The thermal structure of mantle plumes: axisymmetric or triple-junction?
The thermal structure of mantle plumes: axisymmetric or triple-junction?
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地幔柱的热结构：轴对称还是三联结？
DOI：
10.1111/j.1365-246x.1990.tb00527.x
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发表时间：
1990
影响因子：
2.8
通讯作者：
G. Houseman
中科院分区：
文献类型：
--
作者：
G. Houseman
地幔柱可能是一个复杂的三维热结构，当它从下面接近岩石圈底部时具有近似轴对称性，但随后向下朝向层的底部，可能由连接的热片的三结或四结组成。一个相对较弱的热片上升只有部分通过该层可能连接两个相邻的地幔柱。这些结论建议的数值实验上的3-D恒定粘度，平面层与无应力的边界，其中详细的梯度变化的对流层的平面形状从顶部的层到它的基地。对流层的平面形状是该层中主要热异常在水平面上的分布图。这些异常是正浮力（对于热流体）或负浮力（对于冷流体）的主要来源，因此它们驱动对流。它们在横截面上可能表现为轴对称（柱）、平面对称（片）或某种复杂的不对称形式。当对流至少部分地由基底加热驱动时，靠近层顶部的平面形状可以被描述为冷沉降片和孤立的热柱的网络，而靠近层底部的平面形状则表现为热上升片和孤立的冷柱的网络。上表面附近的热柱产生于下表面上的热片网络的顶点或节点，并且类似地，层底部处的冷柱形成于上表面附近的冷片网络的顶点下方。在上表面附近，这个实验的表观平面形状类似于地幔对流，与俯冲带相比的冷片和与地幔柱相比的热柱。热羽流撞击地表，产生近似轴对称的温度异常、地表抬升和拉张应力场。然而，相对较小的偏离轴对称的表面观测反映了地幔柱的深层结构，形成的三个或四个热片的层的基础上的交界处。看来，通常出现的大陆裂谷的三联点形式可能反映了一个潜在的结构，这是隐含在下面的地幔对流环流。
SUMMARY A mantle plume is probably a complex 3-D thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below, but followed down towards the base of the layer, probably consists of a triple-junction or quadruple-junction of connected hot sheets. A relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes. These conclusions are suggested by numerical experiments on a 3-D constant-viscosity, plane layer with stress-free boundaries, which detail the gradational change in the planform of a convecting layer from the top of the layer to its base. The planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer. These anomalies are the main sources of positive (for hot fluid) or negative (for cold fluid) buoyancy, and therefore they drive the convective flow. They may appear in cross-section as structures with either axial symmetry (columns), planar symmetry (sheets) or some complex asymmetric form. When convection is driven at least partially by basal heating, the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns, while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns. The hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface, and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface. Near the upper surface, the apparent planform of this experiment is analogous to that of mantle convection, the cold sheets compared to subduction zones and the hot columns compared to mantle plumes. The hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies, surface uplift and extensional stress fields. However, the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume, formed by the junction of three or four hot sheets on the base of the layer. It seems likely that the commonly occurring triple-junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath.