Rhomb-dominated crystallographic preferred orientations in incipiently deformed quartz sandstones: A potential paleostress indicator for quartz-rich rocks
Rhomb-dominated crystallographic preferred orientations in incipiently deformed quartz sandstones: A potential paleostress indicator for quartz-rich rocks
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早期变形石英砂岩中以菱形为主的晶体择优取向：富含石英岩石的潜在古应力指标
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发表时间：
2018
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通讯作者：
Tyler Gabrielson
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作者：
J. Rahl;Allen J. McGrew;Joshua A. Fox;Joshua R. Latham;Tyler Gabrielson
我们描述了石英晶体的优先取向（CPOs），从早期变形的石英砂岩中，其特征是低强度但明确的极向正{r}和/或负{z}菱形排列。这些独特的cpo出现在最小的菌株和几乎没有改变原始碎屑边界的颗粒中。我们假设这些模式反映了dauphin<s:1>孪晶（关于c轴的180°错取向），优先影响与最大主应力方向成大角的弹性刚性z形取向的晶粒。孪生有利于弹性变形，使更柔顺的r形以高角度对准最大主应力。晶体图显示，大约三分之二的晶粒（按面积计算）是双晶，而非双晶晶粒的取向与推断的缩短方向垂直。我们从三个地点的低品位石英岩中记录了这种模式：美国内华达州东北部的尤里卡石英岩；阿根廷西北部的Mesón群；以及美国弗吉尼亚州中部蓝岭的安提塔姆组。这些cpo在最小变形石英岩中的广泛存在表明它们可能有助于确定古应力轨迹。
We describe quartz crystallographic preferred orientations (CPOs) from incipiently deformed quartz sandstones characterized by low-intensity but unambiguous alignment of the poles to positive {r} and/or negative {z} rhombs. These distinctive CPOs appear at minimal strains and in grains with scarcely modified original detrital boundaries. We consider the hypothesis that these patterns reflect Dauphiné twinning (a 180° misorientation about the c-axis) that preferentially affects grains oriented with the elastically stiffer z-rhombs at high angle to the maximum principal stress direction. Twinning facilitates elastic deformation by aligning the more compliant r-rhombs at high angle to the greatest principal stress. Crystallographic maps show that about two-thirds of all grains (by area) are twinned, and untwinned grains are oriented with an r-rhomb perpendicular to the inferred shortening direction. We document this pattern from low-grade quartzite from three locations: the Eureka Quartzite of northeastern Nevada (USA); the Mesón Group of northwestern Argentina; and the Antietam Formation of the Blue Ridge of central Virginia (USA). The widespread presence of these CPOs in minimally deformed quartz rocks suggests that they may be useful in defining paleostress trajectories.