A preliminary analysis on lithospheric thickness and crustal ages in central basin of South China Sea

The models about lithospheric thickness and thermal conduction inside the lithosphere and the top layer of the asthenosphere have been proposed in this study for four type regions: the midoceanic ridge, the extinct spreading ridge, the lithospheric fault fissure and the mouth of the extinct submarine volcanoes which are in deep sea bottom. The solutions of the models are found to be the same. The formulas of temperature distribution inside the lithosphere and the top layer of the asthenosphere, the lithospheric thicknesses to the heat flow and the crustal ages to the heat flow are obtained. The crustal ages and the lithospheric thicknesses of the central basin are calculated. And they are used to draw the lithospheric thicknesses and crustal ages maps of the central basin (in this paper both the central basin and the basin are the central basin of the South China Sea). According to their characteristics, the central basin is divided into three regions. The lithospheric thicknesses, crustal ages and heat flow distribution characteristics are discussed respectively. The formation and evolution of the South China Sea are analysed and it is thought that the South China Sea has undergone three episode-seafloor spreadings.

Thermal Lithospheric Thickness of the Sichuan Basin and its Geological Implications

Thermal lithospheric thickness is an important parameter in studying the tectonic-thermal evolution of basins and plate dynamics.Based on the measured geothermal data and thermophysical properties of the rocks,the thermal lithospheric thickness of the Sichuan Basin was calculated according to the principles of heat conduction in the crust and lithospheric mantle.The calculation results revealed that the thickness of the thermal lithosphere in the Sichuan Basin is 140-190 km and is unevenly distributed.The thickness of the thermal lithosphere in central Sichuan and southwestern Sichuan is less than 160 km,while that in the western Sichuan depression and eastern Sichuan is larger(~180 km).The distribution of the thermal lithospheric thickness in the basin has a good correlation with the geological units and the thickness of the sedimentary layers.The thickness of the thermal lithosphere in the depression area,which has thick sedimentary layers and the fault-fold zone with shallow crustal deformation and thickening,are larger than that in the basement uplifted area,which has thin sedimentary layers.The calculated thermal lithospheric thickness is in good agreement with the geophysical data and reflects the stable conduction temperature field in the Sichuan Basin.The present thermal regime and thermal lithospheric thickness of the Sichuan Basin indicate that flexural thickening of the lithosphere occurred in the eastern Sichuan fault-fold belt and the Longmen Mountain-Western Sichuan depression foreland basin system,while asthenospheric uplift occurred in the central Sichuan region,which were the result of the expansion of the Xuefeng orogeny from the east and the compression of the Tibetan Plateau from the west.

Three-dimensional admittance analysis of lithospheric elastic thickness over the Louisville Ridge

Using bathymetry and altimetric gravity anomalies, a 1°×9 1° lithospheric effective elastic thickness（Te） model over the Louisville Ridge and its adjacent regions is calculated using the moving window admittance technique. For comparison, three bathymetry models are used： general bathymetric charts of the oceans, SIO V15.1,and BAT_VGG. The results show that BAT_VGG is more suitable for calculating T e than the other two models. T e along the Louisville Ridge was re-evaluated. The southeast of the ridge has a medium Te of 10–20 km, while Te increases dramatically seaward of the Tonga-Kermadec trench as a result of the collision of the Pacific and IndoAustralian plates.

A Thickness Gauge for the Lithosphere Based on Ce/Yb and Sm/Yb of Mantle–Derived Magmatic Rocks

A new method for determining the partial melting depth of mantle-derived magma and lithospheric thickness in continental regions is derived from REE geochemistry. This effective technique uses variations in the Ce/Yb and Sm/Yb ratios found in mainly volcanic rocks in continental China. The ratios change with the depth of origin consistent with the correlation between lithospheric thickness and the Ce/Yb and Sm/Yb ratios found in oceanic basalt. These ratios increase exponentially with the depth of origin, the lithospheric thickness, of a wide variety of Cenozoic volcanic basalt and Paleozoic kimberlite in the North China Craton, northeastern China continent and vicinity. This functional relationship with depth is shown in a plot of the ratios that forms a concordia curve, which is closely expressed by formulas using 8–degree polynomials. These provide a more accurate gage in measuring the lithospheric thickness than the traditional geophysical methods. When applied to volcanic rock of different ages it also reveals how the thickness has changed over time and thus, greatly aids the understanding of the tectonic history. Relations between the COcontent, mineral reactions and pressure in the upper asthenosphere beneath the base of the lithosphere appears to affect the proportions of REE in partial melts and brings about a close correlation between lithospheric thickness and the Ce/Yb and Sm/Yb ratios in mantle–derived magmatic rock. This thickness gauge, for both continental and oceanic lithosphere, provides a new approach in analyzing the lithospheric thickness in different tectonic settings and geologic times.

The effect of altimetry data in estimating the elastic thickness of the lithosphere in the western Pacific Ocean

The elastic thickness of the lithosphere(Te)is a key parameter used to describe the strength of the lithosphere.It is usually estimated by a spectral analysis between gravity and topography.In previous research on the estimation of Te,altimetry data were used on both the gravity data and topography data,which could lead to deviations.The study described in this paper analyzed the effects of using gravity anomalies derived from different data sources on the estimation of Te,Taking the western Pacific region as an example,this study analyzed the impact of the repeated presence of altimetry satellite data on the calculation of the effective elastic thickness and found that if gravity anomalies and topography model both contain altimetry satellite data,they systematically overestimate effective elasticity.For a uniform area,the difference in Te can reach up to 30%.For a Te distribution,the difference can reach up to about16%.After eliminating this effect,the effective elastic thickness of the western Pacific region was found to be 10 km,and the statistical results of the effective elastic thickness distribution showed that the effective elastic thickness of the lithosphere in most areas of the western Pacific is about 12 km.The paper shows the importance of choosing the appropriate gravity model in evaluating the elastic thickness of lithosphere in the oceans.A figure of Te at seamounts with loading ages demonstrates that Te in the western Pacific is generally distributed within the 100-300℃isotherm depth and does not increase with loading age.

Last deglacial relative sea level variations in Antarctica derived from glacial isostatic adjustment modelling

We present relative sea level （RSL） curves in Antarctica derived from glacial isostatic adjustment （GIA）predictions based on the melting scenarios of the Antarctic ice sheet since the Last Glacial Maximum （LGM）given in previous works.Simultaneously,Holocene-age RSL observations obtained at the raised beaches along the coast of Antarctica are shown to be in agreement with the GIA predictions.The differences from previously published ice-loading models regarding the spatial distribution and total mass change of the melted ice are significant.These models were also derived from GIA modelling; the variations can be attributed to the lack of geological and geographical evidence regarding the history of crustal movement due to ice sheet evolution.Next,we summarise the previously published ice load models and demonstrate the RSL curves based on combinations of different ice and earth models.The RSL curves calculated by GIA models indicate that the model dependence of both the ice and earth models is significantly large at several sites where RSL observations were obtained.In particular,GIA predictions based on the thin lithospheric thickness show the spatial distributions that are dependent on the melted ice thickness at each sites.These characteristics result from the short-wavelength deformation of the Earth.However,our predictions strongly suggest that it is possible to find the average ice model despite the use of the different models of lithospheric thickness.By sea level and crustal movement observations,we can deduce the geometry of the post-LGM ice sheets in detail and remove the GIA contribution from the crustal deformation and gravity change observed by space geodetic techniques,such as GPS and GRACE,for the estimation of the Antarctic ice mass change associated with recent global warming.

Crust and upper mantle structure of East Asia from ambient noise and earthquake surface wave tomography

The complex tectonic background of East Asia makes it an ideal region for investigating the evolution of the continental lithosphere,for which high-resolution lithospheric structural models are essential.In this study,we measured Rayleigh-wave phase-velocity dispersion curves at periods of 10-120 s and group velocity dispersion curves at periods of 10-140 s using event records from more than 1,000 seismic stations in and around China.By jointly inverting new and previously published dispersion data from ambient noise and earthquakes,we developed a high-resolution shear-wave velocity model down to a depth of~300 km beneath East Asia.Our model revealed heterogeneous lithospheric structures beneath East Asia,and allowed us to investigate the velocity structure of the entire lithosphere.We also derived crustal and lithospheric thickness models from the three-dimensional(3D)shear-wave model,revealing strong spatial heterogeneity and a general thinning trend of lithospheric thickness from west to east across the study region.Overall,our models reveal important lithospheric features beneath East Asia and provide a valuable baseline dataset for understanding continental-scale dynamics and evolution.

Global volcanic arc magma composition correlates with thickness of both arc crust(Moho depth)and arc lithosphere(LAB depth):A revealing message on arc basement histories and arc magmatism

Past fifty years have seen mounting publications on the genesis of volcanic arc magmas.While details remain debated,it is generally agreed that arc magmas result from slab-dehydration induced mantle wedge melting followed by crustal level differentiation of varying extent and sophistication.Two recent arc magma studies deserve particular attention because they attempt to discuss globally unifying controls on arc magma composition.Both Harvard study(Turner and Langmuir,2015a,b)and Rice study(Farner and Lee,2017)show correlations of arc magma composition with crustal thickness and both ascribe the crustal thickness as the principal control on their observed magma compositional variations,yet the physical role of the crustal thickness in their interpretations is markedly different because of(1)the ambiguous use of“crust”and(2)their different magma compositional ranges chosen in discussion.The Harvard study only uses basaltic samples corrected to MgO=6.0 wt.%to discuss mantle processes and interprets the arc crustal thickness as restricting the mantle wedge melting,i.e.,the extent of melting decreases with increasing crustal thickness.The Rice study uses samples of all compositions(basaltic to rhyolitic),whose extent of differentiation increases with increasing crustal thickness,interpreted as Moho-crossing mantle wedge melts travelling greater vertical distance with greater degree of cooling and erupting more evolved compositions above thicker crust than melts erupted above thinner crust without need of invoking mantle wedge processes.We commend these efforts and approve their different approaches but emphasize that the unifying understanding of global arc magmatism requires clearly defined Moho(the base of the crust)and LAB(the lithosphere-asthenosphere boundary)and their intrinsic controls on mantle wedge melting(Harvard Study model)and crustal level magma differentiation(Rice Study model)beneath global arcs.In this study,we use chemical compositions of 36,945 global arc volcanic samples provided by the Rice study together with the literature data on seismic Moho and LAB depths of these sample locations to establish(1)the correlation of crustal thickness interval averaged magma composition with crustal thickness and(2)the correlation of lithosphere thickness interval averaged magma composition with lithosphere thickness.These correlations reaffirm our understanding that the lithospheric mantle must exist beneath volcanic arc crust with a globally averaged LAB/Moho depth ratio of 3.26±0.63,i.e.,the arc crust is on average about 31.8%±6.1%of the lithosphere thickness.This knowledge forms a solid constraint essential for models of global arc magmatism.

The deep thermal structure of the lithosphere in the northwestern South China Sea and its control on the shallow tectonics

The northwestern sub-basin of South China Sea(SCS)is a unique tectonic unit formed in the early spreading of the SCS.The northwestern Sub-basin has a series of complex geological structures such as seamounts and fault zones surrounded by the Xisha Trough,the Zhongsha Massif,and the Pearl River Valley.These extensional structures and magmatic activity in the northwestern sub-basin are closely related to the lithospheric structure and its deformation.However,details of the deep lithosphere structure are still poorly known.Here,we obtained detailed data of water and Moho depth using sonar buoys,Extended Spread Profiles(ESP),Ocean Bottom Seismometer(OBS),both Multi-beam and land-sea joint seismic surveys in the northwestern sub-basin and its surrounding areas.Then we adopted a thermal isostasy method to calculate the depth of the Lithosphere-Asthenosphere Boundary(LAB)in the northwestern sub-basin of the SCS and its surrounding regions.Results show that the range of LAB depth is~25–110 km.The shallowest burial depth is 25–60 km occurring in the ocean basin.The depth increases to 60–110 km toward the continental margin.The lithospheric structure on the north and south sides of the Xisha Trough is symmetrical and shows the deep structure and thermal features of aborted rifts.The LAB depth in the Zhongsha Trough and the Zhongsha Massif increased from 60 to 70 km southwestwards,consistent with the trend of surface morphology.The LAB depth to the west side of the Pearl River Valley is 60–80 km,and the thinning of the lithosphere is related to the distribution of faults,depressions and the magmatic activity.The LAB depth in the northwestern sub-basin and the eastern subbasin is less than 60 km with the thinnest part being less than 46 km.Combining ocean drilling,seismic investigation,and seafloor topography,we show that the ocean basin of the northwestern sub-basin of the SCS locates within the 46 km isobath of the LAB.The formation of the rifted valleys and discrete blocks surrounding the ocean basins is both controlled by the regional tectonic movement and the deep thermal state,where their lithospheric structures show strong heterogeneity.