Abstract The lower chert unit of the 3.48 Ga Dresser Formation (Pilbara Craton, Western Australia) hosts Earth’s oldest convincing evidence of life in the form of stromatolites in a depositional setting that has been interpreted as either a shallow marine lagoon, an active volcanic caldera, or a basin deposited during regional extension. Understanding the environment where early life thrived is important not only for early life studies on Earth, but in the search for life on Mars, and for understanding the setting for the origin of life. Structural analysis of the extensive network of syn-depositional hydrothermal chert-barite veins that immediately underlies the Dresser was undertaken in order to test if deposition occurred within an evolving caldera system, or during regional extension. Over 190 hydrothermal veins were mapped for their geometry and relative age relationships. Bedding orientations were recorded within nine fault-bounded structural sub-blocks and used for back-rotating the measured veins to their original orientation. The results show that: 1) There are four main vein systems, including three that were emplaced during deposition of the Dresser Formation lower chert unit, and a fourth that was emplaced within syn- to post-depositional listric growth faults; 2) The majority of veins were sub-vertical prior to regional tilting; 3) The first three vein systems show mutually cross-cutting relationships; 4) Vein densities and the number of vein systems vary between sub-blocks; 5) Conjugate vein networks (NE-SW and NW-SE) dominate two middle and northern sub-blocks. These results show that the first three vein systems were emplaced through fracturing under a stress regime with a vertical σ1. Vertical conjugate fractures are consistent with sandbox models of overburden fracturing during emplacement and discharge of a magma chamber in a volcanic caldera. Areas of single vein orientations reflect marginal areas of the caldera system, dominated by radial or by ring faults. The veins in late listric growth faults represent the final period of caldera collapse, which controlled the depositional thicknesses of overlying strata, possibly during subsequent regional extension. These results document an unique setting for earliest life on Earth, within shallow water and terrestrial deposits of an active, low-eruptive caldera setting.

中文翻译：

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澳大利亚皮尔巴拉克拉通 3.48 Ga Dresser 组同沉积热液脉结构分析

摘要 3.48 Ga Dresser 地层（西澳大利亚皮尔巴拉克拉通）的下部燧石单元在沉积环境中以叠层石形式存在地球上最古老的令人信服的生命证据，该沉积环境被解释为浅海泻湖、活跃的火山口、或区域延伸过程中沉积的盆地。了解早期生命繁盛的环境不仅对于地球上的早期生命研究很重要，而且对于在火星上寻找生命以及了解生命起源的环境也很重要。对直接位于德莱瑟 (Dresser) 下方的同沉积热液燧石-重晶石脉的广泛网络进行了结构分析，以测试沉积是否发生在不断发展的火山口系统内，或在区域扩展过程中发生。绘制了 190 多个热液静脉的几何形状和相对年龄关系。层理方向被记录在九个以断层为界的构造子区块内，并用于将测得的矿脉反向旋转到它们的原始方向。结果表明： 1）主要脉系有4条，其中3条在德莱赛组下部燧石单元沉积过程中就位，1条在同沉积到沉积后的纵长断层内就位；2）在区域倾斜之前，大部分矿脉是亚垂直的；3）前三个脉系呈现相互交叉的关系；4) 子块之间的静脉密度和静脉系统的数量不同；5) 共轭静脉网络（NE-SW 和 NW-SE）支配着两个中部和北部的子区块。这些结果表明，前三个脉系统是在垂直 σ1 的应力状态下通过压裂就位的。垂直共轭裂缝与火山喷口岩浆房就位和排放过程中上覆岩层压裂的沙箱模型一致。单脉方向的区域反映了火山口系统的边缘区域，以径向断层或环形断层为主。晚listric 生长断层中的脉代表火山口坍塌的最后时期，它控制着上覆地层的沉积厚度，可能在随后的区域延伸期间。这些结果记录了地球上最早生命的独特环境，位于活跃、低喷发的火山口环境的浅水和陆地沉积物中。垂直共轭裂缝与火山喷口岩浆房就位和排放过程中上覆岩层压裂的沙箱模型一致。单脉方向的区域反映了火山口系统的边缘区域，以径向断层或环形断层为主。晚listric 生长断层中的脉代表火山口坍塌的最后时期，它控制着上覆地层的沉积厚度，可能在随后的区域延伸期间。这些结果记录了地球上最早生命的独特环境，位于活跃、低喷发的火山口环境的浅水和陆地沉积物中。垂直共轭裂缝与火山喷口岩浆房就位和排放过程中上覆岩层压裂的沙箱模型一致。单脉方向的区域反映了火山口系统的边缘区域，以径向断层或环形断层为主。晚listric 生长断层中的脉代表火山口坍塌的最后时期，它控制着上覆地层的沉积厚度，可能在随后的区域延伸期间。这些结果记录了地球上最早生命的独特环境，位于活跃、低喷发的火山口环境的浅水和陆地沉积物中。单脉方向的区域反映了火山口系统的边缘区域，以径向断层或环形断层为主。晚listric 生长断层中的脉代表火山口坍塌的最后时期，它控制着上覆地层的沉积厚度，可能在随后的区域延伸期间。这些结果记录了地球上最早生命的独特环境，位于活跃、低喷发的火山口环境的浅水和陆地沉积物中。单脉方向的区域反映了火山口系统的边缘区域，以径向断层或环形断层为主。晚listric 生长断层中的脉代表火山口坍塌的最后时期，它控制着上覆地层的沉积厚度，可能在随后的区域延伸期间。这些结果记录了地球上最早生命的独特环境，位于活跃、低喷发的火山口环境的浅水和陆地沉积物中。