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Control of pre-existing fabric in fracture formation, reactivation and vein emplacement under variable fluid pressure conditions: an example from Archean greenstone belt, India
Solid Earth ( IF 3.4 ) Pub Date : 2020-07-08 , DOI: 10.5194/se-11-1227-2020
Sreyashi Bhowmick , Tridib Kumar Mondal

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low-Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before.

中文翻译:

在可变流体压力条件下控制现有织物在裂缝形成,重新活化和静脉植入中的作用:以印度太古宙绿岩带为例

在一定的构造应力和流体压力条件下,大多数上地壳流体流动受到岩石中预先存在的裂缝/页岩的强烈影响。在本研究中,我们从Chitradurga剪切带(CSZ)附近的Chitradurga片岩带的含金块状大型玄武岩(近壳岩)中,分析了范围广泛的横切裂缝,这些裂缝充满了取向和厚度各异的石英脉,西部Dharwar Craton,印度南部。该研究包括以下步骤:(1)分析内部磁性结构,使用磁化率各向异性(AMS)研究,确定主偏玄武岩的强度,(2)通过下半球等面积投影定量流体压力条件通过确定驱动压力比(ř â??²),应力比( Ï??)和易感性压裂,和(3)解密使用断层滑动分析古应力条件。我们将NNW’SSE解释为NWâ€??? SE(平均337 / 69â ???NE NE)定向的磁性织物在该地区的岩石中由于NE NESW缩短而在区域D1 / D2变形期间发展。但是,由NWâSE到EâW缩短的D3变形导致了沿CSZ的左弦运动。由于这种正弦剪切,以CSZ为剪切边界的突出取向的裂缝形成了瑞氏剪切分量。随后,所有先前存在的织物以及里德尔剪力组件均被重新激活,并且通过在浅深度处由高到低P f的间歇性流体压力波动来进行静脉定位(“ ¼” →“ 2.4”)。 ?km)。但是,NNW™SSE方向易于在高和低P f下重新激活Â条件,从而沿这些方向获得最大的静脉厚度。从断层滑动分析推导出的古应力以及裂缝和静脉的运动学与先前估计的区域构造学非常吻合。因此,整合多个研究领域有助于在逻辑上解释以前从未涉足的研究领域中的流体流动状况和静脉置入机制。
更新日期:2020-08-20
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