Overprinting of magnetic fabrics in granites by small strains: numerical modelling
Overprinting of magnetic fabrics in granites by small strains: numerical modelling
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DOI：
10.1016/0040-1951(94)90238-0
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发表时间：
1994-05
期刊：
影响因子：
2.9
通讯作者：
K. Benn
中科院分区：
文献类型：
--
作者：
K. Benn
利用磁化率的各向异性（AMS）来确定花岗岩中弱定义的矿物优选取向结构。然而，小应变叠加在这些岩石典型的低强度AMS结构上的影响尚未得到很好的理解。数值模拟研究了小的叠加应变对预先存在的AMS结构的形状、大小和方向的影响，这与文献中未变形花岗岩的研究结果相似。利用March应变响应模型，计算机生成的黑云母岩组构在纯剪切（平面应变）、轴对称缩短和简单剪切机制下承受渐进应变。结果表明，小应变（在纯剪切和轴对称缩短机制下≈10-15%的应变，在简单剪切机制下0.5≥1.0的应变）可以显著地改变或覆盖预先存在的AMS织物。一种新的AMS织物的方向可能与最初各向同性岩石织物的剪切方向相似。在低应变下，AMS椭球形状的演变很大程度上取决于初始岩组构相对于应变框架的取向；在渐进应变过程中，AMS椭球体可能向扁圆形或长条形演化，且椭球体的演化过程可能发生逆转。该模型表明，在岩体就位后影响其的小应变可能会掩盖早期形成的AMS结构。结果还表明，在简单剪切中，即使早期织物不是各向同性，AMS椭球体的方向也可以作为小应变的剪切感指标。
The anisotropy of magnetic susceptibility (AMS) is used to determine weakly defined mineral preferred orientation fabrics in granites. However, the effects of small strains superimposed on the low-intensity AMS fabrics typical of these rocks is not well understood. Numerical simulations are presented which investigate the effects of small superimposed strains on the shapes, magnitudes and orientations of pre-existing AMS fabrics, which are similar to those reported in the literature for undeformed granites. A computer-generated biotite petrofabric is subjected to progressive strain, using March's strain response model, for pure shear (plane strain), axially symmetric shortening and simple shear regimes. Results indicate that a pre-existing AMS fabric might be significantly modified or overprinted by small strains (≈10–15% strain in the pure shear and axially symmetric shortening regimes, 0.5 ⩽ γ ⩽ 1.0 for simple shear). A new AMS fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric. At low strains, the evolution of the shape of the AMS ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework; the AMS ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain. This modelling suggests that small strains affecting a pluton following its emplacement could mask AMS fabrics developed earlier. The results also suggest that, in simple shear, the orientation of the AMS ellipsoid may serve as a shear-sense indicator for small strains even if the earlier fabric is not isotropic.