Detection of the deep crustal structure of the Qiangtang terrane using magnetotelluric imaging

    Detection of the deep crustal structure of the Qiangtang terrane using magnetotelluric imaging Sihong Zeng, Xiangyun Hu, Jianhui Li, Shan Xu, Hui Fang, Jianchao Cai PII: DOI: Reference:

S0040-1951(15)00466-7 doi: 10.1016/j.tecto.2015.08.038 TECTO 126759

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

2 February 2015 17 August 2015 21 August 2015

Please cite this article as: Zeng, Sihong, Hu, Xiangyun, Li, Jianhui, Xu, Shan, Fang, Hui, Cai, Jianchao, Detection of the deep crustal structure of the Qiangtang terrane using magnetotelluric imaging, Tectonophysics (2015), doi: 10.1016/j.tecto.2015.08.038

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ACCEPTED MANUSCRIPT Detection of the deep crustal structure of the Qiangtang terrane using magnetotelluric imaging Sihong Zenga, Xiangyun Hua*, Jianhui Lia, Shan Xua, Hui Fangb, Jianchao Caia a

Hubei Subsurface Multi-scale Imaging Key Laboratory, Institute of Geophysics & Geomatics,

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Institute of Geophysical and Geochemical Exploration CAGS, Langfang, China

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b

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China University of Geosciences, Wuhan, China

ABSTRACT

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To determine the deep electrical structure of the Qiangtang terrane, northern Tibetan Plateau, we reanalysed three broadband magnetotelluric (MT) profiles collected by China University of

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Geosciences (Wuhan) in 1993–1994 and derived corresponding 2-D resistivity models. In these models, a remarkable high-conductivity layer that was divided into northern and southern parts and

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that had formed an antiformal zone of high conductivity beneath the central Qiangtang terrane was

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visible. This high-conductivity layer corresponds well with the trace of the southward subduction of

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the mélange from the Jinsha River suture inferred from surface geology. According to our resistivity models, the crust of the northern Qiangtang terrane is mostly composed of mélange. The underplated

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mélange may be closely related to the formation of the high-conductivity layer and provide a reasonable explanation for the cause of special geophysical features in this area. In addition, the similarity between these resistivity models provides evidence that the Qiangtang anticlinorium extends eastward to at least 92°E, whereas the differences between them may offer an explanation for the gradual narrowing of the metamorphic belt from west to east in the central Qiangtang terrane. Keywords: Magnetotelluric, Qiangtang terrane, High-conductivity layer, Crustal formation, Qiangtang anticlinorium 1. Introduction Since the closing of the Tethys Ocean, the ongoing collision between the Indian and Asian

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ACCEPTED MANUSCRIPT continents created the Tibetan Plateau, which is the most spectacular topographic feature on the surface of the Earth. A wide range of geodynamic models have been proposed for the evolution of the Tibetan Plateau, including the northward injection of the Indian crust (Zhao and Morgan, 1985), block

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extrusion along principal strike-slip faults (Tapponnier et al., 1982, 1990), and flow in a weak lower

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crustal layer (Rodyen et al., 1997, 2008). In the past decades, numerous geological and geophysical studies have been conducted in the Tibetan Plateau (Nelson et al., 1996; Chen et al., 1996; Owens and

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Zandt, 1997; Makovsky et al., 1999; Kind et al., 2002; Wei et al., 2001; Tilmann et al., 2003; Unsworth et al., 2005). In contrast to southern Tibet, the Qiangtang terrane (Fig. 1a) exhibits a high crustal

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Poisson’s ratio (Owens and Zandt, 1997), inefficient Sn propagation (Zhao et al., 2010; Xu et al., 2011), strong SKS anisotropy (Huang et al., 2000), high-conductivity anomalies (Wei et al., 2001; Unsworth

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et al., 2004), and widespread Cenozoic volcanism (Yin and Harrison, 2000; Kapp et al., 2005). At

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present, the contribution of the north–south variations in the Tibetan crustal structure to these

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geological and geophysical features is the subject of heightened research interest. A band of blueschist-bearing metamorphic core complexes approximately 600 km long and 300

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km wide is exposed in the central Qiangtang terrane (Fig. 1b), and the origin of these complexes remains a topic of debate. Currently, there are three main models of its origin: (1) an in situ palaeo-Tethyan suture that separates the northern and southern Qiangtang terrane (Li et al., 2006; Zhai et al., 2013), (2) an intracontinental subduction zone that the southern Qiangtang continental block subducted towards the northern Qiangtang block along the Shuanghu suture (Zhang et al., 2006a, 2006b, 2011), and (3) an early Mesozoic mélange that was underthrust from the Jinsha River suture (JRS) and was then exhumed in the interior of the Qiangtang terrane (Kapp et al., 2000, 2003, 2005). These three models imply fundamental differences in the first-order crustal structure of the northern Tibetan Plateau (Zhang et al., 2006a). Hence, the origin of the central Qiangtang metamorphic belt is

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ACCEPTED MANUSCRIPT important to understand the evolution of the northern Tibetan Plateau, and it is also significant to reveal the deep crust of the Qiangtang terrane. However, due to the inhospitable climate and the difficulty of gaining access to this area, very few geophysical observations have been obtained from

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the Qiangtang terrane.

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Magnetotelluric (MT) exploration can determine the conductivity of the crust and upper mantle by measuring variations of naturally occurring electromagnetic fields at the Earth surface. Previously,

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the INDEPTH 500-line and 600-line were collected traversing the Qiangtang terrane, and two-dimensional resistivity models derived by different researchers based on these data showed that

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the conductive crust found in southern Tibet is also present in northern Tibet (Wei et al., 2001; Unsworth et al., 2004), and the electrical structure in the southern Qiangtang terrane differs from the

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northern Qiangtang terrane. In addition, three north–south trending MT profiles spanning the

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Qiangtang terrane were collected at densely spaced sites by China University of Geosciences (Wuhan)

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in 1993–1994. This survey used state-of-the-art methods for the time, and the interpretation was more focused on the shallow sedimentary structure of the Qiangtang terrane for the purpose of oil and gas

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exploration (Zhang et al., 1996). In this work, we have reanalysed these three MT profiles using modern, more advanced processing, analysis, and modelling techniques to compare the similarities and differences in the electrical structures between the eastern and western Qiangtang terrane and provide additional evidence to resolve the controversial issues mentioned above. 2. Regional Geology The Qiangtang terrane, located in the central and north Tibetan Plateau, was accreted to Asia in the Late Triassic or Early Jurassic. It lies between the Bangong-Nujiang suture (BNS) and Jinsha River suture and is adjacent to the Lhasa terrane in the south and Songpan-Ganze terrane in the north. The Qiangtang terrane can be divided into three second-order tectonic units: the northern Qiangtang terrane,

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ACCEPTED MANUSCRIPT the southern Qiangtang terrane, and the central Qiangtang uplift (or Qiangtang anticlinorium) (Fig. 1). The first-order geologic framework of Qiangtang is characterized by dominantly metamorphic rocks and Late Palaeozoic shallow marine strata in the north and Triassic-Jurassic shallow marine carbonate

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rocks interbedded with terrestrial clastic and volcaniclastic strata in the south (Yin and Harrison, 2000).

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In the northern Qiangtang terrane, the Tanggula Mountain is parallel to the north-northwest strike and has extensive granitic intrusions exposed on the surface. In addition, Cenozoic volcanic rocks are

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widely distributed in the northern Qiangtang terrane. The petrology of these volcanic rocks suggests that they originated from melting of subcontinental lithospheric mantle or the crust-mantle transitional

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zone (Lai, 2000; Ding et al., 2003). The central Qiangtang metamorphic belt is structurally exposed beneath low-grade Carboniferous–Triassic strata in the footwalls of domal Late Triassic–Early Jurassic

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normal faults and is comprised of a strongly deformed matrix of metasedimentary and mafic schists

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(Kapp et al., 2003). The presence of metabasite, metagraywacke, chert, and minor ultramafic material

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within the Qiangtang metamorphic belt is consistent with it having been derived from fragments of, and sediments deposited on, oceanic lithosphere (Kapp et al., 2003).

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3. Collection and analysis of magnetotelluric data Phoenix MTU instruments were used to collect broadband (320–0.00055 Hz) MT data at a total of 215 sites along three north–south trending profiles, which were oriented perpendicular to the dominant geo-electrical strike direction of the northern Tibetan Plateau. Measurements were made at stations spaced approximately 3–5 km apart and a line interval of approximately 150–200 km (Fig. 1b). At each site, the two horizontal electrical field components (Ex and Ey) and the horizontal and vertical magnetic fields components (H x, H y and Hz) were recorded. A robust time series processing technique was applied to obtain the impedance tensor at each observation site. The dimensionality and regional strike of the MT data must be assessed prior to 2-D inversion. In

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ACCEPTED MANUSCRIPT this study, the distortion decomposition code developed by McNeice and Jones (2001), which is based on the Groom–Bailey (GB) decomposition (Groom and Bailey, 1989), was applied to the MT response estimates for each site to check for the presence of galvanic distortion and determine the most

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consistent geo-electrical strike direction over most sites and most periods. First, MT data for each site

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were analysed in every decade for periods 0.001–1000 s, with an assumed error floor of 2° for phase and equivalent 7% for apparent resistivity. From Fig. 2, we inferred that the dominant geo-electrical

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strike remained consistent over a large period for most of the study area. In Fig. 2, areas highlighted with the yellow ellipses indicate locations in which the strikes among the adjacent stations are complex

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and three-dimensional bodies exist in the subsurface. In addition, the average root-mean-square (RMS) error of misfit is generally less than 3.0, and the absolute values of the twist and shear angles for each

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site indicate that the level of distortion is moderate in the study area (Fig. 3).

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Based on the single site multi-frequency GB decomposition results, the geo-electrical strike of

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N90°E, which fits most of the data for most periods, was selected for further analysis. The impedance data were recalculated at a constrained geo-electrical strike direction of 90°. The average RMS error

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values for each site along the three profiles are shown in Fig. 4. The distribution of the RMS misfits shows that most of the sites have an average misfit of less than 2.0, which is considered to be an acceptable value (Jones, 2012). The sites with high RMS misfit are mostly located in the central and northern Qiangtang terrane and correspond well with areas marked with yellow ellipses in Fig. 2. The consistency of the geo-electrical strike along the profiles and the relatively low RMS misfit values suggest that 2-D modelling is a reasonable approach to determine the electrical structure beneath the surface. 4. Two-dimensional modelling Two-dimensional

resistivity

models

of

these

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three

profiles

were

computed

using

ACCEPTED MANUSCRIPT distortion-removed data. In this study, the smooth 2-D inversion method used was that developed by Rodi and Mackie (2001) and implemented in WingLink software. Many 2-D inversions, using various combinations of data components, starting models, and inversion parameters, were conducted to

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determine electrical features that are robust and to derive a final model that could fit the data

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appropriately. Static shift is the displacement of MT apparent resistivity curves by a frequency–independent constant value and is caused by local near-surface inhomogeneities. Many

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methods have been proposed to remove the static shift (e.g., Jones, 1988; Ogawa and Uchida, 1996). Setting a large error floor for apparent resistivity is considered to be an effective method to

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accommodate the static shift in the MT data (Ogawa, 2002). Additionally, TE-mode data are more sensitive to the three-dimensional effects (Wannamaker et al., 1984). Thus, in our model, the error

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floors for phase and apparent resistivity were set to 20% and 50% for the TE mode and 5% and 10%

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for the TM mode, respectively. The final models were generated using a smoothing factor τ = 5 for

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profile A and τ = 3 for profiles B and C. The initial models were constructed with 100 Ωm uniform half-spaces. The weighting functions, alpha and beta, were set to 1.0 and 0.3, respectively. The data

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errors positively influence the MT inversion results (Li et al., 2009), so the real data error was used in our inversion and an error floor was set; data errors that are below this error floor will be reset to this value.

Before looking at the final models, the validity of the inversion models needs to be estimated. Consistency between the model responses and measured data (Fig. S1, S2 and S3 for profiles A, B and C, respectively) confirmed the validity of our final models. The good fit to the measured data is also shown for two typical MT sites for each profile, and it can be seen that the data are generally well fit at all periods (Fig. 5). The final 2-D resistivity models for the three profiles are shown in Fig. 6a, 6c, and 6e; the RMS misfits for each site along the three profiles are shown at the top of the models (Fig. 6b,

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ACCEPTED MANUSCRIPT 6d, and 6f). The data fit is considered to be acceptable for all sites along the profiles, as the individual RMS values are mostly around 2.0, and only at some sites the misfits are up to 4.0. Several inversion models obtained by using different starting models (an initial model of 1000 Ωm and 10 Ωm uniform

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half-spaces) and different data components (TM mode only) are shown in Fig. S4. The general features

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of these models are comparable to the models in Fig. 6; only some differences on the scales and shapes of the anomalies are observed. The robust inversion models lead us to conclude that the features in

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these models are representative of the subsurface structure. The penetration depths beneath each site for both the TE and TM modes were estimated by the Niblett–Bostick inversion on the

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distortion-removed data (Niblett and Sayn-Wittgenstein, 1960; Jones, 2006). From Fig. S5, we inferred that the deep subsurface within 100 km is adequately sampled everywhere with respect to footprint

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size (Muller et al., 2009).

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As seen in Fig. 6, there is an apparent N–S dichotomy in the electrical structure of the southern

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and northern Qiangtang terrane. In the southern Qiangtang terrane, the resistive upper crust extends to a 25–35 km depth along profiles A and B and to a maximum of ~20 km along profile C. In the

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northern Qiangtang terrane, the resistive upper crust extends to an approximately 20–30 km depth along profiles A and B and varies from ~55 km in the south to ~20 km in the north along profile C. Beneath the resistive upper crust, there is a remarkable high-conductivity layer in the middle-to-lower crust. In the southern Qiangtang terrane, this high-conductivity layer dips southwards from the near surface in the central Qiangtang uplift to a depth of ~50 km for profiles A and B and to a depth of ~30 km for profile C. A relatively low velocity anomaly at a 20–50 km depth was detected from receiver function images using the seismic data along the INDEPTH 500-line and was considered to be a detachment fault (Shi et al., 2004). In the subsurface of the northern Qiangtang terrane, the high-conductivity layer has a hook-like shape. It extends upward to the near surface of the central

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ACCEPTED MANUSCRIPT Qiangtang uplift in the south and the JRS in the north and downward to the lower crust or even the upper mantle in the central part of the Qiangtang terrane. In addition, the high-conductivity layer beneath the southern and northern Qiangtang terrane both extends upward to the near surface in the

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Qiangtang uplift and forms an antiformal structure with high conductivity; this anomaly was also

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inferred by Wei et al. (2001). The extensive high-conductivity layer in the middle-to-lower crust of the Qiangtang terrane is similar to that inferred in previous MT surveys (Wei et al., 2001) and corresponds

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well with the low-velocity zone inferred from seismic data (Owens and Zandt, 1997). The characteristic of the N–S dichotomy of the electrical structure between the southern and northern

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Qiangtang terrane is consistent with the model of Zhang et al. (1996). However, our 2-D models are not entirely in agreement with the results of the Zhang et al. (1996). The high-conductivity layer in our

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models is much more widespread within the northern Qiangtang terrane, not within the southern

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Qiangtang terrane. In addition, a high-resistivity anomaly, which corresponds with the Tanggula

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Mountain on the surface, appears in each profile, including the three profiles and the INDEPTH 500 and 600-lines (Wei et al., 2001).

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5. Discussion

5.1 The formation of the central Qiangtang metamorphic belt All three resistivity models of the Qiangtang terrane show a layer of high conductivity in the middle-to-lower crust. This high-conductivity layer extends to a depth of ~50 km (profiles A and B) or ~30 km (profile C) in the south and to the lower crust or upper mantle in the north, forming an antiformal structure beneath the central Qiangtang uplift. One important concern is how the high-conductivity layer with that special shape was formed. Kapp et al. (2000, 2003) proposed that the mélange was underplated beneath the Qiangtang terrane during the early Mesozoic southward subduction of the Songpan–Ganze oceanic lithosphere along the JRS and was subsequently exhumed 8

ACCEPTED MANUSCRIPT to shallow crustal levels. Haines et al. (2003) observed relatively low seismic velocity beneath the Qiangtang crust, which supported the hypothesis that a large volume of mélange has been underthrust beneath the Qiangtang terrane. Comparing our resistivity models with the geological model inferred

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from the seismic data, we see that the shape of this high-conductivity layer and its north–south trend

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beneath the Qiangtang terrane is similar to the assumed mélange subduction path from the JRS (Haines et al., 2003; Shi et al., 2004). Therefore, we suspect that this north–south high-conductivity layer is

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related to the palaeotectonic track of the southward subduction of the mélange during the Triassic Period. The mélange subducted southwards beneath the northern Qiangtang terrane from the JRS, was

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subsequently exposed in the core of the Qiangtang antiformal culmination, and finally moved southwards to the deep part of the southern Qiangtang terrane (Kapp et al., 2000, 2003).

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Additionally, our 2-D inversion models also provided some previously unknown details of the

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mélange underthrusting model. We have sketched the lithosphere structure along the three profiles

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based on our resistivity models in Fig. 7. According to Fig. 7, most of the crystalline basement of the central and northern Qiangtang terrane has been tectonically removed and replaced by mélange, which

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subducted southwards beneath the northern Qiangtang terrane and was subsequently exposed in the core of the Qiangtang antiformal culmination, forming the central Qiangtang metamorphic belt. From westernmost profile B to easternmost profile C, the exposed mélange in the central Qiangtang terrane gradually narrows, and ultimately disappears on the surface of profile C. This feature is consistent with the narrowing and ultimate disappearance of the metamorphic belt on the surface of the central Qiangtang terrane from west to east. We also notice that the dip angle and the depth of the high-conductivity layer from the JRS gradually decrease from west (profile B) to east (profile C). This phenomenon could be explained by the mélange from the JRS being deeply subducted southwards at a steep angle in the west, and subducting southwards to a relatively shallow depth at a relatively gentle

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ACCEPTED MANUSCRIPT angle in the east. The variations in subduction angle and depth between the west and east may partially account for the narrowing and ultimate disappearance of the metamorphic belt on the surface of the central Qiangtang terrane from west to east.

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Fig. 7 also clearly shows the deep structure of central Tibet. Owing to the presence of the

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high-conductivity layer in the middle crust, the MT data provide less rigorous constraints on the lower crust and upper mantle structure of the Qiangtang terrane. However, the inversion model of

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profile A has revealed a subvertical high-conductivity zone, which extends downward from the lower crust to the upper mantle. According to Unsworth et al. (2004), a similar subvertical

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high-conductivity zone was also observed on the INDEPTH 600-line and was caused by the upwelling of mantle-derived melts. As the mantle-derived magma migrated upward through the weak

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tectonic belt, they initiated crustal melting and pervasive low resistivity resulted in the lower crust;

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lower crustal flow can occur for mass balance and transport material to the neighbouring profile or

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farther areas (Clark and Royden, 2000). The absence of the subvertical high-conductivity zone beneath profiles B and C suggests that upwelling of the magma is not prevalent and may relate to

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some special tectonics at deep depths. The aeromagnetic data revealed an ENE trending negative anomaly around 90°E longitude in central Tibet, thought to be a deeply buried aulacogen (He et al., 2007). We suppose this aulacogen is related to the formation of the observed subvertical high-conductivity zone of profile A. Thus, additional long-period magnetotelluric data in our research areas are needed to validate these models. 5.2 The origin of the high conductivity layer in the Qiangtang terrane As we know, to produce a high-conductivity layer as observed in the Qiangtang terrane, a conducting phase should exist to allow electric current to flow through the rock. Interconnected metallic ores, graphite, partial melting, and aqueous fluids can all cause high crustal conductivities 10

ACCEPTED MANUSCRIPT (Jones, 1992). The large spatial extent of the conductive layer excludes metallic mineralization as an explanation. Interconnected graphite film is often an effective conductor in stable cratons, but it is generally not the case in regions with active tectonics such as the Tibetan Plateau (Wei et al., 2001).

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Thus, the explanation for the high-conductivity layer beneath the Qiangtang terrane is interconnected

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fluid, either partial melting or aqueous fluid. The seismic anomalies observed by Owens and Zandt (1997) showed that the lower crust of the Qiangtang terrane is characterized by low P-wave velocity,

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inefficient S-wave propagation and high Poisson’s ratio. Moreover, widespread potassic and ultrapotassic magmatism derived from the mantle or crust-mantle transitional zone was found in the

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northern Qiangtang terrane (Yin and Harrison, 2000; Ding et al., 2003). All of these geophysical and geological properties imply the presence of high temperatures and partial melting in the lower crust

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and upper mantle of the Qiangtang terrane.

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What is the cause of the high-conductivity anomalies observed in the middle or upper crust of the

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Qiangtang terrane? Laboratory measurements have shown that water-saturated rocks will begin to melt at lower temperatures, typically 650 °C (Lebedev and Khitarov, 1964). The analysis of the seismic data

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along the profile 500-line by Mechie et al. (2004) showed that a temperature of 700 °C is achieved at a depth of 18 km under the southern Qiangtang terrane. According to Jiménez-Munt et al. (2008), the northern Qiangtang terrane is much hotter than the southern Qiangtang terrane. Thus, a temperature greater than 650 °C is achieved at a depth of 18 km beneath the whole Qiangtang terrane, where hydrous melting is likely to occur in the mid-crust. This depth is in agreement with the top of the high conductivity middle crust (Solon et al., 2005) and the top of the mid-crustal velocity layer (Zhao et al., 2001). The high-conductivity anomalies shallower than this depth observed in our models are likely caused by aqueous fluids, either atmospheric water in faulted structure fracture zones or water introduced by the subduction of mélange from the JRS (Kapp et al., 2000). Laboratory experiments

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ACCEPTED MANUSCRIPT show that pure melts have an electrical conductivity of 3–10 Sm-1, whereas aqueous fluids have slightly higher conductivity values, with the highest in excess of 100 Sm-1 (Li et al., 2003). The resistivity of the conductivity layer in our research area is approximately 5 Ωm. Using Archie’s Law to

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estimate the fluid fraction, a bulk resistivity of 5 Ωm requires a melt fraction in the range 2–7%

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depending on the resistivity of the melt, and requires a smaller aqueous fluid fraction (Unsworth and Rondenay, 2013).

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According to the geological model proposed by Kapp et al. (2000) and our inversion models (Fig.7), a large part of the crystalline basement of the central and northern Qiangtang terrane has been

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tectonically removed and replaced by mélange that was underthrust southwards from the JRS. The mélange is comprised of a strongly deformed matrix of metasedimentary and mafic schists derived

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from fragments of, and sediments deposited on, oceanic lithosphere (Kapp et al., 2003). The

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underplated water-rich mélange may relate to the formation of the high-conductivity layer and provide

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a reasonable explanation for the cause of special geophysical features in this area. First, the lack of an integrated crystalline basement may offer a partial explanation for the low seismic velocity and high

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crustal Poisson’s ratios of northern Tibet (Owens and Zandt, 1997). Second, the aeromagnetic data revealed an ENE trending negative anomaly around the longitude of 90°E in central Tibet (He et al., 2007), and P-wave tomography shows that beneath the Qiangtang terrane a low-velocity anomaly exists from the mid-crust down to 310 km depth (Zhou and Murphy, 2005); these geophysical features are suggestive of localized upwelling of mantle-derived magma through a tectonically weak zone. The mélange may be weaker than the continental rocks of deeper southern Tibetan crust (Kapp et al., 2000). The upwelling of the mantle-derived magma is likely to occur through the weak zone and result in partial melting in the mid-to-lower crust of the north Qiangtang terrane. Third, the presence of a high-temperature anomaly in the crust of the Qiangtang terrane has been verified through petrological

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ACCEPTED MANUSCRIPT observations (Hacker et al., 2000), joint modelling of gravimetry, topography and geoid data (Jiménez-Munt et al., 2008), and the α-β quartz transition from a seismic reflector (Mechie et al., 2004). A water-rich mélange is prone to partial melting at high-temperatures; the partial melting would

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further enhance the high-conductivity, low-velocity anomaly and explain the widespread Cenozoic

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volcanism in the northern Qiangtang terrane (Yin and Harisson, 2000). Lastly, Mechie et al. (2004) reported that the depth of the α-β quartz transition in central Tibet showed little change, but the high

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conductivities and presumably high degrees of partial melting in the mid-crust are absent south of the BNS in the northern Lhasa block (Solon et al., 2005). This fact may indicate a strong variability of

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water content on either side of the BNS (Mechie et al., 2004), so the underplated water-rich mélange can provide a reasonable explanation for the pronounced N-S change in geophysical features below the

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approximate surface trace of the BNS.

5.3 The eastward extension of the Qiangtang anticlinorium

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As seen from the geological map in Fig. 1b, the central Qiangtang anticlinorium disappears on the surface approximately 90°E. However, some geologists inferred that the Qiangtang anticlinorium is

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eastward-plunging at depth to about 92°E (Yin and Harisson, 2000; Kapp et al., 2000, 2003). However, due to the occurrence of the extensive marine strata of Jurassic age on the surface and the lack of regional geophysical observations, the presence of the central Qiangtang anticlinorium further east of 90°E is not obvious. According to our 2-D models, an antiformal structure with high conductivity beneath the near surface of the central Qiangtang terrane in present in our three profiles and the INDEPTH 600-line (Wei et al., 2001). Hence, we confirm that the central Qiangtang anticlinorium is plunges eastward at depth to at least 92°E. 6. Conclusions Our two-dimensional MT resistivity models not only offer compelling evidence that supports the 13

ACCEPTED MANUSCRIPT geological model proposed by Kapp et al. (2000), but also provide additional previously unknown details about the variation of the southward dip angle and depth of the mélange from west to east. These results completely delineate the palaeotectonic track with regard to the formation of the

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metamorphic belt in the central Qiangtang terrane. It is very important to elucidate the formation of the

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crust in the northern Tibetan Plateau and the cause of special geophysical features in this area. In addition, based on these resistivity models, we also confirm that the Qiangtang anticlinorium extends

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to about 92°E at depth. Acknowledgements

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This research received financial support from the National Basic Research Program of China (973 Program, No. 2013CB733200) and the National Natural Science Foundation of China (No. 41274077,

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No. 41474055). We are especially thankful to the editor Kelin Wang, Martyn Unsworth and one

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anonymous reviewer for their helpful comments and constructive suggestions that significantly

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improved the manuscript. We also thank Profs. Adam Schultz and Qingsheng Liu for their constructive comments on an earlier version.

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Fig.1. (a) Map showing the topography of Tibet. The white rectangle indicates the position of the study region and the detailed map in (b). The orange lines mark the INDEPTH 500 and 600 lines. The green dashed line denotes the limits of the zone of inefficient Sn propagation (Zhao et al., 2010). The light blue line and circle denote the SKS anisotropy (Zhao et al., 2010). (b) Tectonic map with background topography modified from Kapp et al. (2005). The red dots mark the MT stations along the profiles, and the digits are station numbers. The yellow dashed line denotes the supposed trace of the central Qiangtang anticlinorium. Abbreviations are as follows: YZS=Yarlung-Zangbo Suture; JRS=Jinsha River Suture; BNS=Bangong-Nujiang Suture; TTS=Tanggula Thrust System.

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Fig. 2. Maps showing the preferred geoelectrical strike at each site along the three profiles for four decadal period bands (period 0.1–1 s, 1–10 s, 10–100 s and 100–1000 s). The ellipses highlight areas with an inconsistent strike compared with adjacent sites.

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Fig. 3. Distribution of the average absolute values of the twist and shear angles over the whole recorded period range for unconstrained data along the three profiles, (a) and (b) for the profile A, (c) and (d) for the profile B, (e) and (f) for the profile C.

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Fig. 5 Typical apparent resistivity and phase curves from the three profiles, two typical MT sites for each profile are shown for south Qiangtang ((a), (c) and (e) for profile A, B and C, respectively)and northern Qiangtang ((b), (d) and (f) for profile A, B and C, respectively). The continuous lines represent the fit of the 2D inversion model in Fig. 6 to the MT data. Locations of these MT stations are shown in Fig. 1(b). Fig. 6. Two-dimensional resistivity models obtained by joint inversion of TE and TM modes. The RMS misfits for each sites along the three profiles are shown at the top of the models. Fig. 7. Lithosphere resistivity structure along the three profiles (a, b and c for profile B, A and C, respectively) and dynamics model for the formation of the crust of Qiangtang terrane.

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Graphical abstract A high-conductivity layer, which was divided into northern and southern parts and formed an antiformal zone of high conductivity beneath the central Qiangtang terrane, was supposed to be related to the palaeotectonic track of the southward subduction of the mélange from the JRS. A large part of the crust of the Qiangtang terrane has been tectonically removed and replaced by the subducted mélange. The weak mélange may act as a tectonic weak zone for the upwelling of the mantle-derived magma, and then initial the partial melting in the mid-to-lower crust and explain the special geophysical and geological features observed in this area.

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A remarkable high-conductivity layer with special shape was visible beneath the Qiangtang terrane; Most of the crystalline basement of the central and northern Qiangtang terrane has been tectonically removed; The Qiangtang anticlinorium is plunges eastward at depth to at least 92°E.

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