Lithospheric Structure of the Northern Ordos From Ambient Noise and Teleseismic Surface Wave Tomography

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.

Seismic Evidence for Lateral Asthenospheric Flow Beneath the Northeastern Tibetan Plateau Derived From S Receiver Functions

“…In this study, we prefer to interpret it as the LAB rather than a MLD on the basis of previous observations by the surface wave tomography and S ‐RF techniques. The most recently constructed 3‐D shear wave velocity model places the LAB depth at 125 ± 5 km close to the northern end of profile B–B′ (S. L. Li et al, ), which is in good agreement with our interpreted LAB. Meanwhile, our observations are also supported by another independent shear wave velocity model indicating that the lithospheric thickness beneath the Ordos and SW Alxa blocks is 100–140 km (Y. Li et al, ).…”

“…Beneath the WQO, the measured lower layer delay times of 1.0–1.5 s are likely produced by the asthenospheric flow except for the deformation in mantle lithosphere (Huang et al, ). Furthermore, the most recent results from surface wave tomography show an interconnected low‐velocity zone at depths of 100–200 km under the northern Ordos block and its surrounding regions, suggesting that the asthenosphere beneath the NE Tibetan Plateau flows northeast toward the northern Ordos lithosphere (S. L. Li et al, ).…”

Shear Wave Splitting Evidence for Keel‐Deflected Mantle Flow at the Northern Margin of the Ordos Block and Its Implications for the Ongoing Modification of Craton Lithosphere

“…More importantly, the spatial pattern of the low-velocity anomaly in the upper mantle closely matches the regions with large splitting delay times, and delay times decrease from >1.2 s beneath the Yinshan Belt Figure 5. Two cross sections showing shear wave splitting measurements (red for this study's measurements and black for previous studies) and the S wave velocity model from Li et al (2018). The thick black lines in Panels (a) and (b) represent the Moho depth (Li et al, 2018).…”

“…The width of the Fresnel zone is estimated to be~80 km at a (Chang et al, 2008(Chang et al, , 2011(Chang et al, , 2012a(Chang et al, , 2016Li et al, 2011;Zhao, 2005;Zhao et al, 2008;Zhao & Zheng, 2007;Zhao & Xue, 2010). The background indicates the S wave isotropic velocity perturbation at 140 km depth (Li et al, 2018). Gray lines mark the locations of cross sections shown in Figure 5.…”

Three‐Dimensional Crustal Structures of the Shanxi Rift Constructed by Rayleigh Wave Dispersion Curves and Ellipticity: Implication for Sedimentation, Intraplate Volcanism, and Seismicity

“…To investigate whether our model, especially the shallow structures, constructed by joint inversion of dispersion curves and H/V ratios has improvements compared with published models without including H/V ratios in their studies, we compare our model with two recently published models by Li et al (2018) and Ai et al (2019). Li et al (2018) constructed a 3-D Vs model based on the inversion of Rayleigh wave phase velocities at 8-143 s periods, and Ai et al (2019) jointly inverted Rayleigh wave phase and group velocities at 6-45 s periods and P wave receiver functions to build a crust and uppermost mantle Vs model. Figures 8 and 9 summarize 1-D Vs profiles beneath six selected points, located in the Ordos Block, the Datong Basin, the Linfen Basin, the Taiyuan Basin, the Taihang Mountain, and the Lvliang Mountain, from our model compared with those from the models of Li et al (2018) and Ai et al (2019).…”