The Moho structure beneath the Yarlung Zangbo Suture and its implications: Evidence from large dynamite shots

Accepted Manuscript The Moho structure beneath the Yarlung Zangbo Suture and its implications: Evidence from large dynamite shots

Hongqiang Li, Rui Gao, Wenhui Li, Zhanwu Lu PII: DOI: Reference:

S0040-1951(18)30323-8 doi:10.1016/j.tecto.2018.10.003 TECTO 127942

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

26 September 2017 27 September 2018 3 October 2018

Please cite this article as: Hongqiang Li, Rui Gao, Wenhui Li, Zhanwu Lu , The Moho structure beneath the Yarlung Zangbo Suture and its implications: Evidence from large dynamite shots. Tecto (2018), doi:10.1016/j.tecto.2018.10.003

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The Moho structure beneath the Yarlung Zangbo Suture and its implications: evidence from large dynamite shots Hongqiang Lia, Rui Gaob，c *, Wenhui Lib and Zhanwu Lub* a

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SinoProbe Centre, Chinese Academy of Geological Sciences, Beijing 100037,

China.

Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037,

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b

China

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School of Earth Science and Geological Engineering, Sun Yat-sen University,

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Guangzhou 510275, China

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* Corresponding author：[email protected]，[email protected]

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ACCEPTED MANUSCRIPT Abstract: The Yarlung Zangbo Suture（YZS） is the collisional front between the Indian and Eurasian plates. The deep structure beneath the YZS has long been a focus of research. Its particular tectonic location and formation time provide a natural laboratory for the

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study of ongoing continental collisional processes. Lateral variations in the Moho discontinuity provide evidence of basic processes governing plate tectonic evolution.

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The depth and geometry of the Moho provides first-order information for restoration

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of complex geodynamic systems. This paper presents a studying result of the Moho, both adjacent to and beneath the YZS, obtained using five large dynamite shots along

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two profiles. These five large shots were combined to construct two single-fold profiles (YZS-A and YZS-B) and obtain high-resolution images of the Moho beneath

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the YZS. Both profiles show that continental crust thickness beneath the YZS is twice the global average of continental crust, and the depth of the Moho along profile YZS-B is deeper than YZS-A.YZS-A (81°E) indicates a relatively flat Moho without

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offset related to continent-continent convergence beneath the collision zone.

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Conversely, a Moho structure with offset and overlapping reflections was imaged beneath profile YZS-B (88°E). Significant lateral variations in the geometry of the Moho discontinuity along the YZS may indicate that the Indian lithosphere is

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thrusting beneath Tibet at a different angle from west to east.

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Keywords: large dynamite shot; deep seismic reflection; Moho discontinuity; Yarlung Zangbo Suture.

1. Introduction The Yarlung Zangbo Suture (YZS) is situated between the Indian block to the south and the Gangdese forearc basin to the north, and marks the boundary between the Indian subcontinent and Eurasia (Li et al., 2015; Yin et al, 2006). It is regarded as the advancing front of the ongoing collision at the earth’s surface between the Indian and 2

ACCEPTED MANUSCRIPT Eurasian plates (Dewey et al., 1988), and its deep structure has long attracted the attention of researchers from around the world (Hirn et al., 1984). Owing to its particular tectonic location and recent formation time, it provides a natural laboratory for the study of ongoing continental collision processes (Nelson et al., 1996). Lateral variations of the Mohorovicic (Moho) discontinuity, which separates the crust from

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the upper mantle, are thought to provide evidence of basic processes governing plate tectonic evolution (Anderson, 2007; Meissner, 1973), with the depth and geometry of

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the Moho providing first-order information for restoration of complex geodynamic systems (Prodehl & Mooney, 2012). Study of the Moho beneath and adjacent to the

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YZS can therefore improve our understanding of the deep processes involved in uplift

(Haines et al., 2003; Yin et al., 2000).

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of the Tibetan Plateau, formation of the Himalayas, and evolution of the lithosphere

The E-W trending YZS contains Late Jurassic to Early Cretaceous ophiolites and

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constitutes a remnant of the Neo-Tethys Ocean (Allegre et al., 1984; Dewey et al., 1988; Hebert et al., 2012; Yin and Harrison, 2000). Regionally, it is marked by

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dismembered ophiolite fragments, mélanges, and later north-vergent reverse faults

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(Alsdorf et al., 1998; Ratschbacher et al., 1994). Over the past three decades, numerous seismic experiments have been conducted across the YZS, including deep seismic reflection profiles (Gao et al., 2016), deep seismic soundings (Hirn et al.,

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1984; Liu et al., 200; Teng et al., 1983), and broadband observation studies (e.g., H; Kind et al., 2002; Shi et al., 2015, 2016; Yuan et al., 1997; Zhao et al., 2010;).

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Nevertheless, there remains strong disagreement regarding the character of the Moho along the YZS in Tibet. Hirn et al. (1984) proposed an offset of > 15 km along the Moho beneath the YZS, based on wide-angle observations acquired through a Sino-French cooperative experiment. Xiong et al. (1985) showed that the Moho offset below the YZS reaches 6 - 8 km based on the results of a long E-W profile. Conversely, Kind et al. (2002) were unable to discover a Moho offset beneath the YZS through inversion of broadband data, while Wang et al. (2008) suggested that the Moho beneath the YZS is dislocated by up to 6 km (Wang et al., 2008). According to Jiang et al. (2008), the Moho exhibits a 20 km offset, based on multiple broadband 3

ACCEPTED MANUSCRIPT seismic profiles across the YZS. Gao et al. (2016) did not find any significant changes in Moho depth when using deep seismic reflection profile data across the western YZS. The above-mentioned studies all indicate a very deep and complex Moho, with varying characteristics of the YZS, not only on either side of the suture, but also along-strike of the suture. However, different studies have yielded contrasting models,

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and further investigations are therefore needed to determine whether a Moho offset is present, and to investigate variability in Moho characteristics beneath the YZS. The

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above issues cannot currently be effectively resolved without improving the accuracy of available geophysical observations to obtain reliable evidence.

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Rapid development of the technology of deep seismic reflection profiling provides an opportunity to resolve the above controversies. With the support of the SinoProbe-02

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experiment and the Geological Survey Project in China, two deep seismic reflection profiles across the YZS (Fig. 1) were acquired: the western profile of the YZS

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(YZS-A: 81°E) and the middle profile of the YZS (YZS-B: 88°E). The western profile (YZS-A: 81°E) is the same seismic profile as HKT-B in Gao et al. (2016). Three kinds

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of dynamite shots (i.e., 192 kg, 48 kg, and 1000 - 2000 kg) were utilized to obtain

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deep earth reflections. Five large dynamite shots with 1000-2000 kg charges were employed to improve the signal-to-noise ratio (S/N) along the two profiles. This study analyzes images of the Moho, adjacent to and beneath the YZS, obtained

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through these five large dynamite shots along the two profiles. The five shots were processed to construct two single-fold profiles, thereby providing the high-resolution

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images of the Moho beneath the YZS and its surroundings. This enabled analysis of lateral variations in Moho depth and changes in topography at different spatial locations along the YZS.

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Fig. 1. Locations of large dynamite shots and profiles, for the Yarlung Zangbo Suture (YZS), and

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the Bangong-Nujiang Suture (BNS). Blue lines mark the locations of sutures, while red stars mark shot locations, with three shots distributed along YZS-A across the western YZS and two shots situated along YZS-B across the central YZS. Inset shows Tibet Plateau and surrounding areas and

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location of the study area, HI-CLIMB (H) and INDEPTH (I) project seismic profiles.

2. Data and Analysis

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2.1 Seismic data acquisition

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Two deep seismic reflection profiles traversing the YZS were obtained in 2011 and 2015, respectively. The first profile (YZS-A) is situated at 81°E and oriented nearly northeast. The second profile (YZS-B) is located at 88°E and sub-parallel to a north-south trend (Fig. 1). Five large dynamite shots were employed along the two deep seismic reflection profiles, with three along profile YZS-A and two along profile YZS-B. Data acquisition and shot parameters are listed in Tables 1 and 2, respectively. Each large dynamite shot consisted of 10 – 20 boreholes at depths of approximately 50 - 70 m, and each hole was loaded with an explosive charge of 100 kg. Conventional Sercel 428XL geophone sensors with dominant frequencies of 10 Hz 5

ACCEPTED MANUSCRIPT were placed 50 m apart beyond a source offset distance of 1200 m. The geometries of the two profiles are shown in Fig. 2. Fig. 3 shows an example of a shot gather along YZS-B. There are several sets of high S/N reflections in the shallow crust, but reflection from the deep Moho is relatively weak before processing (Fig. 3). It is therefore important to identify and extract deep reflections to appropriately image the

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topography of the Moho.

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Table 1. Data acquisition parameters.

Table 2. Detailed parameters of the large dynamite shots.

Year FFID Station Latitude Longitude Elevation Charge Profile 2011

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20800

30.37° N

81.14° E

4269.5 m

2011

851

22615

31.07° N

81.14° E

5122.6 m

2011

961

21894

30.80° N

81.14° E

4722.2 m

2015

624

4380

29.09° N

88.16° E

4565.6 m

2015

628

5330

29.49° N

88.33° E

4425.7 m

1000 kg

YZS-A

2000 kg

YZS-B

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Fig. 2. Schematic diagrams of observation systems for（a）profiles YZS-A and（b）YZS-B. The red line marks the deep seismic reflection profile, and the light gray line marks the receiving

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arrangement of each shot. The locations of the receiver arrays (light gray line) are mirror images of the deep seismic reflection profile. Note that some of the shots are not properly located along the receiver array line due to deviations in shot positions.

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Fig. 3. Example of shot gather FFID 624 from the southern end of profile YZS-B; shot gather has been processed (e.g. static, denoise, balance, RNA and F-K Power).

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2.2 Recognition of the Moho discontinuity Compared with data acquired through conventional dynamite explosions using small quantities of explosive materials (charge < 500 kg), data obtained using large

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dynamite explosions have higher S/N, broader recoverable frequency bandwidth, higher resolution, a simpler wavelet, and a desirably continuous lateral correlation

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(Jarchow et al., 1990). Large dynamite shots can be used to differentiate the fabrics of the lower crust from the Moho (Stern et al., 2015). Reflections from the Moho can be discerned among the shot gathers (Fig. 3) from the deep seismic reflection profiling experiment of this study. The seismic Moho is defined as a first-order velocity discontinuity wherein P-wave velocities increase abruptly from crustal velocities (< 7.2 km/s) to mantle velocities (> 8.0 km/s); alternatively, seismic velocities increase gradually stepwise across the Moho from 6.5 to 7.1 km/s (Holbrook et al., 1992; Rohr et al., 1988). This remarkable seismic wave impedance boundary is commonly 8

ACCEPTED MANUSCRIPT observed on normal incidence seismic reflection profiles as an abrupt downward decrease in seismic reflectivity. Moho reflections are considered the deepest, relatively strong reflections that appear at a similar depth (i.e., time) along a transect (Carbonell et al., 2013; Cook et al., 2010; Hammer and Clowes, 1997). The crust in the obtained images is generally more reflective than the mantle. Amplitude varies

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sharply (either laterally and/or vertically) due to remarkable wave impedances between the lower crust and upper mantle.

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Notwithstanding these wave impedances, it is difficult to distinguish Moho reflections directly in the raw shot gathers. One way to highlight Moho reflections is to analyze

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amplitude variations versus two-way travel time (TWT) near the shot location using statistical methods, with different frequency bands in the shot records. However, the

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offset distance (i.e., distance between shot and receiver locations) at which the TWT for the Moho can be regarded as a zero-offset TWT (i.e., characteristic of vertical

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incidence reflection) must be determined. This can be estimated using the definition of the first Fresnel zone (Lindsey, 1989; Monk, 2010; Sheriff, 1980). The concept of

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the Fresnel zone proposes an area of influence around a seismic ray that can be

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described both in terms of the travel time to a point in the subsurface and the change in travel time associated with one-quarter of the wavelength (Monk, 2010). According to the first Fresnel zone, the acceptable distance between shot and receiver (i.e., at

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which TWT is regarded as zero-offset travel time) can be estimated using velocity, frequency, and depth of Moho (Li et al., 2017). Frequency can be measured at each

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shot gather and velocities at the Moho can also be estimated from wide-angle and other geophysical data. Using a crustal velocity of 6.0 km/s, a depth of 70 km depth for the Moho, and a dominant frequency of 15 Hz, the first Fresnel zone was determined to be located at approximately 3.741 km. The offset distance is therefore 3.741 km, within the near-vertical incidence zone.

2.3 Characteristics of the Moho in the near-vertical incidence zone Fig. 4 shows the amplitude decay curve for different shots at near-vertical incidences 9

ACCEPTED MANUSCRIPT (i.e., offset distances < 3.741 km) in different bands from profiles YZS-A and YZS-B. Tomography static corrections, amplitude recovery, and trace balancing were applied to the shot gathers to reduce the impacts of wave propagation and attenuation prior to analysis. As shown in Fig. 4, lower frequencies generally result in stronger energies. Abrupt variations in seismic amplitude occur between 22.5 – 24 s in profile YZS-A

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and 21 – 24 s in profile YZS-B. There are no significant amplitude changes beyond 24 s. Combining information regarding the Moho structure for adjacent areas from

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geophysical surveys (Kind et al., 2002; Shi et al., 2015, 2016; Yuan et al., 1997; Zhao et al., 2010) and reflection phases, Moho reflections are the deepest, relatively strong

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reflections that appear at a similar depth (i.e., time) (Carbonell et al., 2013). The depth of the Moho ranged from 67.5 to 72 km, based on an average crustal velocity of 6.0

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km/s. The effective frequency, TWT, and reported depths of the Moho for each shot

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gather are listed in Table 3.

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ACCEPTED MANUSCRIPT Fig. 4. Amplitude decay curves at near-vertical incidence in different frequency bands for profiles YZS-A and YZS-B (after amplitude recovery and trace balancing). Table 3. Two-way travel time, frequency band, and reported depths of the Moho in the

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near-vertical incidence zone for each shot gather.

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2.4 Structure of the Moho from shot gathers

It is difficult to distinguish Moho variations along the profile due to low S/N in the

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raw shot gathers. Stacking is a technique commonly applied to improve S/N (McFadden et al., 1986; Reister et al., 2001). Rather than utilizing a single trace, several traces with the same offset distance for the same shot gather are stacked; if the

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noise is random but the signal is coherent, the average noise level will be less than the

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amplitudes of the individual trace, and the stacked trace amplitude will therefore be improved. Considering the depth of the Moho and the low frequency of the signal, moveout is relatively small between neighbor traces and seismic traces are stacked

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according to the offset distance in the shot gathers (as shown in Fig. 5a). Fig. 5 illustrates stacked images according to the offset in the shot gather. The offset is 0.5

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km, indicating that ten adjacent traces are stacked into a single trace, following which the S/N of the Moho discontinuity is markedly improved. As shown in Fig. 5, the phase of the Moho in the near-vertical incidence zone is consistent with the travel times of the amplitude decay curves in Fig. 4. The spatial patterns of the Moho discontinuity in profiles YZS-A and YZS-B exhibit notable heterogeneities. The Moho discontinuity beneath YZS-A is approximately flat and less variable while that beneath YZS-B dips and changes abruptly.

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Fig. 5. Schematic diagram illustrating the process of stacking using offset distance in the shot

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gathers (Fig. 5a) and stacked images according to the offset distance in each shot gather (YZS-A,

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YZS-B). S/N of Moho reflections improves and the Moho is easily identified. The offset distance is 0.5 km (i.e., ten adjacent traces are stacked into a single trace). Red stars represent shot

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locations. Rectangular boxes indicate Moho reflections in the near-vertical incidence zone.

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2.5 Single-fold deep seismic reflection profile Seismic data processing is employed to reduce the effects of surface waves, eliminate low apparent-velocity noise from scattering effects, and enhance weak primary reflections from the lower crust and upper mantle. We used ProMAX Omega and Echos systems to process large dynamite shots following a special procedure. No deconvolution or velocity analysis were included in the procedure due to weak reflection and poor fold coverage in the lower crust and upper mantle. The focus of processing is to retrieve a high-resolution image of the Moho. The main process involves the following key steps: geometry merge, tomographic static corrections, 12

ACCEPTED MANUSCRIPT amplitude recovery and balancing, denoising (noise suppression), top muting, normal moveout correction, stacking, random noise attenuation, automatic gain control and band-pass filtering. Pre-and post-processing shots are shown in Figs. 6 and 7, respectively.

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Table 4. Large dynamite shot processing parameters (RNA: Random Noise Attenuation, F-K power: the function of F-K power is to enhance the signal in a window of seismic data. When F-K

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samples are raised to a power > 1, energy that is strong and localized in F-K space becomes even stronger. This process enhances continuous and linear seismic energy that maps to a localized

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region in FK space).

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ACCEPTED MANUSCRIPT Two single-fold deep seismic reflection profiles were constructed from the five large shot gathers. The Moho depths near shot locations are consistent with the analyses shown in Fig. 4, and the Moho depth pattern also agrees with the stacking results (Fig. 5). The blue lines in Fig. 8 mark the Moho reflections with amplitude and frequency variations in the near-vertical incidence zone. Gaps between shots at times reaching

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up to 15 s on the sections are the result of sparse shots, and long-distance offset distances. Nevertheless, the Moho is highly reflective throughout both the single-fold

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profiles.

Fig. 6. Raw and processed shots in YZS-A profile; a) and c) are raw shots (AGC (1000 ms) and bandpass filter (4-6-28-30 Hz)), b) and d) are processed shots before stacking (tomographic static correction; amplitude recovery; amplitude balance; bandpass filter; surface wave attenuation; abnormal amplitude attenuation; AGC; muting; normal moveout correction).

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Fig.7. Raw and processed shots in YZS-B profile; a) and c) are raw shots (AGC (1000 ms) and bandpass filter (4-6-28-30 Hz)), b) and d) are processed shots before stacking (tomographic static correction; amplitude recovery; amplitude balance; bandpass filter; surface wave attenuation;

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abnormal amplitude attenuation; AGC; muting; normal moveout correction).

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Fig. 8. Large dynamite shot reflection profiles. The approximate depths on the right side of the plots are estimated using an average crustal velocity of 6.0 km/s (Cook et al., 1998; Gao et al., 2013; Guo et al., 2013). The red stars show the locations of the large dynamite shots. The red line

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shows the Moho geometry along YZS-A and is taken from Gao et al. (2017).

In order to test the reliability of the Moho fabric, we designed a model (Fig. 9A)

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based on the Moho Structure from YZS-B and ran a raytracing program (Fig. 9B). We compared the traveling time of a single shot from the model and actual data; in Fig. 9C, M1 and M2 show calculated travel-time curves from the model, while the black line shows the Moho reflection from actual data. The travelling time of M1 is consistent between the model and the actual Moho reflection, although there are some issues with fit due to velocity variations above the Moho. There is an offset on the travel-time curves of M2 at a distance of 40 km due to velocity changes between M1 and M2, but this is not found in actual data; it is hard to distinguish small changes directly, owing to the low resolution of reflection. 16

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Fig. 9. A) Model of Moho geometry from YZS-B; B) Raypaths through model A (shots located at 10 km); C) Processed shot gather, the black line shows the Moho reflection based on seismic data, while M1 and M2 show calculated travel-time curves from model raytracing.

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ACCEPTED MANUSCRIPT 3. Results and Discussion 3.1 Topography of the Moho discontinuity The Moho discontinuity is the most important boundary within the Earth’s lithosphere that can be assessed using deep seismic reflection data (Hauser et al., 1987; Cook et

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al., 1992). It is distinguished by strong and continuous reflections in large dynamite

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shot reflection profiles. Our two single-fold profiles across the YZS clearly show the existence of a well-imaged Moho; however, obvious differences exist between the

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Moho structures in the two profiles. The Moho discontinuity in profile YZS-A (Fig. 7) is observed at depths between 60 and 70 km (Gao et al., 2016) and corresponds to a

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very clear but narrow band of reflections that are typically <0.4 s thick between 22 24 s. The depth of the Moho is approximately 66 – 72 km across the entire profile

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(assuming an average crustal velocity of 6 km/s), and the Moho is relatively flat without offsets. The depth of the Moho discontinuity in the southern extent of profile

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YZS-B (Fig. 7) is consistent with the deep seismic reflection data collected by Project

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INDEPTH (Zhao, et al., 1993). The reflections from the Moho are clear, with a narrow band of reflections that are typically < 0.3 s between 21 – 25 s. The depth of the Moho is approximately 63 – 75 km across the entire profile (assuming an average

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crustal velocity of 6 km/s). A gap in the Moho is observed approximately 20 km north of the YZS, the amplitude of which is < 6 km, whereas an overlapping Moho is shown

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to the south of the YZS.

3.2 Lateral variations of the Moho along the YZS The 2000 km-long east-west Himalayan-Tibetan Collision Zone is a complex tectonic setting comprised of six major tectonic domains (Yin et al, 2000). Crustal shortening estimated from geological observations within Himalayan orogeny resulting from Cenozoic deformation reaches several hundred kilometers and demonstrates distinct lateral variations (Yin, 2010). This deformation is also confirmed by differential 18

ACCEPTED MANUSCRIPT movements obtained by GPS data (Zhang et al., 2004). Many geophysical observations have provided insights into the significant lateral variations in the subduction-related mantle structure, which are rarely related to lateral variations in the Moho structure along the YZS, beneath the collision zone (Chen et al., 2009; Li et al., 2008, 2009; Zhao et al., 2010).

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While there are no major changes in surficial features between the east (YZS-B) and west (YZS-A) of the Tibetan Plateau, western Tibet is somewhat higher (5 km

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compared to 4.5 km) and significantly narrower (400 km compared to 1000 km) than eastern Tibet (Gilligan et al., 2015). Although both profiles show crustal thicknesses

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that are double the global average for continental crust, and the Moho of YZS-B is deeper than that of YZS-A, and the geometries of the Moho beneath the profiles show

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large variability. Profile YZS-A (western Himalaya) indicates a relatively flat Moho, offset related to the continent-continent collision beneath the collision zone,

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while profile YZS-B (middle Himalaya) shows a Moho structure characterized by offset and overlapping. The overall Moho geometry imaged from the two large

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of the Moho along the YZS.

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dynamite shot reflection profiles reveals big differences and confirms lateral variation

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3.3 Mechanisms for lateral variations of the Moho

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Subduction of the Indian lithosphere under Eurasia plays an important role in the tectonic evolution of the Tibetan plateau and surrounding regions. There are at least three possible geodynamic models for the collision between the Indian and Eurasian plates: (1) horizontal underthrusting of the entire Indian lithosphere beneath Tibet as an integral body, (2) subduction of the Indian lithosphere at intermediate angles similar to oceanic subduction (of either only the Indian mantle lithosphere after delamination from all or part of the Indian continental lower crust or of the entire Indian continental lithosphere), (3) detachment (break-off) of a portion of Indian lithosphere followed by vertical sinking into the deeper mantle. All three may have 19

ACCEPTED MANUSCRIPT occurred at different times during orogenic evolution, and all three may now be occurring in different locations beneath the Tibetan plateau (DeCelles et al., 2002). The seismic profile results reveal a changing Moho angle of the India-Asia collision in the east-west direction, with a steep angle to the east and a shallow angle to the west (Li et al., 2008; Liang et al., 2018; Zhao et al., 2010). We therefore infer that the

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significant lateral variations of the geometry of the Moho discontinuity from YZS-A to YZS-B due to the Indian lithosphere indicate thrusting beneath Tibet at an

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increasing angle from west to east (Li et al., 2008; Zhao et al., 2010). The angle of the subducted Indian plate gradually becomes steeper from west to east, as indicated by

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many previous geological and geophysical observations, such as body wave tomography (He et al., 2010; Li et al., 2008; Liang et al., 2012; Tilmann et al., 2003;

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Zhao et al., 2010; Zhou and Murphy, 2005), surface wave tomography (Brandon and Romanowicz, 1986; Li et al., 2013), receiver functions (Kind et al., 2002; Kosarev et

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al., 1999; Kumar et al., 2006; Nábělek et al., 2009; Yuan et al., 1997; Zhao et al., 2010), and controlled-source seismic investigations (Gao et al., 2017; Zhao et al.,

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1993). It has been suggested that the horizontal sliding distance of the Indian

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lithosphere under the Tibetan Plateau decreases from west to east (Kind and Yuan, 2010) and that the dip angle of the subducting Indian lithosphere varies laterally (Li et al., 2008; Zhou and Murphy, 2005). The latter generates variations in Moho

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topography between profiles YZS-A and YZS-B. A slab-tearing model has been proposed to explain these systematic lateral variations (Chen et al., 2015; Hou et al.,

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2006; Xiao et al., 2007). The tearing of the subducted Indian lithospheric slab is a plausible mechanism to explain the significant lateral variations of the Moho along the YZS.

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Fig. 10. Graphical interpretation of Moho structure across the YZS-A and YZS-B from large

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dynamite shots and other data. (MHT: Main Himalayan Thrust; ILM: Indian Lithospheric Mantle; TLM: Tibetan Lithospheric) Mantle.

4. Conclusions Five large dynamite shots along two profiles across the YZS provide new insight into the Moho structure adjacent to and beneath the YZS， these results are significant in terms of understanding the tectonic evolution of the region. Based on results obtained, a model is proposed to describe the differences in convergent processes (Fig. 10). We 21

ACCEPTED MANUSCRIPT propose the following preliminary conclusions: (1) Moho reflections is difficult to distinguish directly in the raw shot gathers. However, the amplitude decay curve for shots at near-vertical incidence is effective for recognizing Moho reflections. The seismic traces are stacked to enhance their S/N according to the offset distance in shot gathers variations in the Moho can

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consequently be detected. (2) Both profiles show that continental crust thickness beneath the YZS is twice that

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of global average continental crust, and the depth of the Moho YZS-B is deeper than of the YZS-A profile. YZS-A (western Himalaya) results reveal a relatively flat Moho

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without offset related to continent-continent convergence beneath the collision zone. Conversely, a Moho structure with offset and overlapping reflections was observed

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beneath profile YZS-B.

(3) The significant lateral variations of the geometry of the Moho discontinuity from

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Acknowledgments

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a different angle from west to east.

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YZS-A to YZS-B may indicate that the Indian lithosphere is thrusting beneath Tibet at

We thank Jinyi Li, Xiaohui Yuan, Zhigang Shi, Ramon Carbonell and two anonymous

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reviewers for their valuable comments on earlier versions of this manuscript. Fig. 1 was produced using the Generic Mapping Tools software package (Wessel and Smith,

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1998). This study was financially supported by the National Natural Science Foundation of China (grant numbers 41704089, 41574019, 41590863, 41430213，and 41602271).

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ACCEPTED MANUSCRIPT Highlights

- We investigate the Moho discontinuity beneath the Yarlung Zangbo Suture (YZS).

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- Continental crust thickness beneath the YZS is twice the global

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average.

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- Depth of the Moho varies across two profiles taken. - The Indian lithosphere may be thrusting beneath Tibet at

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different angles.

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