Abstract The association of mafic extrusive rocks and serpentinized mantle rocks is a common feature encountered in numerous ultra-distal magma-poor rifted margins as well as in (ultra)slow-spreading oceanic settings where detachment faulting and mantle exhumation occur. Although seismic imaging and high-resolution TOBI radar allow imaging this association, the nature of this interface is poorly understood (e.g. nonconformity vs. tectonic channelized vs. diffuse fluid flow along the interface). The timing between magmatism and active detachment faulting is often poorly constrained and it remains difficult to decipher if mafic extrusives are pre-, syn- or post-exhumation of mantle rocks at the seafloor. In this study, we characterize the nature of this interface from a section of a Jurassic ultra-distal margin preserved in the Platta nappe, SE Swiss Alps. We show that the basalt-serpentinized mantle contact does neither correspond to an Alpine thrust system, nor to a nonconformity or a detachment plane exhuming mantle rocks from underneath already emplaced basalts. Rather, the basalt-serpentinite interface corresponds to a weak decoupling level formed by conjugate low-angle normal faults onto which conjugate high-angle normal faults branch. These structures are diagnostic of co-axial extensional tectonics occurring during and after emplacement of basalts over a formerly exhumed serpentinized mantle. Carbonate extensional and shear veins and foliated ophicalcites formed during this Jurassic extensional event. Their distribution across and along the basalt-serpentinite interface demonstrates that this decoupling interface was a high-permeability layer that channelized high fluid flow responsible for the widespread hydrothermal alteration. A positive feedback between fluid flow and extensional deformation is proposed.

中文翻译：

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大洋领域玄武岩和蛇纹石化地幔之间界面的性质：来自阿尔卑斯山地质剖面的见解

摘要 基性喷出岩和蛇纹岩化地幔岩的组合是许多超远缘岩浆贫乏裂谷边缘以及（超）缓慢扩张的海洋环境中遇到的一个共同特征，其中发生拆离断层和地幔剥脱。尽管地震成像和高分辨率 TOBI 雷达允许对这种关联进行成像，但对该界面的性质知之甚少（例如，不整合与构造通道化与沿界面的扩散流体流动）。岩浆活动和活动拆离断层之间的时间关系通常没有很好的限制，而且如果基性喷出物是海底地幔岩石的剥脱前、同期或后剥落，仍然难以破译。在这项研究中，我们从保存在 Platta 推覆层中的侏罗纪超远侧边缘的一部分表征了该界面的性质，SE瑞士阿尔卑斯山。我们表明玄武岩-蛇纹石化地幔接触既不对应于高山逆冲系统，也不对应于不整合面或脱离平面，从已经布设的玄武岩下方挖出地幔岩石。相反，玄武岩-蛇纹岩界面对应于由共轭低角度正断层形成的弱解耦水平，共轭高角度正断层分支到该水平上。这些结构是在玄武岩就位期间和之后发生的同轴伸展构造的诊断，这些构造是在以前挖掘出的蛇纹石化地幔上发生的。在侏罗纪伸展事件期间形成了碳酸盐伸展和剪切脉以及叶状辉绿岩。它们在玄武岩-蛇纹岩界面上和沿其分布表明，这种解耦界面是一个高渗透层，它引导高流体流动，导致广泛的热液蚀变。提出了流体流动和拉伸变形之间的正反馈。