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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Stability of dike intrusion along preexisting fractures
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Stability of dike intrusion along preexisting fractures

机译：沿既有裂缝的堤防侵入稳定性

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摘要

We investigate the ability of magma to propagate along preexisting fractures oblique to the least compressive stress. Relaxation of the preexisting shear stress to zero over the portion of the fracture dilated by magma (the dike) results in slip for some distance along the closed portion of the fracture ahead of the dike tip and a stress concentration near the dike tip. This could lead to the production of new tensile cracks, oblique to the parent dike, that could capture the flow. If the sheer stress resolved on the fracture plane is perpendicular to the fracture front (mode I-II), the front may deviate along its entire length; if the shear stress is parallel to the fracture front (mode I-III) the front may splay into segments. For mode I-II dikes the maximum tensile stress occurs at the dike tip and is parallel to the dike. If the effective tensile stress exceeds the rock tensile strength, then the intruding magma, rather than dilating the existing fracture, is expected to propagate into a self-generated crack analogous to the "wing cracks" observed to form at the tips of pure mode II fractures. For mode I-III dikes the maximum tensile stress lies within the plane of the dike and is oriented at some angle that depends upon the far-field boundary conditions. Even if the magma pressure exceeds the ambient normal stress, it appears to be very difficult for dikes to intrude into preexisting fractures unless one or more of the following conditions is satisfied: (1) the fracture is nearly perpendicular to the least compressive stress; (2) the resolved shear stress on the fracture is small compared to the excess magma pressure (i.e., the ratio of shear to opening of the dike walls is small); (3) the effective ambient dike-normal stress is small compared to the rock tensile strength. This indicates that it may be quite difficult for dikes emerging from midcrustal to lower crustal depths to follow faults. [References: 38]
机译：我们研究了岩浆沿已存在的斜向最小压缩应力的裂缝传播的能力。在岩浆（堤坝）扩张的裂缝部分上，先前存在的剪应力松弛为零，导致沿堤坝尖端前方裂缝的闭合部分滑移了一段距离，并且应力集中在堤坝尖端附近。这可能导致产生新的拉伸裂缝，该裂缝与母堤倾斜，可以捕获流动。如果在裂缝平面上分解的切应力垂直于裂缝前沿（模式I-II），则该前沿可能会沿其整个长度偏离;反之，如果剪应力平行于裂缝前沿（模式I-III），则该前沿可能会张开。对于模式I-II堤防，最大张应力出现在堤防尖端并与堤防平行。如果有效拉伸应力超过岩石的拉伸强度，那么侵入岩浆将扩展为自生裂纹，类似于在纯模式II尖端观察到的“机翼裂纹”，而不是扩展现有裂缝。骨折。对于I-III型堤防，最大拉应力位于堤防平面内，并以一定角度定向，该角度取决于远场边界条件。即使岩浆压力超过环境法向应力，除非满足以下一个或多个条件，堤坝仍很难侵入先前存在的裂缝：（1）裂缝几乎垂直于最小压应力; （2）与超高的岩浆压力相比，裂缝上的解析剪切应力较小（即，剪切强度与堤壁的开口率之比较小）; （3）与岩石的抗拉强度相比，有效的环境堤防正应力较小。这表明从中地壳到低地壳深度的堤防很难追随断层。 [参考：38]

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