No abstract

We constructed a high‐resolution 3‐D Vs model of the northern Ordos block and its surrounding areas by surface wave tomography, which reveals significant intracratonic heterogeneities. In the western Ordos, the lithosphere thickness is ~200 km with shear velocities comparable to the high velocities of other Archean cratons worldwide. However, the lithosphere thins gradually toward the east and Vs drops by 2–3% in the uppermost mantle beneath the eastern Ordos, coincident with the high surface heat flow (~68 mW/m2 in average) there. This observation suggests that the thick, cratonic keel is only locally preserved beneath the western Ordos, and the eastern part of the Ordos seems to undergo local rejuvenation. At greater depths (>180 km), a low‐velocity channel is observed beneath the high‐velocity keel of the Ordos. Beneath the Datong volcanoes, a low‐velocity anomaly is observed, dipping westward with depth and closely following the slope of the lithosphere beneath the northern Ordos. This prominent low velocity is connected with the low‐velocity zone beneath the northern Ordos, which is further connected with the low‐velocity zone beneath the northeastern Tibetan Plateau (NET). We propose that the asthenosphere beneath the NET flows toward the northern Ordos in response to the continuous northward convergence of the Indian‐Eurasian continents, and the asthenosphere flows upward following the eastward thinning lithosphere which leads to decompression partial melting, which migrates upward to feed the Datong volcanoes. The significant variations of the lithospheric thickness of the Ordos block may control the distribution of the asthenospheric flow.

The Tibetan Plateau is the largest orogenic belt on the Earth with high topography and thickened crust, which extends over 2,000 km from the Himalayas to central Asia (England & Houseman, 1986). It is generally believed that the growth of the Tibetan Plateau is the consequence of the continental collision between the Indian and Eurasia plates since the Cenozoic (Tapponnier et al., 2001). However, how the Tibetan Plateau uplifted to its present high topography is a subject of debate (Spicer et al., 2021).Significant attention has been given to the mechanism of crustal thickening and outward growth of the high terrain along the northeastern (NE) Tibetan Plateau (Yin & Harrison, 2000;Zheng et al., 2017), which is the leading edge of the plateau expansion. NE Tibet is composed of several blocks, including the SGT, Kunlun-West Qinling Terrane (KL-WQL), and Qilian orogen from the south to north, which are separated by several major active faults including the KL-WQL, South Qilian Suture, and Haiyuan Fault (HYF; Figure 1). To the east and northeast, NE Tibet is surrounded by the relatively stable Alax and Ordos blocks. On the surface, NE Tibet is generally bounded by the Haiyuan-Liupanshan Fault system to the northeast and the north Qilian frontal thrust (NQFT) to the north (Gao et al., 2013, Figure 1). However, recent surface geological investigations have found that the Tibetan Plateau continues to grow northeastward across the Hexi Corridor, as evidenced by the uplift of Longshou Shan and Heli Shan (Zhang et al., 2017;Zheng et al., 2013). Receiver function studies also reveal that the thickened crust (>50 km) is present across the Haiyuan and Liupanshan faults, suggesting the northeastward growth of the Tibetan plateau (Guo et al., 2015;Shen et al., 2017). Furthermore, recent studies reveal that rapid

Eastern Eurasia have experienced large-scale and long-term intra-continental deformation in the Cenozoic (Tapponnier et al., 2001;Yin, 2010). The continental deformation in eastern Eurasia is manifested by mountain building (e.g., the growth of the Qinling and Liupanshan mountains) (Yin, 2010), reactivation of large strike-slip shear zones that stretch thousands of kilometers from west to east (e.g., the Qilian and Qinling faults) (Enkelmann et al., 2006;Tapponnier et al., 1990), development of localized rifting systems from northern Tibet to the Trans North China Craton (TNCO) (e.g., the Weihe and Shanxi rifts and South Ningxia-Yinchuan-Hetao basin) (Zhang et al., 1998), and volcanism in the intraplate settings (e.g., the Fansi, Datong, and Hebi volcanoes) (Xu, 2007) (Figure 1a).The Ordos block and Sichuan basin have thick cratonic keels and are considered to be the cores of eastern Eurasian continent. They are often viewed as barriers that divide eastern Eurasia into two broad intra-continental deformation regimes. To the west, the indention of the Indian continent to the Eurasian continents since ∼50 Ma has led to a total of ∼1,000-2,000 km north-south shortening of the Eurasian continent, resulting in the high topography within the Tibetan interior and over 250-1,250 km eastward continent expansion (Molnar & Tapponnier, 1975). In contrast, the tectonic evolution of eastern margins of Eurasia is subject to the subduction and subsequent retreat of the western Pacific plate. The eastern Eurasia is predominated by extensional deformation, manifested

We use 123 temporary seismic stations to determine shear wave splitting patterns beneath the northern Ordos block and its surrounding areas. The mapped pattern of anisotropy shows a dramatic arc-shaped anisotropy contrast beneath the northeastern Ordos block that closely follows the lateral fast/slow-velocity interface seen in a recent surface tomographic model at~140 km depth. Both seismic anisotropy and velocity appear to demarcate the current boundary of cratonic lithosphere in the upper mantle at this depth, located >150 km south of the geological surface boundary of cratonic crust. We suggest that the craton's keel in the northeastern corner of Ordos block has already been eroded and replaced by warmer asthenospheric mantle, while the cratonic crust above this "missing" keel has yet to be destroyed by deformation and/or crustal metamorphism, thus creating the keel divot at the northeastern corner of the Ordos block. The fast polarization direction of anisotropy tends to wrap around the northeastern margin of the Ordos block, changing from predominantly NW-SE beneath the western part of the margin to nearly E-W beneath the east. We suggest this pattern reflects that plate motion-related asthenosphere flow is being deflected by the cratonic keel of the Ordos block. Such keel-deflected asthenospheric flow could enhance the erosion of the craton's keel, leading to the observed lithospheric reworking beneath this region.

The northeastern margin of the Tibetan Plateau (NE Tibet) is a complex transition zone located between the North China Craton (Alxa and Ordos blocks) and the Qiangtang block, consisting of thrust and strike-slip fault systems (Tapponnier et al., 2001;Yin & Harrison, 2000). It is suggested that NE Tibet has undergone a series of orogenic activities since the Cenozoic, resulting in the current NE-SW directional crustal shortening of ∼10-20 mm/yr (Figure 1a). Previous geophysical studies have revealed significant crustal (lithospheric) rheological contrasts between the Tibetan Plateau and its bounding Asian blocks (Chen et al., 2017;Cook & Royden, 2008;Sun & Liu, 2018). How the NE Tibet expanded since the Cenozoic in response to the Indian-Eurasian continents collision is still actively debated. Although numerous studies have suggested that crustal channel flow may control the crustal deformation in the Tibetan Plateau interior, the presence of ductile channel flow at the expansion margins of NE Tibet is still controversial (