Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling
Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling
复制标题
DOI：
10.1144/gsl.sp.1998.135.01.04
复制
发表时间：
1998
期刊：
影响因子：
--
通讯作者：
Shoufa Lin;D. Jiang;P. Williams
中科院分区：
文献类型：
--
作者：
Shoufa Lin;D. Jiang;P. Williams
本文描述了一种具有三斜对称的天然剪切带，根据自然实例提出了三斜剪切带的一般模型，并研究了三斜剪切带内的运动学和应变几何。在加拿大阿巴拉契亚山脉的罗珀湖剪切带，伸展线理的方向在剪切带边界附近大致向下倾斜，并向中心逐渐变浅。剪切带中央部分的构造呈近似单斜对称性，S面和C面的两极、S面的伸展线理和C面的条纹均呈大圆环状分布。然而，来自边缘部分的线理并不在同一条带上，剪切带的整体对称性是三斜的。理论模拟表明，观测到的应变几何形状可以用斜向扭压解释，剪切带中心的单剪与纯剪之比大于边缘。后者表明区域边界平行运动分量相对于边界正常挤压分量具有更高的局部化程度。我们强调，由于大多数天然剪切带的边界位移位于倾滑和走滑之间，它们的运动图景通常是三斜的；单斜剪切带是特殊的端元。具有单斜对称性的构造数据并不一定意味着它们是单斜运动图像的结果；目前的模拟表明，边界平行运动与边界法线运动的高比率的三斜运动图像可以产生明显的单斜构造几何学。模拟结果还表明，对简单剪切带所作的简单陈述，即伸展线理将与之对齐，从而表明剪切方向不能外推到三维扭压(或扭张)剪切带。
Abstract We describe a natural shear zone with triclinic symmetry, present a general model for triclinic shear zones based on natural examples, and investigate the kinematics and strain geometry within such zones. In the Roper Lake shear zone in the Canadian Appalachians, the orientation of a stretching lineation is oriented approximately down-dip near the shear zone boundary and becomes gradually shallower towards the centre. The structures in the central portion of the shear zone exhibit approximately monoclinic symmetry where the poles to both the S- and C-surfaces, the stretching lineation on the S-surfaces and the striations on the C-surfaces all plot in a great circle girdle. However, the lineations from the marginal portion do not plot in the same girdle, and the bulk symmetry of the shear zone is triclinic. Theoretical modelling shows that the observed strain geometry can be interpreted by an oblique transpression with a larger ratio of simple shear to pure shear in the centre of the shear zone than in the margin. The latter suggests a higher degree of localization of the zone boundary-parallel movement component relative to the boundary-normal compression component. We emphasize that, as the imposed boundary displacements for most natural shear zones lie between dip-slip and strike-slip, their movement pictures are generally triclinic; monoclinic shear zones are special end members. Structural data that exhibit monoclinic symmetry do not necessarily mean that they resulted from a monoclinic movement picture; the present modelling demonstrates that a triclinic movement picture with a high ratio of boundary-parallel movement to boundary-normal movement can result in apparent monoclinic structural geometry. The results of the modelling also show that the simple statement made for simple shear zones that stretching lineations will align with, and therefore indicate, the shear direction cannot be extrapolated to three-dimensional transpressional (or transtensional) shear zones.