Mineralogic and Textural Analyses of Drill Cuttings from the San Andreas Fault Observatory at Depth (SAFOD) Boreholes: Initial Interpretations of Fault Zone Composition and Constraints on Geologic Models
Mineralogic and Textural Analyses of Drill Cuttings from the San Andreas Fault Observatory at Depth (SAFOD) Boreholes: Initial Interpretations of Fault Zone Composition and Constraints on Geologic Models
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圣安地列斯断层深度观测站 (SAFOD) 钻孔钻屑的矿物学和结构分析：断层带组成的初步解释和地质模型的约束
DOI：
10.1130/ges00076.1
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发表时间：
2007
期刊：
影响因子：
2.5
通讯作者：
James P. Evans
中科院分区：
文献类型：
--
作者：
K. Bradbury;D. C. Barton;J. Solum;S. D. Draper;James P. Evans
我们检查了圣安德烈亚斯深部断层观测站(SAFOD)钻孔的钻屑，以确定断裂带和围岩中的岩性和变形结构。切割样品代表了与圣安德烈亚斯断层相邻的1.7公里地图距离和3.2公里垂直深度的岩性。我们分析了2.2公里深的垂直先导孔和3.99公里长的主孔，平均30m岩屑取样空间的2个横纹薄片和66个纹面薄片。我们识别出第四系和第三系(？)测深760~1920m为花岗岩，1920~3150m为砂岩和岩屑砂岩，3150~3987m为粉砂岩、泥岩和页岩互层。根据主孔1050、1600-2000、2200-2500、2700-3000、3050-3350和3500m测量深度的碎屑岩丰度百分比、变形颗粒的存在和蚀变相的存在，推断至少存在五条断裂带，包括断裂带和断核区。这些带与井眼地球物理特征相关联，与断层的存在相一致。如果较深的碎裂岩带和蚀变强度与圣安德烈斯断裂的地表痕迹相连，则该断裂带向西南倾斜80-85°，并在厚约250-300m的破坏带中由多个滑动面组成。这一解释得到了井眼地球物理研究的支持，这些研究表明，该地区是一个地震速度低、电阻率降低、孔隙度可变的地区。
We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithol- ogy and deformational textures in the fault zones and host rocks. Cutting samples repre- sent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hun- dred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spac- ing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We iden- tify Quaternary and Tertiary(?) sedimen- tary rocks in the upper 700 m of the holes; granitic rocks from 760-1920 m measured depth; arkosic and lithic arenites, interbed- ded with siltstone sequences, from 1920 to ~3150 m measured depth; and interbed- ded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least fi ve fault zones, which include regions of damage zone and fault core on the basis of percent of cata- clasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600-2000, 2200-2500, 2700-3000, 3050-3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80-85° southwest, and consists of multiple slip surfaces in a damage zone ~250-300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity.