Lateral growth of NE Tibetan Plateau restricted by the Asian lithosphere: Results from a dense seismic profile

Highlights

• High density seismic array revealed fine crustal structures between northern Tibetan Plateau and the Alxa block.

• The pure-shear crust shortening caused the strongly deformed Qilian thrust belt with complex internal structures.

• There is no obvious subduction of the Asian lithosphere beneath NE Tibetan Plateau.

Abstract

We present high-resolution receiver function images along a 700-km long dense seismic array extending from northern Tibetan Plateau to the Alxa block, crossing the entire Qilian thrust belt (QTB). The dense stations, with less than ~2 km station intervals, allow the receiver functions to unveil unprecedented details of crustal structures across the northern frontier of the growing Tibetan Plateau. The migration image shows a thickened and strongly deformed QTB crust, with an uneven Moho and complex internal structures that are indicative of pure-shear shortening. The Alxa block, in contrast, has a thinner crust, a flat Moho, and little internal crustal deformation. These results suggest that the lateral growth of NE Tibetan Plateau is restricted by the strong Asian lithosphere, which shows no visible subduction beneath the Tibetan Plateau as previously suggested.

Introduction

It is well known that the Tibetan Plateau has risen from the Indian-Eurasian continental collision (Powell and Conaghan, 1975; Yin and Harrison, 2000), but how the plateau has grown laterally remains uncertain. Classical models, either based on continuum mechanics (England and Houseman, 1986) or physical experiments (Tapponnier et al., 1982), all predict northward growth of the plateau. But how does the plateau grow laterally, and what controls the present shape of the Tibetan Plateau, are questions without clear answers. Some studies have suggested that the Tibetan Plateau has grown laterally with sequential subduction (underthrusting) of slices of the Asian lithosphere under it (Tapponnier et al., 2001). This process would lead to simple shear deformation in northern Tibetan Plateau, with a dipping Moho and localized deformation along the shear zones (Fig. 1a). Other studies, based on the lack of late Cenozoic deformation in the crust (Yuan et al., 2013) or mantle lithosphere (Shen et al., 2017) north of the plateau-bounding faults, have suggested that the Tibetan orogenesis has been largely limited between the rigid Asian lithosphere and the indenting Indian plate (Yuan et al., 2013; Shen et al., 2015). In this case, the northern margin of the Tibetan Plateau would be dominated by pure-shear crustal deformation, which could cause localized deformation of the Moho, crust buckling, and bivergent fault-propagation (Fig. 1b).

The Qilian thrust belt (QTB), located near the northeastern margin of the Tibetan Plateau (Fig. 2a) and is actively deforming (Li et al., 2018; Zheng et al., 2013), is an ideal place to resolve the different models of the lateral growth of the Tibetan Plateau. Many recent seismological studies, using both active and passive sources, have explored the crustal and lithospheric structures in this region, but the interpretations have been controversial. For example, some have suggested subduction of the Asian lithosphere (Ye et al., 2015), whereas others show passive and limited deformation of Asian lithosphere (Shen et al., 2015; Shen et al., 2017b). A major problem is the lack of sufficient resolution in these studies. Passive-source broadband seismic experiments usually have low resolution for crustal structure images because of large inter-station distance, often >10–20 km. Reducing the station spacing requires large number of broadband instruments and is costly to operate. A more plausible and economic way to achieve high-resolution crustal structures is to record earthquakes with high-density short-period geophone arrays normally used by active-source seismic experiments (Yuan et al., 1997; Zhu, 2000; Liu et al., 2017).

In this study, we collected teleseismic waveforms recorded by a 700 km long seismic profile across the north Tibetan Plateau, QTB and Alxa block of Asian continent. These data enabled us to use teleseismic receiver functions to image fine crustal structures. The dense array, with a station spacing less than 2 km, ensures a higher resolution than previous passive experiments. The new data provides crustal images of dominant pure shear shortening along the NE Tibetan Plateau, which shed lights on the lateral growth of the Tibetan Plateau.