Early‐Stage Lithospheric Foundering Beneath the Eastern Tibetan Plateau Revealed by Full‐Wave P_n Tomography

Abstract

The west-east contrast of magmatism in northern Tibet suggests that the lithospheric root has been removed in the west, following continental collision that led to lithospheric thickening and removal, but not in the east where paradoxically larger convergence occurred. Here we show a full-wave P n tomography model for the eastern Tibetan Plateau, which reveals a high-velocity layer beneath the Moho extending to 150-km depth. The anomalously high velocities and its northward dipping top surface suggest a very depleted and cold mantle consistent with an underthrusted Precambrian Lhasa lithosphere. A high-velocity column connects this layer to another high-velocity layer below 190-km depth, representing early-stage removal of the Tibetan mantle lithosphere and its interaction with the underthrusted Indian lithosphere. The west-east contrast is thus attributed to different stages of lithospheric removal, which may be controlled by varying angles of Indian subduction from the west to the east.Plain Language Summary The northern Tibetan Plateau can be divided into two parts: the western part with abundant young volcanic rock (<17 million years) and the eastern part where young volcanic rock is absent. Using compressional seismic waves that mainly travel in the upper mantle, we model the three-dimensional wave speed structure of the upper mantle of the eastern Tibetan Plateau. Our model reveals that a very old and cold Tibetan mantle layer above 150-km depth is experiencing foundering under eastern Tibet, and those foundered materials have downwelled onto the underlying Indian Plate below 190-km depth. The mantle structure beneath the plateau east of~93°E is different from that in the west where most of the Tibetan mantle lithosphere may have been removed already. This difference is consistent with the lateral contrast in recent volcanic activities and is presumably due to different styles of subduction of the Indian Plate between the west and the east.

Lateral Growth Mechanism of Proto‐Tibetan Plateau in the Late Paleogene: Implications From Detailed Crustal Structures of the Hoh Xil Basin

“…The inefficient Pn and Sn propagation characteristics observed (Barazangi & Ni, 1982; Brandon & Romanowicz, 1986) indicate an LVZ in the uppermost mantle beneath north‐central Tibet, which also features low electrical resistivity (Figure 3e, Unsworth et al., 2004). Its spatial range was further constrained surrounding the HXB and in a thin sub‐Moho layer (Bao & Shen, 2020; Barron & Priestley, 2009; M. Chen et al., 2017). The discontinuous Moho beneath the northeastern Hoh Xil‐Songpan Ganzi was revealed by the seismic reflection profile and was proposed to be induced by the tectonic juxtaposition of various crustal and mantle fragments during the closure of Paleo‐Tethys (Wang et al., 2011).…”

“…The inefficient Pn and Sn propagation characteristics observed (Barazangi & Ni, 1982;Brandon & Romanowicz, 1986) indicate an LVZ in the uppermost mantle beneath north-central Tibet, which also features low electrical resistivity (Figure 3e, Unsworth et al, 2004). Its spatial range was further constrained surrounding the HXB and in a thin sub-Moho layer (Bao & Shen, 2020;Barron & Priestley, 2009;M. Chen et al, 2017).…”

Compositional Variation in the Crust of Peninsular Ranges and Surrounding Regions, Southern California, Revealed by Full‐Wave Seismic and Gravity Joint Inversion

“…Here we introduce different damping (denoted as $${\lambda }_{V}$$ and $${\lambda }_{\rho }$$) for wave speed and density perturbations, respectively, considering the different spatial coverages and noise levels of the seismic and gravity data, as well as a smoothing matrix L with a corresponding coefficient $$s$$. We apply a data‐weighting diagonal matrix $${\boldsymbol{W}}_{\boldsymbol{s}}$$ to account for the nonuniform quality of phase delay measurements, which was introduced in Bao and Shen (2020), and a scalar factor $${w}_{g}$$ to balance the relative weight between seismic and gravity data.…”

“…$$\delta u(t)$$ is the perturbation of the displacement waveform, $$\hspace*{.5em}u(t)-\tilde {u}(t)$$; a tilde represents synthetics based on the reference model; a dot represents the time derivative, and t 1 and t 2 are the upper and lower limits of a finite time window. We note that the above equations are slightly modified from previous publications according to a change in the representation of perturbations (e.g., Bao & Shen, 2018, 2020; Zhao et al., 2005).…”

Constrained Gravity Inversion With Adaptive Inversion Grid Refinement in Spherical Coordinates and Its Application to Mantle Structure Beneath Tibetan Plateau

“…How the northward convergence is accommodated by the lithospheric process is intriguing and controversial. Different competing but possibly complementary mechanisms have been proposed, including crustal shortening and thickening (Bao et al., 2020; England & Houseman, 1986), eastward crustal flow (Bai et al., 2010; Li et al., 2020; Royden et al., 1997), underthrusting of the Indian lithosphere beneath much of the Tibetan Plateau (Zhou & Murphy, 2005), subduction of the Indian lithosphere with an increasing angle from the western part to the eastern part (Li et al., 2008), an intact warping Indian lithosphere with a large dip angle (Wang et al., 2019), a fragmented Indian lithosphere (Ceylan et al., 2012; Li & Song, 2018; Liang et al., 2012, 2016), lithospheric delamination and convective thinning (Bao & Shen, 2020; Chen et al., 2017; Ren & Shen, 2008). In addition, P and S wave tomography suggest that the crust and mantle lithosphere may be strongly coupled (Li & Song, 2018).…”