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

本项目通过对同一时期的板内岩浆活动、长期海平面变化和真极移（TPW）的地质观测，试图约束古生代以来与盘古大陆合并和分裂有关的地幔结构演化。现今地幔以2度球面调和结构为主，其特征为两个对跖体，主要是非洲和中太平洋下方的地震慢异常（即非洲和太平洋超羽流）和环太平洋地震快异常。这种2度长波长地幔结构与板块运动历史有关。近1ga超大陆旋回（即盘古大陆和罗丁尼亚大陆的合并和分裂）的观测和地球动力学的考虑表明，在超大陆合并期间，地幔可能以更长的波长结构为主，可能在1度。由于地幔对流结构控制着地幔内的传热和化学混合，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具有重要意义。本文将建立具有真实地幔流变和板块运动历史的可压缩热化学对流的三维球形模型，该模型受古地理（即盘古大陆的组装和分裂）的约束，以检验以下两个关于板内岩浆作用的假设：(1)板块内岩浆活动在盘古板块合并前和合并后约100 Ma的减弱，是大陆板块（如冈瓦纳板块和劳亚板块）的汇聚使非洲半球地幔降温的结果，而随后与盘古板块分裂相关的岩浆活动增强是由于盘古板块周围俯冲引起盘古板块下方上升流回流而形成的非洲超羽流结构造成的。这表明，在泛大陆组合后约100 Ma的非洲超羽流构造形成之前，非洲半球的地幔可能相对较冷。2)板块内岩浆活动喷发位点与非洲和太平洋地震异常边界在核幔边界附近的相关性具有热化学对流的特征，其中在核幔边界附近的致密成分仅比周围地幔密度中等，而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式再现了当今地球的地幔结构、地表动力地形和大地水准面（即旋转极位）的一般特征，但长波长动力地形和大地水准面控制的长期TPW和海平面变化观测对盘古时期全球地幔结构演化，特别是太平洋半球的地幔结构演化提出了限制。

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.