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

该项目试图通过对同一时期板内岩浆活动、长期海平面变化和真极移(TPW)的地质观测，限制古生代以来与盘古大陆组装和解体有关的地幔结构的演化。现今地球地幔以球谐2级结构为主，以非洲和中太平洋(即非洲和太平洋超柱)下的两个主要地震慢异常和环太平洋地震快异常为特征。这种二级长波地幔结构与板块运动历史有关。对最近1Ga的超大陆旋回(即盘古和罗迪尼亚的组装和解体)的观察和地球动力学的考虑表明，在超大陆组装过程中，地幔可能以更长的波长结构为主，可能处于1级。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，因此了解地幔结构的时间演化具有重要意义。将建立具有真实地幔流变学和受古地理约束的板块运动历史(即盘古组装和解体)的可压缩、热化学对流的三维球状模型，以检验关于板内岩浆作用的下列两个假说：1)盘古大陆板块(如冈瓦那和劳拉西亚)汇聚导致非洲半球地幔冷却的大陆板块(如冈瓦那和劳拉西亚)会聚导致板内岩浆活动水平降低，而随后与盘古大陆解体有关的增强岩浆活动是由于盘古大陆俯冲引起的非洲超强热柱结构的形成而导致的。这意味着在泛古大陆组装后约100 Ma非洲超热柱结构形成之前，非洲半球的地幔可能是相对较冷的。2)板内岩浆活动喷发地点与核幔边界附近非洲和太平洋地震异常边界的相关性具有热化学对流的特征，CMB附近的致密组分只比周围地幔密度中等，但与化学界面平坦的纯热对流或层状地幔对流不一致。虽然所提出的对流模式再现了现今地球的地幔结构、地表动态地形和大地水准面(即自转极位置)的一般特征，但受长波动态地形和大地水准面控制的长期TPW和海平面变化的观测对全球地幔结构演化，特别是在盘古时期太平洋半球的演化构成了限制。

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.