On the origin of mixed-layered clay minerals from the San Andreas Fault at 2.5–3 km vertical depth (SAFOD drillhole at Parkfield, California)
On the origin of mixed-layered clay minerals from the San Andreas Fault at 2.5–3 km vertical depth (SAFOD drillhole at Parkfield, California)
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关于圣安德烈亚斯断层 2.5-3 公里垂直深度的混合层状粘土矿物的起源（加利福尼亚州帕克菲尔德的 SAFOD 钻孔）
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
B. A. Pluijm
中科院分区：
文献类型：
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作者：
A. Schleicher;L. Warr;B. A. Pluijm
本文对圣安德烈亚斯断层观测台（SAFOD）深孔三个关键位置的泥岩取样基质进行了详细的矿物学研究。自生伊利石-蒙脱石（I-S）和绿泥石-蒙脱石（C-S）混合层矿物粘土的特征表明其成岩成因较深。一种随机排列的含20-25%蒙脱石层的I-S矿物是整个圣安德烈亚斯断裂带（在3,066和3,436 m测量深度/MD取样）的主要自生粘土物种之一，而含2-5%蒙脱石层的自生伊利石是断层下的主要相（在3,992 m MD取样）。最富蒙脱石的混合层状组合出现在活跃变形蠕变带约3300 - 3353 m（真正垂直深度约2.7 km）， I-S（70:30）和C-S（50:50）。所有泥岩样品的基质均显示出广泛的石英和长石（包括斜长石和钾长石）溶解，并伴有填孔粘土矿物的结晶作用。然而，岩石变形对基质的影响似乎很小，主要由弯曲和断裂的云母颗粒定义的弱扁平结构。利用现有的埋藏沉积环境中I-S结晶动力学模型，结合当前井深和热结构，可以评价I-S生长的条件和时间。假设沉积盐水中典型的K+浓度为100-200 ppm，目前的地热梯度为35°C/km，采样深度的钻孔温度约为112°C，可以预测大多数I-S矿物是在过去4-11 Ma形成的，可能仍与循环流体处于平衡状态。这种简单埋藏模式的例外是出现了比埋藏模型预测的蒙脱石含量更高的混合层状相。这些矿物是断层活跃爬行部分的特征，是抛光脆性滑动表面上局部薄膜粘土涂层的特征，可以用沿着断层这段循环的较冷流体或K+枯竭盐水流动的影响来解释。
A detailed mineralogical study is presented of the matrix of mudrocks sampled from spot coring at three key locations along the San Andreas Fault Observatory at depth (SAFOD) drill hole. The characteristics of authigenic illite–smectite (I–S) and chlorite–smectite (C–S) mixed-layer mineral clays indicate a deep diagenetic origin. A randomly ordered I–S mineral with ca. 20–25% smectite layers is one of the dominant authigenic clay species across the San Andreas Fault zone (sampled at 3,066 and 3,436 m measured depths/MD), whereas an authigenic illite with ca. 2–5% smectite layers is the dominant phase beneath the fault (sampled at 3,992 m MD). The most smectite-rich mixed-layered assemblage with the highest water content occurs in the actively deforming creep zone at ca. 3,300–3,353 m (true vertical depth of ca. 2.7 km), with I–S (70:30) and C–S (50:50). The matrix of all mudrock samples show extensive quartz and feldspar (both plagioclase and K-feldspar) dissolution associated with the crystallization of pore-filling clay minerals. However, the effect of rock deformation in the matrix appears only minor, with weak flattening fabrics defined largely by kinked and fractured mica grains. Adopting available kinetic models for the crystallization of I–S in burial sedimentary environments and the current borehole depths and thermal structure, the conditions and timing of I–S growth can be evaluated. Assuming a typical K+ concentration of 100–200 ppm for sedimentary brines, a present-day geothermal gradient of 35°C/km and a borehole temperature of ca. 112°C for the sampled depths, most of the I–S minerals can be predicted to have formed over the last 4–11 Ma and are probably still in equilibrium with circulating fluids. The exception to this simple burial pattern is the occurrence of the mixed layered phases with higher smectite content than predicted by the burial model. These minerals, which characterize the actively creeping section of the fault and local thin film clay coating on polished brittle slip surfaces, can be explained by the influence of either cooler fluids circulating along this segment of the fault or the flow of K+-depleted brines.