晚新元古代-早寒武世现代板块构造与现代地球系统的建立

板块构造是维持岩石圈与表生系统之间物质与能量交换循环、保持地球生命力的一级动力学机制,但是板块构造在地球上并非与生俱来。板块构造的演化主要受控于地幔温度下降,并部分受俯冲带富水沉积物的影响。自太古代板块构造启动以来,其经历了热俯冲、浅俯冲为特征的早期板块构造阶段、地球中年期以及冷俯冲、深俯冲为特征的现代板块构造阶段。新元古代晚期,特别是显生宙以来,地球上开始广泛出现低温/压(T/P)比的超高压变质岩;与此同时,同样受控于地幔温度的马里亚纳型大洋初始俯冲蛇绿岩开始出现于地球上了。这显示新元古代以来,由于地幔温度下降,板块构造体制进入深俯冲阶段。在此阶段,随着冈瓦纳古大陆的逐步聚合,在530~520 Ma,其周缘形成了俯冲带并发生了全球性的构造联动。此外,活动陆缘数量、温压比(T/P)、锆石Hf-O同位素演化趋势等都在早寒武世达到了峰值或低谷,表明早寒武世全球俯冲通量达到了一个新高度,俯冲作用达到全板块尺度。因此,启动于新元古代的现代板块构造,很可能在早寒武世才建立起来,在此之前,经历了构造转换阶段。板片的深俯冲造就了冈瓦纳古大陆聚合阶段史无前例、高耸入云的造山带,这些巨型山脉显著改变了地球的气候,为海洋输送了大量Fe,P等营养物质,促进了海洋初级生产力。此外,这一时期的高俯冲通量导致CO_(2)的构造排气量达到峰值。全球性构造过程显著提高了初级生产力,引发新元古代大气增氧事件(NOE),为早寒武世生命大爆发提供了必要条件。在此之后,地球表生气候环境逐渐接近现代地球,多门类高级生命繁盛,多圈层密切协同演化的现代地球系统逐渐建立。

基于GPlates平台的“深时”板块构造重建:数据库现状与应用实例

板块构造重建是大地构造领域一项关键的研究工作,是构建"深时数字地球"(Deep-time Digital Earth, DDE)的重要途径。板块重建工作需要整合地球科学多个研究领域、海量的数据,同时又是地球动力学和气候模拟等研究的重要约束条件。因此,追踪目前国际主流的板块重建所需的数据库以及板块重建工具的最新进展和应用为了解板块重建研究的前沿提供全面的信息。文章系统介绍了支持板块构造重建研究的多个主流数据库,着重描述数据库的数据格式、数据获取方式及数据如何支持基于GPlates平台的板块构造重建。为了阐述大地构造研究相关数据库在板块重建研究中的应用,此次研究从板块构造重建的三个不同层次出发,列举了多个最新的板块重建模型及其在探索地球系统演化方面的意义。最后,结合DDE项目的相关任务,对250 Ma以来全球板块构造和变形的演化、量化大地构造在地球系统演化中的作用、超大陆旋回中的古大陆和古海洋构造重建以及前中生代(>250 Ma)全球板块运动模型重建这四个关键科学问题作出了新的展望。

Global kinematics of tectonic plates and subduction zones since the late Paleozoic Era

Detailed global plate motion models that provide a continuous description of plate boundaries through time are an effective tool for exploring processes both at and below the Earth's surface. A new generation of numerical models of mantle dynamics pre-and post-Pangea timeframes requires global kinematic descriptions with full plate reconstructions extending into the Paleozoic(410 Ma). Current plate models that cover Paleozoic times are characterised by large plate speeds and trench migration rates because they assume that lowermost mantle structures are rigid and fixed through time. When used as a surface boundary constraint in geodynamic models, these plate reconstructions do not accurately reproduce the present-day structure of the lowermost mantle. Building upon previous work, we present a global plate motion model with continuously closing plate boundaries ranging from the early Devonian at 410 Ma to present day.We analyse the model in terms of surface kinematics and predicted lower mantle structure. The magnitude of global plate speeds has been greatly reduced in our reconstruction by modifying the evolution of the synthetic Panthalassa oceanic plates, implementing a Paleozoic reference frame independent of any geodynamic assumptions, and implementing revised models for the Paleozoic evolution of North and South China and the closure of the Rheic Ocean. Paleozoic(410-250 Ma) RMS plate speeds are on average ～8 cm/yr, which is comparable to Mesozoic-Cenozoic rates of ～6 cm/yr on average.Paleozoic global median values of trench migration trend from higher speeds(～2.5 cm/yr) in the late Devonian to rates closer to 0 cm/yr at the end of the Permian(～250 Ma), and during the Mesozoic-Cenozoic(250-0 Ma) generally cluster tightly around ～1.1 cm/yr. Plate motions are best constrained over the past 130 Myr and calculations of global trench convergence rates over this period indicate median rates range between 3.2 cm/yr and 12.4 cm/yr with a present day median rate estimated at～5 cm/yr. For Paleozoic times(410-251 Ma) our model results in median convergence rates largely～5 cm/yr. Globally,～90% of subduction zones modelled in our reconstruction are determined to be in a convergent regime for the period of 120-0 Ma. Over the full span of the model, from 410 Ma to 0 Ma,～93% of subduction zones are calculated to be convergent, and at least 85% of subduction zones are converging for 97% of modelled times. Our changes improve global plate and trench kinematics since the late Paleozoic and our reconstructions of the lowermost mantle structure challenge the proposed fixity of lower mantle structures, suggesting that the eastern margin of the African LLSVP margin has moved by as much as ～1450 km since late Permian times(260 Ma). The model of the plate-mantle system we present suggests that during the Permian Period, South China was proximal to the eastern margin of the African LLSVP and not the western margin of the Pacific LLSVP as previous thought.

From Paleogeographic maps to Evolving Deep-time Digital Earth models

Major advances in computational power,as well as community modelling workflows and an improvement in data availability,has revolutionized Earth sciences over the last decade.Geological data has been synthesized into flexible and open access plate tectonic and paleogeographic reconstructions using the open-source and cross-platform GPlates(www.gplates.org)software since 2008(Muller et al.,2018;Muller et al.,2008).

Reconstructing seafloor age distributions in lost ocean basins

Reconstructions of past seafloor age make it possible to quantify how plate tectonic forces,surface heat flow,ocean basin volume and global sea level have varied through geological time.However,past ocean basins that have now been subducted cannot be uniquely reconstructed,and a significant challenge is how to explore a wide range of possible reconstructions.Here,we investigate possible distributions of seafloor ages from the late Paleozoic to present using published full-plate reconstructions and a new,efficient seafloor age reconstruction workflow,all developed using the open-source software GPlates.We test alternative reconstruction models and examine the influence of assumed spreading rates within the Panthalassa Ocean on the reconstructed history of mean seafloor age,oceanic heat flow,and the contribution of ocean basin volume to global sea level.The reconstructions suggest variations in mean seafloor age of~15 Myr during the late Paleozoic,similar to the amplitude of variations previously proposed for the Cretaceous to present.Our reconstructed oceanic age-area distributions are broadly compatible with a scenario in which the long-period fluctuations in global sea level since the late Paleozoic are largely driven by changes in mean seafloor age.Previous suggestions of a constant rate of seafloor production through time can be modelled using our workflow,but require that oceanic plates in the Paleozoic move slower than continents based on current reconstructions of continental motion,which is difficult to reconcile with geodynamic studies.