Search

We present a new reference model for the evolution of oceanic lithosphere, which incorporates the effects of incomplete viscous relaxation, radiogenic heating, and secular cooling. The new reference model is based solely on thermal conduction, that is, without involving the occurrence of small‐scale convection, and unlike the plate model, it does not contain unphysical boundary conditions. Yet, our model can explain both bathymetry and the heat flow data on the normal seafloor. The success of the new model owes to the use of realistic material properties in conduction modeling as well as the consideration of all of major processes that take place ubiquitously beneath seafloor. The effect of secular cooling on the bathymetry of old seafloor is particularly notable. Whereas secular cooling brings only weak temperature variations with an amplitude of ∼20 K, it can nonetheless affect global bathymetry substantially owing to the deep sensitivity of long‐wavelength topography kernels. We suggest that the well‐known fact that Earth has been cooling, which was not considered in any of previous reference models, may be the key to the long‐standing puzzle of seafloor flattening. The new reference model is expected to be useful to better quantify the impact of the emplacement of hotspot islands and oceanic plateaus, the effect of small‐scale convection, and the regional history of secular cooling in the convecting mantle. Plain Language Summary: Seafloor deepens as it moves away from mid‐ocean ridges, and this subsidence reflects how the suboceanic mantle loses its heat by conduction. When seafloor becomes older than 70 million years old, however, it starts to deviate upward from what is predicted by the simple law of thermal conduction. A common approach to model such a deviation is to adopt the so‐called plate model, which can suppress seafloor subsidence with a constant temperature boundary condition at a shallow depth (120–140 km), although the real Earth does not contain such a boundary. Here we show that, by taking into account all of major processes intrinsic to the suboceanic mantle, from the cold shallow part to the hotter deep part, it is possible to explain the evolution of seafloor topography as well as heat loss, without invoking an unphysical boundary condition. In particular, this study illustrates that the fact that Earth is cooling, which is long known in Earth sciences, can have considerable effects on the large‐scale behavior of ocean basins. Key Points: A new reference model for the evolution of normal oceanic lithosphere is proposedThe model is based solely on thermal conduction and yet free of unphysical boundary conditionsThe model incorporates the effects of incomplete viscous relaxation, radiogenic heating, and secular cooling [ABSTRACT FROM AUTHOR]

We used ambient noise data recorded by the NECESSArray, a portable seismic array deployed in Northeast China, to construct Rayleigh-wave phase velocity dispersion curves and obtain phase velocity maps for periods from 6 to 40 s. S-wave velocity structure beneath NE China is obtained by joint inversion of the phase velocities and receiver functions. Our results show good correlation between crustal velocity structure and geological features. At shallow depths the Songliao Basin is separated into two small sub-basins by a fast velocity anomaly corresponding to a southwest uplifted horst and the southern sub-basin is associated with the Kailu depression. A low-velocity anomaly extending from the lower-crust to uppermost mantle beneath the Changbaishan mountain range is consistent with P- and S-wave tomography models. The mantle low velocity has been interpreted as a local mantle upwelling attributed to the subducting Pacific slab induced return flow. Finally, the S-wave velocity in the mid-and-lower crust is lower beneath the Xing'an–Mongolia Orogenic Belt (XMOB) than beneath the Changbaishan mountain range. This was not expected since the Changbaishan mountain range has experienced more extensive volcanic activity in the Cenozoic, including the currently active Changbaishan and Jingpohu volcanoes, than the XMOB. We infer that the mid-and-lower crust of the XMOB is mainly felsic in composition and propose that the low shear-wave velocity of the mid-and-lower crust beneath the XMOB could be the consequence of removal of deep mafic crust through a continental accretion and subsequent delamination event in the Paleozoic, or via convection removal induced by the rollback of the flat subducted Paleo-Pacific slab during the middle Mesozoic. [ABSTRACT FROM AUTHOR]

Abstract: We analyzed tens of thousands of receiver-function data recorded by 185 broadband seismic stations to study the crustal structure beneath the northeast China. Moho depth and average crustal ratio were measured at each station using the H–κ grid searching technique. For stations located above unconsolidated sediments, we applied the H–β method to remove strong shallow reverberations and generate subsurface receiver functions that allow for effective H–κ analysis. The measurements show that the Songliao basin has a relatively thin crust (∼31 km), and the Moho depth increases significantly from southeast (∼27 km) to northwest (∼35 km). The northwestward tilting of the Songliao basin may suggest that it was initiated by lithospheric flexure due to the load of the Great Xing'an range in the Jurassic before the large-scale extension in the Cretaceous. Moho depth varies from 26.7 km to 42.3 km across the study area, with the shallowest and deepest Moho being located at the eastern flank of the Songliao basin and the Great Xing'an range, respectively. The Moho depth correlates well with the surface topography in the western part, but not the central and eastern parts of the study area. The residual topography computed based on the Airy's isostasy model indicates that the high topography at the eastern flank of the Songliao basin, the Changbaishan volcanic center, and the southern end of the Great Xing'an range is likely dynamically supported by the upper mantle. [Copyright &y& Elsevier]