Low-temperature deformation in calcite veins of SAFOD core samples (San Andreas Fault) — Microstructural analysis and implications for fault rheology
Low-temperature deformation in calcite veins of SAFOD core samples (San Andreas Fault) — Microstructural analysis and implications for fault rheology
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DOI：
10.1016/j.tecto.2011.05.014
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发表时间：
2011-08
期刊：
影响因子：
2.9
通讯作者：
E. Rybacki;C. Janssen;R. Wirth;Kai Chen;H. Wenk;D. Stromeyer;G. Dresen
中科院分区：
文献类型：
--
作者：
E. Rybacki;C. Janssen;R. Wirth;Kai Chen;H. Wenk;D. Stromeyer;G. Dresen
利用光学显微镜和透射电子显微镜研究了圣安德烈亚斯断层深度观测站（SAFOD）4个岩心样品的显微结构。这些样品由钻井活动第三阶段（3141- 3307米MD）的砂岩、粉砂岩和断层泥组成，显示出石英、长石、粘土和无定形材料的复杂成分。显微构造表明，强烈的剪切变形和溶解-沉淀变形是主要的变形过程。这些样品还含有丰富的方解石脉。在被检查的矿脉中，方解石颗粒表现出不同程度的变形，并有孪生和晶体塑性的证据。位错密度（范围从0.33·1012 m-2到0.33·1013 m-2）和孪晶线密度（0.22mm-1到165 mm-1）被用作古压力计。相应的差应力估计值在33 - 132 MPa之间变化，从位错密度推断，从孪晶密度获得92- 251 MPa，可能反映了不同的最大应力状态和/或晶粒尺度应力扰动。应力估计的平均值分别为68± 46 MPa和168± 60 MPa，其中位错密度的估计值可能代表下限，而孪晶密度的估计值可能代表上限。的应力估计也是兼容的残余晶格应变确定与微焦点劳厄衍射产生的等效应力为50- 300 MPa的孪晶方解石。较低的应力边界同意从钻孔崩落测量在导向孔进行的应力估计。根据这些数据，并假设静水孔隙压力和接近上覆岩层应力的低中间主应力，SAFOD现场圣安德烈亚斯断层的摩擦滑动被限制在0.24和0.31之间的摩擦系数。这些低摩擦值可能与存在的粘土，滑石，和无定形相中发现的故障核心，并支持弱圣安德烈亚斯故障的假设。
The microstructures of four core samples from the San Andreas Fault Observatory at Depth (SAFOD) were investigated with optical and transmission electron microscopy. These samples, consisting of sandstone, siltstone, and fault gouge from phase III of the drilling campaign (3141–3307m MD), show a complex composition of quartz, feldspar, clays, and amorphous material. Microstructures indicate intense shearing and dissolution–precipitation as main deformation processes. The samples also contain abundant veins filled with calcite. Within the inspected veins the calcite grains exhibit different degrees of deformation with evidence for twinning and crystal plasticity. Dislocation densities (ranging from≈3·1012m−2to ≈3·1013m−2) and twin line densities (≈22mm−1–165mm−1) are used as paleo-piezometers. The corresponding estimates of differential stresses vary between 33 and 132MPa, deduced from dislocation density and 92–251MPa obtained from twin density, possibly reflecting chronologically different maximum stress states and/or grain scale stress perturbations. Mean values of stress estimates are 68±46MPa and 168±60MPa, respectively, where estimates from dislocation density may represent a lower bound and those from twin density an upper bound. The stress estimates are also compatible with residual lattice strains determined with microfocus Laue diffraction yielding equivalent stresses of 50–300MPa in twinned calcite. The lower stress bound agrees with stress estimates from borehole breakout measurements performed in the pilot hole. From these data and assuming hydrostatic pore pressure and a low intermediate principal stress close to the overburden stress, frictional sliding of the San Andreas Fault at the SAFOD site is constrained to friction coefficients between 0.24 and 0.31. These low friction values may be related to the presence of clays, talc, and amorphous phases found in the fault cores and support the hypothesis of a weak San Andreas Fault.