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

Abstract

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.

Eastward Asthenospheric Flow From NE Tibet Inferred by Joint Inversion of Teleseismic Body and Surface Waves: Insight Into Widespread Continental Deformation in Eastern China

“…Our results with higher resolution at shallow depths clearly illustrates that the cold and seismically fast cratonic keel at the southeastern margin of Ordos block has been replaced by asthenosphere at 60-200 km depths, where an anti-clockwise flow occurs (Yu & Chen, 2016). A similar lithosphere reworking has been reported at the northeastern corner of Ordos (Li et al, 2020), forming a lithospheric divot that influence the mantle flow direction. Thus, we suggest that the lithospheric reworking around the Ordos block is likely to be an ongoing process that is caused by thermal-chemical erosion induced by mantle flow around the Ordos (Figure 10).…”

Panoptic View of Mantle Flow Beneath Trans‐Continental Northeast Asia: Distinct Variation Detected From ∼2,000 km Shear Wave Splitting Profile

“…This mantle flow model can also be used to interpret splitting measurements conducted to the north of our study area (Lu et al, 2020), where a southeastward opening U-shaped pattern in the fast direction, or a rotation pattern, has been revealed, and the authors explained it with complicated flows in both horizontal and vertical direction in the BMW. Although the keel-deflected mantle flow has been reported in other regions, for example, Eastern US and Northeastern Ordos (Li et al, 2020;West et al, 2009;Yang et al, 2017), this is the first time to show it clearly across the NSGL, and we infer that the location of the deflected keel might mark the frontier of the Pacific subduction regime (Figure 4b).…”

Asthenospheric Flow From East Asia to the Philippine Sea Plate Revealed by Rj‐MCMC Inversion of Surface Waves

“…Furthermore, seafloor spreading in the Mariana Trough, which started from the Miocene and has continued to the present day (Seton et al., 2020), creates space beneath the Philippine Sea Plate to draw in asthenospheric materials from NE Asia (see Figure 1a). It has long been suggested that the slab roll‐back of the Pacific Plate and far‐field effects of the Indian‐Eurasian continental collision induces widespread eastward mantle flow beneath the eastern China, although the destination of the mantle flow is controversial (Jolivet et al., 2018; S. Li et al., 2020; Schellart et al., 2019; Yu & Chen, 2016). Our study suggests that at least part of the eastward asthenospheric materials from East China do flow into the Philippine Sea Plate through the slab window beneath the Nankai Trough.…”