Enigmatic crustal and upper mantle structure in the NE Sino-Korean Craton based on nuclear explosion seismic data

摘要

We interpret the crustal and upper mantle structure along-2500 km long seismic profiles in the northeastern part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP) and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination dripping. An intra-lithospheric discontinuity (ILD) at-75 km depth with positive velocity contrast is probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long wave train of scattered phases of up to 4 s duration are observed in the 800-1300 km offset range, which are modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially molten or close to the solidus, which explains both low velocity and the heterogeneous structure.