe, (e.g., Kao et al., 1998) and S-splitting analysis (Rau et al., 2000; 1981, 1984; Wu et al., 1997) and geodynamics (Willett et al., Kuo-Chen et al., 2009). These results contributed in important 2003; Kaus et al., 2008; Lavier et al., 2013) are all very useful intro- ways to tectonic studies of Taiwan. ductions. Some of these will be highlighted in the course of our dis- A number of fundamental characteristics of the Taiwan orogen cussion. A simple geologic map with the main Taiwan provinces is were derived through the previously mentioned works. For exam- shown in Fig. 2. Interpretations of the main geologic features will ple, a thick root of over 50 km under the Central Range was imaged be discussed in context in the following section. tomographically, while seismicity and moment tensor solutions F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 175

Fig. 1. Plate boundaries around Taiwan. Solid red line: subduction boundaries. Dashed red line, the western edge of the subducted PSP. Dashed green line, spreading center for the . Red vectors: measured GPS velocity vectors, relative to the red circle in Island. Blue vector: PSP motion predicted by Seno (1993). The EUP/ PSP boundary in and offshore northern Taiwan follows that of Wu et al. (2009) and the boundary SW offshore is that of Lin et al. (2008). Geological units: DP = Coastal Plain, WF = Western Foothills, HR = Hsueshan Range, BR = Backbone Range, ECR = Eastern Central Range, TV = Tatun Volcanoes, IP = Ilan Plain and CR = Coastal Range. Cities: 1 = Taipei, 2 = Hualien, 3 = Taitung, 4 = Kaohsiung, and 5-Tainan. TTL = Tainan Taitung line. The Chelungpu fault that was activated during the 1999 Chi-Chi earthquake is shown in red to the right of ‘‘CP’’. Note BR and ECR are lumped together as Central Range.

(from which double-couple focal mechanism can be derived) were A comprehensive understanding can undoubtedly be gained by applied to understand the mechanics of both shallow- and deep combining knowledge from Taiwan research with that from old crustal faulting and subduction (Wu et al., 2004; Chen et al., and mature mountains. To advance beyond our present knowledge 2002; Kao and Rau, 1998). Additionally, the absence or presence of Taiwan tectonics it is especially critical to add subsurface and of seismicity noted in different parts of the crust (Wu et al., geodetic data, as detailed as possible, to the existing rich archives 1989, 2004) has been interpreted in terms of the rheological prop- of geological information upon which many hypotheses are based. erties of the crust. Nevertheless, many outstanding questions New subsurface data can be used to effectively test and improve remain: for example, what role does the subduction of the EUP the models if sufficient spatial resolution and lateral coverage play; how much does the PSP deform in the collision; what are can be achieved. Tomographic Vp imaging has contributed a great the mechanical and petrological processes in the core of the oro- deal to tectonic interpretation; with the addition of S-wave veloc- gen; and what changes occur in crustal and mantle processes in ities, the composition of the crust and, pressure and temperature the transition from subduction in southern Taiwan to collision in may now be constrained, as we show later. Furthermore, since off- central Taiwan? shore eastern Taiwan is a part of the collision system, marine Similar questions have been raised regarding mountain ranges observations are necessary. Resistivity and laboratory rock velocity elsewhere in the world, whether they were active a hundred mil- can, in addition to seismic parameters, provide independent infor- lion years ago or only active since the early Tertiary (for example, mation regarding the state of the rock materials. Finally, now that Dabie, Appalachians, Himalayas and Alps). Studying an orogen as numerical geodynamics is capable of modeling complex geological young and active as Taiwan, rare in the world, has distinct advan- processes it would be desirable to explore interpretations that are tages. In particular, the mountain building processes we can guaranteed to be mechanically sound and geologically compatible uncover are most probably directly responsible for the orogeny. with current knowledge. These are actually the design principles of 176 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

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).

2. 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 dif- ferent 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 necessitate lateral interpola- tion in mapping, whereas translating geologic information to struc- tures at depth demands vertical extrapolation. In some cases extrapolation or interpolation can be quite reasonable; tectonic interpolation across stratigraphic gaps of millions of years or extrapolation beyond a few kilometers in depth or horizontally, however, may quickly become problematic. Carefully logged and sampled drill holes provide some key data for correlation of strata and, when coupled with seismics, will lend further assistance. Most of the drill holes in Taiwan, however, were placed in sedimentary basins for economic interests, with depths limited to about 4 km. As always, resampling and remapping may be needed when new concepts are to be tested. The combined information from geologic and tectonic maps of Taiwan yields a view of the basic makeup of Taiwan as a result of cumulated geologic processes from the late Paleozoic to the present (Ho, 1972). The current phase of active orogeny is gener- ally assumed to have begun 5–6 million years ago when the Luzon Arc first approached the continental shelf of the EUP (e.g., Castelltort et al., 2010). Certainly the entire geologic edifice of Tai- wan is built by the ongoing episode of plate tectonic processes; some aspects may be interpreted in terms of plate tectonic struc- tures at the surface, while the products of plate interactions at depth can only be imaged remotely. Fig. 2. Geologic framework and location map of Taiwan. CP = Coastal Plain, The most recognizable plate tectonics-related structures on HR = Hsueshan Range, WF = Western Foothills, BR = Backbone Range, TB = Tailuko land are the juxtaposed Coastal Range (CR) complex and the Cen- Belt, YB = Yuli Belt (TB and YB of Central Range), CoR = Coastal Range, CST = Chel- tral Range across the Longitudinal Valley (LV) (Figs. 1 and 2). The unpu-Sanyi fault, CkT = Chouko fault CST and CkT are representative mountain- andesitic volcanics and forearc/arc sediments in the Coastal Range front faults in western Taiwan), LF = Lishan Fault, CCF = Chauchou Fault, dashed line = deformation front, the assumed westernmost thrust. PP: Prehnite-Pumpelly- became shortened during the post- collision to form the ite; GS: ; BS: Blueschist. mountain, while the volcanic arc continued southward to the inac- tive volcanic islands of Lutao and Lanhsu SE-offshore of Taiwan (Huang et al., 2000; Juang and Bellon, 1984)(Fig. 1). Across the the TAIGER project (2004–2009). The project has thus far produced LV, the pre-Tertiary metamorphic Central Range looms. A simple a large amount of land and marine geophysical data. Ongoing anal- and elegant model of the tectonics of Taiwan by Suppe (1981, yses of this data have produced new images of the lithosphere 1984) raised its status to a type locality for arc–continent collision. under Taiwan that can serve as the basis for model building and Suppe (1981, 1984) assumes that the Luzon Arc and Taiwan are lin- testing. ear and oblique to each other and that they collide obliquely; the In this paper, rather than conducting a comprehensive review of arc is the indenter and Taiwan is a wedge of incoherent material. all aspects of works relevant to Taiwan tectonics, we mainly aim to The resulting ‘‘fold and thrust’’ mountain range of Taiwan has a review issues regarding the structures at depth based on fresh constant width and steadily propagates to the south (Fig. 3). In this results of subsurface imaging from the TAIGER (TAiwan Integrated view, a 2-D trend-perpendicular geologic section in southern Tai- GEodynamics Research) project. In view of the wide acceptance of wan can, in principle, be derived from a section in northern Taiwan Taiwan as an archetype of arc–continent orogeny, identifying all created at a time s = d/v ago, where d is the along-strike distance the possible factors that enter into model construction will benefit between the sections and v is the velocity of propagation. Davis the study of orogeny as a whole. In the following (Section 2)we et al. (1983) and, more emphatically, Huang et al. (1997) made shall first discuss the models that have been proposed. In the sub- the Taiwan fold-and-thrust belt a northern continuation of the sequent section (Section 3) we present a succinct discussion of the accretionary wedge of the Manila subduction system. In most tec- geophysical studies of Taiwan; readers familiar with these subjects tonic discussions a ‘‘deformation front’’ is assumed to exist in wes- may elect to skip ahead. After presenting results of new analyses tern Taiwan (e.g., Suppe, 1984; Teng, 1996; Huang et al., 2000; (Sections 4 and 5) we shall discuss their bearing on different Malavieille et al., 2002; Angelier et al., 2009). While Suppe F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 177

Fig. 3. Two versions of the detachments of thin skin tectonic model of Taiwan in (a and b), and (c) Diachronistic propagation of the Taiwan orogen Suppe (1984), (d) ‘‘Root zone’’ model of Twiss and Moore (1992). See text for explanation.

(1984) considers the front to be the PSP/EUP boundary, presum- that the two sections have a similar exhumation history in the last ably linked to the Manila Trench, others consider the PSP/EUP con- 6 Ma, and to thus question the notion of southward propagation. tact to occur in the LV. Additionally, orogenic sedimentation in western Taiwan and in The pre-collision geometry of the continental shelf is unknown, the Taiwan Strait does not record a diachronistic spatial depen- as the effects of collision have masked it. Most investigators dence (Castelltort et al., 2010; Andrew Lin, personal communica- extrapolate the shelf edge southwest of Taiwan northeastward to tion, 2013). Maps of Taiwan (e.g. Fig. 5) clearly show that a become the pre-collision shelf. One possible clue is offered by collision of the Luzon Arc with EUP in southern Taiwan has not the fairly strong positive magnetic anomaly along the shelf edge occurred. This leads to several questions: How, then, was the to the southwest of Taiwan (Fig. 4 and Hsu et al., 1998); it runs southern Central Range created? Or, if the uplift of southern Tai- directly into the Peikang basement high and disappears upon wan began a few million years before the present, what caused encountering the Central Range. Similar anomalies have been the uplift? These considerations are key to the understanding of observed along passive margins in other parts of the world and southern Taiwan. We shall discuss them later on the basis of are usually attributed to the presence of a strong contrast in the new observations. magnetic properties of the rocks across the juncture between the As young and active as Taiwan is, theories of Taiwan tectonics continent and the ocean basin (Lizarralde et al., 1994). In Taiwan, are diverse. This may be, in part, a function of time; namely, the the anomaly stoppage is most probably a result of deformation ideas were developed as different sets of observations became and the relatively high temperatures in the core of the Central available. With a wide variety of geoscience data and expertise, it Range (see more in Discussion). As shown in Figs. 1, 3 and 5,if is also unavoidable for modelers to use preferentially a subset of the shelf edge follows the course indicated, a part of the eastern the observations available in their own fields. Ideally, a good model and much of the southern Central Range lie on the continental should satisfy all relevant observations, even though they may not slope. The interpreting of recent apatite fission track dating for yet be available at the time of the model’s formulation. Testing southern and northern Taiwan has led Lee et al. (2006) to conclude models with new observations is the essence of scientific progress 178 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 4. (a) Magnetic anomaly map in the vicinity of Taiwan (Hsu, 1998). Note the positive anomaly near the edge of the continental shelf that is termined in central western Taiwan; north of Taiwan a more diffused positive anomaly is present on the continental shelf, and (b) Bouguer gravity of Taiwan (Yen and Hsieh, 2010).

(Popper, 1968). One may expect that diverse hypotheses based on The models of Taiwan tectonics currently under discussion empirical observations will eventually converge as pertinent were mostly proposed since the 1970s as the geology of Taiwan observations become available, which they did in the 1960s with became cast in the plate tectonic framework (Chai, 1972). Most the elucidation of plate tectonics, (Hallam, 1983; Oreskes, 1999). models were formulated at a time when the knowledge of crustal F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 179

Fig. 5. Possible configuration of the shelf edge before collision. ‘‘10’’ and ‘30’’ refer to the crustal thicknesses of the rifted basin and continental shelf, respectively. By closing the Okinawa Trough a nearly linear continental shelf can be defined.

and upper mantle structures under the island only began to accu- drives the deformation of a much thicker critical wedge; in this mulate and critical information was lacking. All the models never- class of models the same subduction geometry persists throughout theless have implications for subsurface structures, often the computation. A trove of ideas was proposed in these papers, illustrated in cross-sections. Thus a simple procedure for model and some of the concepts have been applied to other orogens. testing would be to check the model implications against subsur- For example, Dahlen et al. (1984) included thermal metamorphism face observations. If they do not agree, the ensuing back-and-forth in their model and Willett et al. (2003) included the important checking of observations against model may prompt a new round effect of weathering. Teng (1996) among others, however, concen- of data gathering and refinement of the model. Several explicit trated on evolutionary tectonics, attempting to reconstruct the attempts at testing models of Taiwan tectonics have taken place plate settings in the recent millions of years, a task made difficult in recent years (e.g., Wu et al., 1997; Lee et al., 2006; Beyssac by the absence of young spreading centers in the Philippine Sea et al., 2007; Castelltort et al., 2010). Such tests help sharpen the area. foci on the multiple complex arguments regarding key aspects of Carena et al. (2002) revised Suppe’s (1981, 1984) detachment the hypotheses. Modelers ideally know best the implications of by using a subjective selection procedure to filter seismicity, and their models, and if stated clearly these become criteria for model then fitting the resulting hypocenters to define the detachment. testing. Without clear criteria, experimentalists have to deduce the Huang et al. (1997, 2000) and Malavieille et al. (2002) incorporated implications from models to be tested. Although seldom stated more details of offshore geology in their shallow detachment mod- thus, it is in model testing that relevant laboratory, numerical sim- els. While Huang et al.’s (1997, 2000) model did not specify the ulation or fieldwork becomes a scientific experiment. structures at depth, Malavieille et al. (2002) and Malavieille and Among the commonly discussed models of Taiwan there are Trullenque (2009) depicted deep structures related to subduction, several key ideas that we shall address throughout this paper. Per- based largely on analog laboratory models. haps foremost is the existence of a shallow detachment. Suppe Wu et al. (1997) proposed a model of deep crustal and upper (1981, 1984) elaborated the well-known ‘‘thin-skinned’’ tectonic mantle deformation to account for deformation under the orogeny. theory of Taiwan, based on interpretation of the folds of the Wes- The model was based mainly on the tomography, gravity anoma- tern Foothills and some seismic sections (<10 km) as those over a lies, focal mechanisms and seismicity available at the time, along shallow dipping detachment, which is also the top of the subduct- with the main geologic characteristics of Taiwan. The so-called ing EUP (Fig. 3). A numerical simulation by Dahlen et al. (1984) ‘‘lithospheric collision’’ model for Taiwan includes the mainly examined in a comprehensive manner the dynamics of a shallow ductile (i.e., aseismic) Central Range rooted down to 55 km, a ‘‘ready-to-fail’’ wedge as proposed by Suppe (1981). Although with brittle/ductile sandwiched crust west of the Central Range and variations, many of the ensuing modeling papers (e.g., Willett et al., the brittle Coastal Range. Subsequently, the 1999 Chi-Chi 2003) used the basic concept that EUP subduction under the PSP aftershocks illuminated several important structures related to 180 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 orogeny, including a high-angle reverse fault under the Lishan fault Taiwan. The number of broadband seismic stations has increased (Wu et al., 2004). The recently expanded seismological dataset steadily since 1995, and at present there are two networks: one illustrates more clearly the 3-D structures in the upper mantle as operated by the Institute of Earth Sciences, Academia Sinica, and related to the subduction/collision under northern Taiwan (Wu the other by CWB. A total of about 50 broadband stations (CWBBB) et al., 2009) and the subduction of EUP under southern Taiwan coexist with the short period CWBSN stations, in addition to sev- (Chen et al., 2012). The most active phase of the Taiwan orogeny eral hundreds of strong motion stations. The broadband wave- probably took place in the recent 1–2 million years (Lee et al., forms are routinely inverted for moment-tensor solutions; these 2006); hence, effects of tectonic processes should be clearly pres- results have made advanced tectonic analyses feasible (Kao et al., ent at both the surface and at depth. Such detailed subsurface 1998). information becomes indispensable in tectonic interpretation. Since Taiwan is surrounded by water, with significant offshore Incidentally, most models of Taiwan orogeny are concerned seismicity, a more comprehensive understanding of Taiwan was with structures to the east of the deformation front, but there are hampered by the lack of offshore seismic stations. The deployment significant earthquakes in the Taiwan Strait; for example, an of drop-and-retrieve type ocean-bottom seismometers (OBS) M = 6.5 event in 1994 south of the Penghu Islands yielded a focal became more frequent, and a permanent OBS was established off- mechanism with a strait-parallel T-axis (Kao and Wu, 1996). This shore of northeastern Taiwan in 2011, as a part of the CWBSN. mechanism does not fit the EUP bending model, as expected from Recently the development of drop-and-retrieve type OBS programs a subducting EUP, but rather, the orientation of the T-axis might be in Taiwan is accelerating (Lee, C.S. and Kuo, B.Y., personal commu- regarded as similar to that of graben structures in the Himalayas nication, 2013); they have provided constraints for the Ryukyu and southern Tibet (Kao and Wu, 1996). The Strait is an important subduction zone and the velocity structures offshore of eastern aspect of the Taiwan orogeny. Not much can be said about it at this Taiwan (Kuo et al., 2009, 2012). time, however, as very little work has been carried out there until Worldwide seismic stations began operation in the 1890s. From recently. Multichannel seismic surveys and earthquake recording 1890 to 1960 many stations or networks were poorly timed and in the Taiwan Strait will generate some needed basic data. calibrated, yet the data from this community of heterogeneous net- Geodynamic modeling of orogeny with PSP overriding the east- works led to the discovery of major features of the Earth, such as dipping EUP, a model consistent with our general understanding of the core and the world seismicity. Although focal mechanisms Taiwan, has offered some new perspectives regarding Taiwan orog- were determined during this period, they were not consistent eny. A paper by Yamato et al. (2009) succinctly reviewed efforts of enough for tectonic studies, due mainly to polarity uncertainties. such modeling up to 2008. Two-dimensional modeling has The World Wide Network of Standard Seismographs (WWNSS) advanced to a degree where mechanical and other orogenic pro- began 1962, resulting in synchronized clocks and uniform instru- cesses are coupled. It is still a very challenging task (Lavier et al., ment responses. Thus, focal mechanisms of large earthquakes 2013), especially since many problems in Taiwan have to be trea- became a part of Taiwan studies (e.g., Wu, 1970, 1978; ted as three-dimensional. Pezzopane and Wesnousky, 1989). WWNSS has now been totally replaced by digital quality stations (e.g., Peterson and Hutt, 1989).

3. Evolving facilities and studies of Taiwan subsurface 3.2. Local, teleseismic and global tomography structures The rich local seismicity under Taiwan and the enhanced seismic Most of the recent tectonic studies benefit from three seismic networks enabled Roecker et al. (1987), Rau and Wu (1995),Kim ‘‘observations’’: velocity structures, seismicity and focal mecha- et al. (2005) and Wu et al. (2007a,b, 2009) to obtain more and more nisms. But seismic anisotropy also has important implications for detailed tomography of the island. Undoubtedly few tectonic prov- regional tectonics and for the depth to which orogenic processes inces elsewhere in the world can be similarly studied. The maximum may extend. Magnetotelluric resistivity mapping has also been resolution depth of local earthquake tomography is, however, lim- shown to be an important addition to seismic methods for its abil- ited by the availability of deep earthquakes in the region. Earth- ity to indicate fluid and melting conditions. The gravity field in Tai- quakes deeper than 150 km were recorded in the northern and wan has shown that the Bouguer minimum does not reflect either southern Taiwan subduction zones, but under central Taiwan, no the topography or root configuration. In this section we review the earthquakes below about 60 km have been verified. The depth reso- research relevant to Taiwan in these fields to provide the back- lution of tomography can be extended by including teleseismic ground against which the TAIGER project was designed. arrivals from events at 30° (the angle at the center of Earth between the vector to the station and that to the event), or farther. The usual 3.1. Development of local/worldwide seismic networks and Taiwan assumption that all the anomalies are contained within a ‘‘box’’ research down to a certain depth (typically a few hundred kilometers) may lead to some errors, should strong anomalies exist beyond the box. The first seismic stations in Taiwan were established in the late Global tomography could image the deeper sections of the Earth 1890s to monitor the island’s ample seismicity. The modern net- under an area of interest. The relatively coarse 60 km grid produces work began with the adoption of telemetry in 1973, and the more a low spatial resolution, but for larger scale deep structures under precise arrival pickings of the Taiwan Telemetered Seismic Net- the orogeny it could be informative (Bijwaard et al., 1998; work (TTSN) enabled Roecker et al. (1987) to derive the first Lallemand et al., 2001; Li et al., 2008). Generally, the outer struc- tomography of Taiwan. Seismicity patterns in the Taiwan area soon tures (60 km) of the Earth are not resolved in such global studies emerged. In 1990 a new, unified Central Weather Bureau Seismic because of the coarse grids used and a crust/mantle correction, Network (CWBSN) replaced TTSN. This denser, digital network pro- which, based on existing knowledge, is often applied to the data duced high quality data used in many research projects (Wang and (e.g., Wang et al., 2009); such a problem is avoided if there are Shin, 1998). A recent enhancement includes the downhole instru- enough local earthquakes for simultaneous inversion, as done in ments in metropolitan areas for noise abatement. The more Kuo-Chen et al. (2012). detailed and well constrained seismicity and the series of tomo- In studies of orogeny the change in crustal thickness, as marked graphic images based on the data (Rau and Wu, 1995; Kim et al., by the depth of Moho, is useful for indicating the accumulated 2005; Wu et al., 2004, 2007a,b) contributed to tectonic studies of crustal deformation in studies of orogeny. Defined as a sharp jump F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 181 from a P wave velocity of about 7–8 km/s in the outer part of the Wysession, 2003; Prodehl and Mooney, 2012). Petroleum explora- Earth, it is not determined by the first arrival tomography, even tion mainly employs the near-vertical reflection (or simply ‘‘reflec- though the finite-difference ray-tracing used in modern tomo- tion’’) method, targeting the top 10 km or less. Oliver (1982) graphic inversion codes can model a fairly large velocity gradient promoted broad utilization of the reflection method in deep crustal (Benz et al., 1996). The presence of a thickened low velocity ‘‘root’’, studies. The seismic method employs artificial sources such as associated with overlying high topography, is a useful gage and vibroseis, or small explosions (10’s kg) on land, or airguns in the basis to assess the orogenic deformation. If both Vp and Vs (or ocean. Reflection profiles may yield images of velocity contrasts in Vp and Vp/Vs) tomographic models are obtained, then an explora- the crust down to Moho, and even deeper. Such work, however, is tion of the composition, temperature and properties of the crust or difficult Taiwan, particularly across the mountainous areas, because the lithosphere is possible (Christensen and Mooney, 1995; of the terrain, the road conditions, the scarcity of open land, and the Christensen, 1996; for Taiwan, see Kuo-Chen et al., 2012b). requirement of strong sources to reach the Moho (>50 km). Wang In interpreting tomographic images, one must know the limita- et al. (2002) conducted relatively shallow reflection profiling across tions of the models; Nolet (2008) and others have discussed the the Chelungpu fault following the Chi-Chi earthquake. At sea, mul- desirability, as well as the difficulty, of conducting thorough resolu- tichannel seismics have been successfully employed and, under tion studies. Depending on the basis of model discretization, a pix- favorable conditions, the Moho can be determined. elization-like limit occurs, i.e., features smaller than the grid are Wide-angle studies utilizing either artificial sources or earth- not resolved. The result between grids becomes smooth, however, quakes are more commonly conducted for tectonic research, as when a velocity gradient is employed. The more difficult question refraction profiling is more useful for mapping velocity discontinu- is whether the image of a certain part of the model is well resolved ities, whether mid-crustal or Moho. In areas with slowly varying – or carries real information. The resolution for a cell depends on the crustal structures, the Moho can be readily determined with quality of arrival picks, clock error, the accuracy of station location, refracted waves (‘‘Pg’’ or ‘‘Pn’’) and/or reflected waves (such as the precision of wave arrival picking, and the length and availability PmP). Across the Central Range the distance is too short for Pn to of crisscross seismic rays through the portion of the model. The appear, but Pn from explosions or earthquakes in southeastern results of checkerboard testing are nearly always presented. The China do reach across to Taiwan. Huang et al. (1998) have estimated ‘‘checkerboards’’ refer to 3-D cubic cells in the model space with the overall depths and velocity of the Moho from earthquake alternating perturbations of a few percent around zero; these per- sources, thus providing overall crustal structures away from the turbations are added to a background model, not infrequently the orogeny. A relatively short refraction line in the Longitudinal Valley initial model used in the inversion. The synthetic travel times calcu- determined the depth of the basin to be about 2 km, and lated from the resulting model for all the available stations and called into question the existence of a fault on the east side of the LV event pairs in the dataset are then used as input arrival times for (Wang and Chen, 1997). As Taiwan is surrounded by sea, joint mar- tomographic inversion; if well-defined alternating patterns of the ine and land wide-angle study is an effective approach to determine input anomalies can be recovered, the resolution is considered good. crustal structures (McIntosh et al., 2005). The recent advance in ‘‘Streaking,’’ or distortion of the regular patterns – where cells recording technology and capacity makes it practical during active appear to be linked together – indicates those regions that are not source experiments to leave high sensitivity, closely spaced seismo- adequately sampled by available seismic rays. graphs in continuous mode and to register both active sources and With a series of tomographic models now available, one may earthquakes, thus allowing joint active/passive studies. Such study judge the robustness of the models by the degree of consistency is especially appropriate for seismically active Taiwan (H. van Aven- among the models obtained by using different datasets and inver- donck, personal communication, 2013). sion codes. Although Rau and Wu (1995) and Kim et al. (2005) ‘‘Receiver function’’ played a role in crustal studies beginning in employed data from CWBSN, they used data with different time the early 1980s (Ammon, 1991). A seismogram becomes a receiver periods and amounts of data as well as different codes. Common function containing only the information of the layers under the to both results are the relatively high Vp in shallow (<15 km) parts site when the source effects (usually earthquakes) are removed. and lower Vp in the deeper (>20 km) parts of the Central Range, In practice, it is derived from the radial component P-wave after and a 50 km or thicker crust under the main parts of the Central deconvolution by the vertical component (a proxy for the source- Range. The relatively low velocities in the lower crust under the time function. Central Range shown in these models were detected even in the If the structures under the station are relatively simple and can first local earthquake tomography of Roecker et al. (1987) which be represented adequately by a stack of flat layers, then the veloc- used much less data and a smaller number of stations. The thick- ities and layer thickness can be obtained. In highly 3-D structures ened crust with low velocity compared to the surrounding area like the Central Range of Taiwan, however, the receiver functions was clearly definable as a ‘‘root’’ in Rau and Wu (1995) and Kim are affected by scattered waves and are quite complex, and the et al. (2005). A similar ‘‘root’’ is shown in Wu et al. (2007a,b), albeit Moho depths from different stations vary greatly (Wang et al., imbedded in shorter wavelength variations (Kuo-Chen, 2011). Ma 2010). et al. (1996) used Zhao et al.’s (1992) tomographic codes in which the Moho and Conrad discontinuities were constrained, and their 3.4. Surface wave studies models appear quite different from the ones mentioned above. Although tomographic velocity imaging is the best source of sub- The classical surface wave dispersion curve can be inverted for surface information available, the issues involved in its tectonic inter- crustal and mantle structures (e.g., Stein and Wysession, 2003). pretation are complex. Ultimately it is desirable to translate seismic The dimension of the Taiwan orogen is such that the wavelengths velocities to rocks. Besides the problem of image resolution one must of the commonly used period range (>25 s) are too long to see the also consider temperature, pressure and the presence of fluids. important internal structures. Recent advances (Bensen et al., 2007) in using ambient noise to derive dispersion between any 3.3. Active source crustal studies, joint active/passive onshore/offshore two stations are appropriate for study of Taiwan (Huang et al., profiles and receiver functions 2012, 2014). The tomographically derived phase or group velocity maps, using multiple station pairs at various periods, can be sam- Two general categories of reflection methods are used: (1) near- pled at individual geographical points and then inverted as 1-D vertical reflection and (2) wide angle seismics (e.g., Stein and structures for an approximation; the image at each period has its 182 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 own spatial resolution. When short period surface waves are used 3.7. GPS and leveling in Taiwan the major topographical relief of Taiwan (about 9 km from high mountain to deep ocean) may affect the dispersion and should Following the 1999 Chi-Chi earthquake a concerted effort was be removed (You et al., 2010). made to increase the number of continuous GPS stations and to conduct leveling surveys in Taiwan. GPS stations were deployed both for the measurement of crustal movement and for land use. 3.5. Magnetotellurics, gravity and magnetics By now there are in excess of 386 continuous GPS stations (see http://gps.earth.sinica.edu.tw/), with the main features of the Although essentially all parts of the Earth can be sampled by resulting GPS measurements similar to those discussed by Hsu seismic waves, seismic velocity alone is not sufficient to determine et al. (2009) (Fig. 6). Repeated leveling surveys have also been car- the composition, the physical state or the fluid content of the rocks; ried out since 2000 (Ching et al., 2011)(Fig. 7). The data show that other measurements are needed. It is now almost a standard prac- short-term deformation of the island is closely related to collision tice to include magnetotelluric measurements in large tectonic tectonics and subsurface structures. studies (see Jiracek et al., 2007; Bertrand et al., 2009). In general, the ‘‘skin effect’’ of electromagnetic waves renders the resistivity 3.8. TAIGER project structures less well resolved if a less resistive layer (say 10 ohm m), such as water-saturated sediments, is present near The 1999 Chi-Chi earthquake far exceeded the magnitude the surface; but in a metamorphic terrain, such as the Central Range expected from historical seismicity. The TAIGER project was initiated of Taiwan, where the resistivity is on the order of 100–1000 ohm m, after the earthquake to understand the environment in which such a the structures are resolved down to lower crustal levels. large event could occur. TAIGER was specifically designed to gain sub- Gravity, when coupled with seismic structures, can constrain surface information to test existing ideas of Taiwan tectonics and to the density distribution, allowing us to estimate whether the explore other possibilities. The project included four parts: (1) seismic mountains are in equilibrium or being supported dynamically. imaging and earthquake studies on land and at sea, (2) magnetotellu- Two Taiwan gravity surveys have been published: the surface mea- ric imaging on land, (3) laboratory petrophysics measurements and surements by Yen and Hsieh (2010) and an aero-gravity map by (4) numerical geodynamic modeling. For subsurface seismic imaging, Hwang et al. (2007). It has been shown that when the known we aimed at the velocity structures in the crust and mantle under- and effects of Earth’s elipticity, elevation and the presence of mass around the island along seven transects, down to Moho at sea and to a above sea level are removed from raw measurements (i.e., the Bou- depthofabout200–300kminlandareas.Inthissectionwewill guer gravity anomaly) major mountain belts are shown to be describe only the basic design of the TAIGER project, as the TAIGER regions of negative anomalies, due to the presence of a low-density results will be the foci of discussion in the rest of the paper. root. The high mountains of Taiwan, however, are not regions of a The purpose of the TAIGER project is to derive the best possible negative Bouguer low, even though a root is clearly mapped, as sta- subsurface images of the whole island and its eastern offshore area, ted earlier (Wu et al., 1997). Incidentally, there is a discrepancy under the constraints of available facilities and logistics. Fig. 8 shows between the Bouguer anomalies calculated from measurements the deployment/experiments of the TAIGER project: (1) broadband at the surface and those from the air (Yen and Hsieh, 2010; seismic stations to record waveforms from the local earthquakes as Hwang et al., 2007). This discrepancy awaits resolution. well as teleseisms (Fig. 8a); (2) magnetotelluric sounding and labora- Magnetics is a measurement of variations in rock susceptibility; tory velocity measurements (Fig. 8b); (3a) marine multichannel seis- the measured total magnetic field is a sum of the field induced in mics (MCS) (Fig. 8c); (3b) sea-land joint profiling with airguns the mineral and the ambient field. Magnetic anomaly, i.e., the dif- shooting over OBS and into short period (L22 sensors at 2 km inter- ference between the observed and the ambient field, unlike the val) arrays on land (Fig. 8c); and (4) land wide angle explosions (ver- gravity anomaly, does not directly reveal the shape of the body, tical component ‘‘Texans’’ at 500 m interval). The common features in and the same magnetic body exhibits different anomalies at differ- these deployment plans include the concept of transects (3 marine, 4 ent (magnetic) latitudes. The magnetic properties disappear when on land) and continuous recording; the land transects provide details the local temperature reaches above the Curie temperature of as well as improve overall space resolution by increasing station den- about 570 °C for magnetite at the surface. In Taiwan, magnetic sity; recorders are set in continuous mode in all cases, even for explo- anomaly maps by Hsu et al. (1998) and Yen et al. (2009) respec- sion experiments, so as to record earthquakes, as well. Additional tively show regional and more detailed local features. Because of important design considerations of the TAIGER seismic experiments the temperature effect, main sources for magnetic anomalies are as follows: reside within the top 20 km or so in the crust.

(a) The total number of stations contributing to our tomography 3.6. S-splitting from the TAIGER broadband, BATS and CWBB and the other dense linear arrays listed above, when joined with BATS and The polarization of S-waveforms (an exact analog of light wave CWBBB reaches more than 2800. The picks from the dataset, propagation in anisotropic minerals) can be analyzed in terms of none of which has been used in previous tomography, are crustal and mantle anisotropy, and is a powerful method for tec- adequate for tomography in the desired region. tonics now facilitated by a growing knowledge of rock anisotropy (b) Broadband and coherent dense array waveforms yield more and the advent of digital seismograms. Kuo et al. (1994) first stud- reliable S-wave picks than those from routine processing. ied S-splitting under northern Taiwan and obtained the delay (c) OBS stations offshore of Taiwan effectively enlarge the aper- between the fast- and slow S waves to be <0.1 s. Rau et al. ture of the network and enable the imaging of a wider (2000) used S-phases to determine teleseismic S-splitting; their region, and to a greater depth (200 km). delays were as large as 2 s, with the directions of the fast S-waves (d) To complement the detailed seismic imaging along three sub-parallel to the structural trend. These results are particularly land-based seismic transects (Okaya et al. (2009), TAIGER interesting as they imply that the upper mantle is anisotropic also carried out magnetotellurics for resistivity along the and that the shear deformation, or flow in the upper mantle, is same transects. The long period (10–1000 s) time-varying ‘‘coherent’’ with the deformation of the crust (Silver, 1996). Such EM data is amenable for 2-D and 3-D interpretation data provide a broad framework to the study of regional tectonics. (Bertrand et al., 2009, 2012). F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 183

Fig. 6. Main features of GPS measurements before 1999 (Hsu et al., 2009). The overall horizontal and vertical (see Fig. 7) velocity fields can be explained in terms of plate configurations and collision. See text for discussions.

(e) Laboratory measurements of the seismic velocities of rocks on spherical geometry by Roecker et al. (2006). Two sets of P-wave can provide clues for interpretation, especially regarding tomographic inversions are shown: one based on local earthquakes crustal seismic anisotropy derived from field data. Complete and explosions only, nick-named the ‘‘local,’’ and the other, nick- anisotropic measurements under confining pressures up to named the ‘‘regional,’’ with added teleseismic arrivals. Two differ- 600 Pa were made on 11 metamorphic rock samples from ent grid sizes were used: (1) for the regional tomography, a con- the Central Ranges (Okaya et al., in preparation). The results stant grid (6  6  6km3) model down to 300 km (see Fig. 9a), guide us in the study of S-splitting and anisotropy in general. and (2) for the central area of interest in the local tomography, as shown in Fig. 9b. We used cells with dimensions varying from Numerical geodynamics has been very useful in guiding the for- 2 to 8 km in the radial direction down to 116 km depth and with mulation of many ideas in published TAIGER research (Lester et al., a constant tangential dimension of 4 km  4km (Fig. 9a). The 2012; McIntosh et al., 2013) and in this paper. The methodology regional model targets the larger scale features (10’s of kilometers) and the main results are discussed in detail elsewhere (Lavier in the crust and the upper mantle down to about 200 km while the et al., 2013). local model shows more details in the crust and uppermost mantle. Fig. 10a–c (from north to south) presents three sets of profiles across Taiwan. For each profile, the left panel shows the regional 4. Imaging the Taiwan orogen – TAIGER tomography and S- tomography and the right panel shows the local. First it is noted splitting that although different grid sizes and source sets are used for these two separate inversions, the gross features, such as the thickened Kuo-Chen (2011) and Kuo-Chen et al. (2012a, 2012b) obtained low velocity crust under the mountains and just offshore of eastern subsurface P and S wave images using first arrival picks from local Taiwan and the high velocity rise, as shown by the 7.5 km/s con- earthquakes, and from teleseisms and active sources from the TAI- tour between these two thickened areas are very similar. The GER stations. These were augmented by data from the Taiwan per- velocity changes are detailed in the local results, whereas in manent broadband networks described above. The imaging uses a the regional results the upper mantle down to 200 km are code for combined local earthquake/teleseismic tomography based delineated. 184 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 4000 0 Topo (m)

Fig. 7. Results of repeated leveling survey (2008–2000) (data from Ching et al., 2011). The green circles, plotted with maximum size corresponding to 20 mm/yr, indicate rates of subsidence greater than 20 mm/yr, blue (subsidence) and red (uplift) circles are linearly scaled to a maximum of 20 mm/yr.

The three profile locations are chosen to summarize the major in the pre-collision Taiwan Strait when viewing the crustal images changes in subsurface structures along-strike of Taiwan. The under Taiwan. Recent studies (Huang et al., 2012; Wang, C.Y., per- ‘‘South’’ profile (Fig. 10c) is located in the section where the colli- sonal communications, 2013; Lester et al., 2010, 2012) yielded a sion is presumed to be young (Suppe, 1981) The ‘‘Central’’ profile Strait crust slightly less than 30 km in thickness. (Fig. 10b) images the section where the orogeny has been judged In the central Taiwan local sections (Fig. 10b), the most promi- to be mature (e.g., Wu et al., 1997; Malavieille et al., 2002) The nent feature is the >50 km thick crust under the Central Range. This ‘‘North’’ section (Fig. 10c) is situated near the western end of the thickening can be viewed in several ways. First, the materials with PSP slab, which is subducting northward and colliding with EUP P wave velocities between 5.5 and 6.5 km/s under the Central Range at the same time (Wu et al., 2009). Some argue that northern Tai- are more than twice as thick as those under the Western Foothills, wan is undergoing post-collisional collapse (Teng, 1996), yet Ching and are expanding both with the 5.5 km/s contour convex upward et al. (2011) showed that a part of northern Taiwan in the pre- and the 6.5 km/s convex downward. Secondly, the 6.5–7.5 km/s scribed subsiding area is currently rising. materials also increase in thickness under the Central Range, but We shall describe well-resolved and robust features. More thin out toward the east after reaching the deepest point. The over- details of regional tomography can be found in Kuo-Chen (2011). all shape of the root as marked by the 7.5 km/s contour is highly A Vp/Vs section in central Taiwan will be discussed later in this asymmetric: much steeper on the east side than on the west. section and more selected local tomographic P sections in southern Another remarkable feature in the local sections of central and and central Taiwan will be discussed in the next section together southern Taiwan (Fig. 10b and c) is the presence of the second with seismicity. As stated earlier, in first arrival tomography the ‘‘root’’ in the eastern part of southern (22.8°N) and central Taiwan Moho is not defined. In this paper we use the 7.5 km/s contour (23.8°N); the 7.5 km/s contour under the Coastal Range/Luzon as a marker; it is not the Moho but it is very effective for describing Arc area has a maximum depth of about 40 km. In central Taiwan important features in the velocity images of the crust. In the fol- the eastern root is to the east of the Coastal Range, or the com- lowing we shall first concentrate our attention on the dominant pressed Luzon Arc, but in southern Taiwan it is under the forearc features in the crust and the uppermost part of the mantle in the basin between the Arc and the east coast. Although a similarly local tomography and then on those in the uppermost mantle in thickened crust was also shown in Fig. 13 of Kim et al. (2005), it the regional tomography. It is useful to bear in mind the structures was not discussed in their text. F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 185

Fig. 8. TAIGER land and marine experiments: (a) passive broadband land and ocean bottom seismic stations, (b) magetotelluric stations (white dots) and rock sample sites superposed on simple Geologic Map of Taiwan, (c) marine multichannel ship tracks and land and ocean bottom broadband seismometers. The 7 lines marked (T1, T2, T3, T4a, T4b, T5, T6) are the basic TAIGER transects to map the variations of the Taiwan orogen along strike; lines with circles (OBS instruments) are MCS and wide-angle seismic lines. (d) Short period stations for explosion (at 200 m interval) and for sea-land airguns (at 2 km interval).

The 8 km/s contours are nearly parallel to the contour wrapping ‘‘Central’’ profile (Fig. 10b). Between the two roots and nearly around the central Taiwan root (Fig. 10b), but the 7.5–8 km/s layer under the LV, the 7.5 km/s contour rises to about 29 km, not as between the two roots forms a triangular zone, with the materials high as in central Taiwan. Further south, the resolution of our of lower velocities bending conformably around its apex; in the tomography deteriorates rapidly because only a few stations exist ‘‘central’’ section (Fig. 10b) the apex rose to a depth of 20 km. on the narrowing Peninsula. The tip of this high velocity anomaly lies beneath a fairly narrow In northern Taiwan (Fig. 10a) the crust also thickens (to area under the entire length of the East Central Range, the Longitu- 40 km), but mainly under the Central Range and the NE offshore dinal Valley (LV) and the Coastal Range. This zone was partially area; under the Hsueshan Range the crustal thickness increases imaged in Kim et al. (2005) and interpreted to be shallow crust toward the east, without a ‘‘root’’ to mirror its topography. Again under the collision suture (Liang et al., 2007). the 6–6.5 km/s materials contribute to the thickening. The In the ‘‘south’’ profile of (Fig. 10c) the deepest point of the 7.5 km/s contour continues to hover around 35 km offshore 7.5 km/s contour under the high range (2500 km) is about under the Hoping basin. 41 km. The relative thickening of the 5.5–6.5 km/s materials under The lateral variation in the crustal thickness under Taiwan can the mountain range is, however, much less pronounced than in the be seen more clearly in the 7.5 km/s contour surface plot 186 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 9. Tomographic grids for (a) regional, with constant block size of 6 Â 6 Â 6km3, (b) local models, with variable grid sizes as describe in the text.

(Fig. 11). It shows that the point of maximum thickness is in cen- velocity is not as high; no seismicity is associated with this zone. tral eastern Taiwan. Although elevation of the Hsueshan Range is In the northern profile, there is a hint of some high velocity anom- comparable to that of the Central Range, the 7.5 km/s iso-surface aly patches; they are diffused and the velocity contrasts are smaller does not mirror the topography, or have its own ‘‘root;’’ it sits on than in the south. Fig. 10d–f shows three views of the 3-D velocity the gradually thickening continental shelf. In southern Taiwan, iso-velocity (8.4 km/s) surfaces: first from above (Fig. 10d), then the shape of the 7.5 km/s iso-surface in Fig. 11 shows that the dee- from the southeast (Fig. 10e) and finally from the east (Fig. 10f). per part narrows toward the south near the end of the Hsueshan Fig. 10f shows the association of seismicity with the northward Range as it approaches the edge of the continental shelf. It suggests dipping high velocity anomalies. These images, without post-pro- that the continental shelf does intersect central Taiwan, as shown cessing, very clearly display synoptic views of the major velocity in Fig. 5. The rise of the 7.5 km/s iso-velocity contour under eastern anomalies and their relation to seismicity in the upper mantle. Taiwan is displayed clearly under the Coastal Range and the east- First motion Vp tomography is normally the first order of imag- ernmost Central Range, as is the thickening under southern Tai- ing to be performed, as it is needed for any further refinement of wan; this mirrors the shape of the mountains with the straight imaging with waveforms (e.g., Nolet, 2008). Vp is weakly depen- western edge of the thickened corridor. The thicker crust offshore dent, however, on the rock type, its composition and the pres- of northeastern Taiwan can be seen as extending out from the sure/temperature conditions. The dependence becomes much coast in an inverted triangular shape, wider in the north. stronger with the addition of S-wave velocities, from which Vp/ The three corresponding profiles in the left panel of Fig. 10 show Vs or Poisson ratios can be derived. Laboratory and seismological the development of high velocity anomalies in the uppermost measurements are abundant, so the ratios can become useful tools mantle below 60 km. In the south, the high velocity zone to learn more about the materials and the ambient conditions in (>8.4 km/s) appears as two steeply eastward-dipping (65°) bands. the lower crust. A large amount of S wave data is required for ade- The top zone is aligned with the dipping seismicity (the white quate S-wave imaging. With its rich seismicity Taiwan is a fertile dots) under southern Taiwan, which will be described in more location for such work. details in the following section. The upper zone is also imaged in A typical Vp/Vs cross section (Fig. 12) across central Taiwan the ‘‘local’’ tomography (right panel). In the ‘‘Central’’ profile the shows a region of low ratio under the Central Range, down to about dip angle increases and, in comparison to the south profile, the 1.5 and centered around a depth of 24 km. Laboratory data show F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 187

Fig. 10. TAIGER Regional (teleseisms and local earthquake/explosion sources) and local P-wave tomography (local earthquakes and explosions) along three profiles: (a) the ‘‘north’’ profile is along transect T6, (b) the ‘‘central’’ along T5 and the ‘‘south’’ profile along T6. The 3-D projections of the hypoDD relocated hypocenters are superposed on the 8.4 km/s iso-velocity surfaces are plotted in (d) view from above looking at the east dipping zone extending from southern to central Taiwan, and the north dipping zone under north and offshore of NE Taiwan, (d) view from the southeast; note the alignment of seismicity with the high velocity zone, and (e) the view from the east; note the east dipping zone in southern Taiwan and the north dipping zone in northern Taiwan. that ratios can effectively help distinguish between basic and felsic laboratory measurements (Kern, 1982). Because the a–b rocks (e.g., Kern, 1982; Min and Wu, 1987; Christensen and transformation depends on P and T, and its dependence is well Mooney, 1995; Christensen, 1996; Mechie et al., 2004; Kuo-Chen known, the tomographically located transition can be translated et al., 2012b). When the ratio reaches below 1.7 the rocks are prob- to about 750 °C at a depth of 24 km. As shown by Kuo-Chen ably felsic in bulk composition (Zandt and Ammon, 1995), but for et al. (2012b) the low Vp/Vs anomalies occur widely under the ratios 1.6 or lower a–b quartz transition is called for, based on Central Range. 188 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 10 (continued)

Besides tomographic imaging, TAIGER broadband seismic between the fast and slow S waves are similar to those observed deployment also enabled a more detailed mapping of S-splitting, in South Island, New Zealand (Klosko et al., 1999), and in Tibet using the more desirable SKS related phases and thus ascertaining (McNamara et al., 1994), and they are noticeably larger than those that no influence from source-side anisotropy will be included in in less active orogens. the measurements. Fig. 13 shows Kuo-Chen et al.’s (2009) and In summary, evidently there are orogeny-related features in Rau et al.’s (2000) SKS and other S-splitting results (S and ScS). both the crust and the upper mantle. In the following we shall dis- The fast directions are generally parallel to the structural trend cuss the tectonic interpretations of these observations, together of the orogen, an observation that is common for mountain ranges with seismicity and focal mechanisms and other key non-seismic around the world (Silver, 1996). The magnitudes of delay times TAIGER and non-TAIGER observations. F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 189

geodesy at the surface, and seismic and resistivity images at depth make Taiwan the best observed among the world’s orogens. In this section we will discuss the three units of Taiwan orogen and tran- sition zones in light of the combined observations.

5.1. Southern Taiwan subduction and the transition to collision

Referring to Figs. 1 and 2, south of TTL, the dominant topogra- phy of southern Taiwan is the narrow mountain range, continuing with the Central Range to its north, bordered on the west by the Chauchow fault and the coast on the east. The Coastal Range north of TTL has transitioned into the Southern Longitudinal Trough (SLT) offshore. To the east of SLT is the Luzon Arc and to the west of the southern Taiwan range are, successively, the Pingtung Plain, the southern extension of the Foothills, the Coastal Plain, the accre- tionary prism offshore and finally the Manila Trench. The whole assembly, from the arc to the Manila Trench, but lacking an equiv- alent southern Taiwan range, was compressed to form the Coastal Range. Geologically, southern Taiwan (south of TTL) sees the end of the Pre-Tertiary metamorphics on the surface, but the Lushan formation extends to about 22.2°N where the Kenting for- mation, an accreted terrain from the South China Sea, is now attached to the end of the southern range (Huang et al., 2000). How does southern Taiwan transition into central Taiwan collision? In this and the next section (Discussion), we shall examine the corresponding velocity structures along trend, together with seis- micity and focal mechanisms to search for clues to our question.

5.1.1. Benioff seismicity and subduction under southern Taiwan The coincidence of the top of the high velocity zone with the Benioff seismicity (Fig. 10 ‘‘south’’) renders obvious the interpreta- Fig. 11. Contour map of the 7.5 km/s iso-velocity surface from local tomography. tion of this zone as an active subduction zone in southern Taiwan. The three contours are for 40, 45 and 50 km depths. ‘‘North’’ refers to T6 (Fig. 8C), ‘‘Central’’ to T5 and ‘‘South’’ to T4a. Notice the thickened crust extending to NE Where is the boundary between the seismically active and inactive offshore of Taiwan. zones, and what are the corresponding changes in the crust? Fig. 14 shows the seismicity and the BATS focal mechanisms of southern Taiwan in map view, together with the locations of pro- 5. Taiwan tectonics in light of recent observations files 1–15; the profiles are shown in Fig. 15. No seismicity below the crustal level is present in profiles 1–5, but isolated deeper Taiwan is a relatively compact orogen but the changes in struc- events appear in profile 6, while a deeper inclined zone in profile ture and tectonics, both along- and perpendicular to the strike are 7 begins to take shape. In profile 8–13 the east-dipping seismic quite pronounced, especially when subsurface structures are zone clearly defines the W-B zone, with the transition occurring included in the consideration. With its distinct PSP subduction within 10–15 km around 22.7°N. The active zone starts essentially zone offshore of NE Taiwan and the EUP zone in the south, the oro- at the latitude where the Manila Trench intersects the continental gen can conveniently be divided into three subunits, albeit lacking shelf (Fig. 1). If the high Vp zone down to 200 km under central Tai- sharp boundaries: (1) southern Taiwan, south of a line linking the wan (Fig. 10 ‘‘Central’’) is a subduction slab, it is seismically quies- cities of Tainan and Taitung (TTL in Fig. 1), where the long and cent, while the high Vp zone in Fig. 10 ‘‘South’’ is active. active Luzon subduction zone comes to a stop and the orogen Available BATS focal mechanisms (Mw > 4.5) show mostly dip- grows in elevation northward, (2) central Taiwan, where collision parallel tensile events in the Benioff zone (Fig. 16), indicating that dominates; in this section the orogeny includes the Central Range, the subducting plate is under bending stress. The active subduction Hsueshan Range and the Coastal Range and (3) northern Taiwan tectonics of southernmost Taiwan is further clarified by a series of (north of 24.2°N) where the Ryukyu Arc terminates and the PSP events occurring offshore of southwestern Taiwan in December of subducts northward, consequently colliding with Taiwan at 2006, and the beginning of 2007 (Ma and Liang, 2008). The first increasing depth toward the north. Here the Central Range and mainshock (Mw7.1) has a normal faulting mechanism, with a the Hsueshan Range tail off as subduction–collision deepens (Wu NNE T-axis, whereas the second shock (Mw7.1) 8 min later showed et al., 2009). a mainly strike-slip mechanism and a NNW T-axis (Lee et al., Since the most active phase of the current orogeny is possibly as 2008). Relocation of the main events and several large aftershocks young as 1 or 2 Ma, tomographic images of the crust provide a using depth phases (Chen et al., 2008) or the double difference good subsurface view of the cumulative orogenic deformation, method (Fig. 15; the events on the left side of profiles 14 and 15) which resulted from tectonic forces not too different from those confirmed their depths to be greater than 35 km. Judging from acting today. The seismicity and focal mechanisms, on the other the crustal structure of the area as determined by McIntosh et al. hand, effectively describe the current locus and kinematics of brit- (2005) these earthquakes are intraplate EUP events, and can be tle deformation in the crust. The presence of persistent aseismic understood in terms of similar events in other active subduction areas surrounded by high seismicity points to regions of ductile zones such as the Seattle-Tacoma area (Kao et al., 2008), and Cen- deformation. The combined observations of geology, gravity and tral America (Singh et al., 2002). 190 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 12. Vp/Vs distribution across central Taiwan (Kuo-Chen et al., 2012b) and interpretation: (a) profile location, (b) Vp/Vs section and rays of P and S wave through the model, (c) average Vp/Vs as a function of depth for Central Range (CR), Coastal Range (Co) and the Coastal Plain (location in A), and (d) experimental Vp/Vs as a function of depth for several rocks and the Vp/Vs vs. depth curve for the Central Range (red).

5.1.2. Deformation of the crust from the southern Coastal Range to seismic zone appears on the eastern flank, or the top side, of the Hengchun west vergent high velocity rise between the two roots. The ‘‘root’’ The Coastal Range in central Taiwan is well understood as a on the PSP side, incidentally, is very seismic, in contrast to the ase- result of collision against the EUP (e.g., Huang et al., 1997). Seis- ismic ‘‘root’’ under the high range in southern Taiwan or under the micity sections underneath the southern Coastal Range (Fig. 15, Central Range in central Taiwan. profiles 1–4) show a very distinct east-dipping zone. This zone, Changes in seismicity and focal mechanisms from the colli- projecting to the surface just west of the LV, with a dip of about sional southern Central Range to subduction in southern Taiwan 50°, indicates that the Coastal Range is essentially the tip of a show the different tectonic characteristics of the transition. In thrust sheet. This well-defined zone was mapped prior to the map view (Fig. 14a), a narrow zone of shallow seismicity lies along 2003 Cheng-kung earthquake, which was followed by an extended southern LV, and with the hypocenters forming a steeply east-dip- series of aftershocks with nearly the same spatial distribution ping zone (indicated by an ellipse in Fig. 14 and an arrow in profile (Kuo-Chen et al., 2007) as that shown in Fig. 15, profiles 1–4. Our 6, Fig. 15); most of these foci were aftershocks of a M6.1 earth- relocated events in profiles 1, 2 and 4 (Fig. 15) show a clear shal- quake with a dominantly left-lateral strike-slip mechanism located lower dipping zone at the bottom of the steep zone, where the on the west side of LV, (Wu et al., 2006). The zone is located west of low-angle planes of the mechanisms in the shallow dipping zone the Chengkung seismic zone described earlier. The trend of shallow align with the low-angle seismic zone, as shown in profiles 1, 2 seismicity along the LV can be traced to the east coast of southern and 3 in Fig. 17. We also see in these profiles that the high-angle Taiwan (between profiles 6–10 in Fig. 14a). It then swerves from F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 191

Fig. 13. S-splitting from teleseismic S waves. The fast directions are shown to be parallel to the structural trends. The bars are proportional to the delay times between the fast and the slow S waves. the coast southeastward (near profiles 11 and 12, Fig. 14) to cross slip vectors (Lee et al., 2012). The SW-directed slip vectors for these SLT and join the Luzon Arc seismicity east of the southern tip of events are noticeably different from the EUP-PSP convergence Taiwan (Fig. 14a). The focal mechanisms of the shallow southern direction. These events, sitting near the transition between the col- LV events are mostly thrusts. As the seismic zone swerves into lision and subduction, are tectonically quite curious (see later dis- SLT the mechanisms for the few shallow events show thrust with cussion on ‘‘escape’’). nearly NS P-axis, while the deeper events with several dip-parallel One of the structures in southern Taiwan is related to the obvi- T-axis are associated with the Benioff zone mentioned earlier. East ous topographic lineament designated as the ‘‘Chaochou fault’’ of SLT, an east-dipping seismic zone under the forearc gives rise to (‘‘CCF’’ in Figs. 1 and 14a). It is geomorphically and stratigraphically a seismic zone on the west side of the arc, dipping to the east prominent (Shyu et al., 2005), but no historical earthquake has (Fig. 16, profile 9) or to the west (Fig. 16, profile 13). In terms of been associated with it. In Fig. 14a, ambient seismicity in the vicin- seismicity and focal mechanisms, the changes from southern ity of the lineament is clear; however, it does not show as a well- Coastal Range in central Taiwan to the forearc basin are quite clear. defined zone in cross-sections, except in profiles 9 and 10 (Fig. 15), where events do seem to line up in nearly vertical belts. Most of 5.1.3. Central Range, Foothills and the Coastal Plain – from central to the ambient earthquakes in the Chauchou zone are small, while southern Taiwan the few available focal mechanisms indicate thrusting (Figs. 14b Shallow (12 km) seismicity dominates in the southern Central and 16 in profile 9), differing from Shyu et al.’s (2005) conjecture Range, and a well-delineated narrow aseismic corridor is extant that it is a left-lateral strike-slip fault. between the LV and the higher Central Range (Fig. 14a). North of In the Coastal Plain area, north of 22.7°N near the city of Tainan, TTL, normal and strike-slip faulting in the southern Central Range, several strike-slip events are found, but south of 22.7°N several with fault planes striking sub-parallel to the trend of the Range, is lower crustal or EUP normal faulting events are present common (Fig. 14b). To the west of the Central Range, the Foothills (Fig. 14b). The focal mechanisms of the latter events agree with are populated mainly by thrust type events. The change in mecha- that of the Hengchun mainshock and numerous aftershocks men- nism of shallow (<12 km) events from nearly EW-directed dilation tioned earlier, reinforcing the idea that subduction is ongoing in the Central Range to E–W compression of shallow to mid-crustal south of the 22.7°N latitude and that the northward continuation events in the Foothills region is consistent with a change in the of the Manila Trench intersects southwest Taiwan as depicted in GPS-derived strain pattern in this area (Hsu et al., 2009); this Figs. 1 and 14. observation implies that the strain observed at the surface is com- The transition from Central/Coastal Ranges to the southern Tai- patible with that at depths down to 10–12 km. Two relatively large wan range may be seen in the successive profiles in Figs. 14, 15 and

(MW  6), recent events are shown in Fig. 14b. Event ‘‘A’’ (near pro- 17. The clear east-dipping high-angle (55°) seismic zone (Fig. 15 file 5) took place in 2010 and event ‘‘B’’ (between profiles 7 and 8) profiles 1–4) under the southern Coastal Range begins to de-cluster took place in 2012. These events are located close to TTL at similar (Fig. 15 profiles 5–7) and then becomes organized again into a less depths (greater than 20 km) and have similar thrust fault focal steep (30°) east-dipping zone with a steeply west dipping zone mechanisms with NE-dipping planes and West and WSW directed on the western side of the Arc (Fig. 15 profiles 8–13). The focal 192 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 14. Maps of seismicity, focal mechanisms and locations of profiles in southern Taiwan. (a) Seismicity only. (b) Seismicity with BATS focal mechanisms.

mechanisms change from very well-defined thrusts (Figs. 14b and range compared to the dilatation in the Central Range further north 17 profiles 1–3) to a mixture of different types (Figs. 14b and 17 (Hsu et al., 2009). GPS data seem to show that with the Manila profiles 7–9). Trench clearly defined to the west of southern Taiwan (Fig. 1) and the active subduction of the Eurasian plate, the southern part 5.1.4. Plate motion-related deformation of southern Taiwan of Taiwan island apparently moves as a whole with the PSP. No The rate of horizontal displacement of different parts of Taiwan ocean bottom GPS data is available to show the motion offshore is shown by GPS measurements in Taiwan (Fig. 6 and Hsu et al., of SE Taiwan, but the displacement of the small island south of 2009). The map illustrates quite clearly the rapidly changing dis- Kaohsiung, offshore southwestern Taiwan (Fig. 6) indicates motion placement field on the west coast across the TTL, varying from similar to the nearby onshore measurements, although with a large 0 cm/yr in the north to 5 cm/yr south of the TTL – all with counterclockwise rotation (Fig. 6 and Hsu et al., 2009). In Fig. 6 the respect to Penghu Island. In fact, south of TTL the magnitude of rate difference between stations on Lutao and Lanhsu and those the vectors is noticeably larger on the west coast than on the east onshore of SE Taiwan is about 4 cm/yr, indicating that the forearc coast, causing large EW-oriented dilatation in the southern Taiwan basin is undergoing significant shortening. F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 193

Fig. 15. Seismicity sections for southern Taiwan. Locations of the profiles are shown in Fig. 14. The green colored patches mark the regions with little seismicity. See text for details.

5.2. Central Taiwan – mature collision the largest known event on land. All the events in central Taiwan occurred within the crust; no intermediate depth (>70 km) or dee- Central Taiwan, lying roughly between TTL and 24.3°N, is recog- per earthquakes have been confirmed in this section of Taiwan. nized as the site of the ‘‘mature’’ collision. This section has the Offshore to the east the elevation of the seafloor begins to rise highest topography, where both horizontal shortening and vertical sharply about 30 km from the coastline, and the seismicity begins uplift reach their maximum (Hsu et al., 2009; Ching et al., 2011). to increase from that point westward (Figs. 18b and 19). In recent Here the collision between PSP and EUP is the strongest, and the years, several magnitude 7 or larger shallow (<30 km) events Coastal Range is found only within this section. However north occurred east of Taiwan (Kao et al., 2000). These are apparently of 23.7°N the colliding PSP begins to subduct northward and intraplate events, indicating the internal deformation of PSP. the effects of collision change from mainly shortening to a north- The Chi-Chi earthquake and its aftershocks were the first round ward escape and gradual weakening (see next section) Historically, of major on-land events that were well monitored by both the central Taiwan is known to be the most seismically active section modern CWBSN and GPS networks. The data thus accumulated of Taiwan (e.g., Fang, 1968; Wu, 1978). Since about 1650, when provided many key evidences regarding Taiwan orogeny. Wu well-recorded history began, a series of damaging earthquakes et al. (2004), among others, has relocated and analyzed the seis- have occurred, especially in the southwestern part of this area. micity before and after the mainshock. Here we present seismicity However, the 1999 Chi-Chi earthquake in the northern part was in three periods: (1) from July 1, 1994 to September 20, 1999 194 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 16. Selected southern Taiwan sections with focal mechanisms. See Fig. 14 for locations of the numbered profiles.

(Fig. 18a and b), (2) September 21–December 31, 1999 (Fig. 18c reflect the inter-seismic motion. The Central Range is, from all and d) and (3) January 1, 2000–June 30, 2009 (Fig. 19a and b). datasets, the fastest rising area, where the uplift rate correlates Selected focal mechanisms and P tomographic sections are in with elevation (Fig. 7) Incidentally, the Central Range on the hang- Fig. 20. Together with resistivity and Vp/Vs profiles and the ing wall side of the Chi-Chi thrust fault actually subsided in the co- island-wide geodetic data, the Central Range shows up as the core seismic and post-seismic periods (Wu et al., 2004). of the orogen that drives many geological processes. 5.2.2. Seismicity in the higher ranges, under the Western Foothills and 5.2.1. Deformation, vertical and horizontal, in central Taiwan under the Coastal Plain Recent leveling data of 2000–2008 (Fig. 7 and Ching et al., 2011) The pre-Chi-Chi seismicity for over five years, shown in Fig. 18a present a clear picture of current vertical motions associated with and b, and the post-Chi-Chi seismicity of over 3 months show pat- the orogeny. Available longer-term measurements, such as those terns that illustrate several important tectonic characteristics of from fission-track dating, show that such motions also occurred, the Central Range. Besides the increase in the number of events although at lower rates, over the last 1 or 2 million years (Lee in the Foothills and some shallow parts of the Central Range, sev- et al., 2006) The long-term rates of up to 10 mm/yr in the last mil- eral changes in the seismicity patterns in these figures are particu- lion years (Lee et al., 2006) are somewhat less than the short-term larly significant. First, before Chi-Chi (Fig. 18a) two distinct layers rate of almost 20 mm/yr (Ching et al., 2011), but equally signifi- of seismicity can be discerned in profiles 1–9 under the Foothills cant. Possible reasons for the difference are (1) that the bench- – a typical pattern for continental seismicity (Chen and Molnar, mark-based leveling data reflect the ‘‘rock-uplift’’ rate, not taking 1983). During the 3 months after the mainshock, the aftershock into account the long term denudation, (2) that the faster uplift foci fill in the aseismic region between the two layers and the is only a recent phenomenon, or (3) that the faster rate could deeper layer is not well defined (Fig. 18b profiles 1–4). The double F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 195

Fig. 17. Seismicity and focal mechanisms superposed on tomographic velocity sections in southern Taiwan – selected profiles. See Fig. 14 for locations of the numbered profiles. See text for detailed explanations. layers of seismicity returned after 2000 (Fig. 19a and b). Secondly, and the aftershocks were limited primarily to the west of the Cen- a large portion of the Central Range is in general relatively quies- tral Range, west of the Lishan fault, as marked by a vertical arrow cent before the mainshock, as can be seen in Fig. 18a and b, and in Fig. 19a. gradually returns to the pre-Chi-Chi level after 2000 (Fig. 19). Thirdly, the northern part of the aftershock region did have after- 5.2.3. Crustal seismicity and rheology shocks across the Central Range (Fig. 18a profiles 1–6 and The double-layered seismicity in northwestern Taiwan (profiles Fig. 18b), as did the eastern side of the Central Range. The eastern 1–8, Figs. 18a, b and 19a, b) has been interpreted to be the result of Central Range events are associated largely with normal faulting a rheological ‘‘jelly sandwich’’ crust (Mouthereau and Petit, 2003; during the post-Chi-Chi Central Range subsidence (Wu et al., Wu et al., 1997, 2004), with two brittle–ductile transitions (BDT) 2004). The low seismicity occurs in regions of the crust where in the crust. The phenomenon is found to be true for many conti- 6.0–6.5 km/s materials have thickened and risen to the surface, nental areas of the world and has been similarly interpreted (e.g., forming the pre-Tertiary metamorphic rocks of the Central Range; Chen and Molnar, 1983; Bürgmann and Dresen, 2008). The merg- these aseismic regions are essentially bounded by the Hsueshan ing of the two seismicity layers in profiles 9 and 10 (Fig. 18a) Range, or the Lishan fault, on the west and the LV (e.g., profiles 4, and the disappearance of the bottom layer in profiles 11 and 12 5 and 6 in Figs. 18–20). In 2000–2009 (Fig. 19a and b) the general are coincident with the southern end of the Hsueshan Range; this level of seismicity is still higher than that of the pre-Chi-Chi period, observation agrees with the hypothesis that south of the southern although the pre Chi-Chi pattern of seismicity has returned; note terminus of the Hsueshan Range the crust is no longer continental. that the green patches in Figs. 19a and the corresponding A higher thermal gradient and/or different composition in the (unmarked) areas in 18a are mostly seismically quiescent. The seis- south can account for the disappearance of the lower BDT. micity, focal mechanisms and causative fault of the Chi-Chi main- The filling of the aseismic region immediately after the main- shock sequence have been investigated extensively (Shin and Teng, shock (Fig. 18c and d) is described in Wu et al. (2004) as essentially 2001) The focal mechanism of the mainshock is a thrust fault, but a ‘‘silly putty’’ effect, i.e., the rapid stress adjustment immediately displacement along the 70 km-long fault (Fig. 1) is more complex after Chi-Chi made the media behave like brittle materials, (e.g., Loevenbruck et al., 2004) Very clearly the mainshock faulting whereas during the normal crustal stress-loading cycles they 196 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 18. Seismicity of central Taiwan: (a) seismicity sections and (b) map view with locations of profiles and focal mechanisms, from 1994 to just before the 1999 Chi-Chi earthquake, and (c) and (d) the corresponding sections and maps for the aftershocks of the Chi-Chi earthquake until December 31, 1999 – about 102 days. behave as ductile materials (Chen and Molnar, 1983). From 2000 to presence of many hot springs in the Central Range are consistent June, 2009 (Fig. 19b) some Chi-Chi aftershocks still occurred under with this conclusion. the foothills and a small portion of the eastern Central Range; these areas were not noticeably active before the Chi-Chi. 5.2.4. High angle reverse structure under the Lishan fault The restriction to mostly shallow seismicity in the Central Aftershocks began within an hour of the September 20, 1999, Range has been interpreted as a result of higher temperature Chi-Chi mainshock in the mid- to lower crust between the depths (Wu et al., 1997). The temperature estimate of 750 °Cat24 km of 15–35 km. A NNES-oriented zone about 30 km long and dipping described earlier (see Kuo-Chen et al., 2012b for details) and the steeply toward the west quickly developed to bound the quiescent F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 197

Fig. 19. (a) Sections of seismicity for central Taiwan from 2000 to June, 2009, and (b) in map view. The green shaded area highlight the seismically quiescent areas. mid- to lower crustal region under the Central Range and lie essen- 4, 5 and 6), raising the possibility that this fault facilitates the tially under the Lishan fault (profiles 5–7, Fig. 18c and d). A thickening of the mid-crustal layer. In general, the reverse faulting

MW = 6.4 earthquake, one of the largest Chi-Chi aftershocks, in western Taiwan can perhaps be viewed as an inversion of the occurred in this zone on September 22, 1999. The aftershock foci normal faults created during the Miocene when the continental of this event aligned along a steep zone (indicated in Fig. 18 by hor- shelf went through a stage of rifting (Lee et al., 2002). izontal arrows); a thrust focal mechanism was derived for the

MW = 6.4 event, with its steep nodal plane aligned with the steep 5.2.5. Seismicity associated with the Longitudinal Valley and faulting seismic zone. The mechanisms of several of M < 5 events in the The LV is clearly the locus of the EUP/PSP contact, based on west-dipping zone (Fig. 20, profiles 5 and 6) also show the domi- geology as well as tomographic images. The contact of the arc with nance of thrusting. Notice that on the east side of this fault the 6 the continental shelf was formed when the subduction of EUP led or 6.5 km/s contour makes a large downward turn (Fig. 20, profiles to the destruction of the oceanic or transitional crust between EUP 198 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 20. Seismicity and focal mechanisms superposed on tomographic velocity sections in central Taiwan – selected profiles. For profile locations see Fig. 18. and PSP. The LV is seismically active, based on geomorphic evi- Coastal Range drops from more than 1000 m in the south to about dence (Shyu et al., 2005, 2008) and seismicity. Two M7 events took 500 m on average in the north (Wu et al., 2009). place in northern and central LV in 1951; field evidence indicated The seismic zones under LV in Figs. 18a and 19a generally dip to an oblique thrust with left-lateral motion (Hsu, 1956) for both the east, with the dip angle varying from near vertical to about 35°. events. We have already discussed the southern Longitudinal Val- But the complexity of the seismic zones implies varying fault struc- ley fault as the surface projection of the steep (55°) seismic zone tures. The mechanisms of the events in the vicinity of the northern that defines the causative fault of the 2006 Chengkung earthquake LV are mostly thrust types with a few strike-slips (Fig. 20), even in (Kuo-Chen et al., 2007). But LV is not a simple fault. Shyu et al. very steep zones. It is not uncommon to link this portion of LV (2005, 2008), among others consider that, at least in the southern to strike-slip motion, based on surface mapping of earthquake half, LV is bounded on both sides by thrust faults, with the Central faulting (Hsu, 1956) and geomorphology (e.g., Shyu et al., 2008). Mountain fault on the west side enabling the rise of the Central Northern LV tectonics is further complicated by the Ryukyu Range, and the Longitudinal Valley fault taking up left-lateral and subduction, to be detailed in the discussion on northern Taiwan. thrust motion on the east side. The seismicity in Figs. 18 and 19 show that the relatively high- 5.2.6. High velocity rise under eastern Taiwan angle southern Coastal Range thrust does not continue northward The clearest rise of high velocity materials (7.5–8.0 km/s) under beyond profile10. An east-dipping zone is also discernible in pro- the Coastal Range/LV area, as described in Section 4, occurs along files 5–9, but it is deeper and less steep (35°) and, rather than the extent of mature collision. This rise extends along the entire extrapolating upward to near the LV, it projects toward the Central length of the LV, and the layers above the rise are also strongly Range. We note that the change from the steep thrust to the shal- up-warped (see the 6.5 km/s contour in all profiles of Fig. 20). lower dipping one occurs at a point where the elevation of the We noted earlier that the seismicity and focal mechanisms in the F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 199 southern half of the Coastal Range describe a well-defined east- N-component that drives subduction and a NNW-component that dipping thrust on the top, or east side, of the rise (Fig. 17, profiles causes PSP/EUP collision. Because of the increasing depth of the 1, 2 and 3 and Fig. 20, profiles 7 and 8), but in the northern half subducting PSP plate, the collision of PSP with EUP occurs at dee- (Fig. 20, profiles 4–6) the rise itself is seismic. In southern Taiwan, per levels toward the north until the PSP sinks under the EUP lith- the rise of the 7.5 km/s contour is not so prominent as along the LV, osphere. Below that point PSP is apparently able to overcome the but the rise of the 6.5 km/s contour is quite clear. The tectonic viscous resistance of the EUP in the upper the mantle and advance implications of this high velocity zone include the formation of toward WNW. We note that the deepening collision zone means the LV, normal faulting observed in the eastern Central Range, that the shortening of EUP lithosphere occurs at a deeper level and metamorphism in the eastern Central Range. We shall exam- toward the north, causing the deeper part of the orogen to shorten ine some of these issues in later discussions. and the top to possibly thrust backward, as the SE directed GPS vector indicates (Fig. 6). The orogeny tapers off rapidly only when 5.2.7. Resistivity profile the PSP enters the asthenosphere. Bertrand et al. (2009) presented a 2-D section near 24°N A tectonic incongruity exists above the terminus of the subduct- (Fig. 21). The prominent features in this section include a high ing/colliding PSP. Shown schematically in Fig. 23d, in east–west resistivity zone throughout the upper 30–40 km under the Central cross-section, the EUP straddles across this boundary. But the Range, and a low resistivity zone under the Lishan fault (Fig. 21). EUP is driven on the west side by PSP at depth, while the EUP on The Coastal Plain, with its deep sedimentary basin on the western the east side is coupled to PSP by friction across the EUP/PSP plate side, is a zone of shallow, low resistivity, as expected. The high boundary. The figure summarizes the variations of focal mecha- resistivity core of the Central Range is indicative of its dryness nisms in this area. Interestingly, the deep Hoping basin (Fig. 1)is (Bertrand et al., 2009) and the conductive zone under the Lishan located on the east side above the PSP/EUP boundary. A free-air fault implies the presence of liquid. anomaly of less than À200 mgal (Hsu et al., 1998) implies a deep sedimentary basin, as imaged in Fig. 10a between 135 and 5.3. Northern Taiwan 165 km on the horizontal axis. Kao et al. (1998) and Wu et al. (2009) have shown that focal mechanisms for events just offshore Seismicity maps of northern Taiwan and three sections under of eastern Taiwan in the collision section between Taitung and northern Taiwan are shown in Fig. 22a and b. Profile 3 goes along Hualien are dominated by thrust-type events with trend-perpen- the eastern edge of the northern Central Range and the LV south of dicular P axes, while north of Hualien (Fig. 1) and offshore of east- Hualien (HL in the section and the map). The seismicity in this area, ern Taiwan the mechanisms of shallow (<20 km) events include although at a shallow depth, is too complex to determine the point normal faulting and strike-slip events. Wu et al. (2009) interpreted at which the W-B zone begins to dip northward; the geometry of these offshore events between Hualien and Taitung to occur as a the bottom of the zone, however, is very clear. It starts to bend result of complex deformation in the zone of tectonic incongruity. downward south of HL, at approximately 23.7°N to form a well- The triangular shaped area with thickened crust is located under defined W-B zone (Fig. 22, profile 3). This is actually the end of the Hoping Basin. The triangle lies in the area where the PSP the continuous Ryukyu seismic zone beginning from Kyushu (Wu ‘‘indenter’’ has swept through underneath, thickening the crust et al., 2009). The NNW-oriented dashed red line in Fig. 22b shows along the way as it moved NWW, and causing the apparently coun- the western limit of the active zone in the map view. In cross-sec- terclockwise rotation of northern Taiwan. Northern Taiwan wraps tions parallel to the structural trend and further to the west of the around the end of it to form a bight, similar to the wrap-around at island (Fig. 22a, profiles 2 and 3) only the deeper parts of the zone the east Himalayan syntax. can be seen. Notice also the intersections of the profile lines with The proposed junction of the Ryukyu subduction boundary with the dashed line and the seismicity. Two 3-D views of a block model Taiwan (Wu et al., 2009) is located in the Longitudinal Valley. In are seen in Fig. 23a and b, representing the plate configuration in this model the Coastal Range north of 23.7°N is currently situated northern Taiwan as implied by the W-B seismicity; the blocks were on the subducting PSP. Recently published results of leveling data constructed so as to wrap around the seismicity. for 2000–2008 (Fig. 7)byChing et al. (2011) indicate that the Although relatively simple, the block model in Fig. 23a–c incor- Coastal Range exhibits different behaviors along strike: To the porates the observations we discussed earlier and depicts several north from about 23.5°N, an increasing rate of subsidence is plate tectonic features that correlate well with the orogenic defor- observed, but in the south, a rapid uplift is taking place. The subsi- mation of Taiwan. The WNW directed motion of PSP resolves into a dence of the northern Coastal Range is readily explained from our

Fig. 21. Manetotelluric profile across central Taiwan and resistivity cross-section (Bertrand et al., 2009). CP = Coastal Plain, WF = Western Foothills, HSR = Hsueshan Range, WCR = Western Central Range, ECR = Eastern Central Range and LV = Longitudinal Valley. Note the Central Range is underlain by a high resistivity region while the vicinity of the Lishan Fault (LF) is very conductive. For the locations of MT stations see Fig. 8b. The profile in uses measurements from the stations along two lines in central Taiwan. 200 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 22. Seismicity of northern Taiwan in (a) cross sections and (b) in map view. Notice the red dashed line that crosses the profiles lines – it shows the western termination of the Ryukyu Benioff zone. HL = Hualien, IP = Ilan Plain and TP = Taipei Basin. The focal mechanisms of the slab events show dominant P-axes perpendicular to the profile direction, indicating the lateral stress in the plate as a result of the termination of Ryukyu (PSP) slab and its advance against EUP.

proposed model, i.e., the Coastal Range connected to the PSP side of PSP over active EUP subduction in southern Taiwan. We will first the PSP/EUP boundary and the Okinawa Trough is rapidly opening, discuss the general properties of the plate boundaries and their with flexure taking place as EUP thrusts over the PSP. outstanding problems, then the collision geometry and finally the tectonic characteristics of the three regions. Within each region, 6. Discussion the distinctive features on the PSP and the EUP sides of the plate boundaries will be noted. In carrying out the TAIGER project we The tomographic velocity images, seismicity and focal mecha- were guided by common tectonic models, which we continue to nisms explored in this paper represent a snapshot of cumulative assess as new observations are acquired. Thus, in reviewing the and ongoing deformation resulting from the collision of EUP and models we also present new perspectives and ideas on the Taiwan PSP, and as such can be used as a ‘‘key to the past’’ for deciphering orogeny. the processes of mountain building. The TAIGER tomographic and It is convenient to consider the EUP as essentially stationary, seismicity images of Taiwan in particular highlight the 3-D aspects with the PSP moving in the direction of 305°. This is particularly along-strike of the orogen because of the different tectonic pro- appropriate as the motion of PSP is much larger than that of EUP cesses in operation, namely, (1) the subduction of PSP and its col- (88 mm/yr vs. 23 mm/yr, but roughly the same direction,) seen lision with EUP in northern Taiwan, (2) the full collision of the PSP here in the hotspot reference frame (HS3-NUVEL-1A; Gripp and and EUP lithospheres in central Taiwan and (3) the advance of the Gordon, 2002). F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 201

Fig. 23. 3-D perspective view of plate model of northern Taiwan. The ‘‘plates’’ are constructed by wrapping the seismicity with simple surfaces. (a) A view from NE and above for the overall plate configuration near Taiwan. (b) A view from N to look at the shape of the subduction zone as it gradually emerges from the upper part of the lithosphere where the PSP and EUP are in full collision. (c) A simplified view of the upper part of the model showing the plate boundaries and an ‘‘incongruous zone’’ (in red), over which the EUP is subjected to different stresses. (d) The changes in focal mechanisms in a section across the red surface showing the effects of collision/subduction, while to the east of the red zone the events show mostly EW T-axes, those in the elliptical zone show mixed mechanisms including normal and strike-slip types (Wu et al., 2009).

There is little argument regarding the existence of the two 6.1. Regarding the detachment, the deformation front and the Manila plate boundaries around Taiwan: the Ryukyu Trench about Trench 100 km east of Taiwan is well recognized, and the Manila Trench is quite clear a few tens of kilometers from the continental shelf A deformation front of the collision zone is included in most of SE, offshore SW of Taiwan. The exact geometry of the plate the models discussed in this paper as the western limit of thrust boundaries becomes obscured upon approaching Taiwan, because faulting or mountain front rising from the Coastal Plain, although the collision-induced deformation causes the subsequent details vary. Suppe (1984) extends the mountain front southward sedimentation to mask the internal structures of the orogen and to the Manila Trench, viewing Taiwan as an accretionary wedge the bathymetry of plate boundaries. The overall plate tectonics over a low-angle (6°) detachment; coincidentally this is the EUP/ that lead to the collision of the Luzon Arc with the continent is PSP subduction interface, which continues its propagation west- not, however, under debate. The Luzon Arc on the western margin ward. Carena et al. (2002) remap the plane based on selected seis- of PSP moves northwestward to over-thrust the east-dipping EUP micity. Huang et al. (2000) and Malavieille et al. (2002) develop subduction zone; the PSP between the Arc and the continent more detailed models. They each define an arc-prism boundary, shrinks; and eventually the Arc contacts the shelf near Taiwan effectively the EUP/PSP or the indenter boundary, at the present- and the intense collision begins. As the Arc approaches the day LV, and show that it continues southward along SLT offshore continent, the forearc block becomes shortened to form the of southern Taiwan. In both cases the deformation front is taken Coastal Range and the shelf is compressed to form the Central to be the northward continuation of the Manila Trench. Malavielle Range. A portion of the subducted PSP may be oceanic, while et al.’s model constructs the structural evolution of Taiwan at shal- the part abutting the shelf may contain transitional South China low crustal and upper mantle depths. It emphasizes the breakup Sea (SCS) lithosphere; this is an important consideration and will and the duplexing of the forearc block in the initial collision and be commented on later. the exhumation of the subducted lower crustal rocks in the early We have presented evidence from TAIGER subsurface research stage of the collision. In this model it is assumed that within the relating to the PSP and EUP subduction near Taiwan. Now we turn last 0.5 my EUP continental subduction stopped because of its neg- to discuss the implications of the tomographic images, seismicity ative buoyancy after the breakoff of the oceanic part of the EUP; and other TAIGER data for the current plate interactions and the further development of detachment led to under-plating of youn- cumulated internal deformation of the orogen as a result of the col- ger subducted materials under the EUP materials exhumed earlier. lision. Our discussions are cast in terms of concepts contained On the arc side, a broken (at the arc) piece of the proximal forearc explicitly or implicitly in current tectonic models, while offering block is assumed to have become subducted under the distal part some alternate views where appropriate. (see Fig. 25, adapted from Malavielle’s, 2002 paper). 202 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208

Fig. 24. Schematic diagrams illustrating possible evolution of plate boundaries in the last two million years near Taiwan. In the left panel, the current plate boundaries are shown. T3 is one of the TAIGER transects that provides an estimate of the pre-collision continental margin structure (Lester et al., 2010,2012); the 30 km and 10 km crustal thickness contours are deduced from the results. Note that in the crosshatched area in southern Taiwan the initial crustal thickness should taper off quickly toward the south and the east. The dark grey patches are the positive magnetic anomalies associated with the shelf edge. Note that the Hsueshan Range tapers off quickly south of the magnetic anomaly in central Taiwan. In the right panel, the positions of the trench and arc in 2 Ma, 1 Ma and the present are shown. The yellow area represent the approximate extent of the continental shelf that has been consumed during the collision up to the present. At 2 Ma, the PSP/EUP boundary, the northern extension of the Manila Trench, is near the shelf edge – after collision it developed as the LV. But the current Manila Trench is offset from the LV. If the offset of the Manila Trench an inherent feature? There should be a right-lateral strike-slip fault near TTL, if the offset is developed recently – but no such fault has been found. See text for more discussion.

Many aspects of these models can be discussed in light of new in-sequence and out-of-sequence thrusts on the continental shelf observations. The shallow detachment, which traditionally has should have been initiated (Malavieille et al., 2002), with concom- held a prominent role in discussions on Taiwan orogeny since itant flexure of the EUP. If the formation of the detachment did the early 1980s, is invoked in all three of the hypotheses discussed involve a series of shallow in- and out-of-sequence thrusts, a sub- above and 10’s km or larger displacements are assumed to have stantial ‘‘damaged’’ zone should have occurred in the top 10– taken place in the last few million years. Several consequences of 12 km, and this layer should be detectable in seismic images as a such large displacements can be expected in the detachment zone. lower velocity ‘‘critical wedge’’ of fracture-weakened materials at For example, the alignment of high seismicity, possibly including the top of the crust. One would also expect the Central Range to large events, with low angle (east-dipping) focal mechanisms overthrust the less metamorphosed Foothills terrain and be should be associated with this zone. Duplexing of thrust slices in detected as a velocity reversal in a depth section under the western the zone may lead to higher angle thrust faulting, but sole-thrust Central Range, as explicitly predicted in Malavielle et al.’s (2002) events should be quite frequent; if the detachment cuts across model (Fig. 25). The crustal images presented in Fig. 20 do not the Central Range and the Coastal Range then the seismicity should show features that can be interpreted as such a detachment. occur throughout the Range and be aligned. Relocated seismicity in Detachment thrust in continental crust is often discussed as a Fig. 19a shows flat-lying zones (profiles 2, 3 and 5) or east-dipping flat sole fault or very shallow dipping. Twiss and Moore (1992), zones (profiles 1 and 4) as the top layer of the double seismic layers for example, present the kinematic model for its development. under the Foothills; most probably rheology controls the location But as they point out such models do not consider what happens of these layers. Most Chi-Chi aftershocks occurred in a shallow flat to the continental crust below the detachment and are not com- zone across the northern part of the Central Range, as observed in plete. Of the hypothetical alternatives they offered, the ‘‘root zone’’ profiles 1–5, whereas an essentially aseismic zone in the Central model (Fig. 3d) actually resembles, in a general way, a section from Range is seen in profiles 6–11. The focal mechanisms in the shal- the metamorphic high Central Range to the Coastal Plain; although low zone in profiles 4–7, Fig. 20 are mostly thrust events (also in central Taiwan the ‘‘undeformed basement’’ is a rheological see Wu et al., 2004). In addition, it would be expected that large sandwich with the lower layer being seismic; a 1964 M6.4 blind- persistent displacements in the detachment zone should induce thrust event in the Tainan area was determined to be 23 km deep oriented cracks or alignment of fault or shear zone minerals, such (Wu et al., 1997). that the zone would become strongly anisotropic with fast direc- Is the 1999 Chi-Chi earthquake a detachment event? The focal tions generally in the direction of slip. This zone would be a P-wave mechanism (Kao and Chen, 2000) and finite fault modeling of the reflector, especially for trend-perpendicular wave paths because of mainshock all show oblique slip on an east-dipping (30°) plane. velocity contrasts with rocks above and below. But the only strong It is a major thrust fault, but it does not cross the Central Range reflection seen in the TAIGER explosion data is observed under and the fault dip does make it a good candidate for the hypothe- northern Taiwan, 13 km long and at 18 km depth, whereas profil- sized detachment. ing across central or southern Taiwan did not yield any (Quiros et al., 2013). 6.2. Trench, forearc block and collision To construct the evolution of the detachment in the continental crust after the last bit of possibly transitional EUP was consumed After the Luzon Arc and EUP collide, the arc/forearc becomes and the forearc basin closed, we can assume that a series of compressed to form the Coastal Range, while on the continent shelf F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 203

22.8°N) currently lies offshore to the east. Shortening is taking place between southern Taiwan and the Arc, as the GPS velocity vectors is larger on Lanhsu Island than on the eastern side of south- ern Taiwan (Fig. 6) and seismicity is relatively high in this area (Figs. 14a, b and 15 profiles 7–15). But the Manila Trench lies to the west and the GPS vectors on the west side of the mountains indicate that area moves at about 50 mm/yr west/southwestward – faster than the east side of the island and also much faster than near the ‘‘deformation front’’ to the north. As this part of Taiwan overlies an active EUP subduction zone, while the Manila Trench to its west has not yet come in contact with the continental shelf of southern Taiwan, it is possible that it is moving with the PSP. But evidently the PSP plate-edge complex, extending from the Luzon Arc to the Manila Trench, is drastically different from the forearc block up north. If it does move with PSP then it will even- tually collide with the continental margin, about 160 km to the west. A new Coastal Range would form from all the arc and forearc materials and an additional piece of the southern Taiwan moun- tains. This range would then be quite different from the Coastal Range we now have, where no continental piece is found. A question arises from this ‘‘thought experiment’’. If southern Taiwan continues to move westward with PSP, as the GPS mea- surements seem to indicate, and central Taiwan is engaged in a col- lision in which the ‘‘deformation front’’ is moving much slower than the trench, then there should be an observable right-lateral offset of structures, cutting across the N–S striking structures, such as the Chauchou fault. That no such offset has been observed could mean that no significant displacement of the southern Taiwan Fig. 25. (a) A tomography/seismicity/focal mechanism section across central range as a whole has yet accumulated. Since the former PSP/EUP Taiwan (profile location in Fig. 14a: profile 2). The PSP boundary is an active thrust fault that bottoms at 40 km. At 40 km this thrust intersects with a very low angle subduction boundary stopped in the LV and the Manila Trench is fault. The active part of the low angle fault does not seem to extend too far into the offset from LV, it implies that a bend in the trench may have Philippine Sea basin. (b) Malavielle et al.’s (2002) tectonic model for present day existed, as shown in Fig. 24. Taiwan emphasized the deformation on both sides of the LV, the delamination on the Eurasian side and the formation of a subduction under the western edge of PSP. 6.3. Mountain building and crust/mantle processes in Taiwan The thickening of PSP is seen in (a), although the subduction under the CoR depicted in (b) does not seem to be taking place nor is the shallow overthrust of the older Central Range against Tertiary sediments. See text for detailed explanation. If the geometry of the commonly assumed pre-collision conti- nental shelf is generally correct (see Figs. 5 and 24), the Hsueshan Range and the northern part of the Central Range are situated on the pre-collision shelf (Fig. 24b). In northern Taiwan the collision the Central Range starts to rise. The forearc block east of the has consumed EUP, marked by a blue patch in Fig. 24b. The contin- Coastal Range may have had a width of 100–200 km before the col- ued collision against the shelf has exhumed the Mesozoic Tananao lision, judging from the current distance between the arc and the rocks (Jahn et al., 1992) from mid- or lower crust under the East Manila Trench south of Taiwan, but the Coastal Range is now only Central Range and continued collision has led to the ‘‘pop-up’’ of about 15 km wide. This large degree of shortening led Malavieille the – sediments on the shelf to form the Hsueshan et al. (2002) to argue for plate breakoff and duplexing under the Range. To the west of Hsueshan, the collision also creates the Coastal Range. In a general sense the velocity images of PSP shown tightly folded Miocene Foothills on top of a crustal flexure. The in Figs. 10, 17 and 20 show significant deformation and thickening Hsueshan Range tapers off toward the south as it reaches the edge under the Coastal Range or further to the east. An isolated high of the continental shelf and ends north of 23°N(Fig. 2); south of velocity anomaly (see Fig. 10, ‘‘central’’ profile, 135 km on the x- that point the Central Range is in direct contact with the axis at 90 km depth), in addition to the extensive 7.5–8 km/s mate- Foothills (shelf-slope sediments), and the Hsueshan Range stops rials in nearly all the sections in Figs. 17 and 20 at the depth range near the former continental margin as defined by the magnetic of 30–70 km, could be interpreted as parts of the forearc block. anomaly (Fig. 4). If the shelf edge conjecture shown in Fig. 4 is cor- Taking profile 2 in Fig. 17 as a representative section, the indenter rect, the crust under the shelf at the latitude where the Hsueshan at the PSP/EUR boundary impinges on the Central Range with a Range terminates should have thinned toward the east and the major thrust fault that extends from the surface to about 38 km southeast, tapering off from about 30 km to about 10 km at the with a zone of low-angle thrust at the bottom. While this corrobo- foot, as discussed earlier. Further south, as the island narrows, rates Malavielle et al.’s thickened PSP (the indenter), the Coastal the mountain chain gradually tapers off but does not disappear, Range does not appear to overlie an active mid-crustal detachment even though the crust ought to attain the thickness of the oceanic that stretches from the Foothills to under the Luzon Arc. PSP. In fact the crust near the southern tip is about 30 km thick Thus far we have described the LV as the contact of the PSP (McIntosh et al., 2005). In southern Taiwan, where the arc and indenter with the EUP, where the EUP subduction zone was con- the continent are not in collisional contact, a mechanism for build- sumed prior to the collision, in agreement with Huang et al. ing the mountains in this area with concomitant crust thickening (2000) and Malavieille et al. (2002). As the Manila Trench migrated needs to be sought. westward until the forearc block contacted EUP during the early There are at least two possible answers to this dilemma, assum- stage of collision, more intense deformation of both EUP and PSP ing the pre-collision shelf configuration described earlier is correct: began to occur. The forearc block in southern Taiwan (south of (1) the formation of the southern Taiwan mountains, as well as 204 F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 parts of the Central Range, by inversion of the rifted continental 6.4. Vertical coherent deformation vs. EUP subduction under Taiwan crust on top of the subducting SCS lithosphere (McIntosh et al., 2013) and (2) the southward extrusion of continental materials The fast axes and delay times of the polarized S-waves shown in from the deeply indented and highly compressed northern Taiwan Fig. 13 are similar to S-splitting results in many mountain ranges. (e.g., Wu et al., 1997; Lacombe et al., 2001). Both ideas can be sub- The global data archive (Wüstefeld et al., 2009), shows that many jected to further tests. For example, if the orogen were built from of the compressive or transpressive mountains of the world, e.g., rifted EUP, then the petrology of the Taiwan orogen should reflect South Island (New Zealand), Tianshan (China), Tibet (China) and that of the basin, mixing EUP continental materials with Miocene Zagros (Iran) exhibit the same characteristics, i.e., the teleseismic magmatics and sediments. With regard to the extrusion concept, fast S-wave directions / align with the trend of the structures, Lacombe et al. (2001) and Angelier et al. (2009) linked the focal and the delay times dt are relatively large (1–2.4 s) (Silver, 1996 mechanisms in southwestern Taiwan and in the Central Range to and others). The crust in Taiwan contributes 0.1–0.3 s of delay time major faults in the area and to generally southwest-directed (Chang et al., 2009), while in the mantle, with an average 4% escape. The recent M > 6 earthquakes in the southern Central anisotropy, a dt of 1 s translates to a source layer about 115 km Range (event A and B in Fig. 14b; Huang et al., 2011 and Lee thick (Silver, 1996). Thus for the S-split observations we need an et al., 2012), occurring at 24 km depth and yielding WSW-direc- anisotropic region in the upper mantle at least 100–200 km thick, ted slip on NE-dipping plane, are consistent with such a model. In one that has been deformed coherently with the crust so that the / both cases, more detailed seismic imaging of the Hengchun Penin- lines up with the structural trend. Another feature of the Taiwan sula and Ridge will be useful for deciphering the details of the results is the asymmetry of the delays: Consistently larger delays evolving inversion process or extrusion. These two possible mech- are observed for events from the east than for those from the west. anisms could also be mixed, i.e., inversion and escape could occur A plausible interpretation is that the source zone may be steeply simultaneously if the viscosity of the materials is low enough. east-dipping (Kuo-Chen et al., 2009), and essentially aligned with Escape could take place roughly at the same time as the building the east-dipping high velocity zone in south central and southern of the Central Range. Taiwan. The rapid uplift of the Central Range (Ching et al., 2011 and The plausible association of the high P velocity anomaly shown Fig. 7) indicates that it is apparently the most active part of in Fig. 10 with the source of anisotropy needs further consider- the whole orogen. Ching et al. (2011) found that detachment ation. If the velocity anomaly is part of the subduction zone, then modeling cannot explain such uplift. The strain at the surface cal- the usual assumption of a cold, heavy and partly brittle zone makes culated from GPS is mainly dilatational in the northern and the acquisition of LPO, requiring large strain and low viscosity, less southern Central Range (Hsu et al., 2009), with the maximum likely. One possibility is that the subducted lithosphere is the rifted strain axes at various angles from the structural trend of the Cen- South China Sea lithosphere and that / was inherited from it. The tral Range. This dilatation is probably a result of ‘‘flowering’’ or anisotropy of active continental rifts shows / at an angle with the compression at depth and expansion on top, as demonstrated rift axis and dt greater than 1 s (Gao et al., 1997; Kendall et al., also by the pattern of foliation across the Central Range – gener- 2005). With the generally EW-oriented spreading center in the ally from west to east vergent, going from west to east. Ductile South China Sea (e.g., Li et al., 2011) a N–NNE fast direction could deformation most probably dominates in the core area, as have been formed. But if magma-filled cracks are the main source implied by the general lack of seismicity in the Central Range of anisotropy (Gao et al., 1997; Kendall et al., 2005), then such an area, especially below 12 km or so (Wu et al., 2004). This conclu- effect should diminish under high pressure at mantle depths. An sion is consistent with our temperature estimates from Vp/Vs alternate and internally consistent hypothesis would require the results as described earlier. Studies have found that a–b transi- outer part of the aseismic high P velocity anomaly to be under ele- tion significantly weakens the rocks (Fig. 12; Lowry and Pérez- vated temperature and viscous condition. In the oblique collision Gussinyé, 2011; Peng and Redfern, 2013) and could lead to rapid environment of Taiwan, shearing parallel to the strike or lateral deformation. Overall, the tomographic images show that mid- to flow because of convergence may lead to an LPO parallel to strike. lower crustal rocks have exhumed, consistent with the outcrops The LPO is created under finite strain conditions (e.g., Mainprice, of Pre-Tertiary metamorphic rocks at the surface. The tempera- 2007). In northern Taiwan the continuous rotation of the fast- ture of 750 °C at 24 km, deduced from Vp/Vs, raises the question direction to EW is most probably related to the end flow of the of whether partial melting may have occurred. The likelihood of Ryukyu subduction system, where the westward motion of the melting is highly dependent on the water content. Our MT profile end of PSP has intruded the Eurasian upper mantle (Wu et al., across central Taiwan indicates that it is a very dry region 2007a,b, 2009; Kuo-Chen et al., 2009, 2012a) and subsequently (Bertrand et al., 2009), so the likelihood of melting is low, at least stopped the along-strike flow. in the top 25 km or so. S-splitting results provide important yet incompletely under- With the crustal thickness 40 km or greater under the Central stood constraints on the Taiwan orogeny. Mantle processes most Range, the continental rocks in the lower crust are subjected to certainly accompany those in the crust. In deciphering them we conditions favorable for eclogitization (Ryan, 2001; Leech, advance in our knowledge of the orogeny. 2001). Such transformations have profound effects on the dynamics of orogeny, and the resulting products could lead to material exchange with the upper mantle. The images of the 7. Conclusion deep root under the Central Range show pooling of 7.5–8 km/s materials. Although the layer below this goes into 8.1–8.2 km/s, The young and active Taiwan orogen provides a unique environ- typical Vp for eclogites, the region below this layer is not well ment for detailed studies of orogenic processes. Analyses of enough resolved to answer the eclogite question. The 7.5–8 km/ recently acquired subsurface data have added important con- s layer could also be a mix of some eclogite and granulite or gab- straints to the testing and exploring of models for the collision of bro and retain the lower velocity for the layer. In any case, the plates in Taiwan. The major subsurface structures we have mapped root under Taiwan is a good candidate for addressing eclogitiza- can readily be understood in terms of the EUP/PSP plate geometry tion and mountain building, and also the consequent delamina- and the collision tectonics of Taiwan. tion that has been considered important in orogeny (e.g., Ryan, The rapid uplift in the Central Range, the dramatic crustal thick- 2001). ening, the metamorphism and the possible high temperature at F.T. Wu et al. / Journal of Asian Earth Sciences 90 (2014) 173–208 205 mid-crustal depths all point to the Central Range as the probable belt? Insights from active deformation studies in the Ilan Plain and Pingtung engine of cause. Seismic velocity images of the Central Range indi- Plain regions. Tectonophysics 466, 356–376. Bensen, G.D., Ritzwoller, M.H., Barmin, M.P., Levshin, A.L., Lin, F., Moschetti, M.P., cate a thickening and exhumation of mid-crustal rocks, while the Shapiro, N.M., Yang, Y., 2007. 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