Regional map of Taiwan showing major geographical and tectonic features. The Eurasian and Philippine Sea Plates are separated by the Ryukyu and Manila Trenches. This map was generated using Google Earth 4.0.2737 software, freely available at  

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... collision is widely regarded as a fundamental mode by which continents grow, and the Taiwan orogen has long been considered as a type example of it. In Taiwan, the Luzon Volcanic Arc is actively colliding with the southeast continental margin of China. The Taiwan Orogen is one of the world's most active, with high rates of deformation and seismicity. In addition, due to the oblique nature of convergence between the arc and continental margin, Taiwan offers a unique opportunity to investigate the evolution of arc- continent collision in both time and space (Suppe, 1981). This is possible because variations along strike can be viewed as snapshots of various stages in orogen evolution over time. In this paper, the general tectonics and geology of Taiwan are briefly reviewed. 2-D inversion models of electrical resistivity using the TAIGER (TAiwan Integrated GEodynamic Research) MT data are then presented. TAIGER is an international geological and geophysical research program initiated in 2004 to study the orogenic processes of Taiwan. Finally, the MT inversion models are interpreted with an intent to answer key questions as to how continents grow and mountains evolve. Specifically: 1. Is orogenic deformation ''thin-skinned'' or is basement deformation pervasive throughout the system? 2. How does crustal deformation, as expressed in the surface geology, extend into the mantle? 3. How is deformation partitioned laterally as well as vertically in the lithosphere? The Taiwan orogen has developed in response to the collision between the Luzon Volcanic Arc on the Philippine Sea Plate and the continental margin of the Eurasian Plate (Figure 1), beginning about 3 Ma (Tsai et al., 1977; Teng, 1990). Northeast of Taiwan, the Philippine Sea plate subducts northwestwards beneath the Ryukyu Trench, while south of Taiwan the Eurasian plate is subducting beneath the Philippine Sea plate along the Manila Trench. The convergent boundary (i.e. the suture zone) is marked on land in Taiwan by the Longitudinal Valley (Figure 2; Ho, 1988). The rapid (relative plate convergence is 80 mm/yr; Yu et al., 1997) arc-continental collision is responsible for the dominant geological variations that occur east-west across the island, with rocks generally becoming older and more metamorphosed to the east. The main geological structures are parallel to the strike of the suture in an overall NNE–SSW direction (Figure 2). East of the suture, the Coastal Range (CR) is comprised of accreted Luzon volcanic arc materials that originated on the Philippine Sea plate. West of the suture, the principal geological units are: (1) the Eastern Central Range (ECR or the Tananao schist) that exposes rocks of greenschist facies, (2) the Western Central Range that is comprised of the Backbone (BR) and Hseuhshan Ranges (HSR) and also often referred to as the Taiwan slate belt, (3) the Western Foothills fold-and-thrust belt (WF) and (4) the relatively undeformed Coastal Plains (CP). MT data were collected on three profiles, NN’, CC’ and SS’ in Taiwan during 2006-2007 (Figure 2). Five-component (three magnetic and two electric) long-period (1-10000s) MT data were recorded at sites spaced ~5 km apart using NIMS instruments designed by Narod Geophysics Ltd. in Canada. To reduce the effects of cultural noise, these long-period MT data were remotely referenced (Gamble et al. 1979) using a station located on the Penghu archipelago, ~60km from the west coast of the main island of Taiwan. Figure 3 shows one hour of typical time series data recorded simultaneously at a location in central Taiwan (TGR235) and on Penghu (TGR000). Note the similarity of the orthogonal horizontal magnetic fields recorded at these stations, even though they are separated by over 100 km. This similarity occurs because the magnetic fields originate within the ionosphere, far from the Earth’s surface. In contrast, the electric and vertical magnetic fields show much greater variation, since they reflect differences in the subsurface electrical resistivity structure (Bertrand, 2010). Dimensionality analysis from tensor decomposition has been performed on these data using the McNeice-Jones algorithm (McNeice and Jones, 2001). Figure 4 shows the results of the tensor decomposition algorithm (McNeice and Jones, 2001) applied to each MT station individually, and also averaged over the period bands indicated (Bertrand, 2010). Note that strike directions derived from tensor decomposition have an inherent ambiguity of 90 o . This 90 o ambiguity can be resolved by using external information, such as the regional geological trend. Thus, the geoelectric strike azimuth that closely parallels the average island-strike has been chosen for display. It is clear from the consistent orientation of the bar azimuths that an overwhelming orogen-parallel geoelectric strike direction is determined for these MT data. Furthermore, despite some divergence, at the majority of sites the geoelectric strike remains consistent over all period bands shown. This consistency is an important indication that the MT data on these profiles can be considered 2-D. Closer observation of the strike directions reveals that the dominant azimuth for each profile subtly rotates clockwise from south to north Taiwan. This rotation matches the overall S-shape curvature of the island (Yu et al., 1997) and indicates that the geoelectric strike is consistent with the large-scale 2-D orogen structure and ocean bathymetry (Bertrand, 2010). While the previous map-view plots illustrate the dimensionality of individual MT stations, rose diagrams are advantageous for revealing average data trends (Bertrand, 2010). In Figure 5, rose diagrams are shown that consolidate the results displayed in Figures 4, for data collected on linear profiles. For the main profiles that traverse Taiwan (Lines NN’, CC’ and SS’) the rose diagrams of strike effectively reveal that a dominant 2-D geoelectric strike direction exists collinear to the island long-axis and major geologic features in Taiwan. The results of this analysis indicate that the electrical structures are two-dimensional with dominant strike directions N45°E, N37°E and N30°E in northern, central and southern Taiwan, respectively (Figure 5). As expected, these strike directions are essentially parallel to the regional geology. The dimensionality analysis presented implies that the generation of 2-D inversion models will be appropriate for these MT ...