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

We constructed a high-resolution 3-D V-s 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 similar to 200 km with shear velocities comparable to the high velocities of other Archean cratons worldwide. However, the lithosphere thins gradually toward the east and V-s drops by 2-3% in the uppermost mantle beneath the eastern Ordos, coincident with the high surface heat flow (similar to 68 mW/m(2) 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.