Sedimentary and crustal structures beneath the northern Ordos basin constrained by receiver functions and Pn multiples

Abstract

Knowledge of elastic properties of the Earth's crust provide important constraints on its chemical composition, isostasy and tectonic evolution. However, accurate determination of crustal properties beneath sedimentary basins is challenging. This challenge mainly arises from interference caused by sedimentary reverberations, which may either mask desired signals or cause significant bias in parameter estimates. Some studies attempted to remove the sediment effect by applying wavefield downward continuation or resonance filters to conventional receiver functions, but successful applications were limited. Recently, a novel method utilizing Pn and its multiples, named as PnPn, has been developed and proven effective in imaging the Moho beneath sedimentary basins. Arrival times of Pn multiples are sensitive to crustal thickness (H) and P wave speed (Vp). In contrast, arrival times of converted phases in receiver functions are most sensitive to crustal thickness and Vp/Vs ratio. In this study, we apply a joint analysis of the newly developed Pn multiple method and conventional receiver functions to investigate the sedimentary and crustal structures of the northern Ordos basin along a west-east trending profile. We first apply a multi-frequency receiver function waveform fitting technique to constrain the shallow sediment structure. Then, we combine receiver functions and Pn multiples to determine the thickness, Vp and Vp/Vs ratio of the crystalline crust. Our results show that the interior of the Ordos basin is characterized by thick sediments, with the maximum thickness reaching 4.6 km. The sediment thickness shoals toward the eastern margin of the Ordos basin. The sediment structure in general is consistent with previous findings from active source studies and is of higher resolution than previous passive source studies. For the crystalline crust beneath the northern Ordos basin, the absolute Vp ranges from 6.45 to 6.57 km/s and the Vp/Vs ratio ranges from 1.73 to 1.78. These values suggest an overall intermediate crustal composition beneath the northern Ordos basin, in contrast to felsic crustal composition beneath the eastern North China Craton. The crustal thickness in the interior of the northern Ordos basin is remarkably flat, approximately 40 km, closely aligning with the Airy model. However, a deviation from Airy isostasy of approximately 5 km in crustal thickness is observed at the eastern margin of the Ordos basin, which could be due to increased bulk density of the crust accompanying the thinning of low-density sedimentary layer.