High resolution crustal and upper mantle velocity model of northern Ordos block from surface wave tomography: evidence for the on-going craton reactivation

The Ordos block is the nuclei of the Archean craton of North China Craton (NCC), which had experienced significantly lithospheric reworking since the Mesozoic. A series of rifting zones developed surrounding the Ordos block during the Cenozoic. Here, we present a high-resolution crustal and upper mantle velocity model of the northern Ordos block from surface wave tomography. The seismic data are mainly from the recently deployed portable seismic array, the NODArray (North OrDos Array) with 102 stations, by Peking University. In addition, 221 permanent stations from CEA (China Earthquake Administration) and 106 portable stations from NCISP (North China Interior Structure Project) are also used in the tomography. Phase velocities at periods 6-40 s were obtained from ambient noise tomography and phase velocities between 25 and 143 s were measured from the two-plane-wave tomography with finite-frequency effects included. We then inverted for 3-D S-wave velocity model from the surface down to 250km depth beneath the Ordos block using a MCMC scheme (Markov Chain Monte Carlo). The resulting velocity model reveals significant lithospheric reactivation in the margins of the Ordos block. As expected, the prominent high velocity is shown beneath the interior of the Ordos block, suggesting the preservation of thick lithospheric root. However, beneath the Inner Mongolia Suture Zone (IMSZ), the Precambrian suture zone between the Ordos bock and Yinshan belt to the north of the Ordos block, we observed ultra-low velocity anomaly from the mid-to lower crust to upper mantle. The Yinshan belt and Trans-North China Orogen (TNCO) are also linked to low velocity anomalies in the upper mantle. The most significant feature of our model is the low velocity anomaly beneath the Datong volcano, which extends to at least 250-km depth. This continuous low velocity anomaly can be interpreted by a plume-like upwelling, which originates from the deep upper mantle and ascends into the lithosphere to feed the recent magmatic activities at Datong volcano.