Results and Discussions

Our GPS surveys have provided a complete velocity field of SW Taiwan relative to the Paisha station of the Penghu archipelago, which belongs to the stable continental shelf. Horizontal GPS displacement velocities display the trends of variation in the investigated area. The station velocities range between 31 mm/yr and 56 mm/yr, with azimuths ranging between 245^o and 273^o with respect to the permanent station S01R. Both horizontal and vertical GPS displacement velocities display the trends of variation in the investigated area (Figs. 2 and 4). For the horizontal velocity field, the station velocities decrease westwards and gradually increase southwards. In terms of velocity vector trends, there is a contrast in kinematics in the study area. In the central and western part of the study area, the GPS stations move mostly toward the west, whereas in the Kaohsiung and Pingtung coastal area the displacement vectors show a clear deviation toward the southwest (Fig. 2). As pointed out by Hu et al. (1997; 2001), this significant counter-clockwise rotation of displacement velocity vectors is related to transtensional deformation, and reflects the along-strike southward increase in extensional deformation in southwest Taiwan, in the transition zone from collision to subduction.

The changes in baseline lengths from the repeated GPS surveys in the Pingtung area are used to assess the spatial variation of the horizontal crustal strain over the region. It is assumed that, spatially, the crustal strain accumulates uniformly over each grid, and that the rate of strain accumulation is constant over the time period

considered. The most prominent feature of the strain distribution patterns in the study area certainly corresponds to the significant extension in the coastal region of the Pingtung plain. The southern part of the offshore coastal area show remarkable extension rates of 0.6-2.0 µstrain/year in an azimuth 015°-020°. By contrast, in the north and central part of the Pingtung plain, the small contractions mostly trends WNW-ESE directions. This significant southward increase of the extensional strain rates is attributed to the lateral extrusion of blocks bounded by major discontinuities in the study area (Fig. 1). These results are in general agreement with the previous models of lateral extrusion due to the low lateral confining conditions related to the Manila subduction zone as a free boundary or/and the presence of prominent Peikang High as a rigid indenter (Lu et Malavieille, 1994; Lu et al., 1998; Hu et al., 1997, 2001; Lacombe, 1999; 2001; Bos et al., 2003). Based on the rigid blocks models (Angelier et al., 1999; Lacombe et al., 2001), the escaping area comprises four rigid blocks moving toward the SW along major discontinuities with both lateral and reverse shears. These discontinuities comprise the right-lateral reverse motion of the Deformation Front, right-lateral reverse motion of the Chishan fault, left-lateral reverse motion of the Kaoping Fault and left-lateral reverse motion of the Chaochou Fault (Fig. 1). Based on the inversion of fault slip and continuous deformation deduced from GPS data in Taiwan (Yu et al., 1997), Bos et al. (2003) also presented a surface deformation model for southern Taiwan. This model exhibited a strain pattern with E-W contraction accompanied by a southward increase of predominantly N-S oriented extension.

The vertical deformation revealed by our GPS measurements is distributed in two distinct regions, north and south, with dominant uplift and subsidence respectively (Figs 4 and 5). Chen (1984) pointed out that tectonically the area north of Pingtung area is characterized by uplift whereas the coastal area of Pingtung plain is

dominated by subsidence. On the basis of GPS measurements, a significant subsidence rate from ~11 mm/yr to ~25 mm/yr is observed in the southern Pingtung plain. The maximum subsidence rate is compatible with the value of 25 mm/yr previously calculated based on conventional geodesy from 1914 to 1979 (Chen, 1984).

This significant subsidence partly results from the large groundwater pumping for aquacultural purposes. Taking into account a recent study of groundwater table evolution (Kuo et al., 2001), the total subsidence of the southern Pingtung plain should be attributed to the mixed effects of regional tectonic tilting towards the southwest and local over-abstraction of groundwater. On a longer time scale, Lai et al.

(2002) quantified Holocene subsidence rates in the southern Pingtung plain, based on radiocarbon dating and drill cores (Fig. 6). Furthermore, the Holocene subsidence pattern is consistent with the isopach of fine-grained sediments in the study area (Fig.

7). Based on their study, the average subsidence rate is about 4 mm/yr with a subsidence pattern similar to that of figure 5. This long-term average subsidence rate for the Holocene in southern Pingtung is about 2 to 6 times smaller than the observations derived from GPS measurements from 1996 to 1999. We infer that this regional subsidence is due to both the prominent groundwater level decrease which causes rapid subsidence of the coastal zone of the Pingtung plain, as well as the transtensional deformation associated with the tectonic extrusion.

5. Conclusion

Four years of GPS measurements with three campaigns have shown that subsidence in the Pingtung-Kaohsiung area is dominated by transtentional crustal deformation due to tectonic extrusion. The horizontal station velocities varied from 32 mm/yr to 54 mm/yr in azimuths ranging from 247^o to 273^o with respect to the

permanent station (S01R) located on the stable continental shelf. The southern part of offshore coastal area show remarkable extension rates of 0.6-2.0 µstrain/year in an azimuth 015°-020°. This significant southward increase of extensional strain rates is attributed to the lateral extrusion of blocks bounded by major discontinuities in the study area. For the vertical movement, the station velocities range from 13 mm/yr to -25 mm/yr. There are 20 stations with recorded velocities showing subsidence in the southern part of the Pingtung plain. Significant subsidence rates from ~11 mm/yr to

~25 mm/yr have been observed. These results clearly demonstrate the existence of transtensional deformation and the southward increase of extensional deformation in the along-strike direction of the whole study area. The comparison with the pattern of Holocene subsidence rates (about -4 mm/yr in the same area) and isopach of fine-grained sediments suggests that about 75% of this subsidence may result from the decrease in groundwater levels induced by over-pumping. These localized anthropogenic activities contributed to the natural risk that results from tectonic subsidence associated with tectonic extrusion and lateral extrusion at the southern tip of the Taiwan collision belt.

Acknowledgments

We are grateful to Margot Böse, Alexander Koh and Serge Shaprio for constructive comments that led us to improve the manuscript. The authors would like to thank Chia-Yu Lu, Yu-Chang Chan, Meng-Long Hsieh, Shui-Bei Yu, Ching-Huei Kuo and Benoit Deffontaines for the suggestions and discussions. This research was supported by grants from the National Science Council of Taiwan (NSC 91-2119-M-002-020) and the Central Geological Survey of the MOEA. Some figures were produced using the Generic Mapping Tools written by Paul Wessel and Walter H.

F. Smith.

References

Bos, A.G., Spakman, W., Nyst, M.C.J., 2003. Surface deformation and tectonic setting of Taiwan inferred from a GPS velocity field. Journal of Geophysical Research 108(B10), 2458, doi:10.1029/2002JB002336.

Chen, H.-F., 1984. Crustal uplift and subsidence in Taiwan: an account based upon retriangulation results. Special Publication of the Central Geological Survey 3, 127-140.

Chan, Y.-C., Lu, C.-Y., Lee, J.-C., 2000. Orogen-parallel shearing in on-going mountain building: a case study form the southeastern Central Range of Taiwan. Eos. Trans. AGU, Fall Meet. Suppl., Abstract, 81.

Chiang, C.-S. Yu, H.-S., Chou, Y.-W., 2004. Characteristics of the wedge-top depozone of the southern Taiwan foreland basin system. Basin Research 16, doi: 10.111/j.1365-2117.2003.00222.x, 65-78.

Dixon, T.H., 1991. An introduction to the Global Positioning System and some geological applications. Review of Geophysics 29, 249-276.

Dodson, A.H., 1995. GPS for height determination, Survey Reviews 33, 66-76.

Fan, K.-L., 2001. Some coastal environmental problems in Taiwan, Acta Oceanographica Taiwanica 39, 1-10.

Hix, G.L., 1995. Land subsidence and ground water withdrawal, Water Well Journal 49, 37-39.

Hsu, S.-K., 1998. Plan for a groundwater monitoring network in Taiwan.

Hydrogeological Journal 6, 406-415.

Hu, J.-C., Angelier, J., Yu, S.-B., 1997. An interpretation of the active deformation of southern Taiwan based on numerical simulation and GPS studies.

Tectonophysics 274, 145-169.

Hu, J.-C., Yu, S.-B., Angelier, J., Chu, H.-T, 2001. Active deformation of Taiwan from GPS measurements and numerical simulations. Journal of Geophysical Research 106, 2265-2280.

Huang, C.-C., Chiang, C.-J., Lai, Y.-H., 1998. The hydrogeological framework and groundwater system model of Pingtung Plain, Proceedings of the Symposium on Groundwater and Hydrogeology of the Pingtung Plain, Taipei, 139-152.

Hugentobler, U., Schaer, S., Fridez, P., 2001. Bernese GPS software, Version 4.2, Astronomical Institute, University of Berne, 515 pp.

Kuo, C.-H., Chan, Y.-C., Wang, C.-H., 2001. Subsidence: over withdrawal groundwater, tectonic or both? Eos. Trans. AGU, Fall Meet. Suppl., Abstract, 82, F479.

Lacombe, O., Mouthereau, F., Deffontaines, B., Angelier, J., Chu, H.-T., Lee, C.-T., 1999. Geometry and Quaternary kinematics of fold-and-thrust units of southwestern Taiwan. Tectonics 18, 1198-1223.

Lacombe, O., Mouthereau, F., Angelier J. Deffontaines B., 2001. Structural, geodetic and seismological evidence for tectonic escape in SW Taiwan. Tectonophysics 333, 323-345.

Lai, T.-H., Hsieh, M.-L., Liew, P.-M., Chen, Y.-G., 2002. Holocene rock uplift and subsidence in the coastal area of Taiwan. Eos. Trans. AGU, Fall Meet. Suppl., Abstract, 83, F1280.

Lai, T.-H., Hsieh, M.-L., 2003. Late-Quaternary Vertical Rock-movement Rates of the Coastal Plains of Taiwan, 2003 Annual Meeting Geological Society, Taipei, 119.

Lallemand, S.E., Tien, H.-H., 1997. An introduction to active collision in Taiwan.

Tectonophysics 274, 1-4.

Lin, A.T., Watts, A.B., 2002. Origin of the west Taiwan basin by orogenic loading and flexure of a rifted continental margin. Journal of Geophysical Research 107 (B9), 2185-2203.

Lin, C.-W., Chang, H.-C., Lu, S.-T., Shih, T.-S., Huang, W.-J., 2000. An introduction of the active faults of Taiwan, Central Geological Survey Special Publication,13, 122 pp.

Liu, C.-W. Lin, W.-S., Shang, C., Liu, S.-H., 1999. The effect of clay dehydration on land subsidence in the Yun-Lin coastal area, Taiwan. Environmental Geology 40, 290-296.

Lu, C.-Y., Malavieille, J., 1994. Oblique convergence, indentation and rotation tectonics in the Taiwan Mountain Belt: Insights from experimental modeling.

Earth and Planetary Science Letters 121, 477-494.

Lu, C.-Y., Jeng, F.-S., Chang, K.-J., Jian, W.-T., 1998. Impact of basement high on the structure and kinematics of western Taiwan thrust wedge: Insights from sandbox models. Terrestrial, Atmosphere and Oceanic Sciences 9(3), 533-550.

Malavieille, J., Lallemand, S.E., Dominquez, S., Deschamps, A., Lu, C.-Y., Liu, C.-S., Schnürle, P., 2002. Arc-continent collision in Taiwan: new marine observations and tectonic evolution., in Byrne, T.B., and Liu, C.-S., eds, Geology and Geophysics of an Arc-Continent Collision, Taiwan. Boulder, Colorado, The Geological Society of America Special Paper 358, 187-211.

MOEA (Ministry of Economic Affairs), 1987. Study on the subsidence of land in coastal areas in Pingtung County, 77 pp

MOEA (Ministry of Economic Affairs), 1997. The execution of land subsidence prevention and reclamation plan in 1997 (in Chinese), MOEA, Taipei, Taiwan.

Segall, P. Davis, J.L., 1997. GPS applications for geodynamics and earthquake studies.

Annual Review of Earth Planetary Science 25, 301-336.

Shyu B.H., 1999. The sedimentary environment of southern Pingdong plain since the

last glacial, Master Thesis, National Taiwan University, Taipei, pp 212.

Sun H., Grandstaff, D., Shagam R., 1999. Land subsidence due to groundwater withdrawal: potential damage of subsidence and sea level rise in southern New Jersey, USA. Environmental Geology 34, 290-296.

Teng, L.S., 1990. Geotectonic evolution of late Cenozoic arc-continent collision in Taiwan. Tectonophysics 183, 57-76.

Wessel, P., Smith, W.H.F., 1998. New, improved version of the Generic Mapping Tools Released. EOS Trans. AGU 79, 579.

Wu, L.-C., 1993. Sedimentary basin succession of the upper Neogene and Quaternary Series in the Chishan area, southern Taiwan and its tectonic evolution.

National Taiwan University PhD Thesis, 212pp.

Yu, S.-B., Chen H.-Y., Kuo L.-C., 1997. Velocity field of GPS Stations in the Taiwan area. Tectonophysics 274, 41-59.

Yu, S.-B., Kuo, L.-C., Punongbayan, R.S., Ramos, E.G., 1999. GPS observation of crustal deformation in the Taiwan-Luzon region, Geophysical Research Letters 26, 923-926.

Fig. 1. Tectonic framework and main structural units in Taiwan (after Teng, 1990; Hu et., 2001; Lacombe et al., 2001; Malavieille et al., 2002). Large open arrow shows the direction and velocity of plate convergence of Philippine Sea plate and Eurasian plate relative to the South China block (Yu et al., 1997; 1999). Major thrust faults with triangles are on the upthrust side. Numbers indicate 1, normal fault; 2, thrust fault (active); 3, thrust fault (inactive); 4, strike-slip fault; 5, indenter of rigid promontory at the front of the active belt; 6, back-arc opening; 7, tectonic escape; 8, migration of the thrust front.

Fig. 2. Horizontal velocity field of GPS stations in Pingtung plain relative to Paisha, Penghu (S01R) from 1996 to 1999. Locality of S01R, see Figure 1. The 95%

confidence ellipse is shown at the tip of each velocity vector. Solid star indicates permanent GPS station. Solid circles are survey mode stations. Thick lines are active faults. Shaded topography is based on 40 m x 40 m DEM. CCF: Chaochou Fault; CSF:

Chishan Fault; LKF: Liukuei Fault; HKSF: Hsiaohangshan Fault; FSF: Fengshan Fault.

Fig. 3. Principal strain rates in the Pingtung area. Convergent arrows denote contraction, whereas divergent arrows represent extension. Thick grey lines indicate the fault traces based on data of MOEA (Lin et al., 2000).

Fig. 4. Vertical velocities of GPS stations in the Pingtung plain relative to Paisha, Penghu (S01R) from 1996 to 1999. Symbols are the same as Figure 2.

Fig. 5. Contours of vertical velocities of GPS stations in the Pingtung plain relative to Paisha, Penghu (S01R) from 1996 to 1999. Contour interval is 2 mm/yr. Dotted contours indicate uplift and solid contours are subsidence. Symbols are the same as Figure 2.

Fig. 6. Contours of vertical uplift and subsidence rates (mm/yr) of Holocene based on radiocarbon dating and drill cores (Modified after Lai et al., 2002; Lai and Hsieh, 2003). White line denotes the boundary of uplift and subsidence region. Solid lines indicate subsidence whereas dotted lines represent uplift. Solid triangles indicate the localities of well sites.

Fig. 7. Contours of percentage for depositional isopach of fine-grained sediments in subsidence area (Modified after Huang et al., 1998). Open circles indicate the localities of well sites.