The Deep Electrical Structure of Southern Taiwan and Its Tectonic Implications

Chih-Wen Chiang,Chien-Chih Chen,Edward Bertrand,Han-Lun Hsu,Martyn Unsworth,Chow-Son Chen,Thong Duy Kieu

doi:10.3319/tao.2010.02.25.01(t)

Abstract

The Taiwan orogen has formed as a result of the arc-continent collision between the Eurasian continental margin and the Luzon volcanic arc over the last 5 million years and is the type example of an arc-continent collision. The tectonic processes at work beneath Taiwan are still debated; the available data have been interpreted with both thin-skinned and lithospheric collision models. In 2004, the Taiwan Integrated Geodynamical Research (TAIGER) project began a systematic investigation of the crustal and upper mantle structure beneath Taiwan. TAIGER magnetotelluric (MT) data from central Taiwan favor a thick-skinned model for that region. The Taiwan orogen becomes younger to the south, so the earlier stages of collision were investigated with a 100-km-long MT profile in southern Taiwan at latitude of 23.3°N. Data were recorded at 15 MT sites and tensor decomposition and two-dimensional inversion were applied to the MT data. The shallow electrical resistivity structure is in good agreement with surface geology. The deeper structure shows a major conductor in the mid-crust that can be explained by fluid content of 0.4-1.4%. A similar feature was observed in central Taiwan, but with a higher fluid content. The conductor in southern Taiwan extends to lower crustal depths and is likely caused by fluids generated by metamorphic reactions in a thickened crust. Together the central and southern Taiwan MT profiles show a crustal root beneath the Central Range.

Highlights

The island of Taiwan was formed by the arc-continent collision between the Luzon volcanic arc on the western margin of the Philippine Sea Plate (PSP) and the passive continental margin of southeastern China, beginning some 5 Ma ago (Suppe 1981; Ho 1986)
The polarity of subduction changes across the island; in the northeast the PSP is subducting to the northwest beneath the Eurasian Plate (EUP), while in the south a portion of the continental EUP subducts to the east beneath the PSP (Tsai 1986; Seno 1993; Kao 2000)
The Western Foothills (WF) and Hsuehshan Range (HR) are separated by the Chuchih fault, which merges with the Lishan fault at the southern extent of the HR to form the Laonungchi or Chaochou fault

Summary

Introduction

The island of Taiwan was formed by the arc-continent collision between the Luzon volcanic arc on the western margin of the Philippine Sea Plate (PSP) and the passive continental margin of southeastern China, beginning some 5 Ma ago (Suppe 1981; Ho 1986). The south pointing induction vectors could be related to the coast-effect, where the conductive anomaly is the ocean These indicators of 3D geoelectric structure suggest that a 2D analysis must be applied with caution. Error floors of 20%, 4.35° and 0.1 were used for the apparent resistivity, phase and tipper data, respectively These error floor values were chosen to fit strong distortion in southeastern profile as shown in Fig. 4a and were motivated by the approach used by Booker et al (2004) and Becken et al (2008) to down-weight the influence of 3D induction and static-shift effects. The calculated responses from the best-fitting model (Fig. 8) are in good agreement with the measured MT data, except for the long-period TM phases at the westernmost 7 sites where poor fitting indicates strong 3D effects that cannot be accounted for with a 2D response. It may be the result of the galvanic distortion effects or the coast effect as described in the section on 3D modeling

Three-Dimensional Forward Modeling and the Coast Effect

Geoelectric Structure and Its Interpretation

Conclusions