Deep structure of the Longling Ruili fault underneath Ruili basin near the eastern Himalayan syntaxis: insights from magnetotelluric imaging

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1 Tectonophysics 364 (2003) Deep structure of the Longling Ruili fault underneath Ruili basin near the eastern Himalayan syntaxis: insights from magnetotelluric imaging Denghai Bai a, Maxwell A. Meju b, * a China Seismological Bureau, Institute of Geology, Beijing , China b Department of Environmental Science, Lancaster University, Lancaster LA1 4YW, UK Received 17 July 2001; accepted 3 February 2003 Abstract Magnetotelluric (MT) geophysical profiling has been applied to the determination of the deep structure of the Longling Ruili fault (LRF), part of a convergent strike-slip fault system, underneath thick Caenozoic cover in Ruili basin in southwestern Yunnan, China. The recorded MT data have been inverted using a two-dimensional (2-D) nonlinear conjugate gradients scheme with a variety of smooth starting models, and the resulting models show common subsurface conductivity structures that are deemed geological significant. The models show the presence of a conductive (5 V m) 60 cover sequence that is thickest (1 1.5 km) in the centre of the basin and rapidly pinches out towards the margins. A half-graben structure is interpreted for the Ruili basin. This is underlain by about 7 10 km thick upper crustal layer of high resistivity (>200 V m) 4000 that is dissected by steep faults, which we interpret to flatten at depth and root into an underlying mid-crustal conductive layer at about 10 km depth. The mid-crustal layer does not appear to have been severely affected by faulting; we interpret it as a zone of partial melt or intracrustal detachment. The MT models suggest SE directed thrusting of basement rocks in the area. The Longling Ruili fault is interpreted as a NW-dipping feature bounding one of the identified upper crustal fragments underneath Ruili city. We suggest that MT imaging is a potent tool for deep subsurface mapping in this terrain. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Strike-slip fault; Seismic hazards; Ramp basin; Structure; Magnetotelluric imaging; Partial melt 1. Introduction The Ruili basin and associated fault systems in southwestern Yunnan of China, lie within the rightlateral accommodation zone between the Red River Fault and the eastern Himalayan syntaxis (Fig. 1). The * Corresponding author. Tel.: ; fax: address: m.meju@lancaster.ac.uk (M.A. Meju) /03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi: /s (03) regional geological framework developed in response to the collision between India and Eurasia about 40 my ago (e.g., Molnar and Tapponnier, 1975 ). The Indo-Eurasia convergent zone changes from E W to N S trend in the eastern Himalayan syntaxis in western Yunnan in China. The large-scale tectonic features of the region east of this syntaxis have been investigated by several workers (e.g., Scharer et al., 1990; Holt et al., 1991; Leloup et al., 1995; Wang and Burchfiel, 1997; Wang et al., 1998 ). Wang et al.

2 136 D. Bai, M.A. Meju / Tectonophysics 364 (2003) terrane (or Baoshan tectonic element by Wang and Burchfiel, 1997). The LRF is interpreted as a ductile shear belt (Lin, 1990) along which basement rocks have been thrust from northwest to southeast and is thought to have served as a boundary between different terranes (the Tengchong and Baoshan tectonic elements of Wang and Burchfiel (1997) ). The LRF system maintains a NE trend from south of Ruili to north of Longling (located along strike, about 96 km NE from Ruili), beyond which it curves north to merge obliquely into another N-trending fault (Damengliu fault). The LRF dips to the NW and downthrows thrust sheets believed to be a continuation of a Fig. 1. Map showing the regional tectonic setting (after Tapponnier major shear zone further north outside our study area et al., 1982). The shaded box between the Red River Fault and the and parallel to the Damengliu fault at the border of the eastern Himalayan syntaxis (EHS) shows our study area. The Tengchong and Baoshan terranes (Wang and Burchfiel, 1997, Fig. 4). The LRF is interpreted as having a acronym SG stands for Sagaing fault. left-slip component in the Longling area (Holt et al., (1998) noted that the main fault systems trend southeastward across the Tibetan plateau and its foreland The Ruili area is one of the fastest growing areas of 1991; Wang and Burchfiel, 1997). region. There are also smaller fault systems of NE China and there are concerns about seismic hazards SW trend in the area, some with associated sedimentary basins. The deep structure of one of the latter was the site of an earthquake swarm M = ( ) in posed by these faults; for example, a N-trending fault systems in the Ruili basin constitutes the focus of this the Longling area in 1976 (Holt et al., 1991). The paper. exact structural relationship between the LRF and WF The Ruili basin (Fig. 2) lies within a major fault and their trajectories underneath the Ruili basin are belt at the border between China and Burma (see also poorly understood. It is uncertain whether both faults Wang and Burchfiel, 1997, Fig. ). 4About 150 f 500 merged into a single fault or whether the Wanding m thick Caenozoic cover rocks are found in the central fault was cut off by the LRF under the basin. Wang part of the Ruili basin and these are considered to be and Burchfiel (1997, Fig. 9) found evidence of a underlain successively by Triassic strata (dolomite, recent left-slip displacement activity on the WF that slate and siltstone), Permian rocks (limestone and ends against a thrust fault at Wanding (ca. 25 km ENE dolomite) and Cambrian migmatite and gneiss in that of Ruili city) before reaching LRF and opine that the order. The Caenozoic rock formations are mostly Wanding fault does not extend beyond LRF (their alluvial deposits (sand, gravel, clay and conglomerate) adopted western boundary of the Baoshan terrane). and sandstone. There is an area of high thermal The trajectory of LRF underneath Ruili basin is activity (hot springs and ponds) in the eastern part generally assumed to lie about 2 km south of the of the basin, i.e., to the east of Ruili city Fig. (see 2). Paleozoic metamorphic and sedimentary rocks and Cretaceous granite intrusions occur along the margins of the elongate basin. Two major faults, the NEtrending Longling Ruili fault (LRF) and the ENEtrending Wanding fault (WF), outcrop northeast of the basin and are generally assumed to intersect or join about 4 km northeast of Ruili city as depicted Fig. in 2, with the LRF continuing southwestward under basin cover. The Wanding fault is older than the LRF and lies within what is termed the Baoshan centre of Ruili city as shown Fig. in 2 (see also Wang and Burchfiel, 1997, Figs. 4 and ). 11A recent unpublished interpretation of proprietary seismic exploration data for the Ruili basin showed a linear feature running parallel to the geologically assumed position of the LRF but offset about 1.2 km to the

3 D. Bai, M.A. Meju / Tectonophysics 364 (2003) Fig. 2. Geological map of Ruili basin showing the main faults and distribution of the MT stations. Also shown are the temperature gradient data from shallow wells in the area. The trajectory of the Longling Ruili fault south of its intersection with the Wanding fault is conjectural since the area is covered by Caenozoic sediments. Neogene rocks are in unconformable contact with Cambrian rocks in the northwest margin of the basin. west, i.e., positioned 0.5 km from the centre of Ruili city. The trace of this seismic-inferred feature runs NE from the position of borehole 1, passing between stations R34 and R03, through to borehole 13 in Fig. 2. This interpretation was not favoured by the local field geologists (Z. Liao, personal communication). Other smaller subparallel faults were also suggested on the said seismic map to pass near positions R02, R03 and borehole 11 in Fig. 2. The present study is an attempt to use an alternative (i.e., nonseismic) geophysical technique to image the subsurface near Ruili city with the main

4 138 D. Bai, M.A. Meju / Tectonophysics 364 (2003) aim of locating the position of any major faulting in features (e.g., Kurtz et al., 1986; Wannamaker et al., the basement underneath the basin fill. The magneto 1989; Jones et al., 1992; Ingham and Brown, 1998; telluric (MT) method is an established electromag- Unsworth et al., 1997, 1999; Meju et al., 1999; Bai et netic geophysical technique for investigating the al., 2001; Sakkas et al., 2002 ) and was selected for electrical conductivity signatures of sedimentary this particular study. The results obtained from autobasins, strike-slip faults and other deep-seated tectonic mated two-dimensional (2-D) inversion (or conduc- Fig. 3. Comparison of some dimensionality indicators for three representative sites. The diamond symbols show the skew and rotation angles from the Swift (1967) method while the filled circular symbols are those given by Bahr the (1988) method for stations (a) R02, (b) R04 and (c) R06. The principal axis is for a clockwise rotation from the magnetic fnorth 6j west ( of geographic north).

5 D. Bai, M.A. Meju / Tectonophysics 364 (2003) Fig. 3 (continued). tivity imaging) of the recorded data are presented in this paper. 2. Data acquisition and analysis MT depth soundings were made at seven stations along a NW SE profile (Fig. 2) across the adopted trend of the LRF. An MT field equipment built by METRONIX of Germany was used for the singlestation soundings. At each location, the natural electric and magnetic field variations in two orthogonal (magnetic east west and north south) directions as well as using both the Swift and Bahr methods are in reasonable accord for each site. The azimuths from the Bahr method are accepted as the more reliable in geoelectromagnetic work. In our case, they are mostly frequency-independent and consistently trend N25 je the vertical component of the magnetic field were N37jE in reasonable accord with the dominant simultaneously recorded over a frequency sounding regional geological strike (cf. Fig. 2). The phasesensitive skew values computed using the Bahr tech bandwidth of 256 Hz to 4096 s. The average spacing between neighbouring observational stations was nique are typically about However, at a few about 1 km. The topography is flat along the survey sites (see Fig. 3b,c), the values of the Swift skew at low line. Station R02 is at the foot of a mountain in the frequencies ranged up to about 0.6 while the phasesensitive skew remained small; this may suggest the north of the basin. R07 is on the north bank of Ruili River (whose trace follows the China Burma international boundary in Fig. 2). distribution at depth (possibly a complex three-dimen- presence of strong heterogeneity in the conductivity The field data have been processed using standard sional (3-D) basement structure?), which could cause tensorial techniques (Egbert and Booker, 1986; Bahr, galvanic distortion of the field sounding curves (Berdichevsky and Dmitriev, 1976; Bahr, 1988). Interest 1988). Only the data for the sounding bandwidth 256 Hz to 120 s, which are of the best quality, will be ingly, this signature is not seen at high frequencies, interpreted in this paper. To determine the dimensionality characteristics of the data, we have rotated the impedance tensor using both the methods Swift of (1967) and Bahr (1988). Some of the computed structural indicators are shown in Fig. 3 for three sites assumed to represent the possible fault-zone environment (R04) and the adjoining areas of possibly different basement to its northwest (R02) and southeast (R06). Note that the geoelectric azimuths computed which sample shallow depths (simple basin fill?), and

6 140 D. Bai, M.A. Meju / Tectonophysics 364 (2003) LRF, the average dc apparent resistivity is about V 30 m, while on the SE side, it is about V 50 m for AB = 2000 m. The available dc resistivity data are compared with the TE mode MT apparent resistivity data for station R07 Fig. in 4; to achieve comparability, we have employed the scaling relations suggested by Meju (2002, Eqs. (1) and (2)) for determining the equivalent MT period for a given Schlumberger electrode spacing. In this figure, notice that the dc resistivity and MT data are in good agreement. We also found that the MT apparent resistivity curves for both Fig. 4. A comparison of MT and Schlumberger dc resistivity data the TE and TM polarisation modes are coincident at for station R07. The dc current electrode spacings were converted to high frequencies at all sites (shown later Fig. in 6). A equivalent MT periods using a scaling relation suggested Meju by (2002). the initial segments of the apparent resistivity curves typically have values of ca. 15 V 30 m (cf. Fig. 6). If the source of the apparent 3-D signature is local and near-surface, we would expect it to manifest at high frequencies and cause significant static shift of the sounding curves (Berdichevsky and Dmitriev, 1976; Bahr, 1988). Overall, the data appear to show somewhat regional 2-D characteristics especially at high frequencies, but we note that there is evidence of strong 3-D at depth. Since 3-D numerical modelling of a linear survey line may not lead to any unequivocal solution without additional geological controls, we favour a 2 D approach in this paper. The apparent resistivity and phase data required for 2-D modelling were therefore obtained by rotation of the impedance tensor such that the two principal axes are aligned parallel and perpendicular to the main geological strike (in MT parlance, the transverse electric (TE) mode of electromagnetic field polarisation is in the direction parallel to strike while the transverse magnetic (TM) mode is in the orthogonal direction). Some direct current (dc) resistivity profiling was done in the area for geothermal exploration (Liao and Zhao, 1999). The survey employed the Schlumberger array with a constant current electrode separation (AB). On the NW side of the adopted position of the comparison of the dc data from around station R07 with the MT data from station R06 seems to suggest that the R06 curves may have been shifted downward by about one-third of a decade, thus possibly requiring static correction. This would be in agreement with our analysis of the data presented Fig. in 3c. Based on the scanty dc resistivity data available, it would appear that there are no major static shifts of the MT sounding curves except at station R D inversion models A popular full-domain 2-D nonlinear conjugate gradients inversion scheme (Mackie and Rodi, 1996; Mackie et al., 1997; Rodi and Mackie, 2001) was used to jointly invert the TE and TM mode apparent resistivity and phase data sets. For consistency or nonuniqueness checks, the inversion exercise incorporated several half-space (i.e., smooth or featureless) initial models, and only those reconstructed features that are common to all the models may be considered necessary to fit the observed data and therefore worthy of geological interpretation. The statistically equivalent ( c 1.2 rms error) optimal models from four different initial half-space (50, 100, 300 and 500 V m) models are presented in Fig. 5. For all the models, the match between the calculated model responses and the observed data is very good at six of the sites (see for example Fig. 6); it is less Fig. 5. Sample consistency (and equivalence) analysis of full-domain 2-D imaging results. Shown are four statistically equivalent NW SE resistivity images of the subsurface derived by MT data inversion using featureless models of different initial resistivities: V m, (A)(B) V m, (C) 300 V m and (D) 500 V m. Note that the resistivity distribution at depths greater than ca. 15 km varies for the different initial models and is thus poorly constrained by the inverted MT data.

7 D. Bai, M.A. Meju / Tectonophysics 364 (2003)

8 142 D. Bai, M.A. Meju / Tectonophysics 364 (2003) Fig. 6. Comparison of the fit between calculated 2-D model response and the observed data for the 100 V m initial half-space model.

9 D. Bai, M.A. Meju / Tectonophysics 364 (2003) satisfactory for the TE mode at station R05 (the calculated responses differ from the observed data at long periods) and a complex 3-D structure may be present at depth in this locality. It is pertinent to mention that the average apparent resistivity for the high frequency part (>1 Hz) of the MT sounding 4. Possible geological interpretation of results of curves is less than 50 V m while the typical low MT imaging frequency ( s) data have values above V m. Our interpretative experience from studying A plausible structural interpretation of an optimal over 32 smoothness-constrained models generated 2-D resistivity model is provided in Fig. 7. The from the present data set lead us to favour: (i) those conductive basinal section is asymmetric in our mod conductive features present in the optimal inversion models reconstructed from initial resistive half-spaces and (ii) those resistive features present in optimal models reconstructed from conductive initial halfspace models. All the 2-D models (e.g., Fig. 5) show common subsurface conductivity structures in the top km of the crust, and these are therefore deemed geologically significant and worthy of interpretation. proprietary seismic data for the area coincides with the The models reveal the presence of about 7 10 km northern edge of this half-graben, while the thermal thick resistive (z 200 V m), segmented, upper-crustal anomalies in the area (see Fig. 2) seem to be confined geoelectrical layer (dissected crystalline basement?) to its eastern flank. It is significant that Wang and underneath a conductive (5 60 V m) cover sequence Burchfiel (1997) interpreted the structure of a basin of variable thickness. The cover sequence (basin fill?) associated with a similar NE-trending fault further is thickest (1 1.5 km thick) near station R04 in the north within the same Tengchong terrane as a halfgraben and has associated hot springs. The possible middle of the MT profile and rapidly pinches out to the SE, yielding a graben-like structure betwee n presence of a low-angle dipping contact (a thrust stations R34 and R06. The cover sequence would fault?) in the top 1 km and a steep conductive zone also appear to be of significant thickness northwestward from station R02. The localised surficial zones fault was positioned at this location in the proprietary at greater depth is suggested near station R02; a minor of enhanced conductivity around stations R04 and seismic interpretation, and NW-dipping thrust faults R05 in the sedimentary section of the basin Fig. (see 5C,D) are consistent with the anomalous thermal gradients in that part of the basin (cf. Fig. 2). The resistive upper crustal layer (underneath the conductive cover sequence) appears to be broken into three main segments possibly by through-going faults. models seem to vary with the starting model suggesting that they are not well constrained by the inverted data subset (256 Hz 120 s). els. A half-graben is inferred to be present between stations R34 and R06 with its flanks marked by SEand NW-dipping faults; another concealed fault is suggested between them (i.e., between R04 and R05). Since the structure appears to be deepest around station R04, we suggest a NW or W dip of the strata in this part of the basin. It is interesting that the main fault indicated in the unpublished interpretation of are known to be present in the area (see Wang e.g., and Burchfiel, 1997, Fig. ). 4 There are electrically resistive basement highs or horsts at shallow depths underneath stations R03 to R34 and R06 to R07 within the Ruili basin. We interpret the sub-cropping resistive basement block There is a central moderately resistive c 200 ( 400 west of the graben (block A Fig. in 7) as a possible V m) segment with steep NW- and SE-dipping granitic sheet intruded along the LRF zone as seen on borders underneath the graben-like basin, and flanking zones of higher resistivity ( V m). The tention is correct, it is logical to expect that the NW- outcrops NE of the basin (see Fig. 2). If this con resistive flanking zone to the NW is about 6 7 km dipping borders of this granitic sheet would correspond to the LRF system. We also suggest that the thick while that to the SE is about 8 10 km thick. An underlying mid-crustal layer of low resistivity (5 100 resistive segment (block C in Fig. 7) east of the fault V m) is evident at depths of about km in all the may correlate with the granitic outcrop seen east of models, beyond which the resistivity increases to well the Wanding fault outcrop in the northern part of our over 100 V m. The deeper features (>15 km) in the study area (see Fig. 2), its western margin dips SE and

10 144 D. Bai, M.A. Meju / Tectonophysics 364 (2003) may be a major fault (WF?). Ruili and environs, the fastest developing part of China, may thus be founded on top of fault-bounded granitic blocks; we are concerned that both the Longling Ruili and Wanding faults are still active with left-slip displacements (see Holt et al., 1991; Wang and Burchfiel, 1997 ). It is interesting that our resistivity models (Figs. 5 and 7) suggest that severe faulting is confined in the upper crust in this area. The proposed basement faults underneath the half-graben are quite steep at upper levels but appear to flatten at depth or root into the electrically conductive mid-crustal layer (block D in Fig. 7), which we interpret to be a zone of partial melt or possibly an intracrustal detachment zone. It is perhaps significant that a comparable conductive mid-crust was identified further north in the same Tengchong terrane by Bai et al. (2001) and in southern Tibet by Chen et al. (1996), and that intracrustal detachments are suggested to exist within zones of continental deformation (e.g., Burchfiel et al., 1989; Wang and Burchfiel, 1997, Fig. ). 16 Overall, our 2-D conductivity images evoke a picture that is consistent with geology and provide new insights into the deep structure of the faults located underneath Ruili basin. The MT method is thus a potent structural mapping tool in this terrain (see also Bai et al., 2001 ), and as has been demonstrated elsewhere by many other workers (e.g., Kurtz et al., 1986; Wannamaker et al., 1989; Jones et al., 1992; Ingham and Brown, 1998; Unsworth et al., 1997, 1999), it should be incorporated in routine studies of strike-slip tectonics especially in crystalline basement areas. However, the wide station spacing and the short frequency bandwidth used in this study have limited model resolution to a large extent. It will be highly desirable to carry out large-scale MT surveys in the region using much closer station spacing than was adopted in our study so that short wavelength as well as regional features of geological significance can be detected. Fig. 7. A cross section showing our preferred interpretation of the deep structure across the Ruili basin as revealed by 2-D MT imaging. A half-graben structure is interpreted for the basin. The main faults separating the upper crustal segments (A, B and C) and the mid-crustal conductive zone (D) are indicated by thick white lines. The dashed white lines represent inferred thrusts while the black dashed lines indicate the inferred dip of sedimentary strata in the basin proper. No vertical exaggeration. 5. Conclusion The location and structure of a possible basement fault-zone underneath the Ruili basin appear to have been discerned from conductivity imaging of MT data. The MT models also show the geometry of the

11 D. Bai, M.A. Meju / Tectonophysics 364 (2003) basin; all the 2-D models generated by automatic inversion suggest the presence of a conductive (5 60 V m) cover sequence in the sedimentary section and we identify a possible half-graben structure that is about km deep at the centre of our MT profile. The main thermal activities in the area (see Fig. 2) would appear to be confined to the eastern flank of this half-graben. The resistive upper crust is dissected by through-going faults that root into a mid-crustal layer of low resistivity at about 10 km depth; two of these features possibly bound a NW-dipping granitic sheet and are interpreted as corresponding to the Longling Ruili fault system. Analysis of the results obtained from full-domain 2-D inversion initiated with several nonstructured models suggests that the selected data bandwidth (256 Hz to 120 s) does not fully constrain the subsurface beyond about 15 km depth; lower frequency data are required to resolve the conductivity structure at greater depth. Acknowledgements The authors are grateful to Zhijie Liao of Peking University for encouragement and support of the field study. One of the authors (D.B) acknowledges a 1 year fellowship award at University of Leicester from the Royal Society, London. We gratefully acknowledge the interesting geological discussions with Dickson Cunningham and the constructive review of the manuscript by Malcom Ingham. We thank one anonymous reviewer for drawing our attention to the seminal papers of Wang, Burchfiel et al. on the geology of the study area. References Bahr, K., Interpretation of the magnetotelluric impedance tensor: regional induction and local telluric distortion. J. Geophys. 62, Bai, D.B., Meju, M.A., Liao, Z., Magnetotelluric images of deep structure of the Rehai geothermal field near Tengchong, southern China. Geophys. J. Int. 147, Berdichevsky, M.N., Dmitriev, V.I., Distortion of magnetic and electrical fields by surface lateral inhomogeneities. Acta Geod. Geophys. Montan. Acad. Sci. Hung. 11, Burchfiel, B.C., Deng, Q., Molnar, P., Royden, L., Wang, Y., Zhang, P., Zhang, W., Intracrustal detachment within zones of continental deformation. Geology 17, Chen, L., Booker, J.R., Jones, A.G., Wu, N., Unsworth, M.J., Wei, W., Tan, W., Electrically conductive crust in southern Tibet from INDEPTH magnetotelluric surveying. Science 274, Egbert, G.D., Booker, J.R., Robust estimation of geomagnetic transfer functions. Geophys. J. R. Astron. Soc. 87, Holt, W.E., Ni, J.F., Wallace, T.C., Haines, A.J., The active tectonics of the eastern Himalayan syntaxis and surrounding regions. J. Geophys. Res. 96, Ingham, M.R., Brown, C., A magnetotelluric study of the Alpine fault, New Zealand. Geophys. J. Int. 135, Jones, A.G., Kurtz, R.D., Craven, J.A., McNeice, G.W., Gough, D.I., DeLaurier, J.M., Ellis, R.G., Electromagnetic constraints on strike-slip geometry the Fraser River fault system. Geology 20, Kurtz, R.D., DeLaurier, J.M., Gupta, J.C., A magnetotelluric profile across Vancouver Island detects the subducting Juan de Fuca plate. Nature 321, Leloup, H.P., Lacassin, R., Tapponnier, P., Scharer, U., Zhong, D., Liu, X., Zhang, L., Ji, S., Trinh, P.T., The Ailao Shan-red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina. Tectonophysics 251, Liao, Z., Zhao, P., Yunnan Tibet Geothermal Belt Geothermal Resources and Case Histories. Science Press, Beijing, pp Lin, W., Rock features and types of ductile shear belt, western Yunnan. Yunnan Geol. 9, Mackie, R.L., Rodi, W., A nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion. EOS (Trans. AGU) 77, 155. Mackie, R.L., Rieven, S., Rodi, W., Users manual and software documentation for two-dimensional inversion of magnetotelluric data. Earth Resources Lab. Report, Mass. Inst. Technology, Cambridge, MA. Meju, M.A., Geoelectromagnetic exploration for natural resources: models, case studies and challenges. Surv. Geophys. 23, Meju, M.A., Fontes, S.L., Oliveira, M.F.B., Lima, J.P.R., Ulugergerli, E.U., Carrasquilla, A.A., Regional aquifer mapping using combined VES-TEM-AMT/EMAP methods in the semiarid eastern margin of Parnaiba Basin, Brazil. Geophysics 64, Molnar, P., Tapponnier, P., Cenozoic tectonics of Asia: effects of a continental collision. Science 189, Rodi, W., Mackie, R.L., Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion. Geophysics 66, Sakkas, V., Meju, M.A., Khan, M.A., Haak, V., Simpson, F., Magnetotelluric images of the crustal structure of Chyulu Hills volcanic field, Kenya. Tectonophysics 346, Scharer, U., Tapponier, P., Lacassin, R., Leloup, P.H., Zhong, D., Ji, S., Intraplate tectonics in Asia: a precise age for largescale Miocene movement along the Ailao Shan-Red River shear zone, China. Earth Planet. Sci. Lett. 97, Swift, C.M., A magnetotelluric investigation of electrical conductivity anomaly in the southwestern United States. PhD

12 146 D. Bai, M.A. Meju / Tectonophysics 364 (2003) thesis, M.I.T. In: Vozoff, K. (Ed.), Magnetotelluric Methods, SEG, Tapponnier, P., Peltzer, G., Le Dain, A.P., Armijo, R., Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology 10, Unsworth, M.J., Malin, P.E., Egbert, G.D., Booker, J.R., Internal structure of the San Andreas fault at Parkfield, California. Geology 25, Unsworth, M.J., Egbert, G.D., Booker, J.R., High-resolution electromagnetic imaging of San Andreas fault in central California. J. Geophys. Res. 104, Wang, E., Burchfiel, B.C., Interpretation of Cenozoic tectonics in the right-lateral accommodation zone between the Ailao Shan shear zone and the Eastern Himalayan syntaxis. Int. Geol. Rev. 39, Wang, E., Burchfiel, B.C., Royden, L.H., Liangzhong, C., Jishen, C., Wenxin, L., Zhiliang, C., Late Cenozoic Xianshuihe Xiaojiang, Red River, and Dali fault systems of southwest Sichuan and Central Yunnan, China. Spec. Pap. - Geol. Soc. Amer. 327, 108 pp. Wannamaker, P.E., Booker, J.R., Jones, A.G., Chave, A.D., Filloux, J.H., Waff, H.S., Law, L.K., Resistivity cross-section through the Juan de Fuca subduction zone and its tectonic implications. J. Geophys. Res. 94,