Crustal Anisotropy Beneath the Trans-North China Orogen and its Adjacent Areas From Receiver Functions

Abstract

As an important segment of the North China Craton, the Trans-North China Orogen (TNCO) has experienced strong tectonic deformation and magmatic activities since the Cenozoic and is characterized by significant seismicity. To understand the mechanism of the crustal deformation and seismic hazards, we determined the crustal thickness (H), Vp/Vs ratio (κ) and crustal anisotropy (the fast polarization direction φ and splitting time τ) beneath the TNCO and its adjacent areas by analyzing receiver function data recorded by a dense seismic array. The (H, κ) and (φ, τ) at a total of 309 stations were measured, respectively. The Moho depth varies from ∼30 km beneath the western margin of the Bohai bay basin to the maximum value of ∼48 km beneath the northern Lüliang Mountain, which shows the positive and negative correlations with the elevation and the Bouguer anomaly. The average φ is roughly parallel to the strikes of the faults, grabens and Mountains in this study area, whereas a rotating distribution is shown around the Datong-Hannuoba volcanic regions. Based on the φ measured from the Moho Ps and SKS/SKKS phases, we propose that the crustal deformation and seismic hazards beneath the TNCO could be due to the counterclockwise rotation of the Ordos block driven by the far-field effects of the India-Eurasian collision.

Deformation of the Crust and Upper Mantle beneath the North China Craton and Its Adjacent Areas Constrained by Rayleigh Wave Phase Velocity and Azimuthal Anisotropy

“…The low-velocity anomalies beneath the Datong volcano at periods of 30-40 s (Figure 7c,d) shows the intrusion northwestward in the uppermost mantle with weaker azimuthal anisotropy due to the small range expansion in the strong lithosphere. However, the rotating fast axis around the Datong volcano (Figures 7c,d and S3) agrees well with the results obtained from Ps and SKS/SKKS splitting [22,24], affecting the whole lithosphere, which may represent stress-induced anisotropy caused by the mantle upwelling [22,55]. The low-velocity anomalies of 60-80 s along the southwest side of the Zhangbo fault zone (Figures 7e,f and S2), a noticeable seismic zone with strong azimuthal anisotropy (Figure 7e,f), suggest that the large-scale flow of the upwelling materials enhanced the observed azimuthal anisotropy.…”

“…The fast axis direction of the SKS/SKKS phases correlates well with the absolute plate motion (APM) direction, implying the APM-driven model of the upper mantle anisotropy in the NCC [29]. This model disagrees with the hypotheses inferred from stress-aligned microcracks, frozen fossil anisotropy and asthenospheric flow [22,25,26]. Compared with S-wave splitting, the surface wave azimuthal anisotropy estimated from different periods has a better vertical resolution and can provide a depth independent anisotropy to constrain the deformation of the crust and upper mantle.…”

“…The rotated direction of Paleo-Pacific plate subduction (Figure 1a) might be coupled to the crustal deformation pattern that provide the extensional-transtensional stress environment for promoting the counterclockwise rotation of the Ordos block [6,10]. This counterclockwise rotation pattern was also observed by the fast axis directions of the Ps and SKS/SKKS phases splitting (Figure 9) in the mid-lower crust and upper mantle [22,24,40]. It is concluded that the fast axis directions from S-wave splitting are produced by the extrusion of the lithospheric material of the Tibetan plateau [22,24,40].…”

“…Combined with the previous hypotheses [12][13][14][15], that the thinning of the cratonic lithosphere in the eastern NCC resulted from slab rollback related to the subduction of the Paleo-Pacific plate, we further infer that the asthenospheric flow from the mantle upwelling of the Datong volcano also contributed to this thinning (Figure 10). [22,26,40] and at period 40 s vs. the SKS/SKKS phases splitting [24,38] (a) Distribution of Moho depth (after Li et al [50] and Xu et al [22,40]). The black arrows indicate the absolute plate motion (APM) direction of the GSRMv2.1 model [39].…”

“…The Qinling-Dabie orogen between the Sichuan basin and the Ordos block was proposed as acting as an important channel for the eastward material escape of the Tibetan plateau [17,19]. This extrusion of the Tibetan plateau even affected the deformation of the crust and mantle in the Trans-North China orogen [20][21][22]. Therefore, the NCC suffers the westward subduction of the Paleo-Pacific plate in the east and eastward extrusion of the Tibetan plateau in the southwest, leading to significant deformation of the crust and upper mantle structure [10,11,23,24].…”