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 un- derlying 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 www.pnas.org/cgi/doi/10.1073/pnas.2000015117 PNAS Latest Articles | 1of6 Downloaded by guest on September 29, 2021 84o 86o 88o 90o 92o 949 o Tibet 969 o Qiangtang Terrane BNS India 3232o Naqu JF TYG Lhasa Terrane PXG T5 o YZS A T3 30 B T11 A T8 B Tethyan YGG Himalaya S8 T12 CSG 28o T10 Kathmandu Thimphu MFT T9 26o 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).