The Possible Causes of the Crustal Low Resistive Zone for the Western Foothills, Taiwan

This study attempts to discuss the possible causes of the crustal low resistive zone based on the magnetotelluric observations in the Western Foothills, Taiwan. The depth and resistivity of this low resistive zone (LRZ) have the values, on the average, of 9 km and 30 ohm-meters. According to the independently geological data, the possible causes of the LRZ are re­ lated to the high C02 activity in Taiwan and the dehydration reactions. The existence of a significant amount of HC03 in crustal fluid would produce a consequent impact on resistivity.


INTRODUCTION
The Island of Taiwan is located in the active boundary between the Philippine Sea plate and the Eurasian plate. The relative plate velocity between the Philippine Sea plate and the Eurasian plate is at about 7.1 cm/yr and in the direction of about N50°W (Seno et al., 1993). The overall plate configuration in the vicinity of Taiwan is well defined by seismicity (Tsai, 1986). While the Philippine Sea plate is subducting northwestward from the Ryukyu Trench in the northeast of Taiwan, the Eurasian plate is subducting beneath the Philippine Sea plate along the Manila Trench in the south of Taiwan. Thus, Taiwan lies on the region in which the polarity of subduction changes. The rapid arc-continental collision is responsible for the com plex geological setting and the rugged topography. The geology and tectonics of Taiwan can be found detailedly in the two book-length introductions of Ho (1982Ho ( , 1986. Due to the rarity of arc-continent collisions among the active orogenic belts in the world, Taiwan possesses tectonically a unique position. Because of its interest as a typical example of an active arc-continental collision, the structure of the active Taiwan mountain belt always catches the scientists' attentions. The readers are referred to the special issue on TECTONOPHYSICS (Lallemand and Tsien, 1997) for a recent progress of the studies on the active collision in Taiwan. Many geophysical investigations, including the analysis of seis micity (Wang et al., 1994), the gravity measurement (Y en, 1991), the seismic tomography (Rau and Wu, 1995), the seismic reflection profiling (Shih et al., 1997) and, the most recently, the deep electrical sounding , had been conducted to give a detail figure beneath Taiwan during the last decade. This study, which attempts to discuss the possible causes of the crustal low resistive zone based on the magnetotelluric ob servations, is a supplement for the previous paper by the authors ).

THE LOW RESISTIVE ZONE FROM MAGNETOTELLURIC OBSERVATIONS
As an important property of the Earth's interior, the deep electric resistivity had been investigated actively by the magnetotelluric (MT) method in the Taiwan orogen during the last two years (Chen et al., 1996(Chen et al., , 1997. The MT tech nique makes use of naturally-occurring electromagnetic (EM) wave fields as sources for ex ploring the deep electric resistivity structure (Vozoff, 1991). These EM waves propagate dif fusively into the earth, and so the penetration depths of the signals increase with period and resistivity. A tensor MT survey has been carried out in the field. The measurements of EM fields were made using 5 components, including 2 orthogonal electric field components (Ex and By) and 3 magnetic field components (H.x, Hy and Hz), at a MT site. A remote reference by using 2 additional magnetic components (Rx and Ry) was used to suppress EM noise at some sites. The data were recorded by a real-time MT V5-16 system, Phoenix Geophysics Ltd. (Canada), with a capability of measuring the broad band EM signals ranging from 384 to 0.00055 Hz.
To account for the unpredictable natural changes in EM source field strength over time, the observed electric field is normalized by the orthogonal magnetic field at each frequency.
The resulting quantity, e.g. Zyx:::: Ey/H.x, has units of ohms and is called the impedance of the earth. Practitioners of MT generally convert impedance values to apparent resistivity. Appar ent resistivity vs. frequency usually resembles a smoothed version of the true resistivity over increasing depths below the measurement site. Another complementary to apparent resistivity is the phase of the impedance. More details about the principles and data processing for MT method can be found in the published literatures (e.g. Vozoff, 1972).
The important results from MT observations  have concluded that there exists a low resistive zone at the depths of 10-20 km beneath Taiwan. Beneath the West ern Foothills, the depth and resistivity of this low resistive zone (LRZ) have the values, on the average, of 9 km and 30 ohm-meters. These values are significantly different from those be neath the Central Range, which are 20 km and 80 ohm-meters . Figure   1 shows the electrical models beneath two MT sites located on the Western Foothills. The solid and dashed lines represent the electrical models obtained respectively from sites TST and MLI . Here we consider the static shift effect in MT measurements.
In its simplest form for data from either lD or 20 earth models, this effect manifests itself as a shift of the apparent resistivities by a frequency independent multiplicative constant without affecting the phases. A lD interpretation of the shifted apparent resistivity curve would lead to an erroneous model with both resistivities and depths of the layers have been shifted away from the true earth. As shown in Figure 1 I  I  I  I  I  I  I  I  I  l  I  I  I  I  I  I   I   10 1 10 2 1 0 3 resistivity (oh m-m) MLI, a common LRZ, with a resistivity about 10 ohm-meters and a depth ranging from 7 to 10 km, appears at the two sites.

CAUSES OF THE LRZ
The resistivity is sensitive to the existence of small amounts of fluid, melt, or conductive minerals, and can vary over many orders of magnitude, e.g. from 0.01 ohrn-m to 1,000,000 ohm-m (Jones, 1992). The actual resistivity of rock is usually controlled, not only by the matrix host rock resistivity itself, but also by the connection in the rock matrix of typically a fluid (ionic conduction) or solid (electronic conduction) phase. Thus, the bulk resistivity value reflects the pore fluid resistivity and the rock porosity, and is a function of temperature, pres sure, and ion content of the fluid. Besides fluids, the resistivity of rock is also sensitive to the presence of conductive minerals, such as graphite, sulfide, and magnetite (Jones, 1992). De tailed interpretation of the electrical structure will require independent data from geology or from geophysics. This section is an attempt to interpret the possible causes of the above LRZ, according to the independently geological data (Chou et al., 1989;Liou, 1981).   (Figure 2), this attempt will be restricted in this area, in particularly the middle western Taiwan.
The locations of the above two MT sites, i.e. TST and MLI, are shown in Figure 2. Also included in Fig. 2 are the volcanic rock distribution and the oil-gas fields in western Taiwan. A high content of C0 2 in natural gas produced in these oil-gas fields had been reported by Chou et al. (1989). The C0 2 is mainly trapped under the sealing Piling Shale to charge the reserviors of the Mushan Formation and the Wuchihshan Formation in the several onshore and offshore gas-oil fields. The evidence indicating high C0 2 activity in Taiwan includes not only the high C0 2 content of natural gases in the Western Foothills but also high C0 2 content of thermal waters in metamorphic and sedimental terranes, and high concentration of carbonate minerals in mafic and pelitic rocks of the Central Range and the Western Foothills (Liou, 1981 ).
C0 2 is excluded as a conductive candidate because it is an insulating liquid and a non polar gas, and thus has little or no effect on electrical properties (Olheft, 1981). However, as a major dissolved component in aqueous fluids of the crust, C0 2 in aqueous solutions is, at upper and mid crustal temperatures, largely in the form of C0 2 (aqueous) with a lesser amount of H 2 C0 3 (Fein and Walther, 1987). In solution, two dissociation reactions can occur: C 02(aq) + H20 <=> Hco:; + H+ HCO] <=> coi-+ H+ The existence of a significant amount of HCO] in crustal fluid is probable, with a conse quent impact on resistivity. In addition, the concentration of HCO] is extremely sensitive to pH changes (Nesbitt, 1993). Thermal waters from the metamorphic and sedimental terranes in Taiwan are indeed characterized by pH values of 7-10 and high concentrations of NaHC0 3 and Hco:; (Liou, 1981).
What is the source of the aqueous fluids in the crust? It had been expected by Suppe (1981) that the dehydration reactions are taking place at lower temperature about 220°C at depth of 7 km in the Western Foothills due to the drop in fluid pressure-solid pressure ratio. T1'us, the dehydration reactions may offer these fluids in the crust.

DISCUSSIONS AND CONCLUSIONS
The origin of the high content of C0 2 in natural gas is attributed to the deep foreign inorganic sources which are associated with the crust or mantle (Chou et al., 1989). C0 2 mi grated from the C0 2 -pool derived by the magma from the mantle. Beneath the Western Foot hills, the foreign deep co 2 migrated upwards along the fractures and faults originated by igneous activity and orogeny. Therefore, the C0 2 was sealed under the Piling Shale to charge the reservoirs of the Mushan and Wuchihshan Formation.
Subsurface geologic reports, paleontological data and stratigraphic correlation studies of the 88 onshore and offshore exploration wells in the western Taiwan Basin were incorporated by Shaw (1996). While the top for the Mushan Formation at HTP (Figure 2) is approximately at the depth of 2.5 km (Dr. M.T. Lu, pers. commun.), the evaluated depth at HL (Figure 2) is about 5.2 km (Figure3 in Shaw, 1996). Not only the strata were disturbed during the mountain building in Taiwan, but the Mushan Formation thrust upward in the Western Foothills (Suppe, 1981).
While the resistivity of the whole upper crust maintains the value below 50 ohm-m, a minimum value appears at the depth of about 8 km. A simplest explanation for the relatively low resistivity in the upper crust of the Western Foothills is the thick sediments presented there, which may contain water and/or other fluids (Dr. R.J. Rau, pers. commun.). such as conductivity studies of C02-H20-salt solutions at higher temperature, pressure and C02-salt concentrations, are needed to confirm our concept and to provide the physicochemi cal states beneath Taiwan.