Investigating the consequences of Supercontinent Pangea assembly and breakup on the time evolution of large-scale mantle thermochemical structures and magmatism

该项目试图通过对板内岩浆活动、长期海平面变化和同一时期的真极移（TPW）的地质观测，来限制古生代以来与盘古大陆组装和裂解有关的地幔结构的演化。现今地球地幔以球谐2度结构为主，其特征是非洲和中太平洋下的两个对跖的主要地震慢异常（即，非洲和太平洋超级羽流）和环太平洋地震快速异常。这种2度长波地幔结构与板块运动历史有关。超大陆周期（即组装和解体的盘古大陆和Rodinia）的最后1 Ga和地球动力学的考虑的观测表明，地幔可能已占主导地位，甚至更长的波长结构可能在度1超大陆组装。由于地幔对流结构控制着地幔中的热传递和化学混合，以及地球表面的地质和构造，因此了解地球地幔结构的时间演化是重要的。可压缩热化学对流的三维球形模型，具有受古地理学约束的真实地幔流变学和板块运动历史（即，Pangea组装和解体）将被制定来测试以下两个假设关于板内岩浆活动：1）板内岩浆活动的减少水平在Pangea组装之前和大约100 Ma之后，由于大陆板块的会聚（例如，古大陆板块（Gondwana and Laurassia）的俯冲作用冷却了非洲半球的地幔，而随后与盘古大陆裂解相关的岩浆活动增强则是由于环盘古大陆俯冲作用导致盘古大陆下的上涌回流而形成非洲超级地幔柱结构所致。这意味着在非洲超级地幔柱构造形成之前，非洲半球的地幔可能是相对寒冷的，大约在盘古大陆组装后100 Ma。2)板内岩浆活动的喷发部位与核幔边界附近的非洲和太平洋地震异常边界之间的相关性是热化学对流的特征，其中靠近核幔边界的致密组分仅比环境地幔的密度适中，但与纯热对流或具有平坦化学界面的层状地幔对流不一致。虽然所提出的对流模型再现了地幔结构、表面动力学地形和大地水准面（即，地球自转极位置），长波长动力地形和大地水准面控制下的长时间TPW和海平面变化观测，对全球地幔结构演化，特别是泛古陆时期太平洋地区的地幔结构演化具有制约作用。

This project seeks to constrain the evolution of mantle structure since Paleozoic, associated with the assembly and breakup of Pangea, by using geological observations of intraplate magmatism, long-term sea level changes, and true polar wander (TPW) in the same time period. The present-day Earth's mantle is predominated by spherical harmonic degree-2 structure that is characterized by the two antipodal, major seismically slow anomalies under Africa and central Pacific (i.e., the African and Pacific superplumes) and circum-Pacific seismically fast anomalies. This degree-2 long-wavelength mantle structure is associated with the plate motion history. Observations of supercontinent cycles (i.e, assembly and breakup of Pangea and Rodinia) for the last 1 Ga and geodynamic considerations suggest that the mantle may have been predominated by even longer-wavelength structure possibly at degree 1 during supercontinent assembly. Because mantle convective structure controls heat transfer and chemical mixing in the mantle, and geology and tectonics at the Earth's surface, it is important to understand the time evolution of Earth's mantle structure. 3-D spherical models of compressible, thermochemical convection with realistic mantle rheology and plate motion history constrained by the paleogeography (i.e., Pangea assembly and breakup) will be formulated to test the following two hypotheses regarding intraplate magmatism: 1) The reduced level of intraplate magmatism before Pangea assembly and ~100 Ma after, results from the convergence of continental plates (e.g., Gondwana and Laurassia) that cool the mantle in the African hemisphere, while the subsequent enhanced magmatism associated with Pangea breakup is caused by formation of the African superplume structure as a result of circum-Pangea subduction induced upwelling return-flow below Pangea. The implication is that the mantle in the African hemisphere may have been relatively cold before the African superplume structure is formed ~100 Ma after Pangea assembly. 2) The correlation between eruption sites of intraplate magmatism and the boundaries of the African and Pacific seismic anomalies near the core-mantle boundary (CMB) is characteristic of thermochemical convection in which the dense component near the CMB is only moderately denser than the ambient mantle, but inconsistent with purely thermal convection or layered mantle convection with a flat chemical interface. While the proposed convection models reproduce the general features of mantle structure, surface dynamic topography, and the geoid (i.e., rotational pole position) for the present-day Earth, the observations of long-term TPW and sea level changes which are controlled by long-wavelength dynamic topography and geoid, pose constraints on global mantle structure evolution, particularly that in the Pacific hemisphere during Pangea time.