Folding of a single layer in an anisotropic viscous matrix under layer-parallel shortening

IF 2.6 2区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY
Yuan-bang Hu , Paul D. Bons , Tamara de Riese , Shu-gen Liu , Maria-Gema Llorens , Eloi González-Esvertit , Enrique Gomez-Rivas , Dian Li , Yu-zhen Fu , Xue-lin Cai
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引用次数: 0

Abstract

Folds are common structures that provide valuable insights into the direction and amount of shortening and the rheological properties of deformed rocks. Most thin plate folding theory started from M.A. Biot has historically been applied to isotropic materials, but rocks are often anisotropic due to the presence of tectonic foliations, bedding, veins, dykes, etc. Mechanical anisotropy can enhance partitioning of deformation, resulting in low-strain domains and localised high-strain shear domains. Using the Viscoplastic full-field code coupled with the modelling platform Elle (VPFFT-Elle), we investigate the evolving fold geometries, stress field and strain-rate field differences and redistributions resulting from layer-parallel shortening deformation of an isotropic, competent layer embedded in an anisotropic, weaker power-law viscous matrix. We focus on the effect of the orientation of the mechanical anisotropy relative to the competent layer. The simulation results illustrate that the deformation localisation behaviour, and hence fold geometry, depend on (i) the initial orientation of the anisotropy, (ii) the intensity of anisotropy, and (iii) strength of the competent layer, relative to that of the matrix. Variation in the localisation behaviour resulting from different strain-rate distributions lead to two end-member fold geometries: (1) classical Biot-type buckle folding and thickening of the competent layer coupled to the formation of a new axial-planar crenulation cleavage in the matrix, and (2) what we call ‘shear-band folding’ in which sections of the competent layer are offset due to the formation of shear bands in the matrix with opposite sense of shear. This leads to rapid fold amplification. Classical Biot-type buckle folds dominate when the initial anisotropy is parallel or subparallel to the shortening direction, while shear-band folds dominate when the initial anisotropy is normal or at high angle to the shortening direction. Results presented here contribute to our understanding on how mechanical anisotropy controls folding and the rearrangement of the matrix components. Furthermore, the modelled scenarios can serve as a “virtual glossary” to compare real folds in different tectonic settings, providing insights into the possible pre-fold configuration of the folded layer and its anisotropic matrix.

各向异性粘性基质中单层在层平行缩短条件下的折叠
褶皱是一种常见的结构,它为了解缩短的方向和数量以及变形岩石的流变特性提供了宝贵的信息。从 M.A. Biot 开始的大多数薄板褶皱理论历来适用于各向同性的材料,但由于构造皱褶、层理、脉络、堤坝等的存在,岩石通常是各向异性的。机械各向异性可加强变形分区,从而产生低应变域和局部高应变剪切域。利用与建模平台 Elle(VPFFT-Elle)耦合的粘塑性全场代码,我们研究了嵌入各向异性弱幂律粘性基质中的各向同性能层的层平行缩短变形所产生的不断演变的褶皱几何形状、应力场和应变率场差异及再分布。我们重点研究了机械各向异性相对于能级层的取向的影响。模拟结果表明,变形定位行为以及褶皱几何形状取决于:(i) 各向异性的初始方向;(ii) 各向异性的强度;(iii) 相对于基体的能级层强度。不同应变率分布导致的定位行为变化会产生两种端部褶皱几何形状:(1) 经典的比奥特型扣式褶皱,以及与基体中新的轴向平面齿裂形成耦合的能级层增厚;(2) 我们称之为 "剪切带褶皱",其中能级层的部分由于基体中形成剪切力相反的剪切带而发生偏移。这会导致褶皱迅速扩大。当初始各向异性平行于或近平行于缩短方向时,经典的毕奥型扣褶占主导地位,而当初始各向异性为法线或与缩短方向成大角度时,剪切带褶占主导地位。本文介绍的结果有助于我们理解机械各向异性如何控制褶皱和基质成分的重新排列。此外,所模拟的情况可作为 "虚拟术语表",用于比较不同构造环境下的真实褶皱,从而深入了解褶皱层及其各向异性基质可能的褶皱前构造。
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来源期刊
Journal of Structural Geology
Journal of Structural Geology 地学-地球科学综合
CiteScore
6.00
自引率
19.40%
发文量
192
审稿时长
15.7 weeks
期刊介绍: The Journal of Structural Geology publishes process-oriented investigations about structural geology using appropriate combinations of analog and digital field data, seismic reflection data, satellite-derived data, geometric analysis, kinematic analysis, laboratory experiments, computer visualizations, and analogue or numerical modelling on all scales. Contributions are encouraged to draw perspectives from rheology, rock mechanics, geophysics,metamorphism, sedimentology, petroleum geology, economic geology, geodynamics, planetary geology, tectonics and neotectonics to provide a more powerful understanding of deformation processes and systems. Given the visual nature of the discipline, supplementary materials that portray the data and analysis in 3-D or quasi 3-D manners, including the use of videos, and/or graphical abstracts can significantly strengthen the impact of contributions.
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