Localized Foundering of Indian Lower Crust in the India–Tibet Collision Zone

Localized foundering of Indian lower crust in the India–Tibet collision zone

Danian Shia,1, Simon L. Klempererb,1, Jianyu Shia,c, Zhenhan Wua, and Wenjin Zhaoa

aChinese Academy of Geological Sciences, 100037 Beijing, China; bDepartment of Geophysics, Stanford University, Stanford, CA 94305-2215; and cNational Marine Environmental Forecasting Center, 100081 Beijing, China

Edited by Barbara A. Romanowicz, University of California, Berkeley, CA, and approved August 27, 2020 (received for review January 1, 2020) The deep structure of the continental collision between India and 3TD broadband seismometers. Most of them operated for ∼12 Asia and whether India’s lower crust is underplated beneath Tibet mo. These stations fill the gaps between the Himalayan-Tibetan or subducted into the mantle remain controversial. It is also un- Continental Lithosphere During Mountain Building (Hi-CLIMB) known whether the active normal faults that facilitate orogen- (7), Himalayan Nepal Tibet Seismic Experiment (6), International parallel extension of Tibetan upper crust continue into the lower Deep Profiling of Tibet and the Himalaya (5), and Namche Barwa crust and upper mantle. Our receiver-function images collected (8) passive seismic experiments, and provide much improved parallel to the India–Tibet collision zone show the 20-km-thick In- coverage for the India–Tibet collision zone (Fig. 1). dian lower crust that underplates Tibet at 88.5–92°E beneath the Here we present results from two high-resolution receiver- Yarlung-Zangbo suture is essentially absent in the vicinity of the function (RF) profiles (Figs. 2 and 3) paralleling the India–Tibet Cona-Sangri and Pumqu-Xainza grabens, demonstrating a clear collision zone, constructed with data we recorded from 2015 to link between upper-crustal and lower-crustal thinning. Satellite 2019 on two densely spaced 800- and 600-km-long linear arrays. gravity data that covary with the thickness of Indian lower crust These arrays extend east–west from the eastern Himalayan syn- ∼ are consistent with the lower crust being only 30% eclogitized so taxis to the Tangra-Yumco graben (TYG) in the central-western gravitationally stable. Deep earthquakes coincide with Moho offsets Gangdese belt, with Profile B along the Yarlung Zangbo suture and with lateral thinning of the Indian lower crust near the bottom (YZS) and Profile A located ∼75 km north (Fig. 1). of the partially eclogitized Indian lower crust, suggesting the Indian Previous RF profiles south to north across the collision zone lower crust is locally foundering or stoping into the mantle. Loss of identified an RF “doublet” interpreted as representing the top Indian lower crust by these means implies gravitational instability EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES and bottom (Moho) of Indian lower crust (5–10, 16), allowing us that can result from localized rapid eclogitization enabled by dehy- to tie our observations to the previous interpretations (Fig. 3 and dration reactions in weakly hydrous mafic granulites or by volatile- SI Appendix, Fig. S1A). The (Indian) crust–mantle boundary (or rich asthenospheric upwelling directly beneath the two grabens. We propose that two competing processes, plateau formation by under- Moho), is the deepest prominent positive conversion and is seen along most of both profiles (“m” in Figs. 2 and 3). The Moho plating and continental loss by foundering or stoping, are simulta- – neously operating beneath the collision zone. converter is at 65 70-km depth at the west end of Profile B (Fig. 3) as previously determined along the Hi-CLIMB profile at ∼ ∼ –– Tibetan plateau | continental collision zone | Indian lithosphere | receiver 85°E (7, 17), then deepens dramatically to 90 km the largest –– functions | crustal thinning depth in the study region at 87.5°E. The Moho conversion is absent immediately west of the Pumqu-Xainza graben (PXG), nowledge of the structure of the Himalayan collision zone is Kthe key for understanding all geological processes––geodynamics, Significance tectonics, magmatism, metallogeny––occurring during continental collision. However, the lack of constraints on the geometry and Although plate tectonics explains subduction of oceanic crust, extent of the Indian plate beneath the Tibetan plateau has fueled the areally distributed deformation observed in continental speculation whether the Tibetan plateau was built by underthrusting collision requires more complex explanations of the geometric of Indian crust beneath the entire plateau (1), by discrete subduction interaction of two colliding continents and the mechanics of events between coherent lithospheric blocks (2), or by homogeneous plateau formation. Our seismic images reveal variations of the – thickening of the lithosphere of the entire plateau (3) and wide- India Tibet collision parallel to the Himalaya. Our observation spread viscous flow in the crust (4). Knowledge of the collision zone of localized thinning of Indian lower crust, that is sufficiently has been improved by seismic studies in the past two decades (5–11), extreme as to require material loss presumably into the underlying mantle, is an advance in explaining the apparent lack which suggest that Indian lower crust attached to lithospheric mantle – has underplated to ∼31°N (7), that a west–east transition from of mass balance in the India Asia collision that requires loss of underplating to steep subduction of the Indian plate has occurred in continental material. These zones of lower-crustal thinning and likely foundering represent slab segmentation (slab tears) the central–eastern collision zone (9, 10), and that a large portion of that can geometrically accommodate the curvature of the the Indian crust has been transferred from the lower plate to the Himalayan arc. upper plate via crustal-scale duplexing (11). However, the amount of ∼ ∼ underplating ( 400 km from the Main Frontal Thrust to 31°N Author contributions: D.S., S.L.K., Z.W., and W.Z. designed research; D.S., S.L.K., and J.S. (Fig. 1 and ref. 7) and crustal-scale duplexing accounts for only performed research; D.S. and J.S. contributed new reagents/analytic tools; D.S., S.L.K., and ∼50 ± 17% of the Indian crust underthrust beneath southern Tibet J.S. analyzed data; and D.S. and S.L.K. wrote the paper. since onset of the India–Asia collision (12). The fate of the Indian The authors declare no competing interest. crust (especially the lower crust) beneath southern Tibet during ∼57 This article is a PNAS Direct Submission. My since collision remains largely speculative. Published under the PNAS license. 1 – To whom correspondence may be addressed. Email: [email protected] or sklemp@ Receiver Function Imaging of the Himalaya Tibet Collision stanford.edu. Zone This article contains supporting information online at https://www.pnas.org/lookup/suppl/ Since summer 2011 we have deployed 411 seismic stations in doi:10.1073/pnas.2000015117/-/DCSupplemental. southern Tibet, equipped with either Guralp 3ESPCD or Guralp

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Fig. 1. Topographic map of southern Tibet showing location of seismometers. Yellow-filled diamonds were used to construct RF Profiles A and B in Figs. 2 and 3, and blue diamonds mark our Gangdese seismic stations deployed from 2011 to 2019 (9, 10). Black diamonds indicate other seismic stations (5–8, 13). Dotted cyan line: northern limit of underthrust ILC (mantle suture) (7, 8, 10). Cyan rectangles: locations of maximum thinning of ILC on Profile B, and likely correlative points on Profile A as interpreted on Fig. 3. Dashed red lines: Yarlung-Zangbo (YZS) and Banggong-Nujiang (BNS) sutures. Solid red lines: MFT, dextral Jiali fault (JF), and normal faults in the Tangra-Yumco (TYG), Pumqu-Xainza (PXG), Yadong-Gulu (YGG), and Cona-Sangri (CSG) grabens (14). Cyan quadrant circles: focal mechanisms of earthquakes with epicentral depths ≥50 km (SI Appendix, Fig. S4) verified by waveform modeling (15). (Inset)Map shows full extent of the Tibetan plateau.

whether due to difficulty in imaging a steep structure or due to Profile B, two depressions in the top-ILC converter are obvious, the lack of a seismic impedance contrast, and reappears at <65-km of up to 5 km west of PXG and of up to 10 km beneath CSG. In depth just 30 km further east. From the PXG east to the Cona- these regions and at the western and eastern limits of our profiles Sangri graben (CSG) the Moho on Profile B is subhorizontal at the top-ILC converter is arguably too thin to be resolved from ∼75 km depth for ∼280 km between ∼89° and ∼91.5°E. A second the Moho as a separate converter, and our data are easily modeled Moho disruption occurs beneath the CSG (see ref. 10), then the by a single converter (SI Appendix,Fig.S2C). At the largest scale Moho shallows gently to only ∼55 km in the vicinity of the eastern from west to east the top ILC appears to mirror the Moho Himalayan syntaxis at the east end of Profile B. Profile A shows (i.e., deeper [shallower] when the Moho is shallower [deeper]), the same two zones of Moho disruption as Profile B, less clearly although locally, where best observed from ∼91.5–89°E, it is clear beneath the PXG where data coverage is incomplete, but more that the top and bottom of the doublet both dip north, both obvious at 91.5–92°E where the Moho conversion is locally absent deepening ∼5 km from beneath Profile B to beneath Profile A. (see ref. 10), ∼50 km west of the northern mapped extent of the Common conversion-point (CCP) images of the P-RF multi- CSG (18). On Profile A the two zones of Moho disruption bound ple phases, and of the S-RFs (SI Appendix, Fig. S1 C and D), a subhorizontal section of Moho, just as on Profile B, that on A is corroborate our interpretations of the simple P CCP images narrower (spans only ∼220 km from west to east) and a few ki- shown in Figs. 2 and 3. lometers deeper (implying ∼5° northward dip). The top of the RF doublet, normally interpreted as the top of Gravity Characteristics of the Indian Lower Crust in the (Indian) lower crust (ILC) (5–10, 16), is the prominent positive Collision Zone conversion lying above the Moho. It is well-observed along both Previous work not only identified the doublet layer between “d” of our profiles (marked “d” in Figs. 2 and 3) especially where the and “m,” but also showed that portions of the ILC >300–350 km doublet reaches ∼20-km thickness from ∼89–91.5°E in the region north of the Main Frontal Thrust (MFT), somewhat north of the where it was first identified (5). Immediately west and east on YZS, have density and wavespeed characteristic of partially

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Fig. 2. RF images with no vertical exaggeration, showing the principal contrasts within the entire crust and uppermost mantle along Profiles A and B. Red and blue colors represent interfaces with increasing and decreasing impedance with depth, respectively. The “d” indicates the doublet conversion, or the conversion from the top of the ILC (5–8). The “m” denotes the conversion from the Indian crust–mantle boundary (or Moho). Focal mechanisms from 28.5–30.5°N (15) in A and B are projected onto the profiles along the strike of nearby grabens (SI Appendix, Table S3). Depths are referred to sea level. Thin gray lines at 65-km depth are for reference. Representative RFs (stacked every 5° of back-azimuth with time transformed to depth) are shown in C. Note that the Moho has eluded imaging beneath the station YS91 for some northern azimuthal arrivals (where marked “?”), suggesting the thinning of the ILC may have also occurred on the northern side of the station. (Lower Right) Small blue and green crosses show the piercing points at 65-km depth of all RFs used in Profiles A and B, respectively. (Inset) Maps show the corresponding piercing points for the three representative stations (YS91, YS78, and LM47) identified in A and B and shown in C.

eclogitized mafic crust (7, 16, 17). We next use satellite gravity using constant density contrasts at the top and bottom of the ICL. observations (SI Appendix, Fig. S3) to test this hypothesis. In Because our focus is our seismic data we prefer to show an ac- opposition to simple expectations of lower gravity over thicker ceptable gravity fit from a simple model rather than a perfect fit crust, the relative gravity anomaly reaches +50 mGal (+70 mGal from a very complex model. Our modeling approach and result on Profile B) from ∼89–91.5°E where the Moho is significantly agrees with previous models (16) that permit gravitationally stable deeper than adjacent areas (Fig. 3, black line; SI Appendix, Fig. but chemically metastable mafic ILC beneath Profiles A and B, S3D). Simple modeling with a range of densities finds acceptable incompletely eclogitized but with the potential to rapidly chemi- fits if the doublet layer has a density ∼3,000–3,100 kg/m3 (Fig. 3; cally equilibrate leading to gravitational instability. rms misfit ∼24 mGal, SI Appendix, Table S2), clearly higher than the density of granulitic rocks (2,850 kg/m3, rms misfit = 34 mGal), Localized Foundering of the ILC but lower than the density of the mantle (3,300 kg/m3, rms misfit = Southern Tibet stands out globally for its unusual lower-crustal 34 mGal), or mafic eclogite (3,450 kg/m3) (values taken from ref. and upper-mantle earthquakes (15, 21, 22) (Figs. 1 and 2 and SI 16). The average density of the ILC along Profiles A and B thus Appendix, Fig. S4). These earthquakes require a strong litho- matches mafic granulites with ∼30% eclogitization (6), or meta- spheric mantle (15, 21) capable of brittle failure, and may be stable gabbro (16, 19, 20). We stress that we have modeled the associated with eclogitization of the lower crust (23). When we gravity anomaly from a smooth interpretation of the RF image project earthquakes with depths well-determined by waveform

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Fig. 3. (A and B) show RF profiles with 5× vertical exaggeration, and gravity data and models. Superimposed on RF profiles are approximate top (green lines) and bottom (black lines) of the ILC as used for gravity data modeling. Stars are published depths to top ILC and Moho at ∼85°E (7), ∼87°E (6), ∼90°E (5), and ∼93°E (8). Cyan rectangles on Profile B show locations of maximum thinning of ILC, and on Profile A our interpreted correlative points. Thin gray lines are Moho depths from the CRUST1.0 model. Black curves above each RF profile are relative satellite gravity anomalies at 8,500-m altitude (SI Appendix, Fig. S3D), and other colors are modeled anomalies for different hypothetical densities of the ILC (brown: 2,850 kg/m3, same as overlying crust; thick green: best fit with 3,050 kg/m3; blue: 3,300 kg/m3, same as underlying mantle). (C) Interpretive geologic cartoon along Profile B. Solid lines: clearly observed interfaces; dashed lines: inferred boundaries. Pink lines: proposed direct contact of Indian middle/lower crust with the asthenosphere or the LAB (lithosphere-asthenosphere boundary). Dashed black lines: interpreted Main Himalayan Thrust (MHT) where not following top of ILC. Arrows show relative motions. Focal mechanisms (15) for earthquakes 28.5–30.5°N are projected onto the profile along-strike of nearby grabens (SI Appendix, Table S3). Depths are relative to sea level. Nomogram in A (87–88°E) shows appearance of true dips of 0–20° after 5× vertical exaggeration as used throughout this figure.

modeling (15) to be ≥50 km, along-strike of the grabens onto our shear-wavespeed zones (Fig. 4 and SI Appendix, Figs. S6 and S8), profiles (SI Appendix, Table S3), the earthquakes cluster in two we suggest they enable eclogitization (23) and stoping of foun- small areas where the ILC becomes thinner and the Moho is less dering of newly densified ILC into the mantle (Figs. 3C and 4) as well-imaged (we suggest due to strong crust–mantle interaction the mechanism of this mass removal. Rapid eclogitization to ex- producing a diffuse Moho) (Figs. 3C and 2). These earthquakes ceed the mantle density may be enabled by dehydration reactions show a consistent pattern of east–west extension so are likely in weakly hydrous mafic granulites (16) or by volatile-rich as- related to the rifting process that produces PXG and CSG. thenospheric upwelling directly beneath the rifts (25). The absence However, the degree of thinning observed in the ILC in our of the Indian Moho converter beneath the CSG and PXG could profiles, locally >60% beneath PXG and CSG, and regionally then be attributed to local juxtaposition of eclogitized ILC against [>15% where confidently observed and independently corrobo- asthenospheric mantle with similar seismic impedance (Fig. 3C). rated from 87° (6) to 93°E (8)], far exceeds the 3–6% extension Loss of dense lower-crust foundering into the mantle requires observed at the surface (24). Hence, in addition to focused ex- replacement by an equal volume of material. We speculate that tension of the ILC achieved by decoupling between upper and this replacement is by mantle flow from below (to create observed lower crust (13), material must be physically removed from the Moho shallowing) but also by channel flow of the weakest crustal ILC to produce the observed thinning. Because the earthquakes layer, felsic/intermediate Tibetan middle crust (26), into the re- occur close to the bottom of the ILC and the upper-mantle low gion of delamination, thereby deepening the top of the ILC.

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.2000015117 Shi et al. Downloaded by guest on September 29, 2021 Localized foundering of Indian lower crust into upwelling as- crustal delamination or foundering (25) thereby weakening the thenospheric mantle beneath the CSG and the PXG is consistent crust and focusing surface extension (Fig. 4). The existence of a with the observation in these rifts of primordial mantle 3He in thermal decoupling midcrustal channel means that surface extension is springs that is a signature of incipiently melting (i.e., asthenospheric) not required to be vertically above the foundering lower crust (13). mantle directly beneath the crust (27, 28). Adjoint-tomographic im- Such non-coaxial rifting can explain both the presence of Moho ages suggest lithospheric foundering of high-wavespeed upper-mantle disruption at ∼91.5°E on Profile A ∼50 km west of the CSG (10), “blobs” as small as ∼300 km across, north of the YZS at 88° and 92° and the absence of Moho disruption beneath the YGG on either (29), but lack the resolution to show smaller features. Our own travel- profile. We note that the lateral extent of regions of shallow Moho time tomographic models show ∼100-km length-scale high-velocity and thinned ILC are broader west-to-east on Profile A than on our bodies beneath the two grabens (Fig. 4 and SI Appendix,Figs.S6 southern Profile B. Tomographic images suggest this may be asso- and S8) provide additional support for localized foundering of the ciated with processes at the northern limit of the ILC at the Moho, underthrusting Indian continental lithosphere. where the ILC likely subducts (7, 9, 10) rather than underthrusts The inefficient conversion at the top of ILC beneath the YZS Tibetan crust, allowing Tibetan (noncratonic) mantle at the Moho SI Appendix near the west ends of our profiles (Figs. 2 and 3 and refs. 7, 17) to infiltrate above ILC (Fig. 4 and ,Figs.S6andS8). lies directly above low-wavespeed anomalies in the upper mantle Our results suggest that two competing processes, under- (Fig. 4 and SI Appendix, Figs. S6 and S8 and ref. 30), therefore is plating and localized foundering, are operating simultaneously in likely causally linked to in situ high mantle temperature, which the collision zone, with the former maintaining or increasing crustal may have suppressed eclogitization (19) in the ILC and thereby thickness and the latter reducing crustal thickness. The thickness and northern limit of ILC underplating southern Tibet is the net decreased the seismic impedance contrast at its top boundary. result of these opposing processes. The zones of lower-crustal Non-coaxial Rifting Deformation in Southern Tibet foundering represent slab segmentation (slab tears) that can geometrically accommodate the curvature of the Himalayan arc. Delamination of the lower lithosphere (25) and channel flow in – the middle crust (13) have both been linked to west east ex- Materials and Methods tension of Tibet manifested at the surface by the NNE-trending We focus our images on lower-crustal and upper-mantle structures (Fig. 3). extensional grabens in southern Tibet. Our two regions of steeply We obtained the images using 12974 P-wave RFs derived from 168 seismic dipping and disrupted Moho along Profile B are broadly beneath stations (yellow-filled diamonds in Fig. 1) and the CCP stacking method. The the CSG and PXG (Fig. 1) but because the widths of disruption RFs were calculated with the time-domain iterative deconvolution technique are comparable with the rift spacing in southern Tibet and be- (33), in which the Gaussian filter factor was set to 1.5 to mitigate high- cause we see no evidence of disruption beneath the Yadong- frequency conversions from the upper crust and retain signal from the lower EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Gulu graben (YGG) or TYG (Fig. 3) we have insufficient data crust and upper mantle. All of the RFs were migrated from time to depth by to prove a direct link between surface faulting and Moho faulting tracing the rays from the location of each seismic station through a layered reference model (SI Appendix,TableS1). No lateral or vertical smoothing was (13). Nonetheless the two foci of crustal thinning on Profile B applied to the images, to best image east–west variability. We also used tele- are so close to the CSG and PXG that we speculate their origins seismic travel-time tomography to study upper-mantle processes associated are linked. Tearing of subducting Indian mantle lithosphere (31) with the localized foundering of ILC (Fig. 4 and SI Appendix,Figs.S5–S9). In along inherited Indian basement faults (32) could focus lower- total, 21,731 S-wave (S, sS, ScS, SKS, and SKKS) arrival times from 1,128

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Fig. 4. Interpretive cartoon (with exaggerated topography) of processes currently operating in the India–Tibet collision zone. Beneath the Moho we show mantle S-wave velocity structure from our teleseismic tomography (SI Appendix, Figs. S6 and S8). ILC is simultaneously underplating and foundering beneath the CSG and PXG grabens, and foundering then subducting at the northern end of the collision zone. West–east cuts in the three-dimensional block model are along our Profiles A and B. South–north cut along 85°E from ref. 7. Arrows are inferred directions of lithospheric foundering (dark blue) and asthenospheric upwelling (purple) in the upper mantle.

Shi et al. PNAS Latest Articles | 5of6 Downloaded by guest on September 29, 2021 earthquakes and 725 seismic stations were used to invert the upper-mantle ACKNOWLEDGMENTS. This study is supported by the China National Natural S-wave velocity structure. More details are given in SI Appendix, Text. Science Foundation Grants (41674099 and 41374109), the Chinese Geological Survey Grants (1212011220903 and 1212011120185), and the US NSF Grant (EAR1628282). We thank Jim Mechie for sharing his ideas and Eric Sandvol Data Availability. All RF and travel-time data used in this study are available and two anonymous reviewers for comments that have greatly improved the (34) (https://purl.stanford.edu/jz904sc7304). manuscript.

1. E. Argand, La tectonique de l’Asie. Int. Geol. Congr. Rep. Sess. 13, 170–372 (1924). 20. M. Jull, P. B. Kelemen, On the conditions for lower-crustal convective instability. 2. P. Tapponnier et al., Oblique stepwise rise and growth of the Tibet plateau. Science J. Geophys. Res. 106, 6423–6446 (2001). 294, 1671–1677 (2001). 21. V. Schulte-Pelkum et al., Mantle earthquakes in the Himalayan collision zone. Geol- 3. P. England, G. Houseman, Extension during continental convergence, with applica- ogy 47,1–5, https://doi.org/10.1130/G46378.1 (2019). tion to the Tibetan Plateau. J. Geophys. Res. 94, 17561–17579 (1989). 22. K. Priestley, J. Jackson, D. McKenzie, Lithospheric structure and deep earthquakes 4. M. K. Clark, L. H. Royden, Topographic ooze: Building the eastern margin of Tibet by beneath India, the Himalaya and southern Tibet. Geophys. J. Int. 172, 345–362 (2008). – lower crustal flow. Geology 28, 703 706 (2000). 23. B. Jamtveit, Y. Ben-Zion, F. Renard, H. Austrheim, Earthquake-induced transformation 5. R. Kind et al., Seismic images of crust and upper mantle beneath Tibet: Evidence for of the lower crust. Nature 556, 487–491 (2018). – Eurasian plate subduction. Science 298, 1219 1221 (2002). 24. S. Bian et al., Late Pliocene onset of the Cona rift, eastern Himalaya, confirms east- 6. V. Schulte-Pelkum et al., Imaging the Indian subcontinent beneath the Himalaya. ward propagation of extension in Himalayan-Tibetan orogen. Earth Planet. Sci. Lett. Nature 435, 1222–1225 (2005). 544, 116383 (2020). 7. J. Nábelek et al.; Hi-CLIMB Team, Underplating in the Himalaya-Tibet collision zone 25. Y. Ren, Y. Shen, Finite frequency tomography in southeastern Tibet: Evidence for the revealed by the Hi-CLIMB experiment. Science 325, 1371–1374 (2009). causal relationship between mantle lithosphere delamination and the north–south 8. B. Zurek, “The evolution and modification of continental lithosphere, dynamics of trending rifts. J. Geophys. Res. 113, B10316 (2008). ‘indentor corners’ and imaging the lithosphere across the eastern syntaxis of Tibet,” 26. S. L. Klemperer, Crustal flow in Tibet: Geophysical evidence for the physical state of Ph.D. thesis, Lehigh University, Bethlehem, PA (2008), p. 259, ProQuest Dissertations & Theses Global 304564606. https://search.proquest.com/docview/304564606. Accessed Tibetan lithosphere, and inferred patterns of active flow. Geol. Soc. Lond. Spec. Publ. – 8 September 2020. 268,39 70 (2006). 9. D. Shi et al., Receiver function imaging of crustal suture, steep subduction, and 27. Z. Hou, Z. Li, Possible location for underthrusting front of the Indus continent: Con- mantle wedge in the eastern India-Tibet continental collision zone. Earth Planet. Sci. straints from helium isotope of the geothermal gas in southern Tibet and Eastern Lett. 414,6–15 (2015). Tibet. Acta Geol. Sin. 78, 482–493 (2004). 10. D. Shi et al., West-east transition from underplating to steep subduction in the India- 28. S. L. Klemperer et al, Torn subducting cratonic lithosphere shown by mantle fluids: Tibet collision zone revealed by receiver-function profiles. Earth Planet. Sci. Lett. 452, India does not underplate the Lhasa terrane west of 82° or east of 88°E, and does not 171–177 (2016). underplate the Qiangtang terrane. AGU Fall Meeting Abstracts, T22C-01 (2019). 11. R. Gao et al., Crustal-scale duplexing beneath the Yarlung Zangbo suture in the https://agu.confex.com/agu/fm19/meetingapp.cgi/Paper/545424. Accessed 8 Septem- western Himalaya. Nat. Geosci. 9, 555–560 (2016). ber 2020. 12. M. Ingalls, D. B. Rowley, B. Currie, A. S. Colman, Large-scale subduction of continental 29. M. Chen et al., Lithospheric foundering and underthrusting imaged beneath Tibet. – – crust implied by India Asia mass-balance calculation. Nat. Geosci. 9 , 848 853 (2016). Nat. Commun. 8, 15659 (2017). 13. X. Tian et al., Weakly coupled lithospheric extension in southern Tibet. Earth Planet. 30. H. Zhang, J. Zhao, Q. Xu, Crustal and upper mantle velocity structure beneath central – Sci. Lett. 430, 171 177 (2015). Tibet by P-wave teleseismic tomography. Geophys. J. Int. 190, 1325–1334 (2012). 14. M. H. Taylor, A. Yin, Active structures of the Himalayan-Tibetan orogen and their 31. X. Liang et al., 3D imaging of subducting and fragmenting Indian continental litho- relationships to earthquakes, contemporary strain field, and Cenozoic volcanism. sphere beneath southern and central Tibet using body-wave finite-frequency to- Geosphere 5, 199–214 (2009). mography. Earth Planet. Sci. Lett. 443, 162–175 (2016). 15. W. P. Chen, Z. Yang, Earthquakes beneath the Himalayas and Tibet: Evidence for 32. L. Godin, R. S. La Roche, L. Waffle, L. B. Harris, Influence of inherited Indian basement strong lithospheric mantle. Science 304, 1949–1952 (2004). faults on the evolution of the Himalayan Orogen. Geol. Soc. Lond. Spec. Publ. 481, 16. G. Hetényi et al., Density distribution of the India plate beneath the Tibetan plateau: 251–276 (2019). Geophysical and petrological constraints on the kinetics of lower-crustal eclogitiza- 33. J. Ligorria, C. J. Ammon, Iterative deconvolution and receiver function estimation. tion. Earth Planet. Sci. Lett. 264, 226–244 (2007). – 17. G. Wittlinger, V. Farra, G. Hetényi, J. Vergne, J. Nábelek, Seismic velocities in southern Bull. Seismol. Soc. Am. 89, 1395 1400 (1999). Tibet lower crust: A receiver function approach for eclogite detection. Geophys. J. Int. 34. D. Shi, S. L. Klemperer, J. Shi, Z. Wu, W. Zhao, Localized Foundering of Indian Lower 177, 1037–1049 (2009). Crust in the India-Tibet Collision Zone: Supplemental Data Files of Seismic and Gravity 18. Y. Wang et al., Differential crustal deformation across the Cona-Oiga rift, southern Data. Stanford Digital Repository. Available at: https://purl.stanford.edu/jz904sc7304. Tibetan Plateau. J. Asian Earth Sci. 177, 177–185 (2019). Stanford Digital Repository, Stanford University, Stanford, California, https://purl. 19. A. E. Ringwood, D. H. Green, An experimental investigation of the gabbro-eclogite stanford.edu/jz904sc7304. https://purl.stanford.edu/jz904sc7304 Published ahead of transformation and some geophysical implications. Tectonophysics 3, 383–427 (1966). print.

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