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(A, B, and C) are color-coded elevation maps of the modeled 3-D surfaces of the South Tibet Detachment System (STD), Main Central Thrust (MCT), and Main Himalayan Thrust (MHT), respectively. (D) is a colorcoded thickness map of the high-grade orogenic core of the western and central Himalaya. The gray lines labeled with numbers are thickness contours (in kilometer). Also shown are surface traces of major shear zones. See abbreviations in Figure 1A.
Contexts in source publication
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... compile multiple maps in a common projection system and digitize shapefiles for the STD and MCT. They were exported to MOVE and merged with published cross-sections to create a 3-D database. We also digitized the STD and MCT surfaces from cross-sections in MOVE. For the MHT, we reinterpreted the cross-section published in Caldwell et al. (2013) (Fig. DR2 in Appendix 1; see footnote 1) and made it extend along strike of the orogen to the west of Nepal and adopted the 3-D MHT model published in Hubbard et al. (2016) for the Nepal area. All map-view digitized features were projected to the DEM. The lengths of and the space between the cross-sections vary due to the different sizes of the ...
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... we used the kriging algorithm in MOVE to model the MCT and STD surfaces by making the surfaces pass through all the points of corresponding data from both the map view and cross-sections. Figure 2 shows color-coded elevation maps of the modeled STD, MCT, and MHT (see Fig. DR3 in Appendix 2 for the 3-D view model, see footnote 1). The 3-D view of the geometry of the orogenic core, i.e., the space bounded by the STD from the top, the MCT and MHT from the bottom at the frontal part and hinterland part, respectively, is shown in Appendix 2 (Fig. DR3). ...
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... investigate the strain accumulated in the orogenic core, we calculated its thickness by using the modeled elevation data of the STD, MCT, and MHT (Fig. 2D). The thickness of the core is the elevation difference between the STD and MCT for the frontal part and the difference between the STD and MHT for the hinterland part. The thickness map shows significant along-strike variation in thickness in the hinterland, increasing from ∼25-26 km in the western Himalaya to ∼34-42 km in the central ...
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... model describing deformation of the orogenic wedge should account for the significant thickness variation described above and shown in Figure 2D. ...
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... state between the convergence orthogonal segment and the oblique segment can generate OP extensional shear sense between the two segments in the upper crust (Fig. 3B). This is consistent with the development of the GMH, a large-scale extensional fault system, at the position of the abrupt along-strike change in the thickness of the orogenic core (Fig. 2D). Earth surface studies and seismic reflection profiles in many areas have shown that it is common for shear zones at shallow depth to connect to sub-horizontal detachments at mid-lower crustal depth (e.g., Lemiszki and Brown, 1988;Jones et al., 1992;Hajnal et al., 1996;Jolivet et al., 2001Jolivet et al., , 2004Lavier and Manatschal, ...
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... foreland via subsequently excising LHS sheets from the footwall and incorporating them as horses into the antiformal LHS duplex in the hanging wall. In the central sector, because no THS klippe formed between the GHS duplex and the LHS duplex, the GHS duplex is juxtaposed against the LHS directly causing the abnormally high STD in the 3-D model (Fig. 2). From T3 to T4, in each sector, the higher frontal ramp shifted farther to the foreland and the synformal GHS klippen formed between the LHS duplexes and the location of the new ramp. From T2 to T4, the antiformal and synformal features in the outer wedge developed as the MHT flats (c, d, e in Fig. 6B) above the mid-lower crustal ramp ...
Citations
... The mid-crustal low-velocity region imaged in this study supports the result by Yin and Taylor (2011) but also extends into regions to the west. In westernmost Tibet, west of the KKF a large late Cenozoic basin known as the Zhada basin is interpreted to have formed above a ductility deforming mid-crust undergoing vertical thinning in response to oblique convergence along the Himalayan front between India and Asia (Saylor et al., 2010;Murphy et al., 2009;Fan and Murphy, 2020). ...
The northward extent of subducted Indian plate is a fundamental component of hypotheses explaining deformation and magmatism within the Tibetan Plateau. Yet, these aspects of the plate are debated in west Tibet. Here we report a new three‐dimensional lithospheric structure of seismic velocity and radial anisotropy under west Tibet constructed from Rayleigh and Love wave phase velocity maps at periods of 20–167 and 20–125 s, respectively. Our results show the Indian lithospheric mantle to be subhorizontally subducted under west Tibet across the Bangong‐Nujiang suture to the Qiangtang terrane, as indicated by a prominent fast velocity anomaly accompanied with positive radial anisotropy (Vsh > Vsv). We find a positive spatial correlation of this result with information on the distribution of late Cenozoic potassic‐adakitic rocks in western Tibet. Additionally, we show that the midcrust of west Tibet is characterized by an anomalously low shear wave velocity (3.2–3.4 km/s at ~30‐km depth) and positive anisotropy, which is consistent with an estimated ~3% fraction of partial melt. We suggest that the midcrust of this region is capable of flowing and that its three‐dimensional structure shows it to extend south of the Karakoram fault (KKF), a shear zone interpreted as a barrier to crustal flow. Instead, our results are consistent with the KKF embedded in weak middle crust along with several other structures that display a pattern of distributed deformation in the western portion of the Tibetan Plateau.
Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of lithotectonic units from these orogens. The Gurla Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that the HMC comprises Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) rocks. Conventional interpretation of such provenance data would require the Main Central thrust (MCT) to be also outcropping within the core complex. However, new in situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the MCT. Rocks from the core complex record Eocene and late Oligocene to early Miocene monazite (re-)crystallization periods (monazite age peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) overprinting pre- Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/ Pb monazite petrochronology demonstrates that both GHS and LHS protolith rocks were captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion. Monazite ages do not record metamorphism coeval with late Miocene extensional core complex exhumation, suggesting that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of orogen-parallel hinterland extension at ca. 15–13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith associated with a Main Himalayan thrust ramp below the core complex. We demonstrate that isotopic signatures such as Sm-Nd should be used to characterize rock units and structures across the Himalaya only in conjunction with supporting petrochronological and structural data.
Convergence of the Indian Plate towards Eurasia has led to the building of the Himalaya, the highest mountain range on Earth. Active mountain building involves a complex interplay between permanent tectonic processes and transient seismic events, which remain poorly understood. In this Review, we examine the feedbacks between long-term tectonic deformation (over millions of years) and the seismic cycle (years to centuries) in the Himalaya. We discuss how surface morphology of the Himalaya indicates that the convergence is largely accommodated by slip on the Main Himalayan Thrust plate boundary fault, which developed in the roots of the mountain range over millions of years. At shorter (decadal) timescales, tectonic geodesy reveals that elastic strain is periodically released via earthquakes. We use examples from earthquake cycle models to suggest that partial ruptures could primarily occur in the downdip region of the Main Himalayan Thrust. Great (Mw 8+) Himalayan earthquakes are more commonly associated with complete megathrust ruptures, which release accumulated residual strain. By synthesizing numerous observations that co-vary along strike, we highlight that tectonic structures that developed over millions of years can influence stress accumulation, structural segmentation, earthquake rupture extent and location, and, consequently, the growth of the mountain range.
Documenting the processes that facilitate exhumation of ultrahigh‐pressure (UHP) rocks at convergent margins is critical for understanding orogen dynamics. Here, we present structural and temperature data from the Himalayan UHP Tso Morari nappe (TMN) and overlying nappes, which we integrate with published pressure‐temperature‐time constraints to refine interpretations for their structural evolution and exhumation history. Our data indicate that the 5.5‐km‐thick TMN is the upper portion of a penetratively deformed ductile slab, which was extruded via distributed, pure shear‐dominated, top‐down‐to‐east shearing. Strain in the TMN is recorded by high‐strength quartz fabrics (density norms between 1.74 and 2.86) and finite strain data that define 63% transport‐parallel lengthening and 46% transport‐normal shortening. The TMN attained peak temperatures of ~500–600°C, which decrease in the overlying Tetraogal and Mata nappes to ~150–300°C, defining a field gradient as steep as 67°C/km. Within the overlying nappes, quartz fabric strength decreases (density norms between 1.14 and 1.21) and transport‐parallel lengthening and transport‐normal shortening decrease to 14% and 18%, respectively. When combined with published ⁴⁰Ar/³⁹Ar thermochronometry, quartz fabric deformation temperatures as low as ~330°C indicate that the top‐to‐east shearing that exhumed the TMN continued until ~30 Ma. Peak temperatures constrain the maximum depth of the overlying Mata nappe to 12.5–17.5 km; when combined with published fission‐track thermochronometry, this provides further support that the TMN was not underplated at upper crustal levels until ~30 Ma. The long‐duration, convergence‐subnormal shearing that exhumed the TMN outlasted rapid India‐Asia convergence by ~15 Myr and may be the consequence of strain partitioning during oblique convergence.
