Reproducing past subduction and mantle flow using high-resolution global convection models

Plate subduction drives both the internal convection and the surface geology of the solid Earth.Despite the rapid increase of computational power,it remains challenging for geodynamic models to reproduce the history of Earth-like subduction and associated mantle flow.Here,based on an adaptive approach of sequential data assimilation,we present a high-resolution global model since the mid-Mesozoic.This model incorporates the thermal structure and surface kinematics of tectonic plates based on a recent plate reconstruction to reproduce the observed subduction configuration and Earth-like convection.Introduction of temperature-and composition-dependent rheology allows for incorporation of many natural complexities,such as initiation of subduction zones,reversal of subduction polarity,and detailed plate-boundary dynamics.The resultant present-day slab geometry well matches Benioff zones and seismic tomography at depths < 1500 km,making it possible to hindcast past subduction dynamics and mantle flow.For example,the model produces a flat Farallon slab beneath North America during the Late Cretaceous to Early Cenozoic,a feature that has been geodynamically challenging to reproduce.This high-resolution model can also capture details of the 4-D evolution of slabs and the ambient mantle,such as temporally and spatially varying mantle flow associated with evolving slab geometry and buoyancy flux,as well as the formation of shallow slab tears due to subduction of young seafloors and the resulting complex mantle deformation.Such a geodynamic framework serves to further constrain uncertain plate reconstruction in the geological past,and to better understand the origin of enigmatic mantle seismic features.

Global seismic tomography reveals remnants of subducted Tethyan oceanic slabs in the deep mantle

The Tethyan evolution depicts the continuous process of landmasses separating from the Gondwana continent in the south,drifting northwards,and subsequently colliding with the continents in the north over the past 500 million years.In this process,the Tethyan oceans that formed between the landmass and the southern or northern continents underwent growth,evolution,and eventual closure with the early Cenozoic India-Eurasia collision.However,the Tethyan lithosphere did not disappear but rather continued to evolve after entering into the deep Earth.The current position,morphology,and volume of the subducted Tethyan oceanic slabs in the deep mantle record the latest moment of this continuous evolution,providing critical constraints for Tethyan studies.This paper summarizes and analyzes the results of global-scale whole-mantle seismic tomography in the past nearly two decades,revealing a northwest-southeast seismically high-velocity anomaly,which is linearly distributed at depths of 1000–2000 km beneath the Tethyan realm and referred to as the Tethyan anomaly.By searching for an optimal linear combination of previous global seismic tomographic models to best match the known subducted slabs in the upper mantle,we observe that the Tethyan anomaly extends approximately 8700 km in length and 2600 km in width,exhibiting a parallel structure with northern and southern branches.Combining geological records of oceanic subduction initiation and previous geodynamic studies,this study suggests that the main body of the Tethyan anomaly represents the remnants of the subducted Neo-Tethyan oceanic slabs,which subducted from the Late Jurassic to the early Cenozoic.The northern branch consists of subducted slabs from the Neo-Tethys beneath the southern margin of Eurasia,while the southern branch likely reflects the intra-oceanic subducted slabs of Neo-Tethys during the Cretaceous.The western portion of the Tethyan anomaly may reflect remnants of Paleo-Tethys,while the eastern portion,towards India and the Bay of Bengal,shows signs of subduction towards the core-mantle boundary.Finally,this study discusses the future prospects of whole-mantle seismic tomographic studies focusing on the Tethyan realm.

Global geometrical optics method for vector-valued Schrodinger problems

We extend the theory of global geometrical optics method, proposed originally for the linear scalar high-frequency wave-like equations in [Commun. Math. Sci., 2013, 11（1）： 105-140], to the more general vector- valued Schrodinger problems in the semi-classical regime. The key ingredient in the global geometrical optics method is a moving frame technique in the phase space. The governing equation is transformed into a new equation but of the same type when expressed in any moving frame induced by the underlying Hamiltonian flow. The classical Wentzel-Kramers-Brillouin （WKB） analysis benefits from this treatment as it maintains valid for arbitrary but fixed evolutionary time. It turns out that a WKB-type function defined merely on the underlying Lagrangian submanifold can be obtained with the help of this moving frame technique, and from which a uniform first-order approximation of the wave field can be derived, even around caustics. The general theory is exemplified by two specific instances. One is the two-level SchrSdinger system and the other is the periodic SchrSdinger equation. Numerical tests validate the theoretical results.

Average BER of coherent optical QPSK systems with phase errors over M turbulence channels

The average bit-error-rate(BER) performance is studied for a coherent free-space optical communication system employing differentially encoded quadrature phase-shift keying(QPSK) with the Mth-power phase estimation method. A closed-form expression, considering the combined effects of the Málaga(M) turbulence fading, pointing errors, and phase estimation errors, is derived in terms of Meijer’s G function. Numerical and Monte Carlo simulation results are presented to verify the derived expression.