High-strain deformation of conglomerates: Numerical modelling, strain analysis, and an example from the Wutai Mountains, North China Craton
High-strain deformation of conglomerates: Numerical modelling, strain analysis, and an example from the Wutai Mountains, North China Craton
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DOI：
10.1016/j.jsg.2018.06.018
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发表时间：
2018-09
影响因子：
3.1
通讯作者：
H. Ran;P. Bons;Gen-hou Wang;F. Steinbach;M. Finch;A. Griera;E. Gomez‐Rivas;M. Llorens;Shuming Ran;Xiao Liang;Jie Zhou
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文献类型：
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作者：
H. Ran;P. Bons;Gen-hou Wang;F. Steinbach;M. Finch;A. Griera;E. Gomez‐Rivas;M. Llorens;Shuming Ran;Xiao Liang;Jie Zhou
砾岩被广泛用于研究变形历史和流变学、应变、涡度和粘度。以往的研究表明，几个因素，如卵石的形状和浓度，以及材料的性质，影响砾岩变形。然而，卵石浓度和卵石之间的相互作用如何影响变形还不是很清楚。我们使用二维数值模拟平台ELL与全场晶体粘塑性程序耦合，模拟了不同卵石和基质粘度差、不同卵石浓度、线性(应力指数n= 1)和幂定律(n= 3)粘性流变性的砾岩在简单剪切条件下的变形，最大剪切应变为10。鹅卵石可以表现为有效被动、可变形或有效刚性。鹅卵石浓度/粘度对比度的增加会增强鹅卵石的变形，但会减少它们的旋转。提出了一种平均长宽比(RF)-取向(ϕ)图来估计卵石的变形行为和体应变量。紧密排列的坚硬或可变形的鹅卵石可以形成簇，机械地充当单个包裹体。刚性团簇旋转并仅在较短的应变增量下存活，而更稳定的可变形团簇在较小的旋转下继续伸长。我们提供了中国克拉通北部五台山变形砾岩的一个自然例子。它们由嵌入片状基质中的带状铁形成(BIF)鹅卵石组成。利用平均Rf-ϕ图，可以确定∼6在简单剪切下的有限应变。对于线性流变学(n= 1)，鹅卵石的粘度估计约为基质的5-8倍，如果假定n= 3的幂律流变学，则为2-5倍。
Conglomerates have been widely used to investigate deformation history and rheology, strain, vorticity and viscosity. Previous studies reveal that several factors, such as pebble shapes and concentrations, as well as material properties, affect conglomerate deformation. However, how pebble concentration and interaction between pebbles affect deformation is not understood very well. We use the 2D numerical modelling platform ELLE coupled to the full field crystal visco-plasticity code (VPFFT) to simulate the deformation of conglomerates with various viscosity contrasts between pebbles and matrix and different pebble concentrations, with both linear (stress exponentn= 1) and power-law (n= 3) viscous rheologies, under simple shear conditions up to a shear strain of ten. Pebbles can behave as effectively passive, deformable or effectively rigid. An increase in pebble concentrations/viscosity contrasts enhances pebble deformation, but reduces their rotation. A mean aspect ratio (Rf) - orientation (ϕ) plot is proposed to gain an estimate of pebble deformation behaviour and the amount of bulk strain. Closely spaced rigid or deformable pebbles can form clusters that mechanically act as single inclusions. Rigid clusters rotate and survive for only short strain increments, whereas the more stable deformable ones keep on elongating with minor rotation. We provide a natural example of deformed conglomerates from the Wutai Mountains, North China Craton. These consist of banded-iron-formation (BIF) pebbles embedded in a schistose matrix. Using the meanRf-ϕplot, a finite strain of ∼6 under simple shear could be determined. The viscosity of the pebbles is estimated at about 5–8 times that of the matrix for a linear rheology (n= 1), or 2 to 5 times if a power-law rheology withn= 3 is assumed.