Absence of Stress-Induced Anisotropy During Brittle Deformation in Antigorite Serpentinite
Absence of Stress-Induced Anisotropy During Brittle Deformation in Antigorite Serpentinite
复制标题
叶蛇纹石脆性变形过程中不存在应力引起的各向异性
DOI：
10.1029/2018jb016255
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发表时间：
2018
期刊：
影响因子：
3.4
通讯作者：
David E
中科院分区：
文献类型：
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作者：
David E
蛇纹岩在变形过程中的地震学特征的知识是解释俯冲带地震观测的基础，但这还有待于实验的约束。我们测量的压缩和剪切波速度在脆性变形多晶叶蛇纹石，在室温和不同的围压高达150 MPa。超声波速度测量，在不同方向的压缩轴，结合机械测量的轴向和体积应变，在直接加载和循环加载三轴变形试验。为了进行对比，还对西风花岗岩进行了变形试验。在所有围压下，叶蛇纹石的脆性变形与应力诱导各向异性的显著缺失相关，并且在岩石破坏之前，波速对轴向压应力没有明显的依赖性。叶栅岩的强度与花岗岩相当，但在高应力（岩石强度）下，其力学行为是弹性的，并且不变形。微裂纹仅在叶蛇纹石试样中观察到，即使在其压缩强度的90-95%下，也没有观察到。微裂纹损伤是非常局部化的断层附近，并由剪切微裂纹，形成专门沿着叶蛇纹石晶体的解理面。我们的观察表明，叶蛇纹石的脆性变形完全发生的“模式II”的剪切微裂纹。这比叶蛇纹石中预先存在的微裂纹群更显著，与花岗岩中的微裂纹群相当。因此，叶蛇纹石脆性变形的力学行为和地震特征在结晶岩中似乎是独特的。
Knowledge of the seismological signature of serpentinites during deformation is fundamental for interpreting seismic observations in subduction zones, but this has yet to be experimentally constrained. We measured compressional and shear wave velocities during brittle deformation in polycrystalline antigorite, at room temperature and varying confining pressures up to 150 MPa. Ultrasonic velocity measurements, at varying directions to the compression axis, were combined with mechanical measurements of axial and volumetric strain, during direct loading and cyclic loading triaxial deformation tests. An additional deformation experiment was conducted on a specimen of Westerly granite for comparison. At all confining pressures, brittle deformation in antigorite is associated with a spectacular absence of stress‐induced anisotropy and with no noticeable dependence of wave velocities on axial compressive stress, prior to rock failure. The strength of antigorite samples is comparable to that of granite, but the mechanical behavior is elastic up to high stress ( of rock strength) and nondilatant. Microcracking is only observed in antigorite specimens taken to failure and not in those loaded even at 90–95% of their compressive strength. Microcrack damage is extremely localized near the fault and consists of shear microcracks that form exclusively along the cleavage plane of antigorite crystals. Our observations demonstrate that brittle deformation in antigorite occurs entirely by “mode II” shear microcracking. This is all the more remarkable than the preexisting microcrack population in antigorite, is comparable to that in granite. The mechanical behavior and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks.