Abstract The objective of this study is to explain the occurrence and paleo-stress significance of 3D joint clustering in chert nodules (inclusions) within a layered pelagic limestone sequence. The difference in stiffness between chert and limestone is about one order of magnitude. Field observations show that fracture localization occurs mostly in chert nodules as opposed to the limestone matrix. We show with a novel three-dimensional geomechanical modelling analysis how the inclusion (ellipsoid) axes ratio influences fracture intensity and propagation within and outside the chert nodules and how the nodules record different deformation phases under different remote stress conditions. From field observations, we recognize two joint sets in the chert nodules: joints parallel and normal to the plane containing the two major axes of the nodule (bedding plane). In the nodules, the normal-to-bedding joints are interpreted to be younger than the parallel-to-bedding ones. The modelling of the 3D Eshelby solution for the stress field inside the chert nodule and in the surrounding matrix is consistent with our field observations and it suggests a strong differential stress during deformation (σminr/σmaxr

中文翻译：

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远洋石灰岩中的燧石结核作为古应力指标：3D 地质力学分析

摘要 本研究的目的是解释层状远洋灰岩层序中燧石结核（包裹体）中 3D 节理聚集的发生和古应力意义。燧石和石灰石之间的刚度差异大约是一个数量级。现场观察表明，裂缝定位主要发生在燧石结核中，而不是石灰岩基质中。我们用一种新颖的三维地质力学建模分析展示了包裹体（椭圆体）轴比如何影响燧石结核内外的断裂强度和扩展，以及结核如何在不同的远程应力条件下记录不同的变形阶段。通过实地观察，我们在燧石结核中识别出两个联合组：关节平行并垂直于包含结节两个主轴的平面（层理平面）。在结节中，正常到层理的关节被解释为比平行于层理的关节年轻。燧石结核内部和周围基质中应力场的 3D Eshelby 解的建模与我们的现场观察一致，它表明变形过程中存在很强的微分应力 (σminr/σmaxr