Disparate behaviour of deformation patterns beneath northeast Indian lithosphere inferred from shear wave splitting analysis

Journal Pre-proof Disparate behaviour of deformation patterns beneath northeast Indian lithosphere inferred from shear wave splitting analysis

Debasis D. Mohanty, Poulommi Mondal PII:

S0031-9201(18)30346-7

DOI:

https://doi.org/10.1016/j.pepi.2019.106315

Reference:

PEPI 106315

To appear in: Received date:

6 December 2018

Revised date:

13 September 2019

Accepted date:

13 September 2019

Please cite this article as: D.D. Mohanty and P. Mondal, Disparate behaviour of deformation patterns beneath northeast Indian lithosphere inferred from shear wave splitting analysis, (2018), https://doi.org/10.1016/j.pepi.2019.106315

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© 2018 Published by Elsevier.

Journal Pre-proof

Disparate behaviour of deformation patterns beneath northeast Indian lithosphere inferred from shear wave splitting analysis Debasis D Mohanty*, a,b and Poulommi Mondala,b a

Geosciences and Technology Division, CSIR - North East Institute of Science and Technology, Jorhat, Assam, India b Academy of Scientific and Innovative Research (AcSIR), India

Abstract

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New shear wave splitting measurements are obtained from the North East Indian region utilizing the core refracted PKS/SKS/SKKS phases, suggesting the region as more complex

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and anisotropic in nature. The splitting parameters namely time delays (δt) and fast

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polarization directions (ϕ) are computed at nine stations and the results are found to be more consistent with the geologic/tectonic structures and present mantle deformation patterns

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under this particular region. The result streamlines the deformation patterns strictly into three categories. Significant anisotropy in the western fringe of Shillong plateau and surrounding

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regions implies an asthenospheric flow related strain dominating the splitting direction inline the direction of absolute plate motion (APM) of the Indian plate in a no net reference frame

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and discards the effects from lithospheric strain. The stations in Himalayan collision zone redefines an actual deformation pattern in ENE-WSW direction in line with the maximum

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shear plane construing the Himalayan arc at the northeast corner as a consequence of N-S Indo-Eurasian collisional derived lithospheric strain. The anisotropic effect at the Himalayan foredeep section signifies the lithospheric strain induced E-W deformation from N-S continental collision. Mapping the anisotropic layer depth under these stations also strengthens our anisotropic observations and geodynamics understanding of this region with a similar thought of ideas where the deformation patterns are supposed to be dominated by huge forces like APM and lithospheric strains with negligible effects from the local geological structures. ---------------------------------------------------------------------------------------------------------------*Corresponding author: Email: [email protected] (Debasis D Mohanty)

Key Words: Seismic anisotropy, shear-wave splitting, Mantle deformation, Northeast Himalayan lithosphere.

Journal Pre-proof 1. Introduction North East India is considered as one of the most intense seismic zones of the world due to an extremely complex tectonic and geologic setup caused by the south-north and west-east movement of the Indian plate. Comprising of the Himalayan mountain belt in north, Naga Patkoi mountain range in south, Mishmi thrust in east to Dhubri fault in west, this region consists Brahmaputra plain at the middle, along with Shillong plateau, Mikir Hills, Assam valley, the Burmese arc and Tripura folded belt, etc., defining it as an extremely complex

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tectonic entity. The anisotropic nature and mantle deformation studies of Northeast (NE)

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India region hitherto remain less seismologically explored compared to the other parts of the

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Indian sub-continent. A few earlier studies [Singh et al., 2006; Hazarika et al., 2013; Roy et

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al., 2014] throw some insights regarding the anisotropic nature of the NE Indian lithosphere till date but remain unable to comprehend the full scale deformation and anisotropic patterns

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of this diverse tectonic unit. These partial studies suggest the present deformation patterns in

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NE India are governed by complex amalgamation of Indian plate motion related strain in and around of Shillong plateau and collisional tectonics derived stress in the Himalayan arc and

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its foredeep sections, though a broader picture of present mantle dynamics under the whole NE region in a high resolution scale is still missing. Very limited splitting measurements (SKS/SKKS) from this region restricts to complement the full scale ideas of lithospheric dynamics of this most complex and active seismic region, which is critically jawed in between three massive tectonic entities like Indian plate (from west), Burmese silver plate (from east) and Bengal plate (from south). Hence the present need of hour is to understand the critical tectonics and present day mantle deformation patterns, strengthen the quantity of splitting measurements to enhance the quality of geodynamics research of this region, characterization/estimation of seismic vulnerability or hazard estimation of NE India by

Journal Pre-proof understanding the present deformation patterns, etc., for which seismic anisotropic analysis and interpretation plays a significant role in present research context. Seismic anisotropic measurements serve us to image the deep structure of the mantle and causative mechanisms of deformation patterns which in term enhances our knowledge of the past and present geodynamic processes of a particular tectonic region. Seismic anisotropy in Earth’s interior is treated as a signature of numerous processes capable of causing spatiotemporal internal changes governed by huge mechanical internal flow of materials. The

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upper mantle consisting of dominant minerals such as olivine and orthopyroxene [Nicolas

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and Christensen, 1987] is the major source of azimuthal anisotropy, arises primarily from the

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strain-induced lattice preferred orientation (LPO) of these minerals with an alignment of their

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a-axis parallel to the fast polarization direction of anisotropy (ϕ). Shear wave Splitting (SWS), as a consequence of seismic anisotropy is associated with the simple shear strain of

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olivine aggregates caused by the movement of lithosphere relative to the underlying

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asthenosphere. Absolute plate motion (APM) related strain due to shear at the base of lithosphere [Vinnik et al., 1989, 1992] contributes majorly towards the LPO caused azimuthal

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anisotropy as olivine and orthopyroxene deformed by diffusion creep at depth >200km within the lithospheric or sub-lithospheric mantle [Savage 1999]. The presence of frozen anisotropy in the cool lithospheric mantle due to last significant tectonic events is also a causative mechanism for fordeep formation patterns in many cratonic regions [Silver and Chan, 1991]. Traditionally core refracted phases (SKS, SKKS, PKS) are used to quantify the azimuthal anisotropic effects in terms of fast polarization direction (ϕ) and delay time differences (δt) between fast and slow components of the shear wave, collectively known as seismic anisotropic parameters. These parameters are often used to decipher the mantle deformation patterns with a lateral resolution of less than 50 km [Savage, 1999]. The fast axis direction (ϕ) of seismic anisotropy acts as an indicator of lithospheric vertically coherent deformation

Journal Pre-proof for a fault system or mountain chains due to the sub-parallel alignment of this deformation pattern along the strike for the orogenic sutures and regional scale faults [Silver et al., 2001]. In the areas like continental rifts, the anisotropic fast axis direction is a predictionof the alignment of extensional stresses derived by melts pockets and dikes sub-parallel in direction of the rift [Gao et al., 1997, 2010; Hammond et al., 2014]. During the span of last two decades, shear wave splitting measurements and deformation analysis have become a major study to understand the geodynamics and

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tectonics of various continents [W𝑢̈ stefeld at al., 2008]. More than 1500 individual splitting

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results from around the Indian subcontinent are obtained till date using different phases like

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SKS, SKKS, PKS, direct S, etc. from various studies [Singh et al., 2006, northeast Himalaya;

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Singh et al., 2007; Sikkim Himalaya; Kumar and Singh, 2008; Indian shield; Oreshin et al., 2008, Western Himalaya; Heintz et al., 2009, Indian shield; Kumar et al., 2010, Godavari

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graben; Saikia et al., 2010, Indian shield; Mandal, 2011, northwest India; Rao et al., 2013,

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northwest Deccan volcanic province; Hazarika et al., 2013, northeast India; Roy et al., 2014, northern and northeast India; Kumar et al., 2015, southern India; Mandal, 2016, Singhbhum

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craton and Roy et al., 2016, northern part of southern granulite terrain] to decipher the anisotropic and deformation patterns of Indian mantle and crust. The current study presents first results of shear wave anisotropic measurements of northeast India from core refracted PKS phases using the data from nine (9) permanent stations operated by CSIR-National Geophysical Research Institute (NGRI) and CSIR-North East Institute of Science and Technology (NEIST), India. Along with the PKS measurements, new SKS/SKKS splitting parameters (PKS, SKS and SKKS collectively called as XKS here after) from these 9 seismic stations are also obtained to scale out the geodynamics of the region accordingly. The current study fills the gap of anisotropic measurements from PKS phases to understand the complex anisotropy, deformation and mantle flow patterns in the

Journal Pre-proof northeast part of Indian lithosphere and aims to complement the present and previous results from SK(K)S measurements to upgrade our understanding regarding complex tectonics of this region. More over the path difference between PKS and SK(K)S phases make them able to trace the anisotropic effects in a little different fashion. In the epicentral distance range of 130o to 180o, majority of the PKS phases bottom into the core and converted to S at the coremantle boundary (CMB), where the maximum of amplitude is stressed upon the horizontal components with a very less energy devoted to vertical ones, making them more suitable for

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azimuthal anisotropic effects. Contrary to this, the SKS wave fronts traverse through outer

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core as P waves. A considerable path difference in the propagation directions of PKS and

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SK(K)S waves in the mantle along with their characteristic behaviour in the core, collectively

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can attribute towards a little differences in anisotropic behaviour of materials imaged by both these core refracted phases. The present study concentrates upon the earthquakes with

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epicentral distances in between 85o to 180o, which is a best suited case for XKS splitting and

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emphasizes the changes/similarities with previous SK(K)S splitting measurements and enhances the quantity of splitting observations for a quality research on deformation patterns

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of this complex region.

2. Splitting parameter measurements The shear wave splitting analysis of core refracted XKS phases has been performed using waveforms recorded at 9 broad band seismic stations (BBS) throughout the northeast Indian region [Fig. 1]. Data from 6 permanent stations are acquired from CSIR- National Geophysical Research Institute (NGRI), India, whereas the data from rest 3 permanent stations are provided by CSIR-North East Institute of Science and Technology (NEIST), Jorhat, India. The details of seismic networks are mentioned in Table 1. The data ranges a period span of 2002-2004 based on the availability of the networks controlled by NGRI,

Journal Pre-proof whereas the data from NEIST are of the time frame from 2009 to 2010 [except for TZR in the time frame of 2002-2004]. XKS waveforms are selected from the earthquakes with magnitudes ≥ 5.5 occurred within the epicentral distance range varying in between 85o to 180o [for SKS/SKKS phases the epicentral range of 85o – 145o has maintained whereas the range of 130o – 180o is maintained for PKS splitting] [Fig. 2] to avoid any phase interferences and biases as well as to ensure that XKS phases from such distances are ahead of direct S with well recognizable arrivals and suitable for anisotropic analysis. The obtained

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waveforms are resampled to 30 Hz to avoid the aliasing effects. A butterworth band-pass

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filter of 0.04 to 0.2 Hz is used to refine the signal to noise ratio (SNR) in order to avoid the

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noise effects from splitting analysis. In very few occasions we adjusted the upper and lower

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cut off frequencies, to get the meaningful results. We select only those waveforms which have SNR ≥ 3.0. The waveforms with clear XKS phases have been chosen and visually

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inspected for the measurement purposes. To enhance the robustness of measurements, all the

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splitting measurements are performed in manually picked windows and the results are

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validated by applying several filters of different frequency bands. Splitting measurements are performed using two techniques such as rotation correlation (RC) method [Bowman and Ando, 1987] and transverse energy minimization (SC) method [Silver and Chan, 1991] implemented in the Splitlab analysis tool [W𝑢̈ stefeld et al., 2008], which is a robust graphical user interface to handle a large data set with minimizing the time of analysis and moreover uses above two techniques simultaneously to check the reliability of splitting results. RC method follows a rotation of the seismogram into a right handed LQT coordinate system and searches for the best cross-correlation between Q and T components to determine the splitting parameters, fast axis of polarization (ϕ) and delay time (δt) [Fukao,1984; Bowman and Ando, 1987]. SC method is based upon a standard procedure of estimation of splitting parameters by minimizing the energy on transverse component of

Journal Pre-proof the split waveforms. A grid search method is applied with different choices of splitting parameters and in each step an inverse splitting operator is applied to rotate and shift the unsplit waveform in time to remove the effect of anisotropy and the correction is calculated in terms of a misfit or residual transverse energy surface. The optimum pair of splitting parameters corresponds to the minimum misfit function or the minimum transverse energy surface in this grid search with a highest confidence level [Silver and Chan, 1991]. In the present study, a grid search with an increment of 1o in ϕ and 1 sec. in δt has been chosen to

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remove the effect of splitting from Q- T plane in both RC and SC methods. However the final

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results for splitting parameters are considered on the basis of minimization of transverse

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component method (SC) due to its capability of providing more stable results over a wide

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range of back azimuths [W𝑢̈ stefeld et al., 2008] compared to RC method. The non-null measurements are characterized and rated on the basis of qualities of data and results such as

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Good, Fair and Poor on the basis of error estimation.

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A2σ error estimation is considered with a close observation of XKS splitting

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measurements corresponding to a particular confidence region. For a given time series, consisting of n points, the confidence region is calculated from the transverse component 𝑢𝑇 as per the relation,

E(ϕ,𝛿𝑡) = ∑ 𝑢2𝑇 (ϕ,𝛿𝑡)

………

(Equation 1)

where ϕ and 𝛿𝑡 are fast polarization direction (FPD) and delay time respectively. The minimum value of 𝑢𝑇 (ϕ,) is considered as a 𝜒2 variable with degree of freedom n which holds true for a Gaussian white noise process. In case of rotation correlation method (RC), the correlation coefficient r is related either to the sum of squares function [Bokelmann, 1992] or transformed into a new correlation coefficient z [Fisher transformation, Fisher, 1925] following the mathematical relation:

Journal Pre-proof 𝑧 = arctan ℎ(𝑟) =

1 1+𝑟 log( )…..… 2 1−𝑟

(𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛2)

The standard deviation of z distribution 𝜎𝑧 = √1/(𝑛 − 3), where n is the number ofsample, is used to estimate 2𝜎 confidence level (𝜇𝑧) and re-projected into r-space to obtain the original 2𝜎 confidence level (𝜇𝑟), where 𝜇𝑟 = tanℎ(𝜇𝑧). In minimization energy method (SC) [Silver and Chan,1991], the error is estimated

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from the minimum second Eigen value 𝜆𝑚𝑖𝑛 2 of the covariance matrix, determined from the pair of ϕ and 𝛿𝑡 for which, energy in the transverse component of seismogram is minimized

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in the best way. Like RC method, 𝜆𝑚𝑖𝑛 is also determined from a sum of square function 2

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S(ϕ,𝛿𝑡) and approximated as having 𝜒𝜈2 distribution where the degree of freedom 𝜈 is a

≤ 1+

𝑘 (1 − 𝛼) … … … 𝑓 𝑛 − 𝑘 𝑘,𝑛−𝑘

(𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛3)

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𝜆𝑚𝑖𝑛 2

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𝜆2

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function of noise spectrum and instrument response. The confidence region is assessed from

Where f is the inverse of F probability distribution; number of estimated parameter (k) = 2 in

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the present case, confidence level 𝛼 = 0.05, corresponds to the 95% confidence region is demarcated by a grey shaded zone. In SC method, degree of freedom is 1 per second, irrespective of sampling interval and station. The 1𝜎 uncertainty is obtained from a quarter of width and length of the confidence region. Our measurements are based upon 2σ errors less than ±25o for ϕ and ±0.6s for δt, with a proper validation of linearization of the originally elliptical particle motion after correction of anisotropy in a horizontal plane. In addition to this the individual measurements with delay time errors larger than half of the delay time itself, are also discarded. To further streamline the best results, we define two parameters as ρ (= δtRC / δtSC; where RC and SC represent the measurements corresponding to Rotation Correlation (RC) and Transverse Energy

Journal Pre-proof Minimization (SC) methods respectively) and Δϕ (= ϕSC-ϕRC). In addition to above criteria, the good results satisfy if 0.8 < ρ < 1.1 and Δϕ ≤ 8o. For the ‘Fair’ results, all the above criteria for 2σ errors remain same where as ρ and Δϕ are redefined as 0.7 < ρ < 1.2 and Δϕ ≤ 15o respectively. Remaining the criteria for error as same, ‘Good Null’ measurements are characterized by 0 < ρ < 0.3 and 36o ≤ Δϕ ≤ 53o whereas ‘Fair Null’ measurements are characterized by 0 < ρ < 0.4 and 32o ≤ Δϕ ≤ 58o. All other measurements not fulfilling these above criteria are discarded and characterized as ‘poor’ measurements, which are not

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mentioned in the present study (Table 2 and Table S2, representation of all the splitting

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measurements).

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3. Results

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A total of 560 teleseismic events have been identified for the XKS splitting analysis purposes

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at these 9 broad band seismic stations (BBS). A careful and manual observation has made to visualize the waveforms, calculation of SNR and sharp phases of XKS arrivals for the same

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purpose, which restricted the waveform counts to 272 only after the scrutiny. Further wards

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the splitting measurements are characterized by visual quality inspection procedure and the results are divided into five categories such as Good, Fair, Good Null, Fair Null and Poor based on their error surface, particle motion and energy on transverse components along with the criteria mentioned in the previous section. Following such a quality check, we are left with only 38 good and fair results and 25 good null results from 63 events (details about all the teleseismic events selected after primary scrutiny and pre-processing steps for further XKS splitting analysis are mentioned in Table S1 in supplementary materials). All the individual significant splitting results (good and fair results only) are plotted in Fig 3 and these splitting details at particular stations are mentioned in Table 2 (for detail results with a comparison of measurements between RC and SC methods, refer to Table S2 in supplementary material). Though data from 9 BBS have been analysed for splitting

Journal Pre-proof measurements, only 7 stations are able to produce significant results. Responses from stations DMK and SJA are not taken into consideration and discarded due to poor quality and noisy data present in a limited narrow range of azimuthal coverage. Some examples of extremely good splitting results and null measurements are shown in Fig 4 and Fig 5 respectively. The rest results (poor and fair Null) are not discussed in this paper. The stiff scrutiny for valid results from a narrow band of epicentral range (130o to 180o, in this study for PKS splitting and 85o -145o for SKS/SKKS splitting) may be attributed towards qualifying a less number of

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significant XKS splitting measurements. The results gain support from a wide range of back

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azimuthal coverage for events analysed at seven permanent stations [TZR (5.3o), NGL

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(337.6o), RPA (217o), BKD (351.2o), JPA (354.3o), BOKO (143.9o) and TURA (150.4o)]. The

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delay time mainly ranges from 0.4s to 2.30s along with a variation in fast directions at different stations [Fig 6]. However the variations in delay times and fast polarization

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directions remain consistent for measurements at individual stations. The criteria as discussed

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in earlier section has been enforced for good and fair results and the results having minimum

retained only.

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variations in splitting parameters, even between good and fair measurements, have been

The summarisation of splitting results is represented in Fig. 6. The measurements from 7 BBS suggest a majority of delay times in between 0.5s - 1.7s along with the average fast polarization directions (FPD) for maximum measurements in between 40o – 60o, suggesting the results in well coincidence with the strain related to Absolute Plate Motion (APM) of the Indian plate in a no net rotation reference frame. A close observation of results in a scattered plot [Fig 6d,6e] suggest the splitting measurements are clustered into three major ranges of back azimuths such as 0o – 30o, 110o – 150o and 330o – 360o. For further discussion and interpretation purpose, only the good results from minimization energy method (SC) are mentioned here after.

Journal Pre-proof For stations in the western fringe of Shillong plateau, a similar trend of deformation patterns are observed with FPDs in the Northeast (NE) direction. The mean delay times at stations JPA, NGL are 1.66s and 0.86s respectively. Very new results of XKS splitting parameters are obtained at two stations TURA and BOKO. The FPDs from 13 individual measurements at these two stations [Fig 6] are in NE direction coinciding the APM direction of Indian plate, with average delay times centred at 1.49s and 1.41s respectively [Fig 7]. Station TZR in the foredeep region yields delay times in a range from of 0.40s to 1.55s with

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the mean FPD of N85oE. Further north in the sub-Himalaya, station BKD represents a delay

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time in the range 0.53s – 2.07 and mean FPD of N75oE close to the East-West (E-W)/East

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North East- West South West (ENE-WSW) trend of Main Boundary Thrust (MBT). Station well constrained XKS

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RPA situated in the Himalayan collision zone yields seven (7)

splitting results with delay times of 0.45s,0.87s, 1.4s, 1.13s, 0.5s and 0.6s. The FPDs at this

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4. Discussion:

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station are approximately coincidence with the strike direction of Himalayan arc.

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4.1 Synthesis of PKS splitting results

The FPDs or fast anisotropic directions in any area are the manifestations of the directions of major geological strikes or fabrics or the absolute plate motion direction [Silver and Chan, 1991]. The major structural trends in northeast India are in the directions of E-W, ENE-WSW, N-S, etc. While the comparatively smaller structures like Kopili fault lineament, Dudhnoi fault, etc., are striking in an north-south (N-S) direction [Nandy, 2001], the major structures like Dauki fault, Brahmaputra fault, etc. are nearly E-W in their existence. In the Himalayan collision zone, the N-S huge compression due to under thrusting of Indian mass beneath the Eurasian counterpart [Le Fort, 1975; Molnar & Tapponnier, 1975; Tapponnier et al., 1986] produces maximum shear in the E-W direction and responsible for the present day

Journal Pre-proof alignment of Main Boundary Thrust (MBT) and Main Central Thrust (MCT). As part of the eastern Himalayan collision belt, the E-W trend of MBT and MCT slightly changes to ENEWSW in the vicinity of north-eastern corner, north of 27oN latitude, as a result of variations in collisional stresses throughout this bending region [Angelier and Baruah, 2009]. The trend of directions of major geological features and sources of XKS splitting measurements assumed to be associated with the LPO of strained minerals predominantly the Olivine minerals of uppermost mantle [Silver, 1996; Christensen and Crosson, 1968]. The

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orthorhombic symmetry and anisotropic nature of olivine minerals tend to deform by

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dislocation creep under a high pressure and temperature condition in the upper mantle and

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produces a LPO effect as examined from the laboratory experiments [Ribe, 1992; Zhang and

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Karato, 1995]. Such LPO during deformation allows the a-axis of olivine crystal to be aligned in the direction of maximum shear direction, which is the direction of FPD along the

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maximum shear plane in the mantle [Holt, 2000].

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The results represented in Fig 3 show two major trends in the direction of anisotropy

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such as: northeast-southwest (NE-SW) direction in Shillong and in its western fringe regions and ENE-WSW directions in the Himalaya and its foredeep areas. Understanding these significant anisotropy and their sources in these regions has major implications in the IndiaEurasia collision. Consequence to the modifications of anisotropic characters due to active and continuous collision in this particular NE Indian region, the splitting measurements could be discriminated based upon the correlation of azimuths of fast axis with the major and local geological strike/structure directions and APM directions, etc., for understanding the present day deformation patterns and dynamics of this region. Far away from the active Indo Eurasian collision zone, 22 individual significant splitting measurements from 4 stations in the Shillong Plateau and in its western fringe resulted in NE oriented FPDs with larger delay times. The similar trend of FPDs at these 4

Journal Pre-proof stations, (JPA, TURA, BOKO and NGL) implies the major source of anisotropic contribution is from asthenospheric flow of materials under this region, contrary to the lithospheric strain, in the direction of absolute plate motion (APM) of Indian mass in a no net rotation frame. In the Himalayan collision zone and its foredeep region, the directions of anisotropy observed at two stations BKD and RPA can be attributed towards the maximum shear plane direction due to the sustained N-S collision of India and Eurasian mass. The E-W to ENE-WSW trend of Eastern Himalaya collision belt towards the north of 27o N is in well argument with the

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present computed directions of FPDs at the station RPA. Comparatively larger delay time but

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the same directional deformation is observed at BKD, in close southern proximity of RPA, at

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the edge of MBT, which can be attributed towards the same effect of maximum shear strain

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due to collisional tectonics at this region. A well coincidence of anisotropic directions of these two stations with the geologic fabric of this region clearly argues in favour of

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collisional induced lithospheric strain. The similar trend of decreasing in the magnitude of

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anisotropy i.e. delay time from indenting Indian mass towards the Asian mass and further north up to BNS is maintained, as found in earlier studies using SKS and SKKS phases [Gao

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and Liu, 2009; Singh et al., 2006]. Station TZR in the foredeep region presents a composite average significant anisotropy of 0.93sec with a fast axis direction of N85oE, suggesting a consequence of an interaction with lithospheric strain due to N-S collision of Indo-Asian plates, though influence of APM related strain may not be avoided as represented from individual results. Despite the presence of several Northwest-Southeast (NW-SE) and EastWest (E-W) local geologic and tectonic structures like Dudhnoi Fault, Dauki Fault, Brahmaputra Fault, Kopili Fault, etc., the net anisotropic effects at these seismic networks are majorly effected by the mantle flow pattern or asthenospheric mass movement produced by APM related strain and the India-Asia collision derived lithospheric strain.

Journal Pre-proof Null measurements in the present study emphasize the possibility of complex anisotropic pattern in this region. The null measurements ideally represent an isotropic mantle or presence of multiple layers of very heterogeneous layers or coincidence of initial polarization with the observed FPDs [Saltzer et al., 2000]. The present good null results [Table S2, Fig. 5] strengthens the logic for presence of most complex and multiple anisotropic layers in the study region, as the conditions for isotropic nature of NE Indian lithosphere are discarded following the significant splitting parameters at the same stations as

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discussed earlier. Null measurements in the Himalayan collision zone and its foredeep may

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be attributed towards the cancellation of anisotropy caused by APM related asthenospheric

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flow (NE-SW) and E-W compressional effects due to N-S India-Asia collision.

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4.2 Comparison and analysis of present study with previous works

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A close overlook, discussion and comparison have made for the present XKS splitting

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results with the published SK(K)S splitting parameters [Singh et al., 2006 and Roy et al., 2014] in the same area of study [Fig 7]. The main emphasize has been given upon the very

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new PKS splitting parameters, as it was not covered by any previous research and a quality analysis has carried out to understand the correlation/deviation of these results with the SKS/SKKS parameters from the present study as well as from previous published results, leading towards a robust understanding of the geodynamics of NE lithosphere. Our PKS splitting parameters are in very good correlations with the SK(K)S results at the Shillong plateau and its western fringe area. For stations having more than one splitting results from the present study, a composite measurement is taken into consideration by taking the average at that particular station. The very new XKS results at two stations BOKO and TURA produce similar trends of anisotropic effects in comparison to stations like JPA, MND and NGL, where both the XKS splitting results from this study along with SK(K)S results from

Journal Pre-proof previous study are available. A close correlation of all the results summarizes the anisotropic effect as a consequence of APM related strain of Indian plate in the Shillong and nearby regions. Two nearby stations at Tezpur (TZR and TEZ) in the Himalayan foredeep region slightly deviate in terms of splitting parameters when compared in between the PKS and SK(K)S results. SK(K)S results from the present study are absolutely similar to the previous results [TEZ, Singh et al., 2006], suggesting the near E-W FPD direction both at TZR and TEZ as a result of lithospheric strain due to N-S collision. The present study at TZR reveals

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a mean fast direction of N85oE with the delay time of 0.93s, approximately similar to

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previous published results. The contrast in between PKS and SK(K)S measurements can be

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attributed towards lateral heterogeneity beneath this station as a consequence of superposition

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of APM strain of Indian mass, E-W lithospheric strain due to N-S collisional tectonics and strike direction of Kopili lineament at the vicinity of the station. Traversing towards north, a

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good correlation between the published SK(K)S results and present XKS splitting

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measurements can be established at two BBSs, BKD and RPA, which are situated in the continental collision belt in between Indian and Asian mass. In overall, though a similar trend

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of delay time is maintained in compared to previous SK(K)S results, a slight variation in fast axis directions from E-W to ENE-WSW is observed from the present XKS splitting study. The present result can be stressed as a better correlation with the geologic/tectonic strike of Himalayan arc at the north-eastern corner of India, beyond north of 26.5o N [Angelier and Baruah, 2009], which defines the plane of maximum lithospheric strain more accurately compared to previous/published SK(K)S results. Though exact correlation between present splitting results can be established at 4 stations at the western fringe of Shillong plateau mass, the slight difference in FPDs at three stations like TZR, RPA and BKD may be attributed towards the consequence of variation in the ray paths of PKS and SK(K)S from a different cluster of events corresponding to different back azimuthal ranges. Also towards a small

Journal Pre-proof extent, the different path (PKS and SK(K)S) of waveforms in mantle and core alongside with the application of different techniques for splitting analysis compared to previous studies [Singh et al., 2006] are responsible for deviations in the results. Based on the splitting observations from station SHIO, northeast India [Vinnik et al., 1992] and HYB, south India [Chen and Özalaybey, 1998; Barruol and Hoffmann, 1999], Indian lithosphere was initially assumed to be isotropic in nature. This consideration was in

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line with those null or insignificant anisotropic measurements obtained from different

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projects like Hi-CLIMB [Chen et al., 2010]; ANTILOPE-2 [Zhao et al., 2013]; INDEPTH III [Huang et al., 2000] or central Tibet [Yin and Harrison, 2000; McNamara et al., 1994] up

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to southern Tibet. But a gradual increment in splitting delay time is evident from Indus-

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Tsangpo Suture Zone (ITSZ) to Bangong-Nujian Suture (BNS) across Lhasa Terrain; δt

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becomes prominent at BNS and further north (~ 2.0s) as suggested by INDEPTH I and II as well as Franco-Chinese expedition [Hirn et al., 1995; McNamara et al., 1994; Sandvol et al.,

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1997]. By assuming the Indian lithosphere isotropic, such higher delay time at north of BNS

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was attributed to the tectonic fabric of highly anisotropic Eurasian mantle at 33oN latitude where Indian lithospheric mantle terminates [Chen and Özalaybey, 1998; Chen et al., 2010; Sandvol et al., 1997]. On the contrary, significant splitting results from recent studies discard the concept of isotropic Indian lithosphere and clearly point towards anisotropic nature of Indian lithosphere up to the Himalaya [Singh et al., 2006, 2007; Heintz et al., 2009; Singh et al., 2015]. In a recent analysis by Singh et al., 2016, south of ITSZ produces a comparatively lower but significant delay time (~0.8s), whereas a larger delay time (> 1s) is observed for north of ITSZ. Presence of multiple layers of anisotropy in the lithospheric and sublithospheric mantle and crust was invoked as the probable cause behind such observations. Similar observation for anisotropic parameters [Gao and Liu, 2009; Zheng and Gao, 1994] also made in southern Lhasa Terrain and concluded that the region is highly anisotropic

Journal Pre-proof (delay time ~1.5s). A double layer model was considered to explain the azimuthal variation of splitting parameters [Gao and Liu, 2009] beneath the Lhasa station based on the observations of splitting parameters from XKS phases. Splitting analysis carried out in the southern part of collision zone [Singh et al., 2006] suggests that the delay time increases gradually away from collision zone, which has been also achieved by the analysis of results from this study, concentrated on sub-Himalaya, Assam foredeep and Shillong Plateau, aims to solve the more complex geodynamics beneath this region. The observed pattern of

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anisotropy from the current study (E-W/ ENE-SWS) for the Himalaya and foredeep region

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together with the geological fabric pattern in a predominant E-W strike direction indeed

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argue in the favour of N-S collision induced lithospheric strain as the primary cause of

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anisotropy, which is purely consistent with the finite deformation of Asian lithosphere due to the indentation of the Indian plate with the fast polarization directions aligned with the

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principal axis of elongation [Davis et al., 1997]. The present XKS splitting results are

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supposed to be more robust and accurate in compared to previous studies as the results are based upon two methods with a maximum correlation and the geological/tectonic structures

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are more accurately describe the direction of FPDs under the stations. 4.3 Azimuthal Dependence of Splitting parameters In general, insignificant azimuthal variation in splitting parameters is assumed to be the reflection of simple layer of anisotropy having horizontal symmetry axis. But in some complicated tectonic set-up, differences in fast polarization direction as well as delay time occur even for the same seismic phase in a particular station which is attributed to the presence of complex layer of anisotropy including double or multiple layers [Savage and Silver, 1993; Silver and Savage, 1994] or tilted transverse isotropic layer [Yang et al., 2014]. A close observation has made to analyse the dependence or variations in splitting parameters

Journal Pre-proof of XKS phases with respect to the backazimuthal distribution of events, to stabilise the cause of little differences between the PKS and SK(K)S phases from these 7 stations. Hence to decipher the reason behind such disparate results of XKS parameters at a particular station, we have categorised the station locations into two broad groups based on their tectonic setup: stations located in the geodynamically very complex and unstable Indo-Eurasia collision zone (BKD, RPA, TZR: Sub-Himalaya and Collison group) and stations in Shillong Plateau (BOKO, TURA, NGL and JPA: Shillong group) which are away from the collision zone, and

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then the backazimuthal distribution of results are analysed.

The XKS measurements in Shillong group show azimuthally invariable splitting

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parameters [Fig 8], with a mean delay time of 1.39s and a mean fast direction of 46.27o,

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which is sub-parallel to the Indian plate motion corresponding to the asthenospheric flow of

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this region. There is no incongruity in results between PKS and SK(K)S phases in Shillong Plateau since the backazimuthal coverage of events are similar here [Fig 8a]. The

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insignificant azimuthal variations of splitting parameters beneath Shillong group is suggestive

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of a single layer of anisotropy or multiple layers with similar or orthogonal fast directions, representing the anisotropic feature beneath the stations. Though a simple single layer of anisotropy is suggested, an approach towards double/multiple layer is also tested to clarify the anisotropic complexity [Figs. 8a, 8b, 8c, 8d] as discussed later. On the contrary, in SubHimalaya and collision zone, consistent fast polarization directions ranging from 32-62o (NESW) are obtained from both PKS and SK(K)S phases over a narrow back azimuth coverage 0-25o, 217-218o, 350-360o, while strikingly different results (oriented in EW to NW-SE direction) corresponding to SK(K)S phases are found from a specific backazimuth (114118o), where no PKS results are available [Fig 8, Sub-Himalaya group]. To validate the reason for these disparate results, the test for double/multiple layer cases are represented.

Journal Pre-proof At first, we tried to explain such observation by the approach of two layer anisotropy where splitting parameters show strong backazimuthal dependence with 90o periodicity. Along with backazimuthal representation, modulo-90o plots are used to identify possible π/2 periodicity which is expected for two-layer anisotropy. A grid search approach with an increment of 1o and 0.5s to find out suitable fast axis and delay time respectively for upper and lower layer corresponding to the minimum misfit function [Fontaine et al., 2007] has adopted to establish a two-layer model. The extent to which two layer model is suitable to

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describe the backazimuthal variation in Φ and δt in compared to single layer anisotropy is

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2 also quantified by two coefficients, 𝑅 2 [Walker et al., 2004] and 𝑅𝑎𝑑𝑗𝑢𝑠𝑡𝑒𝑑 [Walker et al.,

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2005], where R is the standard misfit reduction. But the best fitting solution [Table S3,

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supplementary material] in our case has very high misfit value (473.10, for Shillong and

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surrounding region; 504.63, for Sub-Himalaya and collision zone), which means this is not in agreement with the observed results for Sub-Himalayan [Fig. 8e-h] and Shillong group of

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stations [Fig. 8a-d], and hence clearly indicates that the azimuthal variation in results is not at

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all an outcome of double layer anisotropy. The above discussions and representations are in strong argument of a simple single layer anisotropy beneath the western fringe of the Shillong plateau corresponding to the concerned stations there. The anisotropy beneath this region can be attributed towards the presence of heterogeneity beneath these stations derived as a consequence of asthenospheric flow in the APM related directions, as represented from the fast directions and delay time parameters. Similarly, though the double layer prediction beneath sub-Himalaya and collision zones is avoided, the disparate behaviour between PKS and SK(K)S phases for the similar stations in this region, can be attributed towards the azimuthal variations of events corresponding to the ray path differences of both these phases. The striking differences and the corresponding anisotropy beneath collision zone and sub-Himalaya can be attributed

Journal Pre-proof towards the presence of highly lateral heterogeneity in upper mantle beneath these stations [Barruol and Ben Ismail, 2001], or the presence of multiple layers with different fast directions, in which both of the cases are so prone in an active collision zone like IndoEurasian collision. 4.4 Estimation of depth of anisotropy and geodynamical consequences Considering the advantage of near vertical incidence angle of XKS phases, shear wave

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splitting provides excellent lateral resolution of the sampled area but at the same time, it has

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very poor vertical resolution, hence, it cannot trace the depth of anisotropy in a straight

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forward way (like tomography). Moreover, such characteristics incidence angle of these seismic phases result a change in splitting parameters as a function of backazimuth, epicentral

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distance and event depth for a particular depth of anisotropy, which in turn can be inferred

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from this lateral variability [Gao et al., 2010; Liu and Gao., 2011; Gao and Liu, 2012]. The seismic wave is influenced by the properties of materials present in the vicinity of

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geometrical raypath, hence the ray is often considered as a tube. For XKS phases, diameter of

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the tube (first Fresnel zone) depends upon dominant period and depth [Alsina and Sneider, 1995; Margheriti et al., 2003; Salimbeni et al., 2008]. By discarding the possibility of double layer anisotropy, it is assumed that the splitting parameters in Shillong Plateau are governed by a single anisotropic layer. To discern its depth, the entire region is divided into square blocks having sides of 0.35o by 0.35o(where each side defined as dx=0.35o ) overlapping gridwith centres spaced at 0.05o( in an another experiment the blocks are divided into 0.3o X 0.3o overlapping grids with central distance of 0.05o between two consecutive blocks, this arrangement has performed to test the robustness and complement the depth of anisotropy determination) and then arithmetic standard deviation of delay time and circular standard deviation of fast axis are measured along with the geographic coordinate of ray piercing points inside the blocks for each depth from 0 to 400 km at an interval of 0.5 km, based on

Journal Pre-proof IASP91 earth model. The average standard deviations calculated over all the blocks produce a dimensionless variation factor (Fv) which reflects the spatial coherency of splitting parameters as a function of depth [Gao and Liu, 2012]. 𝑀𝑖

𝑁

𝑀𝑖

1 1 𝐹𝑉 = ∑ √ 𝑤𝛷 √∑(𝛷𝑖𝑗 − ̅̅̅ 𝛷і )2 + 𝑤𝛿𝑡 √∑(𝛿𝑡𝑖𝑗 − ̅̅̅̅ 𝛿𝑡і )2 … … (𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 4) 𝑁 𝑀𝑖 − 1 𝑖=1 𝑗=1 𝑗=1 ( )

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Where N is the number of blocks used in the study, 𝑀𝑖 indicates number of measurement in the ith block, 𝛷𝑖𝑗 and 𝛿𝑡𝑖𝑗 correspond to the jth fast axis and delay time in the ith block, 𝜙̅i

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̅ iare the average fast axis and delay time calculated by considering all the and 𝛿𝑡

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measurements in the ith block. 𝑤𝛷 and 𝑤𝛿𝑡 are the weighting factors of 𝛷 and 𝛿𝑡 where 𝛷 =

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1/180o (since 𝛷 varies from 0 to 180o) and 𝛿𝑡 has maximum value 2.3s hence 𝑤𝛿𝑡 is

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considered as 1/2.5s.

The optimal depth of anisotropy is reflected by the minima of variation factor. The

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smaller the difference between assumed and true depth of anisotropy, the higher is the spatial

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coherency. In the present study, splitting results obtained from stations BOKO, TURA, NGL and JPA located at Shillong Plateau yield minimum variation factor corresponding to the depth of 320km [Figure 9]. The spatial coherency depth estimator suggests the possible source of depth of anisotropy at an average of 320km below the western fringe of Shillong mass, suggesting the LPO of olivine driven asthenospheric flow as the major contributor of this observed anisotropy. Previous studies [Singh et al., 2006; Roy et al., 2014] are in support of this asthenospheric driven anisotropy mechanism and suggested the APM strains as a consequence of horizontal asthenospheric mass flow as the major controlling factor for deformation in this region which is also supported by the fast directions of SK(K)S splitting measurements. Present individual fast direction measurements of the stations from western Shillong fringe coincide with the previous results and suggestive of the subparallel direction

Journal Pre-proof to APM of Indian plate in a no net reference frame with an average FPD of 46.2 o, in the NESW direction, hence proves the deformation mechanism governed in the area as a consequence of APM related strain pertained to asthenospheric flow. The success of depth estimation from the spatial coherence is dependent upon several factors satisfying the norms as; (a). There must be spatial variations in splitting parameters; (b). The source of anisotropy must be a prediction of a single layer as the complex multiple

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layers/ dipping axes will lead to increased standard deviations with azimuthal dependence in

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splitting parameters; (c). The contributing stations are in a close spatial range compared to the predicted depth of anisotropy to satisfying the overlapping sensitive kernels from different

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ray-paths; and (d). The azimuthal coverage is a decent one. The present study satisfies all

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conditions for the stations at western fringe of Shillong plateau and hence a particular minima

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corresponding to the variation factor curves [Fig 9a] as well as systematic coherency of piercing points [Fig 9b] for the proposed depth (320km) are in well support of the depth

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estimation. The piercing points plotted at the depth 320 km for western fringe of Shillong

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plateau show a similar trend of splitting parameters. A symmetry is maintained for the piercing points plotted on the basis of events from the northern and southern azimuthal range of the station respectively [Fig. 9b], hence strengthens our understanding of the anisotropic depth.

Unfortunately, the stations at Himalayan collision zone and sub-Himalaya region

(RPA, BKD and TZR) don’t satisfy the conditions (criteria b. and d., as discussed above), showing azimuthal variations of splitting parameters and hence excluded from computation of Fυ. The present anisotropy beneath this complex subduction zone may be attributed towards the presence of highly lateral heterogeneity in upper mantle beneath these stations. Particularly for the stations, BKD and RPA, presence of a single layer of anisotropy may be discarded as per the high complexity of this region, affecting a regular mantle motion churning in very unusual cycle to restrict a single layer formation in the upper mantle. A

Journal Pre-proof consistent single layer formation, attributions from multiple layers may also be neglected as per the complex dynamics of this region, hence only supporting the huge lateral heterogeneity due to extensive N-S lateral strain corresponding to the Indo-Eurasian collision, responsible for the anisotropy in this region. In the sub-Himalaya (TZR), a consequence from E-W lithospheric strain corresponding to Indo-Eurasian N-S collision along a lateral heterogeneity may be attributed towards the mixed response of anisotropy with a fast direction of N85oE, though a possibility of superposition with the APM strain related to Indian plate motion may

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not be discarded.

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5. Conclusions

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New results of shear wave splitting of XKS phases from the northeast Indian lithosphere strengthens the understanding of complex anisotropy and deformation patterns in

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this complex, more heterogeneous and active convergent zone. The PKS results from this

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study fills the gaps constrained from the limited studies with previous SK(K)S results in this region with many new measurements of SK(K)S parameters from this study. Mantle derived

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anisotropy measurements from 9 broad band seismic stations seem to categories the deformation and anisotropy patterns throughout the north-eastern region broadly into three divisions. The major anisotropic effects in the western fringe of Shillong plateau seems to be in direction of high strain accumulation related to asthenospheric mass movement attributed towards the absolute plate motion of Indian plate in no net rotation reference frame. The deformation patterns in Himalayan collisional zone is in line with the direction of maximum shear plane due to N-S collisional tectonics between India and Asian continental mass, suggestive of governed by complex lithospheric strain. A close observation of magnitude and direction of splitting results at the Himalayan foredeep region suggests an effect of complex lithospheric strain created from collisional tectonics at Himalayan collision zone, though a

Journal Pre-proof superposition of the effects governed by absolute plate motion strain from Indian plate movement can’t be ignored also. The disparate behaviour of deformation patterns in the north eastern corner of Indian mass emphasizes the complexity in strain partitioning and larger heterogeneity of the mantle front beneath this region which is responsible for critical and highly seismically active geological set up in the world.

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Acknowledgements

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DDM acknowledges the permission and support provided by Director, CSIR-NEIST for utilization of data for publication. The National Geophysical Research Institute (NGRI)

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and Ministry of Earth Sciences (MoES) are acknowledged for providing seismological data

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for this study. This study is supported by an Early Career Research grant from Science and

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Engineering Research Board (SERB), India, vide project grant no. ECR/2018/001293. DDM acknowledges the help provided by Dr Saurabh Baruah, Geosciences Division, for making

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the data available. Authors are thankful to the Editor Prof. Vernon Cormier and all the

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reviewers for their extremely useful comments that improved the quality of the manuscript. PM is thankful to Department of Science and Technology (DST) and Academy of Scientific and Innovative Research (AcSIR) for financial (Inspire Fellowship no: IF180255) and academic supports respectively. The figures are made using Generic Mapping Tools software [Wessel and Smith, 1998] and greatly acknowledged.

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Roy, S., Kumar, M., & Srinagesh, D., 2014. Upper and lower mantle anisotropy inferred from comprehensive SKS and SKKS splitting measurements from India. Earth Planet. Sci. Lett., 392, 192-206. doi.org/10.1016/j.epsl.2014.02.012. Roy, S. K., Kumar, M. R., Bhaskar Rao, Y. J., Srijayanthi, G., Srinagesh, D., Satyanarayana, H. V., & Sarkar, D., 2016. Imprints of diverse mantle deformational episodes in the Cauvery Suture Zone, South India. Precambrian Res., 278, 207-217. doi.org/10.1016/j.precamres.2016.03.0 22. Saikia, D., Kumar, M., Singh, A., Mohan, G., & Dattatryam, R., 2010. Seismic anisotropy beneath the India continent from splitting of direct S wave. J. Geophys. Res., 115, B12315. doi.org/10.10 29/2009JB007009. Salimbeni, S., Pondrelli, S., Margheriti, L., Park, J., & Levin, V., 2008. SKS splitting measurements beneath the Northern Apennines region: A case of oblique trench retreat. Tectonophysics, 462(1-4), 68-82. doi.org/10.1016/j.tecto.2007.11.075. Sandvol, E., Ni, J., Kind, R., & Zhao, W., 1997. Seismic anisotropy beneath the southern HimalayaTibet collision zone. J. Geophys. Res., 102(B8), 17,813-17,82. doi.org/10.1029/97JB01424. Saltzer, R., Gaherty, J., & Jordan, T., 2000. How are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth? Geophys. J. Int., 141(2), 374-390. doi.org/10.1046/j.1365-246x.2000.00088.x. Savage, M., & Silver, P., 1993. Mantle deformation and tectonics: constraints from seismic anisotropy

Journal Pre-proof in the western United States. Phys. Earth Planet. Inter., 78(3-4), 207–227. doi.org/10.1016/00 31-9201(93)90156-4. Savage, M., 1999. Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting? Rev. Geophys., 37(1), 65-106. doi.org/10.1029/98RG02075. Silver, P. G., & Chan, W. W., 1991. Shear wave splitting and subcontinental mantle deformation. J. Geophys. Res., 96(B10), 16,429-16,454. doi.org/10.1029/91JB00899. Silver, P., & Savage, M., 1994. The interpretation of shear-wave splitting parameters in the presence of two anisotropic layers. Geophys. J. Int., 119(3), 949–963. doi.org/10.1111/j.1365246X.19 94.tb04027.x. Silver, P., 1996. Seismic anisotropy beneath the continents: probing the depths of geology. Annu. Rev. Earth Planet. Sci., 24, 385-432. doi.org/10.1146/annurev.earth.24.1.385.

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Singh, A., Kumar, M. R., Raju, P. S., & Ramesh, D., 2006. Shear wave anisotropy of the northeast Indian lithosphere. Geophys. Res. Lett., 33, L16302. doi.org/10.1029/2006GL026106.

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Singh, A., Eken, T., Mohanty, D. D., Saikia, D., Singh, C., & Kumar, M. R., 2016. Significant seismic anisotropy beneath southern Tibet inferred from splitting of direct S-waves. Phys. Earth Planet. Inter., 250, 1-11. doi.org/10.1016/j.pepi.2015.11.001.

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Tapponnier, P., Peltzer, G., & Armijo, R., 1986. On the mechanics of the collision between India and Asia. Geological Society Special Publication, 9, 115-157.doi.org/10.1144/GSL.SP.1986 .019.01.07. Vinnik, L., Farra, V., & Romanawicz, B., 1989. Azimuthal anisotropy in the Earth from observations of SKS at GEOSCOPE and NARS broadband stations. Bull. Seism. Soc. Am., 79(5), 1,5421,558. Vinnik, L., Makeyeva, L., Milev, A., & Usenko, A., 1992. Global patterns of azimuthal anisotropy and deformations in the continental mantle. Geophys. J. Int., 111(3), 433-447. doi.org/ 10.1111/j.1365-246X.1992.tb02102.x. Walker, K., Nyblade, A., Klemperer, S., Bokelmann, G., & Owens, T. J., 2004. On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau. J. Geophys. Res, 109, B08302. doi.org/10.1029/2003JB002866. Walker, K., Bokelmann, G., Klemperer, S., & Bock, G., 2005. Shear-wave splitting around the Eifel hotspot: Evidence for a mantle upwelling. Geophys. J. Int., 163(3), 962–980. doi.org/10.1111 /j.1365-246X.2005.02636.x. Wessel, P., & Smith, W. H., 1998. New, improved version of Generic mapping tools released. Eos, Transactions American Geophysical Union, 79(47), 579-579. doi.org/10.1029/98EO00426.

Journal Pre-proof Wüstefeld, A., Bokelmann, G., Zaroli, C., & Barruol, G., 2008. SplitLab: a shear-wave splitting environment in Matlab. Comput. Geosci., 34(5), 515-528. doi.org/10.1016/j.cageo.2007.08.00 2. Yang, B., Gao, S., Liu, K., Elsheikh, A., Lemnifi, A., Rafayee, H., & Yu, Y., 2014. Seismic anisotropy and mantle flow beneath the northern Great Plains of North America. J. Geophys. Res., 119(3), 1,971–1,985. doi.org/10.1002/2013JB010561. Yin, A., & Harrison, T., 2000. Geologic Evolution of the Himalayan-Tibetan Orogen. Annu. Rev. Earth Planet. Sci., 28(1), 211-280. doi.org/10.1146/annurev.earth.28.1.211. Zhang, S., & Karato, S., 1995. Lattice preferred orientation of olivine aggregates deformed in simple shear. Nature, 375, 774-777. doi.org/10.1038/375774a0

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Zheng, S., & Gao, Y., 1994. Azimuthal anisotropy in lithosphere on the Chinese mainland from observations of SKS at CDSN (in Chinese). ACTA Seismol. Sin., 16, 131–140.

Journal Pre-proof Figure Captions: Fig 1: Simplified tectonic and topographic map of northeast India and adjoining regions. Location of broadband seismic stations (BBS) are shown by red inverted triangles along with their name codes, whose data are used in the present shear wave splitting measurements. Inset map shows the location of study area in Indian subcontinent, demarcated by black rectangle. Major tectonic divisions are labelled in bold black letters. MBT: Main Boundary Thrust, MCT: Main Central thrust, SHL: Shillong Plateau, KF: Kopili Fault, DF: Dudhnoi Fault,

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DAUKI.F: Dauki Fault, AF: Assam Foredeep.

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Fig 2: Epicentral distribution of teleseismic earthquake events used for XKS splitting measurements. Only those events are mentioned which are utilised after primary scrutiny and

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processing steps to produce splitting measurements. Red circles represent the events and

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inserted black filled box represents the study area.

Fig 3: Representation of XKS splitting measurements at individual stations. Red solid bars

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represent the PKS measurements whereas the black bars represent the SK(K)S splitting parameters. The direction and length of the lines represent the fast polarization directions (FPD, ϕ) and delay times (δt) respectively at those particular stations. Results from SC

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method has shown only. The light brown arrows represent the APM directions in a no net rotation frame [NUVEL-1A, De Mets et al., 1994]. Blue circles represent two stations (SJA and DMK) where no good splitting results has been found due to noisy data and less data set constrained to a narrow back azimuthal range. The right panel represents rose diagrams of ϕ measurements for XKS events at the corresponding stations.

Fig 4: Examples of best XKS splitting measurements at 6 broadband seismic stations showing radial (dashed blue) and transverse (solid red) XKS waveforms and particle motion diagrams. Column 1: radial and transverse components of XKS waveforms with station names (upper left) and event date (year and Julian date, below left), phase (bottom right), Column 2: anisotropy corrected radial and transverse components after the waveforms undergone through SC technique for minimization of transverse energy in Q-T coordinate system. Back azimuth (BAZ, left below) and signal to noise ratio (SNR sc, right below) are

Journal Pre-proof shown, Column 3: particle motion diagrams for XKS before and after application of the inverse splitting operator indicated at bottom (left and right) of each box, Column 4: contour plot of energy during grid search with a minimum value indicated by shaded region corresponding to 95% confidence zone with a crisscross of two blue lines corresponding to two coordinates.

Fig 5: Examples of significant good null results derived from the present study. All the

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criteria remain same as discussed in Fig 4.

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Fig 6: Summarization of good and fair splitting results from present XKS splitting analysis. (a) Stereographic plot of XKS splitting results as function of back azimuth (BAZ) and

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epicentral distance. The solid bars represent splitting measurements for particular teleseismic

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events with the length and direction of bar denotes the delay time (δt) and FPD (Φ) respectively at a particular epicentral distance. (b) and (c) Histogram plots for FPDs (Φ) and

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delay times (δt) respectively for the present XKS splitting measurements. (d) and (e) Variation of FPDs (Φ) and delay times (δt) with BAZs respectively for various events of

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XKS phases with 2σ errors represented by vertical bars. (f) Scattering plot showing variation

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of FPDs with delay times. Only good and fair results are taken into consideration.

Fig 7: Analysis and comparison of present XKS splitting measurements (blue bars) with the previous SK(K)S studies from Singh et al. (2006) (red bars) and Roy et al. (2014) (cyan bars). Present measurements show similar fashions compared to the SK(K)S measurements. XKS measurements at RPA and BKD are supposed to be more robust and accurate towards the trend of collisional belt as compared to previous SKS results. All the abbreviations are similar as used in Fig 3. APM directions are represented by light brown arrows in a no net reference frame [NUVEL -1A, DeMets et al., 1994]. Only the composite results of particular stations are plotted in this figure.

Fig 8: Azimuthal variation of splitting parameters for Shillong Plateau and surrounding regions (upper panel) and Sub-Himalaya and Collision Zone (lower panel). (a) and (b)

Journal Pre-proof Variation of FPD (Φ) and delay time (δt) with respect to back azimuth. (c) and (d) Variation of FPD (Φ) and delay time (δt) respectively plotted against modulo 90o of back azimuth, which is calculated by using α90 = α – (n-1)*90o; where α is the back azimuth of a particular event located at nth Cartesian coordinate (which is in clockwise direction stating from upper right panel) and α90 is its corresponding modulo 90o. The blue line represents theoretical result in presence of double layers of anisotropy (Φ = 50o and δt = 1.0s for lower layer and Φ = 28o and δt = 0.5s for upper layer). 0.2 Hz is used as dominant frequency to obtain the two layers model which is not in agreement with the results obtained by splitting analysis. (e), (f), (g), (h) represent similar plots for Sub-Himalaya and Collision Zone. Here the two layers

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model yield Φ = 25o and δt = 0.5s for lower layer and Φ = 61o and δt = 1.0s for upper layer.

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The theoretical curve is not matching with the observed splitting parameters, discarding the

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presence of double layer.

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Fig 9: (a) Spatial variation factor as a function of depth of anisotropy calculated by using a

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block size of 0.30o (red circle) and 0.35o (blue diamond). The minima of the variation curve is centred around 320 km which is assumed to be the depth of anisotropy for Shillong Plateau and surrounding regions. (b) XKS splitting measurements are plotted above ray-piercing

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points at a depth of 320 km for stations located in Shillong Plateau and surrounding regions (BOKO, JPA, TURA and NGL). The length and direction of the bar represent delay time (δt)

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and FPD (Φ) respectively for each station. The solid thick bars indicate those PKS phases coming from northern azimuthal side of a station while thin bars represents PKS phases coming from southern azimuthal range of the station. Similarly, the dashed thick bars are used for SK(K)S phases coming from north and thin ones for south. The colour of the bars indicate respective stations.

Journal Pre-proof Table 1: List of Broadband seismic stations used in this study

Rupa-RPA Seijosa-SJA Bhairabkunda-BKD Tezpur-TZR Jogighopa-JPA Boko-BOKO Dokmukh-DMK Tura-TURA Nangalbibra-NGL

Lat. (deg.) 27.16 26.94 26.92 26.57 26.13 25.96 26.21 25.54 25.23

Lon (deg.)

Elevation (m)

Sensor type

Geological Unit

1489 172 174 77 46 56 37 560 320

RT-151-120B RT-151-120B RT-151-120B RT-151-120B RT-151-120B RT-151-120B RT-151-120B RT-151-120B RT-151-120B

SH SH SH AF SPS SPS MK WGH SP

92.38 93.00 92.11 92.74 90.34 91.24 93.06 90.24 90.42

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Station

Operating Institution CSIR-NGRI CSIR-NGRI CSIR-NGRI CSIR-NEIST CSIR-NGRI CSIR-NEIST CSIR-NGRI CSIR-NEIST CSIR-NGRI

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Lat: Latitude; Lon: Longitude; deg: degree; m: meter; SH: Sub-Himalaya; AF: Assam Foredeep; SPS: Shillong Plateau and surroundings, MK: Mikir Hill; WGH: West Garo Hills, Meghalaya; SP: Shillong Plateau; CSIRNEIST: CSIR-North East Institute of Science and Technology, Jorhat, Assam, India; CSIR-NGRI: CSIRNational Geophysical Research Institute, Hyderabad, India.

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Table 2: Individual splitting results of PKS, SKS, SKKS phases at particular stations Station

Event

BAZ

Phase

ϕSC

ϕerror

δtSC

δterror

(deg)

(2σ,deg)

(s)

(2σ,s)

Q

(deg)

BKD

25/08/2003 06:28

4.7

SKKS

62

1

2.07

0.08

Good

BKD

30/09/2003 14:08

117.5

SKS

123

16

0.53

0.22

Good

BKD

25/12/2003 07:11

351.2

PKS

40

8

1.53

0.27

Good

BKD

17/07/2003 19:57

24.5

PKS

43

10

1.17

0.24

Fair

BOKO

01/10/2009 06:13

101.3

SKKS

42

3

1.35

0.09

Good

BOKO

31/07/2009 10:09

143.9

PKS

45

8

0.60

0.05

Good

BOKO

13/08/2009 09:37

354.5

PKS

52

9

1.20

0.25

Good

BOKO

19/10/2009 22:49

101.2

SKS

53

3

2.30

0.15

Good

BOKO

01/08/2009 13:33

149.6

PKS

45

3

1.60

0.13

Fair

JPA

17/07/2003 19:57

22.6

SKS

56

3

1.87

0.13

Good

JPA

23/12/2002 13:46

354.5

PKS

54

5

2.07

0.31

Good

JPA

09/11/2002 00:14

2.3

SKS

42

16

0.83

0.27

Good

JPA

23/12/2003 14:02

318.5

SKKS

36

7

1.60

0.27

Good

JPA

19/05/2003 16:27

21.1

PKS

48

5

1.93

0.23

Fair

NGL

19/02/2003 00:42

144.8

PKS

41

10

1.00

0.16

Good

NGL

08/09/2002 13:15

111.4

SKS

46

16

0.40

0.15

Good

NGL

01/12/2002 07:56

102.0

SKS

42

8

1.10

0.16

Good

NGL

05/11/2003 00:58

337.6

PKS

61

8

0.93

0.19

Fair

RPA

23/12/2002 13:46

357.0

SKS

37

4

1.13

0.10

Good

RPA

15/11/2002 13:05

217.5

PKS

44

10

0.60

0.09

Good

RPA

15/11/2002 19:58

217.0

PKS

50

13

0.50

0.13

Good

RPA

05/02/2003 19:01

4.6

PKS

32

4

1.40

0.14

Good

RPA

25/08/2003 23:24

24.0

PKS

37

8

0.87

0.13

Good

RPA

12/11/2002 01:46

217.8

SKKS

52

3

0.60

0.05

Good

RPA

27/12/2003 22:38

114.8

SKS

130

10

0.45

0.09

Good

TURA

23/02/2010 15:16

2.4

SKS

38

7

1.60

0.25

Good

TURA

22/05/2009 19:24

11.5

PKS

60

12

0.60

0.20

Good

TURA

01/08/2009 13:33

150.4

PKS

27

2

2.20

0.15

Good

TURA

06/05/2010 11:35

147.7

PKS

21

2

1.00

0.08

Good

TURA

25/07/2009 19:03

116.7

SKS

45

9

1.53

0.27

Good

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(dd/mm/yyyy hh:mm)

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04/04/2010 22:40

24.1

SKS

59

7

1.50

0.28

Good

TURA

19/01/2010 14:23

348

PKS

57

4

1.83

0.18

Fair

TURA

26/11/2009 19:08

0.2

PKS

48

10

1.70

0.34

Fair

TZR

22/01/2003 02:06

21.3

SKS

58

11

1.55

0.44

Good

TZR

25/08/2003 06:28

5.3

PKS

54

7

1.15

0.18

Good

TZR

15/11/2002 13:05

217.1

PKS

41

17

0.40

0.19

Good

TZR

02/11/2003 05:32

136.8

SKS

125

6

0.73

0.08

Good

TZR

28/12/2003 23:47

115.1

SKS

147

7

0.83

0.15

Good

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ϕ - Fast Polarisation Direction; δt- Delay Time; error: 2σ errors; BAZ- Back-azimuth; Q- Quality; deg-degree; ssecond; SC- Transverse Energy Minimization Method. The results are based upon SC method only. 2σ errors are presented in both the ϕ and δt cases. Only good and fair measurements are noted here, other results are not discussed. For details of results along with RC measurements and their correlation, please refer Table S2, supplementary material.

Journal Pre-proof Highlights

 XKS (PKS/SKS/SKKS) results characterize the NE Indian lithosphere into 3 distinct anisotropic zones  Stations in Himalayan collision zone redefines deformation patterns in ENE-WSW

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 PKS splitting results fills gap in understanding deformation patterns beneath NE Indian lithosphere

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9