Dominant Lid Tectonics behaviour of continental lithosphere in Precambrian times:Palaeomagnetism confirms prolonged quasi-integrity and absence of supercontinent cycles

Although Plate Tectonics cannot be effectively tested by palaeomagnetism in the Precambrian aeon due to the paucity of high precision poles spanning such a long time period,the possibility of Lid Tectonics is eminently testable because it seeks accordance of the wider dataset over prolonged intervals of time;deficiencies and complexities in the data merely contribute to dispersion.Accordance of palaeomagnetic poles across a quasi-integral continental crust for time periods of up to thousands of millions of years,together with recognition of very long intervals characterised by minimal polar motions(～2.6-2.0,～1.5-1.25 and～0.75-0.6 Ga)has been used to demonstrate that Lid Tectonics dominated this aeon.The new PALEOMAGIA database is used to refine a model for the Precambrian lid incorporating a large quasiintegral crescentric core running from South-Central Africa through Laurentia to Siberia with peripheral cratons subject to reorganisation at～2.1,～1.6 and～1.1 Ga.The model explains low levels of tidal friction,reduced heat balance,unique petrologic and isotopic signatures,and the prolonged crustal stability of Earth's"Middle Age",whilst density concentrations of the palaeomagnetic poles show that the centre of the continental lid was persistently focussed near Earth's rotation axis from～2.8 to 0.6 Ga.The exception was the～2.7-2.2 Ga interval defined by～90°polar movements which translated the periphery of the lid to the rotation pole for this quasi-static period,a time characterised by glaciation and low levels of magmatic activity;the～2.7 Ga shift correlates with key interval of mid-Archaean crustal growth to some 60-70%of the present volume and REE signatures whilst the～2.2 Ga shift correlates with the Lomagundiδ～(13)C and Great Oxygenation events.The palaeomagnetic signature of breakup of the lid at～0.6 Ga is recorded by the world-wide Ediacaran development of passive margins and associated environmental signatures of new ocean basins.This event defined the end of a dominant Lid Tectonic phase in the history of Earth's continental lithosphere recorded by the quasi-integral Precambrian supercontinent Palaeopangaea and the beginning of the comprehensive Plate Tectonics which has characterised the Phanerozoic aeon.Peripheral modifications to the lid achieved a symmetrical and hemispheric shape in Neoproterozoic times comparable to the familiar short-lived supercontinent(Neo)Pangaea(～350-150 Ma)and this appears to be the sole supercontinent cycle recorded by the palaeomagnetic record.Prolonged integrity of a large continental nucleus accompanied by periodic readjustments of peripheral shields can reconcile divergent tectonic analyses of Precambrian times which on the one hand propose multiple Wilson Cycles to explain some signatures of Plate Tectonics,and alternative interpretations which consider that Plate Tectonics did not commence until the end of the Neoproterozoic.     

普若岗日冰原西侧及念青唐古拉山西坡侧碛垄形态差异及气候构造成因

冰碛垄的几何学特征反应了它形成时的各种影响因素,包括构造状况、气候条件.而对冰碛垄进行高程测量,并应用数值模拟技术反演其形态,可以荻取高度、坡度、坡向等数据进行分析.文中应用该方法对青藏高原腹地普若岗日冰原一条侧碛垄以及东南部念青唐古拉山西端南坡的三条侧碛垄进行处理,结果表明普若岗日的侧碛垄规模比念青唐古拉山的侧碛垄规模小得多,普若岗日处侧碛垄的坡度也较缓,且坡向数据表明侧碛垄几何性质不对称.结合气候、构造等资料对它们的形态学参数进行对比分析表明,气候是造成两地侧碛垄规模差异的主要原因,坡度的差异受气候与构造双重作用影响.从两地坡度的差异可以推导出念青唐古拉山侧碛垄的形成年代比普若岗日地区的要晚.根据普若岗日侧碛垄几何性质的不对称,结合遥感资料解译,判断该区末次冰期以来有正断层活动,断层面倾向北,上盘在南边.     

中国南海天然气水合物沉积成藏条件初探及其分布

中国南海是西太平洋最大的边缘海之一 ,具备良好的天然气水合物成藏条件和勘探前景。根据沉积条件 (沉积速率、沉积相和含砂率等 )分析 ,综合多方面的地质条件 ,认为气水合物存在于水深30 0～ 35 0 0m的区域 ;陆坡和靠近沉积中心的高沉积速率的厚层沉积为天然气水合物聚集的有利部位 ,与BSR分布的吻合率在 6 0 %～ 80 %之间 ;等深流、重力流沉积和各类扇体 (如三角洲、扇三角洲、低位扇等 )的前缘为水合物沉积的有利相带 ,与BSR分布的吻合率在 5 0 %～ 75 %之间 ;赋存气水合物的沉积物的含砂率通常在 35 %～ 70 %之间。依据多方面资料的研究 ,在南海海域划分出 1 1个有利的天然气水合物勘探远景区块 ,面积共 1 2 .5 (± 1 .5 )× 1 0 4km2 ,约占整个南海总面积的 3.6 %。     

地质学的若干问题

对地质学几个领域近几十年的发展做了扼要的总结,指出了一些有突破性的进展,并提出了这些学科今后若干年可能的发展展望,论文涉及的学科包括矿物学、花岗岩、铁镁－超铁镁岩、变质岩、沉积学、多重地层分类与地质年代学、构造这以及板块构造学与大陆动力学。对于各学科并非全面的论述,而是指出其有重大意义的突破性进展,在矿物学方面强调了由于新技术的运用而大大地促进了矿物学的进展,矿物材料学和环境矿物学都是今后着重发展的方向。总结了当前对花岗岩的基本认识,指出应开展“岩浆系统”与“火山系统”的深层次研究;对于铁镁－超铁镁岩今后要加强壳幔物质的相互作用与深部过程的研究;总结了变质岩研究的三个阶段,今后要在重点地质区坚持长期深入系统的研究,特别是前寒武纪麻粒岩区和造山带。沉积学最近有较大发展,有三方面的意义;在构造地质学方面指出在大陆造山带伸展构造、韧性剪切等研究热点以及从定性到定量的研究趋势;板块构造学是地球科学有史以来最伟大的成就,90年代是大陆构造研究的大发展时期。     

http://www.sciencedirect.com/science/article/pii/S1674987112001065

It has been thought that granitic crust,having been formed on the surface,must have survived through the Earth’s evolution because of its buoyancy.At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction,subduction erosion, and continental subduction.Granitic rocks,the major constituent of the continental crust,are lighter than the mantle at depths shallower than 270 km,but we show here,based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone,and thus can be subducted in the depth range of 270-660 km.This suggests that there can be two reservoirs of granitic material in the Earth,one on the surface and the other at the base of the mantle transition zone(MTZ).The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust.Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle.This could explain the anomalous seismic-wave velocities observed around 660 km depth.The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation,chemical discontinuities between granitic material and the surrounding mantle,or a combination thereof.     

A late Cenozoic Earth's crust and climate dynamics record from Lake Baikal

The sedimentary record from Lake Baikal (Siberia, Russia) has been an important source of information about paleoclimatic variability in the northern hemisphere and dynamics of continental rift development. A lack of reliable chronology has, however, been a major obstacle to fully utilizing the Baikal archive for time scales beyond about 4-5 Myr. In this paper we use the distribution of ¹⁰Be to establish a new chronology for the longest core drilled in Lake Baikal so far. The ¹⁰Be-based chronology spans the last 8 Myr and provides better constraints on sedimentation rates and consequently on an east-west tectonic extension in the lake, which has been apparently coeval with other rifts in Asia that are related to the Tibetan plateau uplift. Our data also show higher ¹⁰Be flux in the sediment section older then 5 Myr compared with the younger period. This can be explained partly by warm and humid climatic conditions of the Miocene and partly by a high cosmic ray flux to the Earth resulting from possible low geomagnetic field intensity during that time.