McNamara Allen K, van Keken Peter E, Karato Shun-Ichiro
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA.
Nature. 2002 Mar 21;416(6878):310-4. doi: 10.1038/416310a.
Seismological observations reveal highly anisotropic patches at the bottom of the Earth's lower mantle, whereas the bulk of the mantle has been observed to be largely isotropic. These patches have been interpreted to correspond to areas where subduction has taken place in the past or to areas where mantle plumes are upwelling, but the underlying cause for the anisotropy is unknown-both shape-preferred orientation of elastically heterogeneous materials and lattice-preferred orientation of a homogeneous material have been proposed. Both of these mechanisms imply that large-strain deformation occurs within the anisotropic regions, but the geodynamic implications of the mechanisms differ. Shape-preferred orientation would imply the presence of large elastic (and hence chemical) heterogeneity whereas lattice-preferred orientation requires deformation at high stresses. Here we show, on the basis of numerical modelling incorporating mineral physics of elasticity and development of lattice-preferred orientation, that slab deformation in the deep lower mantle can account for the presence of strong anisotropy in the circum-Pacific region. In this model-where development of the mineral fabric (the alignment of mineral grains) is caused solely by solid-state deformation of chemically homogeneous mantle material-anisotropy is caused by large-strain deformation at high stresses, due to the collision of subducted slabs with the core-mantle boundary.
地震学观测揭示了地球下地幔底部存在高度各向异性的斑块，而大部分地幔在很大程度上被观测为各向同性。这些斑块被解释为对应于过去发生俯冲的区域或地幔柱上升的区域，但其各向异性的根本原因尚不清楚——弹性非均质材料的形状优选取向和均质材料的晶格优选取向都已被提出。这两种机制都意味着在各向异性区域内发生了大应变变形，但这些机制的地球动力学意义不同。形状优选取向意味着存在大的弹性（因而化学）非均质性，而晶格优选取向需要在高应力下变形。在此，我们基于结合弹性矿物物理学和晶格优选取向发展的数值模拟表明，深下地幔中的板块变形可以解释环太平洋地区强烈各向异性的存在。在该模型中——矿物组构（矿物颗粒的排列）的发展完全由化学均质的地幔物质的固态变形引起——由于俯冲板块与核幔边界的碰撞，各向异性是由高应力下的大应变变形导致的。
