Lateral Variation of Crustal Structure Beneath a Stable Craton: Seismic Evidence from Ordos, China

Abstract:

The Archean North China Craton (NCC) has undergone significant reactivation since the Mesozoic in its eastern part, whereas the western part, composed mainly of the Ordos Block, has maintained its stability as a craton. As expected for a cratonic block, receiver function (RF) and seismic tomography studies suggest that Ordos has a much thicker lithospheric root than the eastern part of NCC.

Both RF and Virtual Deep Seismic Sounding (VDSS) have been applied to three E-W linear broadband arrays across north (~39.5°N), central (~37.5°N) and south (~36°N) Ordos. Along the central profile, the Moho suggested by VDSS is ~20km deeper than the RF Moho in the eastern part of Ordos but generally agrees with the RF Moho at west and east ends of the profile. The ~20km thick lower crust between the RF Moho and VDSS Moho is interpreted to be mafic or eclogitic in composition. Along the northern profile, the VDSS Moho agrees well with the RF Moho across the array. Along the southern profile, beneath Shanxi Graben and the Ordos block the VDSS Moho generally follows the same westward deepening trend as the RF Moho though is slightly shallower than the RF Moho beneath the Shanxi Graben at ~112ºE. The shallower VDSS Moho could be due to the fact that the VDSS reflection points are more than ~30km south of the RF conversion points at the Moho, which causes VDSS to sample different Moho depths from RF in the rift zone where strong lateral variation in crustal structure is expected. Further west, across the boundary between Ordos and the NE Tibet Plateau, the VDSS Moho rapidly shoals and then starts deepening again westward, whereas the RF Moho stays at the depth of ~50km.

Our observations suggest that the mafic/eclogitic lower crust revealed by the central line may be limited to the central-eastern part of Ordos, rather than extending beneath the entire Ordos block. The westward thickening of the crust in the southern profile as well as the discrepancy between RF and VDSS Moho beneath the NE Tibet Plateau might be related to the lateral growth of the Tibet Plateau.