Magnetotelluric images of deep crustal structure of the Rehai geothermal field near Tengchong, southern China

Broadband (0.004-4096 s) magneto telluric (MT) soundings have been applied to the determination of the deep structure across the Rehai geothermal field in a Quaternary volcanic area near the Indo-Eurasian collisional margin. Tensorial analysis of the data show evidence of weak to strong 3-D effects but for approximate 2-D imaging, we obtained dual-mode MT responses for an assumed strike direction coincident with the trend of the regional-scale faults and with the principal impedance azimuth at long periods. The data were subsequently inverted using different approaches. The rapid relaxation inversion models are comparable to the sections constructed from depth-converted invariant impedance phase data. The results from full-domain 2-D conjugate-gradient inversion with different initial models are concordant and evoke a picture of a dome-like structure consisting of a conductive (< 10 <Omega>m) core zone, c. 2 km wide, and a resistive ( > 50-1000 Omegam) cap which is about 5-6 km thick in the central part of the known geothermal field and thickens outwards to about 15-20 km. The anomalous structure rests on a mid-crustal zone of 20-30 Omegam resistivity extending down to about 25 km depth where there appears to be a moderately resistive (> 30 Omegam) substratum. The MT images are shown to be in accord with published geological, isotopic and geochemical results that suggested the presence of a magma body underneath the area of study.