Mantle Transition Zones (MTZ) discontinuities beneath the Andaman Subduction Zone

Journal Pre-proofs Mantle Transition Zones (MTZ) discontinuities beneath the Andaman Subduction Zone Santosh Mishra, Srichand Prajapati, S.S. Teotia PII: DOI: Reference:

S1367-9120(19)30454-7 https://doi.org/10.1016/j.jseaes.2019.104102 JAES 104102

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

31 December 2018 17 September 2019 18 October 2019

Please cite this article as: Mishra, S., Prajapati, S., Teotia, S.S., Mantle Transition Zones (MTZ) discontinuities beneath the Andaman Subduction Zone, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/ j.jseaes.2019.104102

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Mantle Transition Zones (MTZ) discontinuities beneath the Andaman Subduction Zone Santosh Mishra1*, Srichand Prajapati2, S. S. Teotia3 1National 2Centre 3

Geophysical Research Institute (NGRI), Hyderabad, India.

for Seismic Imaging, Universiti Teknologi PETRONAS, Malaysia.

Department of Geophysics, Kurukshetra University, Kurukshetra, India.

Corresponding Author e-mail: [email protected]

Abstract We study the mantle transition zone discontinuities beneath the Andaman Subduction Zone using receiver functions generated from broadband seismic stations. The receiver functions were geographically binned using the Common Conversion Point (CCP) technique. Moveout corrected receiver functions shows that P-SV phase conversions from 410 km discontinuity arrives around ~5 to 7 seconds earlier than expected (~44 seconds). Migrating and projecting these receiver functions along a depth profile shows that the mean apparent depth of 410 and 660 km discontinuities is 357 ± 5 km and 660 ± 7 km, respectively. This translates to around ~53 ± 6 km elevation in 410 km discontinuity. We attribute this elevation to

* Currently at PETRONAS Research Sdn. Bhd. (PRSB), Exploration Technology, Group Research and Technology, Malaysia. Open

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the perturbation in isotherm caused by the young and steep subducting slab, which results in phase transformation of Olivine (α- phase) to wadsleyite (β-phase) at much earlier depth. There is no substantial topographical variation in depth of discontinuity at 660 km. We infer this flat topography of the 660 km discontinuity due to the termination of the descending slab, as evidenced in recent tomographic studies. The apparent mean MTZ thickness beneath the Andaman Subduction Zone (ASZ) is 296 ± 6 km. Key Words: The Andaman Islands, Mantle Transition Zone (MTZ), Andaman Subduction Zone (ASZ), Receiver Function, Common Conversion Point (CCP), 410 & 660 km discontinuities.

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Introduction

The Andaman Islands, represents the central part of the 5000 km long Burma-Sunda-Java subduction complex region, displaying a major tectonic-stratigraphic element striking approximately parallel to the trend of subduction Trench. They are the exposed units of an accretionary prism that marks the eastern margin of the Indian plate and forms an important transitional tectonic link between the Eastern Himalaya and the highly active Sunda arc (Fig. 1). The region attracted global attention after the occurrence of the great 2004 earthquake (Mw=9.3), and then another major earthquake (Mw=8.3) in 2005. Complexity and current tectonics beneath the region have been intriguing. Presence of a subducting slab beneath the island is undoubted, but the continuance of this slab has been a topic of debate (Richards et al., 2007).

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In the Western part of the Andaman Islands, the oceanic part of the Indian plate is subducting below the South-East Asian plate (Burmese plate). The subduction in the region is presumed to have started in the early Cretaceous along the western Sunda Arc (Scotese et al., 1988). Soft collision initiated at about 59 Ma ago, when the north eastern corner of the Indian plate hit this subduction zone (Curray, 2005). The nature of the convergence varies from continental type in the Burmese arc to oceanic type in the Andaman arc (Subrahmanyam et al., 2008, Mishra and Prajapati, 2019). Subduction slab shows variability in the dip, slab segmentation and its penetration depth from north of Sumatra (near the Andaman and Nicobar region) to the south of Sumatra. This has been clearly mapped in the recent tomographic images (Richards et al., 2007; Bijwaard et al., 1998; Shapiro et al., 2008; Replumaz et al., 2010). The geometry of slab changes dramatically at the north of Sumatra (Andaman region), where subducting Indian plate no longer exists as a single continuous slab. Recent tomographic images in the region (Replumaz et al., 2004; Pesicek et al., 2008, 2010; Liu et al., 2018) shows a steep dipping slab up to ∼500 km and presence of a major break in slab between the upper mantle and the transition zone. The presence of cold material (or anomalies) within the transition zone will manifest itself in the change of arrival timings of converted phases. The arrival times of this converted phase could help in mapping the depth of the slab penetration and understating the transition zone discontinuities. Seismic discontinuities at depth 410 km and 660 km forms a transitional zone between the upper mantle and the lower mantle. This is studied to understand the dynamics and mineral physics of the mantle. These discontinuities have been widely used to study the interaction of subducting slab (Vidale and Benz, 1992; Richards et al., 1990; Niu and Kawakatsu, 1995; Collier and Helffrich, 1997; Collier et al., 2001). The zone between these two discontinuities is termed Open

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Mantle Transition Zone (MTZ). The thickness of the MTZ constitutes an important constraint on geodynamic and geochemical models for mantle processes. These discontinuities are caused by phase transformation in the olivine component of the mantle (Bernal, 1936; Ringwood, 1969) that affects the Clapeyron slope associated with mineral phase changes. At 410 km, (Mg, Fe) 2SiO4 Olivine (α-phase) transforms into denser structure Wadsleyite (β-phase) which is sometimes also referred as modified Spinel. At 520 km, Wadsleyite transforms into Ringwoodite (γ-phase) termed as silicate Spinel. At 660 km, Ringwoodite breaks down to an assemblage of Perovskite-structured (Mg, Fe) SiO3 and (Mg, Fe) O Magnesiowstite, which marks the beginning of the lower mantle (Katsura and Ito, 1989; Ito and Takahashi, 1989). Nearly, all determinations of the Clapeyron slope for α (410 km) and β (520 km) phase transformation are positive, whereas for γ (660 km) phase transformation is negative. This results in 410 and 660 km discontinuities to be anti-correlative in nature i.e. 410 km discontinuity moves up (or elevates) in colder parts of the mantle, while 660 km discontinuity moves down during slab penetration. A recent study beneath Alaska shows an elevation in the apparent depth of 410 and 660 km discontinuities to be around 417 ± 12 km and 665 ± 12 km (Dahm et. al, 2017). A maximum topographical variation has been reported by Collier and Helffrich (1997), who observed ∼60 km upliftment of 410 km discontinuity and ∼30 km depression of the 660 km discontinuity below Izu-Bonin subduction zone.

This is the first study in this region, where we investigate the upper mantle transition zone (MTZ) discontinuities beneath the Andaman subduction zone using high-quality P-to-S receiver functions. We map the topography of 410 and 660 km discontinuities along the NS profile between 11oN and 13oN, which forms a part of highly active Sumatra Subduction Zone. To Open

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provide a reliable estimate, the receiver functions were move out corrected, geographically binned and stacked along with their piercing point locations. The results are constrained with the recent tomographic studies in the region.

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Data and Methodology

The teleseismic data utilized in this study were recorded by 10 Guralp broadband seismograph that operated in two different phases. The first phase of data was recorded from Nov-2003 to Feb-2004 and then again from Jan-2005 to May-2005. The seismographic station’s configurations included CMG-3T Guralp sensors with a flat velocity response between 0.008–50 Hz and REFTEK 130–01 data loggers. Data were continuously recorded at 20 samples and corresponding GPS time were logged. To ensure high signal-to-noise ratio, events with Mb >5.6 and epicentral distances (∆) between 30◦ to 96◦ were used to avoid any phase contamination by upper discontinuities and diffracted wave field.

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Receiver Functions

The recorded teleseismic seismograms were used to calculate receiver functions (RFs) which isolates P- to S-wave conversions (Langston, 1979; Ammon, 1991). These receiver functions were calculated by deconvolving the radial component from the vertical component of teleseismic recordings. The resulting RFs are sensitive to sharp changes (discontinuities) in shear wave velocities. This method is well established and has been widely used for studying the crust and upper mantle (Phinney, 1964; Langston, 1977; Owens et al., 1984). The deconvolution of selected teleseismic seismogram having a high S/N ratio was performed in the time domain

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using Liggoria and Ammon (1999) approach. To further control the quality of waveform, we accepted the RF waveforms with variance reduction cut-off above 80%. Out of Total 1329 RFs recorded during this period, only 205 RFs finally passed our quality criteria which are used for further analysis. To equalize the effect of variable distances, RFs were moveout corrected to a reference slowness of 6.4 s/deg corresponding to an epicentral distance of 67o (Yuan et al. 1997) using IASP91 model (Kennett and Engdahl (1991). Moveout corrected RF’s receiver function for selected stations were sorted according to their back azimuth and presented in Figure 2.

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Common Conversion Point (CCP) Binning and Stacking

To achieve a reliable depth conversion; Common Conversion Point stacking technique (Dueker and Sheehan, 1997; Owens et al., 2000; Gilbert et al., 2001) is used, which is based on geographical binning and stacking scheme. CCP technique enables the coherent P-SV conversion stacks in-phase, resulting in the contiguous feature to become clean and obvious. The incoherent random noise cancels out to improve the S/N ratio of the data. When projected along the profile, this technique can be used to track the spatial continuity and depth of P-SV conversion. This method involves computation of geographical piercing point locations. The ray path of the converted phase and its arrival time relative to P-timings is calculated using the IASP91 velocity model. The P -to- SV phase conversion for each source pair is calculated at every 2 km depth increment between 200 km and 840 km. The piercing points between 410 km and 660 km depth are shown in Figure 3. The region sampled by piercing point is discretized with 50% overlapping rectangular windows of varying width of around 1◦ to 2◦ in latitude (Fig. 3). Migrated and depth converted waveforms are then stacked and projected along the profile (AA’) across different latitudes from their corresponding bins (Fig. 4b). To produce the depth image of discontinuities, Open

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the amplitudes of the receiver functions sampling the piercing point of a particular sector (bin) are summed using equation (1): 1

𝑁

𝐴(𝑑) = 𝑁∑𝑖 = 1𝐴𝑖(𝑇𝑝𝑑𝑠)

(1)

Where A (d) is the stacking amplitude at the depth d, N is the number of receiver function piercing particular depth d. Tpds is the moveout time of the corresponding receiver function for the discontinuity at depth d. Ai is the amplitude of the ith receiver function. The stacked waveform obtained after CCP binning and stacking along profile AA’ at different latitude is presented in Figure 4b.

The uncertainty estimation is one of the important factors to consider while determining the depth of a discontinuity. In the present study, this uncertainty is estimated using the bootstrap method (Efron and Tibshirani, 1986). For the set of receiver functions in a bin, we create 20 bootstrap samples, each containing randomly chosen receiver functions with repetition. The standard deviation of the mean depths, σ410 and σ660 is calculated using:

1

𝜎𝑑 = 𝑁 ― 1∑

𝑁

(𝐷𝑖 ― 𝐷)2

𝑖=1

(2)

Where N is the number of bootstraps, Di is the depth of the 410 and 660 km discontinuities corresponding to maximum stacking amplitude from the ith bootstrap, 𝐷 is the mean depth of discontinuity in each rectangle bin. The standard deviation of the mantle transition zone (MTZ) thickness given by: 𝜎𝑀𝑇𝑍 = 𝜎2410 + 𝜎2660 Open

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(3)

We observed that the mean variation for the 410 and 660 km discontinuities are about 357 ± 5 km and 660 ± 7 km, and apparent mean thickness of Mantle Transition Zone (MTZ) is 296 ± 6 km.

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Observation

Figure 4b shows the Common Conversion point (CCP) plots of migrated RFs along strike and dip of subduction trench along the profile AA’ (Fig. 3). Positive conversions between 410 km and 660 km discontinuity can be clearly traced along the profile. The result shows an average elevation of ~53 ± 6 km in the 410 km discontinuity. We didn’t observe substantial depth variation in 660 km discontinuity, which has been reported to be depressed in other subduction zones (Richards et al., 1990; Niu and Kawakatsu, 1995; Collier et al., 2001; Collier and Helffrich, 1997; Collier et al., 2001; Takashi et al., 2005). This flat topography of 660 km discontinuity is quite contrary to global observations in other parts of subduction zones (Collier and Helffrich, 1997). To focus on the mantle transition zone, we restricted our study to image the depth of interfaces from 200 km to 850 km. The bins were chosen to have enough coverage to provide a reliable stack of data points. After stacking the waveforms from individual bins, we observe a strong coherency (bluish/green with the yellow background) and positive conversions from 410 and 660 km discontinuities (Fig. 4b). Dominant and notable apparent depth variation of around 41-72 km is observed in 410 km discontinuity, increasing southward from 11oN to 13oN.

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Results and Discussion The principal objective of this study is to map 410 and 660 km discontinuities. We observe a

mean apparent elevation of ∼53 ± 6 km in the topography of the 410 km discontinuity along the profile AA’ (Fig. 4b). This elevation can be attributed to the perturbation in isotherm due to cold descending slab penetrating beneath the Burmese plate, resulting in olivine-spinel transition at a much shallower depth (Vidale and Benz, 1992). As comparatively recent cold Indian slab subducts, the source of colder (than the normal) temperature is intruded in the mantle, resulting in lowering the mantle temperature and shifting α-β olivine phase transformation isotherm to a shallower depth. The decrease in the depth of 410 discontinuity from north to south may be attributed to variation in slab geometry, dip and slab penetration in the transition zone as observed in the recent tomographic images (Pesicek et al., 2008; Replumaz et al., 2004). Similar to the present study, Collier and Helffrich (1997) have reported an upliftment of ∼60 km in 410 km discontinuity beneath Izu Bonin subduction zone.

Generally, in the subduction zones, anti-correlative relation between 410 and 660 discontinuity exists (Collier and Helffrich, 1997; Collier et al., 2001) i.e. with the elevation of 410 discontinuity, we generally observed a depression of 660 km discontinuity (Revenaugh and Jordan, 1991; Collier and Helffrich, 1997; Collier et al., 2001; Niu and Kawakatsu, 1995, 1996; Takashi et al., 2005). However, beneath the Andaman Island, we do not observe any significant depression in 660 km discontinuity, which remains almost flat along the profile AA’. We believe one explanation for this could be, a possible detachment (tear) of slab much before reaching 600 km depth during the complex oblique seduction in this area as suggested by (Richards et al., 2007

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and Kumar et al, 2016), and hence isotherms at this depth (660 km) are not affected. This is well supported by recent tomographic images (Replumaz et al., 2004; Pesicek et al., 2008, 2010), which clearly resolves steeply dipping slab up to 500 km and then the presence of major tear (see Fig. 4a). This observation is supported by the gap in slab between ∼500 and ∼800 km depth in the tomograms (Pesicek et al., 2010 and Liu et. al, 2018). Seismicity studies by Engdahl et al., 2007, also shows no earthquake hypocenters recorded below ∼250 km in the Andaman region, whereas further south in the Sumatra Region, the hypocenters are as deep as 600 km. Other workers (Shapiro et al., 2008; Kennett and Cummins, 2005; Bijwaard et al., 1998; Voo et al., 1999, Mishra and Prajapati, 2019) have also discussed the complexity, dip variability and segmentation in slab geometry as a move from north to south along the Andaman – Sumatra arc.

In such an area with complex convergence, steep dip, high anisotropy, and increasing obliquity there always exist an inherent ambiguity in the interpretation. We have tried to overcome these challenges by following a systematic approach in data processing and interpretation. Firstly, by sorting the raw receiver function according to their increasing back azimuth, then migrating them to the common delta, and then by dividing them into different azimuthal sectors to avoid ray multi pathing effect. We then project these migrated RFs along profile AA’ to see the continuity and spatial extent of the discontinuities. Finally, stacking the depth converted RF’s them from their corresponding bins provided us a reliable estimate of 410 km and 660 km discontinuities (Fig. 4b).

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Conclusion

The transition zone, between 410 and 660 km, is an excellent region to study the slab interaction with the transition zone discontinuities. In this study, we observe an elevation of the 410 km discontinuity of ~ 41 and 72 km (between 13o N and 11o N latitude) beneath the Andaman Subduction Zone. The mean apparent elevation in 410 km discontinuity is ∼53 ± 6 km. This elevation increases southward, having the maximum uplift at 11o N. We attribute this elevation to young (cold) Indian slab subducting steeply beneath the Burmese plate, which causes the perturbation in thermal anomalies, resulting in Olivine (α-phase) transformation much earlier than expected. There is no significant topographical variation in 660 km discontinuity, as it is not affected by the slab penetration which detaches itself detaches much before 600 km discontinuity. Observations from our study are well supported by recent tomographic images (Pesicek et al., 2008; Replumaz et al., 2004).

It could be concluded that in the Andaman Nicobar region, the geometry and dip of the slab changes dramatically. It undergoes multiple segmentation in both horizontal and vertical direction during its complex oblique subduction (Shapiro et al., 2008; Kennett and Cummins, 2005; Bijwaard et al., 1998; Voo et al., 1999, Mishra and Prajapati, 2019). Considering the complexity of the region, large scale multi-parametric observation and detailed seismic data acquisition are required to better image the 3-D structures of the subducting slab, which is a vital approach to better understand the deep subduction processes in the region.

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Acknowledgment

The research was supported by the Department of Science and Technology (now Ministry of Earth Sciences), New Delhi. Special thanks to Jeremy Pesicek (University of Wisconsin-Madison, U.S.A) for sharing his (Pesicek et al., 2010) tomography results. We are thankful to two anonymous reviewers for their constructive comments to improve the manuscript. Thanks to the associate editor Dapeng Zhao for their comments and valuable suggestions.

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Figure 1: Simplified regional tectonic framework of the Burma and Andaman arc-trench system, superimposed on GEBCO bathymetry and SRTM data. Andaman backarc spreading centre (ABSC), Andaman Island (AI), Nicobar Island (NI), Andaman Subduction Zone (ASZ), West Andaman Fault (WAF), Sagaing Fault system (SgF), Sumatra fault system (SFS), Seulimeum strand of SFS (SEU) (after Curray (2005)). Black inverted filled triangles are Narcondam (NI) and Barren (BI) island volcanoes. Red (larger) star is the December 2004 Tsunami generating earthquake and smaller Red star is March 2005 aftershock. BB’ is tomography profile by Pesicek et al. (2008) shown in Figure 4 (a).

Figure 2: Moveout corrected receiver functions plotted according to their backazimuth for selected stations, top box shows stacked waveform. Solid black lines show the global times for 410 and 660 discontinuities.

Figure 3: Piercing points of P-SV phases at 410 and 660 km depth plotted for the study region along with topography and bathymetry data. Profile AA0 was used for Common Conversion Point (CCP) binning and stacking of RFs from their corresponding bins. The solid triangle is the seismological stations used for the present study.

Figure 4: (a) Tomography depth cross-section along the Andaman region (from (Pesicek et al., 2008)). (b) Common Conversion Point (CCP) binned and stacked receiver functions from their corresponding bins at a different latitude, dash line is the 1σ error bounds. (c, d) Bootstrap histograms for the error analysis of 410, 660 km discontinuities.

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Highlights We investigate the upper mantle transition zone (MTZ) discontinuities beneath the Andaman subduction zone using high-quality P-to-S receiver functions. Receiver functions and Common Conversion Point (CCP) technique results shows that mean apparent elevation of ∼53 ± 6 km in the topography of the 410 km discontinuity is observed beneath Andaman Subduction Zone. There is no substantial topographical variation in the topography of 660 km discontinuity. The geometry and dip of the subducting slab varies abruptly and undergoes multiple segmentation during its complex oblique subduction beneath the Andaman Subduction Zone.

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Graphical abstract

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