Crustal Density Structure And Lithospheric Effective Elastic Thickness Of The Ross Sea,west Antarctica And For Their Geodynamical Implications

Crustal density structure is a fundamental parameter to reveal its composition and tectonics which provide key information on understanding the basin's evolution and its geodynamics.Lithospheric effective elastic thickness(T_e)depends primarily on the thermal structure,composition and thickness and it reflects the responds of loads in long terms to the lithosphere.T_evalues refer to the lithospheric rheological structure and corresponds to the ability to resist its deformation.These two significant geophysical parameters,density andT_ecould be to analyze the crustal and rheological structure of Ross Sea,West Antarctica and to reveal the implications of thickness variations,thermal structure and mechanical strength to the tectonic evolution of the Ross Sea.The first phase of extension occurred in Late Cretaceous and may be related to the separation of New Zealand.During this period,the main N-S trending basins are formed.Until to Cenozoic,the opening of Adare Basin in the north study region as a remark,the second phase of extension commenced.The studies of airborne magnetic data show the anomalies continues from the Adare Basin to the Northern Basin,revealing the kinematic link between them.The youngest tectonic unit is located in the Victoria Land Basin,the Terror Rift,which started in 17 Ma.The evidence of seismic reflection profiles shows the fault cut through the seafloor,suggesting the activities may continue to the present.Compared with the first phase activity,the second one is only confined to the front of Transantarctic Mountains(TAMs)and it is possible that the newly activities in Cenozoic transits from the Adare Basin in the north to the Terror Rift in the south.In addition,in this period,the TAMs entered into phase of rapid uplift.The juxtaposition of TAMs and western Ross Sea indicates the relation of origin of geodynamics.The multi-phases of extension caused complicated geodynamic characteristics to the Ross Sea and further contributed to the underlying crustal structure and lithospheric deformation.Consequently,the recovering fine crustal density structure and estimating the variations ofT_ewill help shed new light onto the crustal structure and lithospheric strength of Ross Sea and the analyzing its implications to the tectonic evolution,lithospheric deformation of Ross Sea basins and the uplift of TAMs.In this study,based on the elevation,gravity anomaly,sediment thickness and other geophysical data,the three-dimensional gravity inversion algorithm and coherence technique to achieve the crustal density anomaly andT_evariations.By comparing the inverted density anomaly model with the published seismic refraction profile,the Moho depth and intra-crustal boundary are extracted to obtain the whole,upper and lower crustal thickness.The corresponding stretching factors are further estimated,?_w,?_u,and?_l.Moreover,as for theT_evalues,we tested the effect of gravitational noise to the results.Finally,by combining our results with geophysical and geological evidence,some conclusions are made as follows:(1)The inverted density structure shows that the Moho depth varies between 13and 18 km in the Ross Sea basin,while it increases to 24?30 km in the basin highs.The crustal thickness in the Eastern Basin is more flat and more broadly than other areas.In general,the thickness of lower crust is larger than that of upper crust and the difference is small in the sedimentary basins.The stretching factors indicates?_l>?_w>?_u,suggesting the non-uniform extension in the crust of Ross Sea basin,which is named as depth-dependent extension.The relatively highT_evalues in the central and eastern Ross Sea shows the lithosphere has the strength to support the loads during the sedimentation,indicating it is strong enough to resist the further extension of lower curst,we then rule out the possibilities that depth-dependent extension occurred in the post-rifting period.We discussed the evidence of petrology and numerical modeling,we speculated that the depth-dependent stretching is likely related to the presence of relatively low-viscosity lower curst during the syn-rift period.(2)Comparing the depth of the Moho with that of Curie point indicates that the depth of the Curie point exceeds that of the Moho in the Victoria Land Basin(VLB)and Central Trough,implying that the uppermost mantle magnetic.The high?_w values(>3)in these basins suggest that curst as a whole tends to be brittle based on the numerical models.During this process,faults that cut completely through the curst serve as conduits by which sea water enters the mantle,and this process can contribute to the serpentinization of the uppermost mantle.Additionally,the seismic refraction profile shows that the uppermost mantle beneath the VLB and Central Trough is characterized by reduced velocities,which is significant evidence of serpentinization.(3)The variations of lithospheric strength is different with that of crustal structure.In the western Ross Sea,a lowT_ezone parallel to the TAMs front,below10 km,suggesting the lithosphere is very weak due to the Cenozoic activities.The other areas in the Ross Sea basin,T_evalues are relatively high and change little,averaging 35 km.As for the East Antarctica,T_evalues increase to the hinterland of TAMs,showing the high strength in the craton.(4)Many studies indicate theT_evalues are related to the lithospheric strength when loads occur.The highT_evalues in the central and eastern Ross Sea reveal lithosphere is strong during sedimentation,unlikely the western Ross Sea where is re-weakened by the subsequent Cenozoic activities.We interpret the reason of high_T_evalues as the unique basin evolution history.After the basin opened,the basin has been“starved”for a very long time until the uplift of TAMs brings the sediments to the basins.And during the sedimentation,the underlying lithosphere becomes strong because of the thermal diffusion.(5)The low value lowT_ezone in the western Ross Sea is correlated to the Cenozoic tectonics.Based on the heat flow values,we divided the tectonic boundary between two phases of extension.Moreover,the lithospheric thermal structure of Ross Sea is classified as cold,hot and warm in the East Antarctica,western Ross Sea and central and eastern Ross Sea.(6)Similar to the western Ross Sea,the density and strength of TAMs is low,showing TAMs cannot be supported by its own strength.The studies of tomography indicate the roots of TAMs is small or even absent in some parts and is under less compensated.We summarized the main points about the uplift mechanism of TAMs and suggest the thermal driven mechanism is more consistent with our results.We favor the theory that the lateral thermal transition from hot western Ross Sea to the cold East Antarctica and the positive buoyancy beneath the TAMs supports is high elevation.