Crustal structure beneath Namche Barwa, eastern Himalayansyntaxis: New insights from three-dimensionalmagnetotelluric imagingChanghong Lin1,2 , Miao Peng1,2 , Handong Tan1,2 , Zhiqin Xu3, Zhong-Hai Li4,5 ,Wenxin Kong1, Tuo Tong1,2, Mao Wang1,2, and Weihua Zeng1,2

1School of Geophysics and Information Technology, China University of Geosciences, Beijing, China, 2Key Laboratory ofGeo-detection (China University of Geosciences, Beijing), Ministry of Education, Beijing, China, 3School of Earth Sciencesand Engineering, Nanjing University, Nanjing, China, 4Key Laboratory of Computational Geodynamics, College of EarthSciences, University of Chinese Academy of Sciences, Beijing, China, 5Laboratory for Marine Geology, Qingdao NationalLaboratory for Marine Science and Technology, Qingdao, China

Abstract The eastern terminations of the Himalayan orogeny, named Namche Barwa, are considered avital natural laboratory in the Tibetan plateau for geodynamics due to its distinctive geological andgeomorphological characteristics. Magnetotelluric (MT) data measured at 83 sites around the Namche Barwaare imaged by three-dimensional (3-D) inversion to better reveal the crustal structure of the easternHimalaya. The results show a complex and heterogeneous electrical structure beneath the Namche Barwa.The electrical conductors distributed in the middle and lower crust around the Namche Barwa provideadditional evidence for the “crustal flow”model if they are considered as some parts of the flow in a relativelylarge-scale region. The near-surface resistivity model beneath the inner part of Namche Barwa conforms withthe locations of hot spring and fluid inclusions, the brittle-ductile transition, and the 300°C–400°C isothermfrom previous hydrothermal studies. Relatively resistive upper crust (>800 Ωm) is underlain by a moreconductive middle to lower crust (<80 Ωm). The electrical characteristics of the thermal structure at shallowdepth indicate an accumulation of hydrous melting, a localized conductive steep dipping zone fordecompression melting consistent with the “tectonic aneurysm” model for explaining the exhumationmechanism of metamorphic rocks at Namche Barwa. The results also imply that both surface processes andlocal tectonic responses play a vital role in the evolution of Namche Barwa. An alternative hypothesis that theprimary sustained heat source accounts for the local thermal-rheological structure beneath Namche Barwa isalso discussed.

1. Introduction

The Himalayan range provides significant constraints on the collision between Indian and Asian continents[Argand, 1924; Yin and Harrison, 2000; Tapponnier et al., 2001; Royden et al., 2008; Xu et al., 2011; Bai et al.,2010; Li et al., 2011]. At the eastern and western terminations of the Himalayan orogen, a pair of syntaxesnamed Namche Barwa and Nanga Parbat strongly bend around a vertical axis and form a distinctive colli-sional belts [Burg et al., 1998] (Figure 1). The Namche Barwa, located near the eastern Himalayan syntaxis(EHS), is marked by strong tectonic stress, rapid rock uplift and exhumation, sudden transition in theYarlung Zangbo rivers, and intense Cenozoic metamorphism and anatexis [Burg et al., 1998; Ding et al.,2001; Booth et al., 2009; Xu et al., 2008; Priestley et al., 2008; Wang et al., 2014].

Another unique feature of the Namche Barwa is an active antiformalmetamorphicmassif developed in HigherHimalayancrystallineof the Indiancrust. Themassif hasexhibitedextremelyhigherosional exhumation ratesof5mm/yr over the past ~3Ma [Burg et al., 1998;Ding et al., 2001; Booth et al., 2009; Zeitler et al., 2014]. High seis-micity [Zeitler et al., 2014; Peng et al., 2016] and an extremely young cooling ages [Zeitler et al., 2014] observednear theNamcheBarwaare consistentwith a rapid exhumationpulse.Moreover, high-pressuregranulite faciesrocks and garnet pyroxenite have been found in the region suggesting Paleogene high-pressure (HP) orultrahigh-pressure (UHP) metamorphism and deep subduction of the Indian plate beneath the TibetanPlateau occurred as early as circa 40 Ma [Zhang et al., 2007; Yang et al., 2009; Zhang et al., 2011; Xu et al., 2012].

Several plausible models have been proposed to explain the evolution of the Namche Barwa, includingindenter corner [Koons, 1995], crustal folding [Burg and Schmalholz, 2008], duplex thrusting [Ding et al.,

LIN ET AL. MT IMAGING OF THE CRUST OF NAMCHE BARWA 1

PUBLICATIONSJournal of Geophysical Research: Solid Earth

RESEARCH ARTICLE10.1002/2016JB013825

Key Points:• We present new reticular MT data and3-D inversion results of Namche Barwa

• Our results at Namche Barwa provideadditional evidence to the crustalflow model

• Our results on the inner part of theNamche Barwa support the “tectonicaneurysm” model

Supporting Information:• Supporting Information S1• Data Set S1

Correspondence to:M. Peng,pengm@cugb.edu.cn

Citation:Lin, C., M. Peng, H. Tan, Z. Xu, Z.-H. Li,W. Kong, T. Tong, M. Wang, and W. Zeng(2017), Crustal structure beneathNamche Barwa, eastern Himalayansyntaxis: New insights from three-dimensional magnetotelluric imaging,J. Geophys. Res. Solid Earth, 122,doi:10.1002/2016JB013825.

Received 2 DEC 2016Accepted 6 JUL 2017Accepted article online 8 JUL 2017

©2017. American Geophysical Union.All Rights Reserved.

2001], channel flow [Jamieson et al., 2004], tectonic aneurysm [Zeitler et al., 2001, 2014], and 3-D subductiongeometry [Bendick and Ehlers, 2014]. Among these models, both surface processes and tectonic factors seemto account for the mechanism. Although they have provided some constructive conceptions about thegeodynamic processes, the metamorphic and tectonic evolution of the Namche Barwa remains a long-debated subject.

Magnetotellurics (MT) has been widely used to reveal deep structures in various geological tectonicregions in the Himalaya and Tibetan Plateau in the past two decades [Nelson et al., 1996; Wei et al.,2001; Li et al., 2003; Tan et al., 2004; Unsworth et al., 2004, 2005; Bai et al., 2010; Zhao et al., 2012]. MT issensitive to conductive electrical bodies that correspond to the presence of interconnected fluids or partialmelt and can give vital constraints on crustal rheology. In the region around the Namche Barwa, however,MT surveys have been limited due to steep terrain and difficult field work conditions. In this paper, wepresent a model of the electrical structure produced using a MT 3-D inversion for the purpose of revealingthe crustal structure of this region. The new results focus more on the inner part of the Namche Barwawhere the data were collected and mainly discuss the implications of detailed electrical characteristicsfor exhumation mechanism of metamorphic rocks.

2. Geological Setting and Previous Geophysical Studies2.1. Geological Setting

The Himalayan slices (including the Tethyan Himalayan sequences and the Greater Himalayan sequences) arecharacterized by a nearly 180° bending of the Himalayan structural stratum, giving rise to the inverse U-turnof the Yarlung Zangpo River (Figure 2) [Geng et al., 2006; Zhang et al., 2014]. This region can be divided intothree tectonic units from south to north: (1) the Indian Terrane, (2) the sharply curved Indus-Yarlung suturezone (IYSZ), and (3) the Lhasa Terrane. In the core of the Namche Barwa, the metamorphic terrane was con-sidered to be a complex antiformal structure that consists of four superposed tectonic slices from SE to NW:(1) the Duoxiong-La Migmatitic Dome (DMD), (2) the Nanpaixiang tectonic slice, (3) the Zhibai tectonic slice,

Figure 1. Sketch of a tectonic map modified from Taylor et al. [2003] and Yin and Harrison [2000], showing surface relief,main suture zones, and large active faults of southeastern Tibet. MBT, Main boundary thrust; MCT, Main central thrust;STD, South Tibetan detachment; IYSZ, Indus-Yarlung suture zone; JSZ, Jiali shear zone; and BNS, Bangong-Nujiang suture.The main tectonic elements include the Qiangtang terrain (QT), the north Lhasa terrain (NLS), the south Lhasa terrain (SLS),the eastern Himalayan syntaxis (EHS), the Tethyan Himalayan sequences (THS), Greater Himalayan sequences (GHS),Lesser Himalayan sequences (LHS), and the Indian block (INDB). The insert map shows the location of Namche Barwa andNanga Parbat on a larger scale.

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and (4) the Bilu tectonic slice [Xu et al., 2012]. These units are separated by three ductile shear zones. Someresearchers also differentiate the metamorphic terrane between Namche Barwa massif (NBM) in the north,including the peaks Gyala-Peri (GP) and Namche Barwa (NB), and the Namche Barwa antiform (NBA) in thesouth [Craw et al., 2005; Enkelmann et al., 2011; Zeitler et al., 2014] but not refer to the Namche Barwasyntaxis. Based on a fabric kinematics study, it is indicated that the NBM moved northward andunderthrusted beneath the Lhasa terrain relative to the Himalayan terrane and was restrained by twostrike-slip faults (Figure 2) [Zheng et al., 2003; Xu et al., 2008]. The two strike-slip faults are the western left-lateral Dongjiu-Milin shear zone (DMSZ) and the eastern right-lateral Aniqiao-Motuo shear zone (AMSZ).

Namche Barwa was also considered to be one of the 11 major HP and UHP metamorphic belts with eclogite-facies rocks in China [Yang et al., 2009]. In an earlier study, high-pressure basic granulite was reported in thecore of NBM [Zhong and Ding, 1996]. Some high-pressure mineral assemblages with the granulite faciesmetamorphism have been found within the basic granulite at Namche Barwa [Ding and Zhong, 1999;Zheng et al., 2003; Sun et al., 2004]. Zhang et al. [2007] identified garnet pyroxenite lenses in granulite-faciesmetamorphic pelitic schist and granitic gneiss, which indicates that the Indian plate may subduct to a depthof >80–100 km beneath the Tibetan Plateau. Numerous studies in recent years show that these HP rockshave experienced partial melting or anatexis in the middle and lower crust caused by dehydrated biotiteand muscovite [Booth et al., 2009; Xu et al., 2010; Zhang et al., 2010; Su et al., 2012; Xiang et al., 2013].

Although various geologists had different viewpoints on the composition, formation age, and temperatureand pressure conditions of the metamorphic rocks, they commonly agreed that rocks outcropped in theNamche Barwa region experienced high-pressure metamorphism in the deep Earth [Liu and Zhong, 1997;Burg et al., 1998; Ding and Zhong, 1999; Booth et al., 2009; Qi et al., 2010; Zhang et al., 2015].

2.2. Previous Geophysical Studies in the Region

Previous geophysical studies have focused mainly on large-scale mantle geodynamics in this region for thepurpose of understanding the density structure of the India-Eurasian collision [Bai et al., 2013], the variationalseismic anisotropy [Wang et al., 2008; Lev et al., 2006; Sol et al., 2007], the shape of the contrasting Indian slab[Lei et al., 2014; Peng et al., 2016], and the distribution of hydrothermal fluids [Unsworth et al., 2004; Tan et al.,2004] around the EHS. Seismic anisotropy shows that the deep dynamic mechanism has been greatly

Figure 2. Sketch of a geological map of the Namche Barwa antiform (NBA) and surrounding region (modified after Panet al. [2004] and Xu et al. [2012]). Locations of the 83 magnetotelluric sites used in this study are shown by black trian-gles. Main suture zones and tectonic elements are shown in Figure 1. White dotted lines show locations of cross sections inFigure 6. Yellow triangles show the locations of the Gyala-Peri peak (GP) and the Namche Barwa peak (NB). DMD, theDuoxiong-La Migmatitic Dome; DMSZ, Dongjiu-Milin shear zone; and AMSZ, Aniqiao-Motuo shear zone.

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changed beneath the Namche Barwa, which might be considered as a transverse transition zone of the tec-tonic stress field. Crustal structure imaged from receiver functions indicates a gap at great depths under thelocal region, which is likely related to delamination and asthenosphere upwelling [Xu et al., 2013]. Similar tothe slab tearing in the western Himalayan syntaxis [Negredo et al., 2007], a recent passive seismic studyrevealed significantly contrasting Indian subduction styles with slab fragmentation and induced astheno-spheric mantle upwelling beneath the Namche Barwa [Peng et al., 2016], which has a similar distributionbetween the seismic stations and MT sites in the paper. Although the teleseismic velocity model revealeda large-scale tectonic background of mantle dynamics, it is difficult to resolve the crustal structure due tothe limited resolution.

Previous MT studies mainly concentrated on the conductive partially melting layer at middle to lower crustaldepths with the possible presence of accompanying hydrothermal fluids beneath southeastern Tibet [Nelsonet al., 1996;Wei et al., 2001; Unsworth et al., 2005; Spratt et al., 2005; Jin et al., 2010].Wei et al. [2010] deployedone profile in EW direction parallel to the IYSZ and passed through the Namche Barwa to image transversalvariations of the lithospheric electrical structure beneath Himalayan orogen. MTmeasurements by the EHS3Dproject around the EHS detected two banded zones of crustal low electrical resistivity at a depth of 20–40 km[Bai et al., 2010]. The only available 3-D resistivity model near the region was inverted from SinoProbe MTarray data mostly to the north of the Namche Barwa and proposed a new “Extensional Extrusion” model[Dong et al., 2016]. Unlike previous studies, our new MT data focus on a relatively small region of the easternHimalaya with rather dense stations and thus can image crustal structure beneath the Namche Barwa withhigh resolution.

3. Data Acquisition and Analysis3.1. Acquisition and Processing of MT Data

MT data were recorded at 83 stations (Figure 2) spaced about 5 km in and around the Namche Barwa areafrom 2009 to 2014. The MT field data (320 Hz–0.00034 Hz) were collected with the V5-2000 manufacturedby Phoenix Geophysics Limited of Canada. Five field components were recorded at each station, includingelectric field components Ex and Ey as well as the Hx, Hy, and Hz components of the magnetic field wherex denotes the north-south direction and y for the east-west direction, while z is into the Earth. The field datawere processed using the Phoenix processing software SSMT 2000. This program, according to the officialguidance, takes original time series files, calibration files, and site parameter files as input and in an intermedi-ate step produces Fourier coefficients using robust routines. The Phoenix software MTEditor was also used togenerate the corresponding industry-standard EDI files. After careful data processing, data below 0.00275 Hzwere abandoned because their MT records suffered from strong noise pollution and were therefore not usedin the 3-D inversion which is introduced in section 4.1.

3.2. Quality of Data

After abandoning the data contaminated with strong noise, remaining data were examined using two stan-dards. First, the number of the frequency points with apparent resistivity errors greater than 10% and impe-dance phase errors larger than 5° should be less than 25% of the total frequency point number for more than80% of all stations. Second, the continuity of both apparent resistivity and impedance phase should be clearand change regularly with frequency in the sounding curves. More than 80% of all the data meet the abovetwo requirements. This demonstrates that quality of the data used in the 3-D inversion is of high quality.

3.3. Induction Vector

The induction vector is defined as the complex ratio of the vertical magnetic field to the horizontal magneticfield. It is very sensitive to lateral variation of the subsurface conductivity structure. The real, in-phase induc-tion vector points at the current-gathering area, namely conductive structures. Under the condition of anideal 2-D structure, induction vectors should be perpendicular to the strike direction because conductivestructures extend to infinity along the structural trend. The induction vectors of all 83 stations at a frequencyof 0.01 Hz are displayed in map view in Figure 3. It can be seen that most of their orientations vary substan-tially, and consequently, a consistent strike in this area cannot be found. This gives support for the 3-D prop-erty of these MT data.

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4. Three-Dimensional Resistivity Imaging4.1. A 3-D Inversion of MT Data

A 3-D conjugate gradient inversion algorithm [Lin et al., 2011b] is used for the 3-D resistivity imaging.Moreover, apparent resistivity and phase data were added to the input data. The XY and YX mode apparentresistivity and phase data were inverted together for joint imaging. Surface topography was not incorporatedin the inversion for two reasons. First, the measuredMT stations in this region are too sparse to have sufficientconstraints on recovering the topographic structure in 3-D inversion. Second, our current focus is deep struc-ture (where topographic effects are reduced).

In the 3-D inversions, a range of half-space initial models were used in order to check model consistencyor nonuniqueness. The average value of the input apparent resistivity data was 919 Ωm. Figure 4 showsthe initial root-mean-square (RMS) misfit for different half-space initial models. The smallest RMS misfitwas obtained when the inversion started from a 310 Ωm half-space. Initial models of 50, 100, 200, 310,500, 1000 Ωm were selected for our investigation. Only common recovered structural features of all theinverted models related to different initial models may be considered essential to fit field data, on whichbasis reliable geological interpretation can be realized. In all cases, the 3-D inversion used the same grid(the number of cells of the 3-D grid is 46 × 73 × 52 along the x, y, and z coordinates, respectively), a

regularization parameter (λ) ofbetween 0.001 and 100, and anRMS error of 1.0 as the misfitthreshold. We also investigatedseveral error floors and appliedthem in the 3-D inversion,through which the degree ofmutual consistency can be deter-mined for the various data com-ponents. Figures 5 and 6 showthe inversion model correspond-ing to a 310 Ωm half-space initialmodel, which was obtained after127 iterations with a final RMSmisfit of 1.574812. In the inversionprocess, the error floors were 10%for apparent resistivity and 5% forXY and YX phase. The regulariza-tion parameter (λ) was 0.07.Figure 4. The RMSmisfits for the initial half-spacemodels of different values.

Figure 3. Real induction vectors at 0.01 Hz.

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4.2. Data Fit

The fit between the observed data and the synthetic responses of the best fitting model is shown in Figure 7.In these pseudosection maps, the horizontal axis indicates the site number and the vertical axis representsthe logarithm of each frequency. Maps in the left column are plotted with field data, while those in the rightcolumn are constructed from synthetic data. The rows from top to bottom correspond to ρxy, φxy, ρyx, and φyx(ρ for apparent resistivity and φ for phase), respectively. It can be observed that high resistivity and low phaseare present in most of the shallow structure while low resistivity and high phase concentrate in the deepstructure. The RMSmisfit versus the station numbers is shown in Figure 8. Figure 9 shows the fit for eight spe-cific measuring sites, from which generally good fit can be seen between the responses and field data.

4.3. Inversion Results

The models in Figures 5 and 6 show distinct variations in the subsurface electrical structure in the upper tomiddle lower crust beneath the Namche Barwa. The common conductive structures to all 3-D inversionresults are labeled from A to P. Images of horizontal depth slices, from 2 to 40 km, are shown in Figure 5.The shallow slices of 2 and 5 km depth in the crust are relatively resistive except for some discontinuouslow-resistivity (< 30Ωm) blocks around the NBA. The 12 km depth slice demonstrates different features, withmuch more continuous low-resistivity blocks than those present in the 2 or 5 km depth slices. A relativelywidespread low-resistivity zone around the NBA can be found in the deeper slices (20 km, 30 km, and

Figure 5. Map views of the best fitting model from the 3-D inversion results. Red and blue colors indicate low and highresistivity, respectively, with the scale shown at the bottom. The slice depth is written below each map. The black solidlines depict the large active faults and suture zones as in Figure 1. Locations of hot springs and fluid inclusions on 2 kmdepth slice are from Craw et al. [2005].

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40 km). All these features can also be seen in the vertical cross sections in Figure 6. It is difficult to getinformation from depths >60 km due to the limited frequency range used in the 3-D inversion. Therefore,we focused mainly on the contrasting deep structure in the middle and upper crust beneath NamcheBarwa in the current study.

Previous works [Siripunvaraporn et al., 2005; Lin et al., 2011a, 2012] have proved that reliable 3-D structureclose to the sparse lines can be obtained by 3-D inversion. To clearly understand the complex deep structuresbeneath Namche Barwa, four vertical cross sections (Figure 6) close to the MT lines are plotted along both theSW-NE direction (AA0) and NW-SE direction (BB0, CC0, and DD0). Interpretation of those structures (especiallythe shallow structures) in the regions far away from the MT stations may be perilous because the results inthese regions are poorly constrained and are therefore not considered. The AA0 section starts along thesouthern part of DMSZ and extends to the northwestern part of NBA (near the NBM). The crust underDMSZ is relatively resistive (> 300 Ωm) on the top and is underlain by the conductor E (approximately10 Ωm) below 10 km. The profile in the southwestern NBA is characterized by high upper crustal resistivities(≥ 150Ωm). Conductors I and J (<10Ωm in the center) seem interconnected in the upper crust and are likelyto extend to the surface in the Zangbo gorge. In northeastern part of AA0 section, the resistive crust is dis-played under NBM. The deep part (below 20 km) is likely resistive as verified in the following synthetic test,while the shallow part is questionable since there is a large distance to the nearest MT station. The BB0 sectionstarts from the south Lhasa terrain (SLS) and then passes through the western part of the GP-NB massif to the

Figure 6. Cross-section diagrams of the best fitting model from the 3-D inversion results along four profiles as shown inFigure 2 and also on 2 km depth slice in Figure 5. Red and blue colors denote low and high resistivity, respectively, withthe scale displayed in the middle. The top of each cross section also shows the topography, suture zones, and largeactive faults.

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NBA. The shallow resistive part underSLS is also questionable because ofthe large distance to the nearest MTsite. The deep part under SLS isalso likely conductive (possibly anextension of conductor H). At adepth of 20–40 km, conductor H(<10 Ωm in the center and locatedat the northwest of GP) is connectedto conductor J (which is located atthe west of NB). In the southeasternpart of BB0 section, conductor K(<10 Ωm in the center) seems to bemuch shallower than conductors Hand J. The crust below conductor Kis relatively resistive. The CC0 sectionstarts along the SLS, crosses the Jialishear zone (JSZ), and extends to thenorth Lhasa terrain (NLS). In the

Figure 7. (a) Site by site measured MT data in pseudosection format. (b) Response generated from the inversion results inFigures 5 and 6. White areas represent bad or missing MT data.

Figure 8. RMS misfit versus site numbers of both XY and YX modes for theinversion model.

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Figure 9. Illustration of the fit between the field data (shown with error bars) and the predicted MT responses (solidline) of the final inversion model shown in Figures 5 and 6 for the (left) XY mode and (right) YX mode, showingeight typical stations.

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crust under SLS, there is a conductor C (<10Ωm) in the resistive top layer (> 150Ωm) which is underlain by alarger conductor G (<10 Ωm in the center). The crust under the JSZ is similar to those under the SLS, with arelatively resistive layer on the top which is underlain by conductors M (approximately 10Ωm), N (<10Ωm inthe center), and O (approximately 20 Ωm). In the crust under NLS, there is also a conductor P (approximately20 Ωm). It is shallower and smaller than conductors G, N, and O. The DD0 section starts from the SLS andextends to the NBA. In this section, conductors B (<10 Ωm in the center), D (approximately 20 Ωm), I, andK are underlain by more resistive crust. A resistive layer is present on the top.

4.4. Synthetic Test

The above 3-D results were obtained from the inversion of the data from the 83 sparse MT sites. A synthetictest is presented to investigate whether the observed conductive zones and resistive regions can beconstrained by these sparse data and limited frequency range used. Although previous synthetic and fieldexamples [Siripunvaraporn et al., 2005; Lin et al., 2011a, 2012] have demonstrated that reasonable 3-D

Figure 10. The (a) input model and the (b) inverted result of the synthetic data for the solution examination. Four different horizontal depth slices (10, 25, 30, and40 km) and four different vertical cross sections (AA0–DD0) of both models are shown.

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structure close to the sparse lines can be obtained by 3-D inversion, the targets in those examples areshallower than the structures in this study. In order to assess the reliability and resolution capability of ourobserved data, we designed the model in Figure 10a and computed the synthetic responses with Gaussiannoise at the same frequencies of 194–0.00275 Hz at the same 83 sparse sites. With the same inversionparameters as for the field data, the model in Figure 10b was obtained by inverting the synthetic dataafter 112 iterations, having a final RMS misfit of 1.6. By comparing the input model with the invertedresult, the spatial extent and depth of conductors 1–8 with different depths (the shallowest depth of theconductor top is 4.3 km and the deepest depth of the conductor bottom is 47.6 km) are well recoveredexcept for conductor 5 which is affected by the top conductor 1. Thus, the reliability of the 3-D structureunder and near the MT lines in the models shown in Figures 5 and 6 is validated. It can be seen that theresistivities of the recovered conductors are somewhat higher than the real values. This indicates that thetrue resistivities of the conductors beneath the Namche Barwa may be lower than the values shown inFigures 5 and 6.

Figure 11. (a) The 40 km depth slice of the model with a 1 Ωm conductive block at a depth of 20–57 km in the central region of Namche Barwa added to the 3-Dinversion results in Figures 5 and 6. (b and c) The fit between the field data and the predicted MT responses for the XY (left) mode and YX (right) mode at stations 34and 69, respectively. The field data are shown with black error bars. The predicted responses of the inversion model shown in Figures 5 and 6 are shown with blacksolid lines. The predicted responses of the conductive block model are shown with red dashed lines. RMS, misfit between the field data and the predicted MTresponses of the inversion models shown in Figures 5 and 6. RMS1, misfit between the field data and the predicted MT responses of the conductive block model.

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Figures 5 and 6 also indicate that the central region of Namche Barwa is resistive. There are no data sites inthis region. Therefore, a test is presented to investigate whether the central part of Namche Barwa is resistive.We put a conductive block shown in Figure 11a and a resistive block to the 3-D inversion results in Figures 5and 6 and calculate the misfits, respectively. The resistivity of the conductive block at a depth of 20–57 km inthe central region of Namche Barwa is 1 Ωm. The 1000 Ωm resistive block is in the same position as theconductive block. The misfit for the conductive block model is 1.66. The misfit for the resistive block modelis 1.574849, which is almost the same as the misfit (1.574812) for the 3-D inversion results shown inFigures 5 and 6. It was found that the misfit for the conductive block model is greater especially at lowfrequencies at sites 30, 32, 33, 34, 35, 36, 37, 38, 39, 67, 68, and 69, which are around the position of the con-ductive block. Figures 11b and 11c illustrate the fit between the field data and the predicted MT responses forthe XY (left column) mode and YX (right column) mode at stations 34 and 69, respectively. Figures 11b and11c clearly show that the data fit for these two stations is poor at low frequencies when there is a conductiveblock in the central region of Namche Barwa. This test suggests that the central part of Namche Barwais resistive.

5. Middle-Lower Crustal Conductors and Comparison With SeismicTomographic Results

We now compare the MT model with models produced by seismic data. In Figure 5, the deeper slices (30 kmand 40 km) show a relatively widespread low-resistivity zone around the Namche Barwa. A similar zone char-acterized by low P wave seismic velocity in the lower crust was also found from the teleseismic tomographicimages [Peng et al., 2016] (Figure 12b), although the range of the tomographic result is much larger than thatof our MT result. For more clear comparison, both the P wave velocity image and the resistivity image in the40 km depth slices are shown in Figure 12. Both the low-velocity and low-resistivity discontinuous zones canbe seen at almost the same locations along the IYSZ in the southwestern, northwestern, and northeasternparts of the slices. The difference between the velocity and resistivity results is that the low-velocity structurecan also be found in the southeastern part of the Namche Barwa, while there is no corresponding low-resistivity zone. This is probably due to the lack of data constraint for the MT result in this region.

6. Discussions6.1. Additional Evidence for the Eastward Crustal Flow in Eastern Himalaya

MT soundings image the electrical resistivity (or conductivity) of the Earth using natural electromagneticwaves generated in the Earth’s atmosphere and magnetosphere that contain different frequency compo-nents. The frequency range spans from high frequencies (> 1 Hz) due to lightning activity, to intermediatefrequency (0.01–1 Hz) signals from ionospheric resonances, and down to low frequency (0.00001 Hz–0.1 Hz) signals generated by variations in the solar wind. The depth of penetration in the Earth is determinedby its skin depth, which decreases as the signal’s frequency increases.

Conductivity is sensitive to the presence of fluids, which is a valuable tool for studies of crustal structure. Ingeneral, upper crustal dry rocks (e.g., craton basin) usually exhibit a relatively high electrical resistivity(2000–5000Ωm) as they are igneous, metamorphic, and/or tectonically stable. Elevated crustal conductivities(reduced resistivity) are related to thermal effects, aqueous fluids, partial melt, mineralization, or graphite.However, an increase in temperature during melting can also cause a significant conductivity increase.Laboratory studies show that pure melts have an electrical resistivity in the range of 0.1–0.3 Ωm while aqu-eous fluids have slightly lower resistivity values [Nesbitt, 1993]. A combination of aqueous saline fluids and/orpartial melt is thought to be the source of high conductivity regions in the middle crust of southern Tibet, asopposed to graphite and metallic mineralization [Li et al., 2003]. It has been reported that estimating meltfraction, temperature, and flow velocity with resistivities derived from MT data can provide constraints oncrustal strength and effective viscosity [Le Pape et al., 2015]. In order to examine conditions that allow crustalmaterial to flow, Rippe and Unsworth [2010] developed a relationship linking conductance and crustal flowvelocity within the conductive layer of partial melt, which was used to quantify crustal flow in Tibet. Their cal-culations predict that sustained crustal flow was likely to occur beneath the High Himalayas due to effectiveviscosities of 2.5 × 1018 to 3 × 1020 Pa s, which corresponds to flow velocities between 0.02 and 4.5 cm/a.

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In our resistivity images, the electrical structure beneath the Namche Barwa is complex and exhibits hetero-geneities in the crust. The middle and lower crust contains fairly widespread conductors (20 km slice and30 km slice in Figure 5), which are mainly distributed to the west and north of the NBM shown as thick yellowlines in Figure 13. Similar low-resistivity zones have also been previously observed throughout the middle tolower crust along several transects across most regions of the entire Tibetan Plateau [Nelson et al., 1996; Weiet al., 2001; Li et al., 2003; Unsworth et al., 2004, 2005; Rippe and Unsworth, 2010].

In southern Tibet, the N-S cross section of resistivity models (100 line, 700 line, 800 line, and 900 line) all showa low-resistivity layer (<10 Ωm) at a depth of 30–60 km in the middle and lower crust beneath the SLSextending into the Tethyan Himalayan sequences (Figure 14) [Unsworth et al., 2005; Spratt et al., 2005;Rippe and Unsworth, 2010]. The conductors have been interpreted as a thin layer of aqueous fluids overlyinga thick partial melting layer [Li et al., 2003], which indicates the existence of a warm and weak middle to lowercrust. The presence of aqueous fluids in the region is significant because it weakens the crust, which is con-sistent with the rheological conditions required for crustal flow [Unsworth et al., 2005].

A hypothesis of the “crustal flow”model was first proposed by Clark and Royden [2000] to explain the defor-mation of the eastern margins of the Tibetan plateau where the ductile flow was blocked by the surroundingrigid Yangtze terrane. The model was also compatible with the gentle topography of southeast Tibet inferred

Figure 12. Comparison between the (a) teleseismic Pwave velocity image [Peng et al., 2016] and (b) our resistivity image atthe 40 km depth slice.

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LIN ET AL. MT IMAGING OF THE CRUST OF NAMCHE BARWA 13

from MT studies [Unsworth et al., 2005; Rippe and Unsworth, 2010; Bai et al., 2010], but it is hard to explain allthe deformation beneath the entire Tibetan plateau. Two types of localized crustal flow in southern Tibet andthe surrounding regions have been suggested. One type of the flow was the previously described channelflow in the Himalaya, driven by topography-induced pressure gradients and accommodated by surfaceerosion [Nelson et al., 1996; Beaumont et al., 2001]. The other one was proposed on a larger scale that canaccommodate the ongoing eastward extension on the plateau and maintain a gentle topography buttectonically active plateau. It can explain unobstructed east-west extension crustal flow in southern Tibetbut not exhumed and extruded southward to the surface. Unlike the geodynamics mechanism of theprevious models of crustal channel flows [Clark and Royden, 2000; Beaumont et al., 2001; Jamieson et al.,

2004], a spatially more complexmodel proposed by Bai et al.[2010] imaged two extensiveconductive crustal flows at adepth of 20–40 km distributed infinite space of southeast TibetanPlateau. The model suggestedthat both ductile deformation ofthe weak material flow in themiddle and lower crust and strike-slip structural deformation ofblocks along boundary faults inthe upper crust play a central rolein the deformation dynamicmechanism of Tibetan plateau.

The MT sites in this paper are closeto one of the large-scale channelsto the west of the region shown

Figure 14. Interpretive geologic cross section overlaid on segment of the BB0

resistivity model (shown in Figure 6) in the vicinity of the GP-NB massif.

Figure 13. Map showing crustal flows in the southern and southeastern Tibetan Plateau derive from our results and severalprevious MT profiles. These profiles include the 100 line, 500 line, and 600 line by INDEPTH-MT Project [Unsworth et al.,2005; Spratt et al., 2005; Rippe and Unsworth, 2010]; 700 line, 800 line, 900 line, and 2000 line by extensive studies [Wei et al.,2010]; and the 1 line, 2 line, 3 line, and CZMT-WE by EHS3D Project [Bai et al., 2010]. Wemark red colors if there exists a high-conductive layer in the middle and lower crust beneath each profile, and our results are marked with thick yellow lineswithin a dotted box representing the study area. White arrow represents crustal flows. A black shadow to the southeast ofNamche Barwa represents a region characterized as a rigid crust inferred from seismic tomographic studies [Li et al., 2008;Peng et al., 2016].

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LIN ET AL. MT IMAGING OF THE CRUST OF NAMCHE BARWA 14

by Bai et al. [2010]. Our results provide a clear geophysical proof of electrical conductors at a depth of 20–50 km surrounding the Namche Barwa and appear in a junctional zone between south Tibet and easternHimalaya as shown in Figure 13. The crustal flowmay come from central Himalaya, extending eastward alongthe IYSZ, then turn north moving around the Namche Barwa, and finally extend southward into westernYunnan area. As presented in section 4.4, the central part of Namche Barwa is shown to be resistive as astrong and hard block moving northward. In addition, other geophysical evidence including both MT[Dong et al., 2016] and tomographic results [Li et al., 2008; Peng et al., 2016] suggest a large-scale high-velocityand resistive crust to the southeast of Namche Barwa, whichmay explain why the flow turns north andmovesaround the Namche Barwa. Therefore, our results provide additional evidence to the crustal flowmodel. Sinceour results are limited in a local area, more detailed information about the crustal flow model in a broaderregion can be found in previous studies [Beaumont et al., 2001; Bai et al., 2010]. In fact, we do not claim thatcrustal flow can directly explain the process leading to the uplift of metamorphic rocks at Namche Barwabecause of its unique deep structure. The geodynamic processes controlling Namche Barwa are likely muchmore complicated than those present in the middle segment of the Himalayan orogenic belt. In the follow-ing, we now focus on the inner part of the Namche Barwa.

6.2. Relationship of Near-Surface Resistivity Model to Thermal Structure

The striking thermal anomaly and elevated temperatures at relatively shallow depths have been observed atboth Namche Barwa and Nanga Parbat [Craw et al., 1994, 2005]. This thermal structure was closely related torapid uplift of gneissic rocks from high-grade metamorphic episode at great depths. Abundant hot springsand fluid inclusions in and near the NBM reported by Craw et al. [2005] provide evidence for the presenceof a fluid-flow system and thermal anomaly at shallow depth beneath the NBM. Conductive anomalousblocks in the near-surface resistivity model from our inversion results (the 2 km slice in Figure 5) are consis-tent with the locations of hot spring and fluid inclusion, especially to the southwest of NB and in the north ofNBM. In contrast, the regions near the other locations seem to be resistive. However, as discussed insection 4.3, these shallow resistive regions are poorly constrained by existing data due to few MT sites inthese regions. It is important to note that the structure is still resistive near the GP at depth of 2 km, whichare inconsistent with the locations of hot spring and fluid inclusions. Nevertheless, conductive feature (con-ductor J) appears to be prominent at the same position in the slice at depth of 5 km. These low-resistivityblocks at shallow depth can be interpreted as a conductive thermal anomaly (or boiling water?) whichmay provide the heat for the spring systems, suggesting wide distribution and deep penetration of meteoricwater into the thermally anomalous crust [Craw et al., 2005]. The presence of shallow conductive thermalanomaly demonstrates that the local steep geotherm is likely to occur at Namche Barwa by rapid uplift ofhot rocks and tectonic-erosion interaction [Zeitler et al., 2001, 2014; Koons et al., 2002, 2013].

The results also provide evidence for a brittle-ductile transition at a certain depth (~ 8–10 km below surface)[Craw et al., 2005]. More resistive features can also be found in the shallow slices at depths of 2 km and 5 km,while predominant low-resistivity zones are distributed around the NBA and NBM (conductors H, J, and k) inthe deeper part in the slice at depths of 12 km and 20 km (Figure 5). This resistive upper crust at shallowdepth can be interpreted as brittle rocks or a Mesozoic overthrust arc near the GP-NB massif observed byKoons et al. [2013]. The continuous conductive feature is related to ductile rocks below the brittle-ductileinterface as shown in Figures 6 and 13. Furthermore, a possible 300°C–400°C isotherm drawn by the near-surface electrical property that separates the upper brittle layer and hot ductile rocks also coincides withthe thermal model [Craw et al., 2005].

6.3. Implications for Exhumation Mechanism of Metamorphic Rocks

The results not only indicate that the upper crust is characterized by variant thermal structure at shallowdepth but also prove a rheological property of the middle crust at Namche Barwa. The resulting resistivitymodel from northwest to southeast near the GP-NB massif (Figure 14) shows a relatively resistive(>800 Ωm) upper crust (0–10 km), underlain by a more conductive (<80 Ωm) middle to lower crust (10–40 km). A dull-red spot, delineated by dotted line and marked by “J” in the BB0 section as well (Figure 6), likelyrepresents an accumulation of partial melt, as this anomalous region exhibits much more conductive charac-teristics with a resistivity of approximately 1–5 Ωm. The estimated 400°C–550°C isotherms between 13 and17 km depth below surface [Craw et al., 2005] suggest that it was likely to undergo hydrous melting by meta-morphic reactions, instead of dry melting in the dry rocks that begin to melt at about 1200°C according to

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LIN ET AL. MT IMAGING OF THE CRUST OF NAMCHE BARWA 15

temperature-conductivity curves [Li et al., 2003]. A steep northwest dipping low-resistivity zone (<80 Ωm) ata depth of 8–30 km is also imaged throughout most of upper and middle crust beneath the Yarlung ZangboRiver (Figure 14). Such a weak zone may be interpreted as localized interconnected paths or large reverse-fault systems as proposed by Koons et al. [2013] for devolatilization and rapid decompression melting.Bounded by active and steep faults, the conductive narrow domain present near the surface among the brit-tle layer (shown in Figure 14) is probably weakened by the underlying bodies of high-temperature graniteand migmatites. In addition, the thermal structure at shallow depth indicates that surface processes mightalso be an important factor in the rapid rock uplift and exhumation and may also be coupled to the localizedtectonics at Namche Barwa. Eroded materials of the local system evacuated out through crossing YarlungZangbo River could be a possible contribution to the dynamics of rapid erosion and rock uplift.

What mechanisms may result in the uplift of metamorphic rocks at Namche Barwa? Among several plausiblemodels summarized in the first section, the conceptual “tectonic aneurysm”model [Zeitler et al., 2001; Koonset al., 2002], which describes local-scale feedback between surface erosion and crustal tectonic processes,coincides well with our MT inversion results discussed above. The model needs specific crustal features,including high heat-flow anomalies at shallow depth, an effective path for rapid decompression melting,and a weak and hot middle and lower crust. More detailed information about multidisciplinary studiesand discussions of the tectonic aneurysm model can be found in Koons et al. [2013] and Zeitler et al.[2014]. In fact, the previous MT observation from Nanga Parbat also supported the aneurysm model in thatthe unusual resistive

midcrustal rocks would correspond to young, hot, and dry granulites or water-undersaturated decompres-sion melting, instead of the presence of large magma bodies and interconnected aqueous fluid phase tobe electrically conductive [Park and Mackie, 1997, 2000]. In contrast, the electrical structures beneathNamche Barwa present relatively conductive middle crust near the GP-NB massif (conductors J and K) andcontain massive conductors in the lower crust distributed to the west and north of the NBM, although theyresemble upper crustal structures at Nanga Parbat including a shallow resistive brittle layer, a hydrothermalsystem above the brittle-ductile transition, and conductive regions for devolatilization driven by high strains.Compared with the MT data at Nanga Parbat, our 3-D model is formed from a denser distribution of MT sitesfocused on the inner part of Namche Barwa and provides much more detailed electrical characteristics.

6.4. How Does the Region Remain Hot and Anatectic?

Unlike the fairly widespread low-resistivity layer under the IYSZ and Lhasa terrain in southern Tibet [Weiet al., 2001; Li et al., 2003; Unsworth et al., 2004, 2005], the electrical structure at greater depths beneaththe Namche Barwa in eastern Himalaya exhibits heterogeneities in the crust. The predominant electricalcharacteristic appears to be a mass of conductors developed in the middle crust, which are mainly distrib-uted to the west and to the northeast of the NBM (20 km slice and 30 km slice in Figure 5). In addition, con-ductors J and K can be seen to be connected to conductor I in the slice of 12 km depth (Figure 5). ConductorI is also connected to conductor H which extends to more than 30 km depth in the slices of 20–40 km depth(Figure 5) and the BB0 section (Figure 6). These conductors (J and K) could be possible links between the con-tinuous conductive layer in the lower crust and weak zones for decompression melting at shallower depths.We infer that shallower thermal-rheological structures would be closely related to such conductive weakzones in the middle to lower crust. What mechanism can account for such stable thermal-rheological struc-ture beneath Namche Barwa?

Recent geological studies on high-pressure granulites provide evidence of an additional heat source andlong-lived metamorphism at Namche Barwa [Guilmette et al., 2011; Zhang et al., 2015], so what is the primarysustained heat source of the local system? The eastward crustal flow discussed above may be an alternativehypothesis for the additional heat source. The widespread conductive zones may be favorable conditions forcrustal flow to develop beneath the southeastern Tibetan Plateau [Royden et al., 1997; Beaumont et al., 2001;Copley and McKenzie, 2007]. The continuous crustal flow can provide transitive ductile and warm materialfrom southern Tibet and form long-lasting partial melting of enriched metasomatic layers beneath theNamche Barwa.

The other alternative mechanism may be an upwelling of asthenospheric mantle. Widespread low-velocityzones also exist in the middle to lower crust around Namche Barwa in the light of the results from passive

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seismic tomography (Figure 12a) and receiver function data [Peng et al., 2012, 2016]. The lower crustal layer ofaqueous fluids and/or partial melting may be derived from mantle materials due to the slab fragmentationand induced upwelling of hot materials from mantle asthenosphere [Yao et al., 2010; Liang et al., 2016].The upwelling of asthenospheric mantle under eastern Himalaya probably results from the lateral contrastingsubduction of the underlying Indian plate in the subduction-collision system [Peng et al., 2016]. Furthermore,an uplifted Moho observed at Namche Barwa from receiver function analysis [Peng et al., 2017] supports theupwelling mechanism as well.

7. Conclusions

Based on the new MT data, we present the 3-D resistivity images to reveal the electrical structure of thecrust beneath the Namche Barwa. Our resistivity images of the MT inversion results show apparent varia-tions and heterogeneities in the crustal electrical structure beneath the Namche Barwa. The electrical con-ductors extending along IYSZ and turning north around the Namche Barwa provide additional evidence tothe crustal flow model if considered from a relatively large-scale aspect. Most of the features of our near-surface resistivity model are associated with the thermal structure, such as conductive thermal anomalies,the brittle-ductile transition, and the position of the 300°C–400°C isotherms reported by Craw et al. [2005]and Koons et al. [2002, 2013]. The resistivity model near the GP-NB massif shows relatively resistive uppercrust underlain by a more conductive middle to lower crust. The dull-red spot J probably represents anaccumulation of hydrous melting at a depth of 13–17 km. The steep northwest dipping conductive zonein the middle crust may be appropriately interpreted as localized interconnected paths or large reverse-fault systems for rapid decompression melting. The tectonic aneurysm model proposed by Zeitler et al.[2001, 2014] and Koons et al. [2013] may be a reasonable model for exhumation mechanism of meta-morphic rocks at Namche Barwa. We agree with the view that both surface processes and local tectonicresponses play a vital role in the evolution of Namche Barwa. A possible mechanism is also discussedfor primary sustained heat source of the local thermal-rheological structure beneath Namche Barwa. Thecrustal flow model or the localized upwelling of asthenospheric mantle may be an alternative hypothesisfor the additional heat source.

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AcknowledgmentsThis study has been jointly supportedby the National Natural ScienceFoundation of China (41374078 and41674134), National Key Research andDevelopment Program of China(2016YFC061104 and2017YFC0602204),the Open Fund (GDL1602) of KeyLaboratory of Geo-detection (ChinaUniversity of Geosciences, Beijing),Ministry of Education, the Special Fundfor Basic Scientific Research of CentralColleges, and Beijing Higher EducationYoung Elite Teacher Project.Constructive comments by D.H. Bai andan anonymous referee significantlyimproved this manuscript and aregreatly appreciated. We thank DenysJames Grombacher for his help forimproving the text of English.Supporting information data for the 3-Dmodel are freely available accompany-ing this paper as supporting informationon Journal of Geophysical Research-Solid Earth. Raw data are also availablefor academic purposes and may beobtained fromC.H. Lin orM. Peng (email:linchh@cugb.edu.cn or pengm@cugb.edu.cn).

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