Dynamic topography and the nature of deep thick plumes
Dynamic topography and the nature of deep thick plumes
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DOI：
10.1016/j.epsl.2021.117286
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发表时间：
2021-11
影响因子：
5.3
通讯作者：
W. Frazer;J. Korenaga
中科院分区：
文献类型：
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作者：
W. Frazer;J. Korenaga
地震层析成像成像的深地幔柱的半径（10400公里）比传统地球动力学模型预测的（1000公里）大得多。从表面地形估计的羽浮力通量与预期的高温低粘度的狭窄羽流一致。如果断层摄影成像的羽流很厚，浮力通量估计可能需要非常粘稠或热化学羽流。在这里，我们评估的动态可扩展性的替代模型，积水羽流，这已被建议解释厚羽流以及估计从表面地形的浮力通量。在积水地幔柱模型中，下地幔中的厚管道在穿过地幔过渡带后显著变窄，在该过渡带下方，来自厚下地幔柱的过量物质横向扩散，而这些物质不能被狭窄的上地幔柱容纳。然而，中地幔中的这种多余物质仍然会在表面形貌中表现出来，其幅度可以通过形貌核来量化。我们发现，一个纯粹的热羽的积水将导致不切实际的多余的地形，与积水材料的规模大到足以通过地震层析成像检测。如果地幔柱的厚度如地震层析成像所示，似乎有必要偏离传统的温度依赖粘度或纯粹的热起源的假设。
Deep mantle plumes imaged by seismic tomography have much larger radii (∼400 km) than predicted by conventional geodynamic models (∼100 km). Plume buoyancy fluxes estimated from surface topography concur with narrow plumes with low viscosities expected from their high temperatures. If plumes are thick as imaged by tomography, buoyancy flux estimates may require very viscous or thermochemical plumes. Here we assess the dynamical plausibility of an alternative model, a ponding plume, which has been suggested to explain thick plumes as well as buoyancy fluxes estimated from surface topography. In the ponding plume model, a thick conduit in the lower mantle narrows significantly after passing through the mantle transition zone, below which excess material from the thick lower-mantle plume, which cannot be accommodated by the narrow upper-mantle plume, spreads laterally. Such excess material in the mid-mantle, however, should still manifest itself in surface topography, the amplitude of which can be quantified via topography kernels. We find that the ponding of a purely thermal plume would lead to unrealistic excess topography, with the scale of ponding material large enough to be detected by seismic tomography. If mantle plumes are as thick as indicated by seismic tomography, it appears to be necessary to deviate from either conventional temperature-dependent viscosity or the assumption of purely thermal origins.