Abstract

In ductile shear zones, the strain shown by the rocks depends much on the composition and shape of the mineral constituents. Under simple shear, quartz grains commonly reorient themselves in the direction of tectonic transport or flow. In ductile shear zones, quartz grains are elliptically stretched in the direction of mylonitic foliation to accommodate the imposed ductile strain. Our observations on the rocks of a crustal scale shear zone, the Main Central Thrust (MCT) of the Himalaya, however, reveal that at several places of the shear zone the quartz grains are polygonal and show planar boundaries. The fabric of rocks at such places is not compatible with that of the prevailing fabric of rocks, and can be described as strain insensitive fabric. Following the Panozzo (J. Struct. Geol. 6:215–221, 1984) method, we have estimated strain from quartz grains that show planar boundaries. Our results show that in the MCT zone, the areas of high ductile strain, as existing near the trace of the MCT, the amount of strain shown by such grains of quartz is low, while in areas of low strain, as existing in areas away from the MCT, the amount of strain is relatively higher. As such, the method holds importance in those cases where grain shapes (i.e., planar boundaries) put constraint on estimation of strain because the conventional methods of strain estimation require elliptical shape of objects. This is possibly the first application of the Panozzo method on deformed rocks from India.

Research highlights

• The general fabric of rocks of ductile MCT zone of Himalaya is dominated by elliptically deformed quartz grains.

• However locally the fabric, not compatible with prevailing ductile fabric, contains polygonal quartz grains with flat boundaries.

• Strain has been estimated for polygonal grains by digitizing their outlines and analysing data by computer software.

• Such grains show lower strains near MCT and higher strains away. This is reversely shown by elliptically deformed grains.

• This suggests that the quartz grains with polygonal shapes remained rather insensitive to ductile strain.

中文翻译：

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高延性流动状态下石英颗粒的晶形控制应变：来自印度库玛恩喜马拉雅山主要中央推力带的观测

摘要

在韧性剪切带中，岩石所表现出的应变在很大程度上取决于矿物成分的组成和形状。在简单剪切作用下，石英颗粒通常会沿着构造传输或流动的方向重新定向。在延展性剪切带中，石英粒在my状叶形成的方向上椭圆形拉伸，以适应所施加的延性应变。然而，我们对地壳尺度剪切带的岩石（喜马拉雅山的主要中央推力（MCT））的观察表明，在剪切带的多个位置，石英颗粒是多边形的，并显示出平面边界。在这些地方的岩石织物与主要的岩石织物不兼容，可以说是应变不敏感织物。继Panozzo（J.Struct.Geol。6：215–221，1984）方法，我们已经估计了具有平面边界的石英晶粒的应变。我们的结果表明，在MCT区域中，MCT痕迹附近存在高延性应变的区域，此类石英晶粒显示的应变量较低，而在低应变区域，则存在于远距离区域从MCT来看，应变量相对较高。这样，该方法在晶粒形状（即，平面边界）限制了应变估计的那些情况下很重要，因为传统的应变估计方法需要对象的椭圆形状。这可能是Panozzo方法在印度变形岩石上的首次应用。

研究重点

• 喜马拉雅韧性MCT带岩石的一般结构以椭圆形变形的石英颗粒为主。

• 然而，局部地，该织物与流行的延性织物不相容，其包含具有平坦边界的多边形石英颗粒。

• 通过将多边形的轮廓数字化并通过计算机软件分析数据，可以估算出多边形晶粒的应变。

• 这样的晶粒在MCT附近显示出较低的应变，而在MCT附近显示出较高的应变。相反地​​，这由椭圆形变形的晶粒表示。

• 这表明具有多边形形状的石英颗粒对延性应变仍然不敏感。