The lithosphere－asthenosphere： Italy and surroundings

The velocity-depth distribution of the lithosphere-asthenosphere in the Italian region and surroundings is imaged, with a lateral resolution of about 100 km, by sur-face wave velocity tomography and non-linear inversion.Maps of the Moho depth, of the thickness of the lithos-phere and of the shear-wave velocities, down to depths of 200 km and more, are constructed. A mantle wedge, iden-tified in the uppermost mantle along the Apennines and the Calabrian Arc, underlies the prmctpat recent votca-noes, and partial melting can be relevant in this part of the uppermost mantle. In Calabria, a lithospheric dou-bling is seen, in connection with the subduction of the Ionian lithosphere. The asthenosphere is shallow in the Southern Tyrrhenian Sea. High velocity bodies, cutting the asthenosphere, outline the Adria-lonian subduction in the Tyrrhenian Sea and the deep-reaching lithospheric root in the Western Alps. Less deep lithospheric roots are seen in the Central Apennines. The lithosphere-asthenos-phere properties delineate a differentiation between the northern and the southern sectors of the Adriatic Sea,likely attesting the fragmentation of Adria.

Mantle Ichor HACONS——Evidence of Its Existence(Ⅰ)

A HACONS,mantle fluid, has been proposed in the light of current knowledge concerning Diwa,rift ,asthenosphere,anomalous and depleted mantle,plume, percolating magma fluid, degassing,inorganic origin of some oil and gas,hypogeic salt deposits,alkali-metasomatism in hydrothermal depos-its and experimental studies of basalts.The HACONS fluid originated from the mantle and is composed of hydrogen and halogens(H), alkalis(A), carbon(C), oxygen(O),nitrogen(N) and sulfur(S) com-pound systems .Aluminosilicate magmas are expected when the HACONS fluid reacts with solid rocks.Many geological processes,including tectonism, magmatism,volcanism,metamorphism,hydrothermal activity,thermal sedimentation and related mineralizations are thought to be controlled to a certain extent by large -scale mouement of HACONS fluid in the earth's interior.The asthenosphere is essentially a layer with abundant HACONS fluid.

New discoveries challenge traditional view on the homogeneity of the asthenosphere

A recent study published in 20 March issue of the prestigious journal Nature by Dr. LIU Chuanzhou from the CAS Institute of Geology & Geophysics and his co-workers challenges the conventional wisdom that the Earth's upper mantle is homogeneous. Teaming up with his colleagues from both Germany and US, Dr. Liu have conducted mineral, petrological and geochemical studies on abyssal peridotites selected from two dredge hauls in the ultraslow spreading Gakkel ridge, Arctic Ocean. Peridotite samples from one of these two dredge hauls are rarely fresh.

Study on crustal, lithospheric and asthenospheric thickness beneath the Qinghai-Tibet Plateau and its adjacent areas

Based on the results of pure dispersions of Rayleigh wave tomography in the Qinghai-Tibet Plateau and its adjacent areas, taking S wave velocities from previous linear inversion as the initial model, us-ing the simulated annealing algorithm, a nonlinear simultaneous inversion has been carried out for S wave velocity and thickness of different layers, including the crust, the lithosphere and the astheno-sphere. The results indicate: The crustal thickness shows strong correlation with geology structures sketched by the sutures and major faults. The crust is very thick in the Qinghai-Tibet Plateau, varying from 60 km to 80 km. The lithospheric thickness in the Qinghai-Tibet Plateau is thinner (130―160 km) than its adjacent areas. And two blocks can be recognized, divided by an NNE strike boundary running between 90°E―92°E inside the plateau. Its asthenosphere is relatively thick, varies from 150 km to 230 km, and the thickest area is located in the western Qiangtang. India has a thinner crust (32―38 km), a thicker lithosphere of 190 km and a rather thin asthenosphere of only 60 km. Sichuan and Tarim basins have the crust thickness less than 50 km. Their lithospheres are thicker than the Qinghai-Tibet Plateau, and their asthenospheres are thinner. A discussion has been made on the character and formation mechanism of the typical crust-mantle transition zone in the western Qiangtang block.

Dynamics of thinning and destruction of the continental cratonic lithosphere: Numerical modeling

Thinning of the cratonic lithosphere is common in nature, but its destruction is not. In either case, the mechanisms for both thinning and destruction are still widely under debate. In this study, we have made a review on the processes and mechanisms of thinning and destruction of cratonic lithosphere according to previous studies of geological/geophysical observations and numerical simulations, with specific application to the North China Craton(NCC). Two main models are suggested for the thinning and destruction of the NCC, both of which are related to subduction of the oceanic lithosphere. One is the "bottom-up" model, in which the deeply subducting slab perturbs and induces upwelling from the hydrous mantle transition zone(MTZ). The upwelling produces mantle convection and erodes the bottom of the overriding lithosphere by the fluid-meltperidotite reaction. Mineral compositions and rheological properties of the overriding lithospheric mantle are changed, allowing downward dripping of lithospheric components into the asthenosphere. Consequently, lithospheric thinning or even destruction occurs. The other is the "top-down" model, characterized by the flat subduction of oceanic slab beneath the overriding cratonic lithosphere. Dehydration reactions from the subducting slab would significantly hydrate the lithospheric mantle and decrease its rheological strength. Then the subduction angle may be changed from shallow to steep, inducing lateral upwelling of the asthenosphere. This upwelling would heat and weaken the overriding lithospheric mantle, which led to the weakened lithospheric mantle dripping into the asthenosphere. These two models have some similarities, in that both take the subducting oceanic slab and relevant fluid migration as the major driving mechanism for thinning or destruction of the overriding cratonic lithosphere. The key difference between the two models is the effective depth of the subducting oceanic slab. One is stagnation and flattening in the MTZ, whereas the other is flat subduction at the bottom of the cratonic lithosphere. In the NCC, the eastern lithosphere was likely affected by subduction of the Izanagi slab during the Mesozoic, which would have perturbed the asthenosphere and the MTZ, and induced fluid migration beneath the NCC lithosphere. The upwelling fluid may largely have controlled the reworking of the NCC lithosphere. In order to discuss and analyze these two models further, it is crucial to understand the role of fluids in the subduction zone and the MTZ. Here, we systematically discuss phase transformations of hydrous minerals and the transport processes of water in the subduction system. Furthermore, we analyze possible modes of fluid activity and the problems to explore the applied feasibility of each model. In order to achieve a comprehensive understanding of the mechanisms for thinning and destruction of cratonic lithosphere, we also consider four additional possible dynamic models: extension-induced lithospheric thinning, compression-induced lithospheric thickening and delamination, large-scale mantle convection and thermal erosion, and mantle plume erosion. Compared to the subduction-related models presented here, these four models are primarily controlled by the relatively simple and single process and mechanism(extension, compression, convection, and mantle plume, respectively), which could be the secondary driving mechanisms for the thinning and destruction of lithosphere.