Greenstone‐Up Shear Sense at the Margin of the Mt Edgar Dome, East Pilbara Terrane: Implications for Dome and Keel Formation in the Early Earth
Greenstone‐Up Shear Sense at the Margin of the Mt Edgar Dome, East Pilbara Terrane: Implications for Dome and Keel Formation in the Early Earth
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DOI：
10.1029/2021tc007042
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发表时间：
2022-04
期刊：
影响因子：
4.2
通讯作者：
N. Roberts;B. Tikoff;Rossella Salerno
中科院分区：
文献类型：
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作者：
N. Roberts;B. Tikoff;Rossella Salerno
东皮尔巴拉地体的古太古代埃德加山圆顶长期以来一直被研究为太古代圆顶和龙骨圆顶中的原型圆顶，但其形成的历史存在争议。本研究中提供的运动学数据为埃德加山圆顶和东皮尔巴拉地体的后期发展提供了新的见解。石英晶体择优取向（CPO）、光学显微结构和场结构都表明，埃德加山穹隆的花岗岩-绿岩接触在形成优势岩体后经历了反向剪切（绿岩向上，穹隆向下）。这种反向的剪切感在沿着环绕埃德加山圆顶的剪切边缘的整个范围内都可以观察到，但最好的记录是沿着沿着沃拉沃纳绿岩带的西南边缘。此外，来自剪切边缘外部圆顶三重连接点的石英CPO数据主要表明收缩应变几何形状，与之前的解释一致，即该区域代表浮力不稳定驱动系统中的垂直下沉区。然而，浮力-不稳定性模型不一定能预测沿着沿着穹隆边缘的固态组构中保存的绿岩向上剪切感的发生。考虑了几种地质解释，包括圆顶扩张或后圆顶变形。数据是最一致的解释，直接涉及到圆顶的形成，特别是当考虑在最近发表的结构数据从山埃德加圆顶。这些运动学数据表明，晚期穹隆的发育发生在近静态地壳环境中，而不是伸展或收缩环境。
The Paleoarchean Mt Edgar dome in the East Pilbara Terrane has long been studied as an archetypal dome within Archean dome‐and‐keel terranes, but the history of its formation is debated. Kinematic data presented in this study provide new insights into the late‐stage development of the Mt Edgar dome and East Pilbara Terrane. Quartz crystallographic preferred orientation (CPO), optical microstructures, and field structures all indicate that the granite‐greenstone contact of the Mt Edgar dome experienced reverse (greenstone‐up, dome‐down) sense of shear after the formation of the dominant schistosity. This reverse sense of shear is observed at localities along the entire extent of the sheared margin that rings most of the Mt Edgar dome, but is best documented along the southwest margin in the Warrawoona Greenstone Belt. Additionally, quartz CPO data from a dome triple junction outside of the sheared margin dominantly indicate a constrictional strain geometry, consistent with the previous interpretation that this area represents a zone of vertical foundering in a buoyancy‐instability driven system. However, buoyancy‐instability models do not necessarily predict the occurrence of greenstone‐up sense of shear preserved in solid‐state fabrics along the dome margin. Several geologic explanations are considered, including dome expansion or post‐doming deformation. The data are most consistent with explanations that directly relate to dome formation, especially when considered in tandem with recently published structural data from within the Mt Edgar dome. These kinematic data suggest that late dome development occurred in a near‐static crustal environment rather than an extensional or contractional setting.