The Deep Electrical Structure of Southern Taiwan and Its Tectonic Implications

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.

Evaluation of seismo-electric anomalies using magnetic data in Taiwan

“…This suggests that the crustal structure changes at approximately 30 km depth, thus, indicating the Moho discontinuity in accordance with Wang et al (2010) at a depth between 30 and 40 km beneath Taiwan. These results also suggest the existence of relatively high conductivity materials beneath the central mountain range, which is in agreements with the magnetotelluric survey reported by Chiang et al (2010). Figure 4a illustrates the anomalous azimuth deviations of P a (15 km) versus the epicentre azimuth (70 • ) of EQ2 covering a period of 110 days.…”

“…The relationship between ρ and D ep ( Fig. 2a) at the CCU station is retrieved from previous magnetotelluric studies conducted nearby (Bertrand et al, 2009;Chiang et al, 2010) and is utilised to compute f in Eq. (1).…”

“…The geomagnetic method is a common technique used to understand and/or monitor changes in the underground conductivity (Chiang et al, 2010) and its susceptibility ) of a specific area. Parkinson (1959) proposed that a linear relationship exists between short-period variations in the geomagnetic field, specifically among the vertical component Z, the horizontal component H and the declination D. The complex and frequency-dependent coefficients A and B are computed from H , D and Z via the magnetic transfer function.…”

Preliminary Studies on Unusual Magnetic Diurnal Variation on Hengchun Peninsula

“…The variations in the geomagnetic field caused by solar activity are most likely the same within a small region generally considered to be global effects (Chapman and Bartels 1940). Other sources such as induced geomagnetic fields caused by differences in electrical structure (Lilley and Arora 1982;Lilley 1984;Horng et al 1992;Hsu et al 1998;Lilley et al 1999;Yen et al 2009), pre-earthquake magnetic anomalies resulting from stressed rocks (Yanagihara and Nagano 1976;Rikitake 1979;Shiraki 1980;Chen and Fung 1985;Gong 1985;Chen et al 2006), and the magnetic coast effect due to the distinct conductivity between continental rocks and sea water (Parkinson 1959(Parkinson , 1962(Parkinson , 1983Bennett and Lilley 1971;Lilley and Bennett 1972;Parkinson and Jones 1979;Delaurier et al 1983;Yukutake et al 1983;Ogawa et al 1986;EMSLAB Group 1988;White et al 1990;Kellett et al 1991;Hitchman et al 2000;Armadillo et al 2001;Lee et al 2007;Bertrand et al 2009Bertrand et al , 2012Chiang et al 2010) affect the geomagnetic field within a local area. The GZH, HC, and PHU stations have similar geomagnetic latitudes from 10.92 -13.27°N.…”

An Investigation of the 3D Electrical Resistivity Structure in the Chingshui Geothermal Area, NE Taiwan

“…Previous MT studies applied one-and/or two-dimensional inversion approaches (Tong et al 2008;Chang et al 2014) without dimensionality analysis which may be unable to delineate the geothermal structures if the MT data is mixed with 3D effects, galvanic distortions (Bertrand et al 2009(Bertrand et al , 2012Chiang et al 2010Chiang et al , 2011 or topographic effects (Wannamaker et al 1986;Jiracek 1990). We attempted to carefully reprocess these MT data step by step to evaluate whether the MT data can be appropriately inverted using 1D or 2D inversion.…”