Globally, an orogenic process generally leads to linear mountain construction that is parallel to the dominant collision zone. However, in the Tibetan Plateau, besides its prominently high topography and low relief, a very unique morphology is presented in the east-southern Tibetan Plateau, where the high Nyainqentanghla Range (NR) (peak at 7,016 m) and the Yardong-Gulu rift zone (YGR) strike in a northeasterly direction that is nearly perpendicular to the dominant collision zone between the Indian and Eurasian plates (Figure 1A). More interestingly, the onset of the mountain construction along the Nyainqentanghla Range and rifting along the YGR has been geochronologically constrained to take place during the Late Miocene (Pan and Kidd, 1992; Harrison et al., 1995; Kapp, 2005; Chevalier et al., 2020), a time that post-dated the time when southern Tibet has been unchanged in altitude since the Mid-Miocene (Spicer et al., 2003). Moreover, uplift of the Tibetan Plateau, driven by the collision of India, is widely regarded to rise as a coherent entity (Tapponnier et al., 2001). Thus, what drove the northeast-oriented topographic construction has been ambiguous, as to how the deep process of Indian penetration is coupled to the deformation in the shallower crust in east-southern Tibet remains unclear.

While plate tectonic theory provides a general explanation of geophysical and surface structural features, the details of how deep geodynamics and other kinematic parameters of continent-continent collision cause specific features at local scales are uncertain. Many studies have interpreted surface structures within the Tibetan plateau as accommodating lateral extrusion or anisotropic features as highlighting mantle geodynamics (England and Houseman, 1989; Royden et al., 1997; Clark and Royden, 2000; Beaumont et al., 2001; Shapiro et al., 2004). Few studies have addressed the mechanical origin of surface structures within a consistent framework of kinematic evolution, and crustal-scale responses are missing in the prevailing explanations. The east-southern Tibetan Plateau is an excellent setting for connecting surface features with regional mechanisms due to the post collisional landform construction and, meanwhile, the long term and salient effects of the advancing and deeply rooted front of the Indian slab around the Eastern Himalayan Syntaxis.

In this study, we describe a 120 km-long short-period dense array profile collected across the northwestern corner of east-southern Tibet. The combined image is interpreted along with previous geologic and geophysical data to evaluate regional kinematic processes associated with the ongoing eastward extrusion of the Tibetan Plateau and indentation of the Indian Plate around the Eastern Himalayan Syntaxis. This Late Miocene chaotic but active tectonic events in EST will thus shed new lights on understanding tectonic interactions between India and Eurasia after amalgamation.

The currently ongoing India-Eurasia collision uplifted the Himalayan-Tibetan orogenic belt and further constructed the whole Tibetan Plateau to its current elevation (Blisniuk et al., 2001). The Lhasa terrane is located along the collision front between the Indian and Eurasian Plates (Figure 1A) and all collision-related events will thus be recorded within it. Previous geochemical and petrological studies have divided the Lhasa terrane into three east-west oriented sub-terranes, including the central Lhasa micro-continent, and the southern and northern juvenile sub-terranes that have gone through episodic magmatism (Zhu et al., 2011). These three subterranes are separated from south to the north, respectively, by the east-west-striking Luobadui-Milashan fault and the Shiquanhe-Namco fault (Zhu et al., 2011). Meanwhile, north-striking rift systems are widespread within the Lhasa terrane, which were initiated owing to slab tear of the subducting Indian plate (Hou et al., 2006; Chen et al., 2015; Guo et al., 2018) and subsequent eastward extrusion of the plateau during the Middle Miocene (Blisniuk et al., 2001; Lease et al., 2011). Controlled by the arc structure that protrudes to the south of the Yarlung-Zangbo suture (YZS), orientation of the rift system in the Lhasa terrane changes from a northwesterly direction in the west to a northeasterly direction in the east (Figure 1A).

In the eastern Lhasa terrane (east-southern Tibet, EST), it shows a typically different topography relative to other parts of the Lhasa terrane (Figure 1B). First of all, there is the largest rift zone, i.e., the NE-striking Yardong-Gulu rift system (YGR). Along its footwall, it exhumed the highest NE-striking Nyainqentanghla Range (NR) at 10–8 Ma in the plateau’s interior (Pan and Kidd, 1992; Harrison et al., 1995; Kapp, 2005; Chevalier et al., 2020). The NR is in turn bounded to the west by the Namco Lake (Figure 1B), the second largest lake in the plateau’s interior that was originated at 8 Ma as being a pull-apart basin (Schulze, 2021). Morphologically, both the NR and YGR change coherently from a northerly direction to northeasterly and easterly direction from south to the north (Figure 1B). Additionally, the YGR progressively decreases in vertical slip rate southward from 3 to 6 mm/yr along the northern part to 1.3 ± 0.3 mm/yr in the south (Chevalier et al., 2020; Zuo et al., 2021). Coherently, the NR decreases in elevation along strike to the southwest where the trend changes to a northerly direction (Figure 1B). Field geology shows a SE-dipping low-angle normal fault with mylonites along the range front of the NR, while ⁴⁰Ar/³⁹Ar thermochronolgic studies have indicated the in-situ lower crust partial melting and an emplacement to the midcrust (Kapp, 2005). No features in association with core-complex are yet seen within the NR (Kapp, 2005), and the relationships resulting from the coherence in deformation between YGR and NR needs to be confirmed.

Besides the NR and YGR, there is the NE-striking Namche-Barwa Mountain Range (NBR) locating along the easternmost segment of the YZS to the south (Figure 1A). It is part of the subducted Indian crust and has contributed dominantly to construction of the Eastern Himalayan Syntaxis (EHS) (Xu et al., 2012). The NBR has experienced crustal-scale exhumation through the deep crust at 22–18 Ma to the shallower crust at 6–2 Ma (Xu et al., 2012). Rapid exhumation of the NBR was constrained to commence since ∼8 Ma of Late Miocene in age (Ding et al., 2001). Moreover, the maximum compression stress in association with construction of the EHS from indentation of the Indian slab fans radially throughout the EST (Huchon et al., 1994; Schellart et al., 2019) (Figure 1A). To both northern and southern sides of the EST, there locates, respectively, the east-west striking YZS and the Jiali fault zone (Figure 1B). The dextral Jiali shear zone to the northern edge of EST possesses a faster slip rate of 5.0 ± 0.5 mm/yr in its westernmost segment, where cuts through the YGR, and then decreases to 2.4 ± 0.8 mm/yr in the middle part (Zhang et al., 2021). Thus, the northwestern corner of EST possesses more active tectonics relative to the other parts.

The P wave receiver function method is widely used to study the structure of the crust and upper mantle by inversion of broadband seismic station data (Langston, 1979; Kumar et al., 2006). The dense nodal array was employed in this study. The teleseismic waveforms used in this study were collected in the north section of the YGR from October to November 2019 (Figure 2A). The observation system was a nearly NW nodal array composed of 92 three-component nodes (Magseis Fairfield ZLand 3C) with 5 Hz corner frequency spaced at 1 km. Nodal seismometers record signals at higher frequencies than that of the broadband seismometers, which resulted in a higher theoretical vertical resolution (Wu et al., 2017). Moreover, the shorter station spacing improves the lateral resolution, compensating for fewer recorded teleseismic events (Tian et al., 2015). Numerous experiments have demonstrated that the receiver functions calculated from seismic data with dense nodal arrays can image more details than those by the broadband station (Leahy et al., 2012; Liu et al., 2017; Saikia et al., 2017; Ward and Lin, 2017; Farrell et al., 2018; Liu et al., 2018; Ringler et al., 2018; Sweet et al., 2018; Tian et al., 2021).

A total of 98 teleseismic events with Ms≥5.1 was recorded during the observation period of ∼40 days. Poor quality dataset was removed by signal-to-noise ratio calculation and manual selection, and 32 available seismic events were retained (Figure 2B) (Ammon, 1991). The 60 s continuous waveforms, the 10 s before and 50 s after the theoretical P arrival time, had been resampled (100 Hz sample rate) to write the information of station and event. The frequencies of band pass filter were used between 0.1 and 8 Hz. The horizontal component (northern and eastern) had been rotated around the theoretical back azimuth angle into the radial and transverse system (Langston, 1979; Zhu, 2000a). A frequency domain deconvolution algorithm was used to calculate receiver functions from radial and vertical components of waveforms. Finally, the 2,709 high-quality RFs were calculated.

In order to highlight the regional crustal structure (Ward et al., 2018) and compare with the results from previous study (Zhang et al., 2013), the receiver functions were smoothed for a stacking radius window of 1 and 3 km (Figures 3B,C). The stacking of the RFs has a good consistence of the main structures with those obtained by broadband data, but also has more details. The common conversion point (CCP) stacking (Zhu, 2000a; Chen et al., 2005) was used to image the lithospheric structures (Figure 3D).

The migrated receiver function image is shown in Figure 4A. Combined with previous studies on Moho depth from broadband receiver function (Zhang et al., 2013; Tian et al., 2015), the interface with high amplitude at 70–78 km in our study represents the Moho geometry. It is at depths of 78 km beneath the NR and then lifts to approx. 70 km to the east of the YGR (Figure 4B). More importantly, the Moho geometry is offset right beneath the NR (Figure 4B), a scenario that is consistent with Ps analysis for an identification of a weakening to disappearing Moho amplitude (Zhang et al., 2013). A sequence of weak interfaces with low amplitude are presented as well beneath the NR, which can be traced from the mantle depths, through the offset Moho, to the upper crust. They show strong amplitude against the overlying shallower crust. Additionally, a prominent intracrustal interface appears at depths of 50 km throughout the entire profile, which shows as being a low velocity layer in previous tomographic studies (Tian et al., 2015). This intracrustal interface appears with a typically weak amplitude of concave downward in the middle of the profile. Along with surface observations of the YGR, the RF imagery connecting the YGR with a sequence of interface terminations can be traced downward to merge with the identified intracrustal shear zone at approx. 50 km. On the other side, a sequence of strong interface terminations right beneath the NR is seen from depths of 50 km, through the offset Moho, downward to the mantle depth. At further deeper depths of approx. 120 km, combined with previous studies by Shi et al. (2015), the discrete strong negative interfaces on both sides of the YGR possibly represents the torn lithosphere-asthenosphere boundary (Figure 4B), but the western segment beneath the NR is much deeper than that of the eastern segment.

Previous tomographic studies by Liang et al. (2016) has documented the significant lithospheric-scale thermal influence from the asthenospheric upwelling beneath this NR (Figure 5A). We superimpose the tomographic results onto our RF results (Figure 5B) and a synthetic view drives to interpret that the domain outlined by weak interfaces may have been affected by the tectonothermal activities from the upwelling asthenosphere through the torn LAB dominantly beneath the NR (Figure 5C). Meanwhile, the original ductile normal fault of the YGR is seen to have cut through into the mantle depth, which was later displaced into two parts by the intracrustal detachment atop the crystalline basement (Figure 5D). The overlying segment has moved eastward for about 100 km from the original segment (Figure 5D). Meanwhile, the concave downward interface beneath the NR is highly derived from stress accumulation to accommodate the uplift of the NR.

Previous tomographic studies have outlined a weakly decoupled lithospheric extension of southern Tibet (Tian et al., 2015). In our study, the overall RF imagery further displays a 100 km-eastward displacement of the upper part of the YGR original normal fault and the dominant influence to the mid-to-lower crust beneath the NR and adjacent regions on both sides (Figures 5D, 6A). This displacement between the surface exposure and the Moho offset is however not evidenced beneath the southern part of the YGR in the MT profile (Wang et al., 2017) (Figures 1B, 6B). The overall architecture implies a geodynamic adjustment from extension through compressive stress to extension for landform construction during extrusion. The next step is to formulate the mechanism behind.

Southern Tibet has remained in a constant elevation since 15 Myr ago (Spicer et al., 2003), while rapid uplift of the Nyainqentanglha Mountain Range (NR) in east-southern Tibet (EST) occurred around 10–8 Ma (Harrison et al., 1995; Ding et al., 2001; Kapp, 2005; Xu et al., 2012). Meanwhile, monsoon in SE was enhanced about 10–8 Myr ago, owing to the contemporaneous increase in elevation of the Tibetan Plateau (An et al., 2001). The driving mechanism for this puzzling topographic uplift in the plateau’s interior and tectonic interactions with climate change remains unclear. In this study, first step toward solving this problem is to have obtained a 120 km-long crustal-scale seismic profile across the northern Yardong-Gulu rift zone (YGR). The results show the obvious offset Moho beneath the NR is 100 km west away from the surface exposure of the YGR. A mid-crustal detached shear zone appears to displace the original normal shear zone of the YGR into two parts and the upper part was displaced eastward. Moreover, seismic features indicate uplift of the NR was associated with stress accumulation above the detachment shear zone syn-or post-dating the decoupled extrusion. Considering the eastward extrusion of the plateau and progressive northward compression from penetration of the Indian slab, we believe their tectonic interactions are the key responsible for this diversity in surface structures. As a result, approx. 56%–78% less extrusion rate of southern segment of YGR was produced than that of the northern segment (Chevalier et al., 2020) and more stress accumulated in the northeastern part of EST to increase the elevation. That resulted in the maintenance of the regional southwest-flowing of the Lhasa river in the northeastern EST, although there is active surface construction to the south along the Yarlung-Zangbo suture zone by the intense collision between India and Eurasia (Laskowski et al., 2019) and could have also partly contributed to the enhanced monsoon in the SE Asia (An et al., 2001).