南太平洋地区两类典型三联点岩石层热结构及其地球动力学研究

The triple junctions, as the intersections of the related three different plates, are important components of the global plate motion system. There are now 7 types of triple junctions located in 19 different area of the global surface. The spatial migrations of these triple junctions are generally with the continuous ridge expansion, fault strike or trench subduction processes, cause deformations of the related plates or form new microplate, and thus deeply influence on the plate motions and the corresponding geodynamical processes..In this study, we mainly select the Macquarie triple junction with RRF type and the Chile triple junction with RTT in the southern Pacific area as our research object. The new transform fault is formed by two oceanic ridges with different spreading rates for Macquarie triple junction，and the spreading oceanic ridge is subducting underneath the South America plate for Chile triple junction. The constraints include those geo-data coming from the geological tectonics, seismology, geomagnetism, gravity, and remote sensing etc. We then construct the corresponding 3D geological tectonics models for the two triple junctions to describe their morphological features, simulate their different geodynamical features including the lithospheric thermal structures and the historical spatial and temporal evolution. We also simulate their plate strain and stress fields around the two triple junctions, and thus study their deformation and dynamic processes, we can thus try to explore the mechanisms of earthquake mechanisms generation and occurrence in the corresponding areas, and to well understand the geodynamical driving mechanism of the plate motions related to the two triple junctions.

作为三个不同板块边界的交点，“三联点”构成全球板块运动系统的重要部分，目前全球已发现19处共7种不同类型的三联点。三联点的空间迁移一般伴随有持续的洋脊扩张、断层滑移或海沟俯冲等过程，导致其周边板块产生变形或生成新的微板块，因而深刻影响着全球板块运动及动力过程。选取南太平洋地区RRF型麦夸里（Macquarie）三联点与RTT型智利（Chile）三联点作为研究对象，考虑麦夸里三联点之不同扩张洋脊速率生成新的转换断层及智利三联点之扩张洋脊俯冲进入南美板块下方等不同情况，利用地质构造、地震地磁与重力遥感等观测数据作为约束，分别构建描述这两类三联点相关运动形态特征的三维地质构造模型，对其岩石层热结构及其地质历史演化等，进行动力学数值模拟。通过这两类三联点周边板块应变应力场等变化规律研究其变形与动力过程，探索其地震孕育与发生机理，并更好地了解这两类三联点周边板块运动的动力驱动机理。

项目执行期间，针对申请书设定的三个研究目标，通过地球动力学建模及相应的数值模拟研究，我们获得了一系列有意义的研究成果。通过对太平洋南部边界RRF型三联点附近地区热结构进行的数值模拟，我们获得了三联点附近地区岩石层的温度分布和地幔流动方式，以及如何进一步控制其地表地形与热流变化的一些新认识；关于智利三联点的时空迁移与演化机理方面，地球动力学模拟结果显示，洋脊位置、板块相对汇聚速度及上覆大陆板块均对俯冲区域海洋板块的温度结构构成显著影响，并且进一步影响了扩张洋脊俯冲过程中的岩石层变形机制及其地表热流的分布特征，同时也有可能导致智利三联点南北两侧，纳兹卡板块俯冲和南极洲板块俯冲的震源深度范围即孕震区特征相差较大；由于智利三联点相关的板块间相对运动，导致智利三联点的性质在RTT型与FTT类型之间不断转变，并且在三联点交替向北向南迁移的位置附近，可以发现存在多处弧前酸性岩；与此同时，GRACE重力数据反演结果也同步揭示了智利三联点附近地区存在明显的重力场长波变化及莫霍面起伏变化。麦夸里三联点临近地区相关板块的相对运动，使得其转换断层边界浅部产生剪应力集中，导致震源深度集中在15-25km，由此产生的地幔流动引起温度结构变化，并同时控制着该地区的地表地形变化。