Lithospheric structure of the Ordos Block and its boundary areas inferred from Rayleigh wave dispersion

Abstract

We have measured Rayleigh wave phase velocities of the Ordos Block and its southern and western boundary areas and inverted these data to describe the upper mantle structure. The Ordos Block was found to be characterized by distinctly high seismic velocities at depths commonly associated with a low velocity asthenosphere. To the south, abnormally slow velocity anomalies appeared in the Weihe rift, probably indicating subsolidus grain-boundary weakening effect under high temperatures near solidus instead of partial melt in the asthenosphere. An intermediate seismic velocity structure was observed beneath the Northern Qinling orogen (NQL) to the south of the Weihe rift. A thin, high velocity lid, which was probably a cratonic relic, was found under the Haiyuan arcuate tectonic region (HATR) west of the Ordos Block. A thick lithosphere was present beneath the Ordos Block (> 200 km) and the NQL (~ 140 km), while a thinner lithosphere (~ 100 km) was found beneath the HATR and the Weihe rift. The observed structural differences were likely associated with different lithospheric tectonics in the study region. Whereas the Weihe rift and the HATR had been strongly affected by Cenozoic lithospheric modification and thinning, the Ordos Block and the NQL appeared to have been much less influenced. Combined with previous studies, our results suggested that the far-field effect of the Indo–Asia collision may have played an important role in the unique lithospheric process in the study region, such as the extension of the Weihe rift, the preservation of a cratonic relic under the HATR, and the localized intense lithospheric modification of these two regions. The substantial variations of deep structure and thus contrasting rheological properties between the Ordos Block and the surrounding notable seismic belts (the HATR and Weihe rift) may be a candidate mechanism for the contrasting seismicity of the region. The lateral variations in the lithospheric structure estimated here generally agreed with changes in upper mantle deformation patterns previously derived from SKS splitting measurements.