Collaborative Research: What is the Strength of Low-Angle Normal Faults?

合作研究：低角度正断层的强度有多大？

• 批准号: 0809220
• 负责人: Jane Selverstone
• 金额: $ 14.86万
• 依托单位: University of New Mexico
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-06-15 至 2012-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0809220&HistoricalAwards=false
• 关键词: Collaborative; Research; What; Strength; Low

Low-angle normal faults have been a puzzle since they were first discovered because they appear to slip while at a high angle to the maximum compressive stress, which should be an unfavorable orientation. Some strike-slip faults, such as the San Andreas, share this enigmatic trait. Several hypotheses have been proposed to explain the apparent mechanical weakness of these unfavorably oriented faults: (1) the stress field rotates as low-angle normal faults are approached; (2) low-angle normal faults are weak because inherently weak materials or well-developed flow fabrics exist in the fault core; (3) low-angle normal faults are weak due to lowering of the effective normal stress by high pore fluid pressure. This project is testing these hypotheses through a combination of structural, petrographic, and fluid inclusion studies of fault rocks formed around the Whipple and west Salton low-angle normal faults in southern California. Structural studies combine outcrop-scale data on shear and tensile fractures with microscopic data on tensile microcracks and fluid-inclusion arrays to define the orientation of the paleostress field, and its temporal and spatial variations. Microthermometry of oriented fluid inclusion arrays constrain the pressure and temperature conditions of fluid entrapment during brecciation and subsequent fracturing. Possible fluid sources are evaluated using exploratory whole-rock and stable-isotope data. Both faults have quartzofeldspathic footwalls, display evidence of paleoseismicity, and have well-constrained slip and footwall-cooling histories. This project focuses on the upper footwalls where macroscale structural rotations relative to the low-angle normal faults are known to be minor. The two faults are complementary because they allow study of slip gradients (finite displacements ranging from 5-50 km) and allow comparison of fault-zone rocks and structure developed at different crustal levels, from the base of the seismogenic zone for the Whipple detachment to in and above the upper seismogenic zone for the west Salton detachment.Low-angle normal faults (i.e., extensional faults that slip at angles of less than 30 degrees to the earth?s surface) have economic and societal relevance because (a) they host ore deposits and control regions in which petroleum accumulates, and (b) they pose a potential seismic risk to communities like Salt Lake City, Utah, and Mexicali, Baja California. Because of their apparently anomalous orientation with respect to major stresses in the earth, the mechanical conditions under which they slip are not well understood. As a result, their seismic hazards are also not well understood. Major strike-slip faults, such as the San Andreas fault, pose a clear seismic threat to many large population centers and are the targets of several efforts (e.g., San Andreas drilling project) to characterize the nature of the fault zone rocks, fluids within/near the fault zones, and nearby stress orientations. This study will provide a complementary data set from another class of faults that can ultimately be combined with data from the San Andreas and other faults to (1) document fault rock features and histories that are unique to different types of major faults, and (2) gain clearer understanding of earthquake mechanics in different settings in the crust.

低角度的正常断层自第一次被发现以来一直是一个难题，因为它们似乎在高角度与最大压缩应力的角度滑动，这应该是不利的方向。一些滑移缺陷，例如圣安德烈亚斯（San Andreas），具有这种神秘的特征。已经提出了几种假设来解释这些不利方向的断层的明显机械弱点：（1）应力场随着低角度正常断层而旋转； （2）低角度正常断层是弱的，因为故障核心存在固有的材料固有的材料或发达的流动织物； （3）由于高孔隙流体压力降低了有效的正常应力，因此低角度正常断层较弱。该项目正在通过在南加州的Whipple和West Salton低角度正常断层周围形成的断层岩石的结构，岩石学和流体包容性研究来检验这些假设。结构研究结合了有关剪切和拉伸骨折的露头数据，以及有关拉伸微裂纹和流体包含流体阵列的显微镜数据，以定义古森特场的方向及其时间和空间变化。定向流体包容阵列的微热计限制了在接值和随后的破裂过程中流体夹带的压力和温度条件。使用探索性的全摇滚和稳定异位数据来评估可能的流体源。这两个断层都有石英丝状脚壁，显示了古震震的证据，并具有良好的滑动和脚壁冷却历史。该项目的重点是宏观尺度结构旋转相对于低角度正常断层的上墙。这两个断层是互补的，因为它们允许研究滑动梯度（从5-50 km）进行研究，并允许比较断层区的岩石和在不同的地壳水平上发展的断层区和结构，从鞭thip脱离的地震区域的基础，以使西萨尔顿的较高偏移量的正常状态（i。地球表面的学位）具有经济和社会意义，因为（a）他们拥有石油积聚的矿石沉积物和控制区域，以及（b）它们对盐湖城，犹他州和墨西哥裔社区和加利福尼亚州的墨西哥市构成潜在的地震风险。由于它们在地球上的主要应力方面显然是异常的方向，因此无法很好地理解其滑动的机械条件。结果，他们的地震危害也不太了解。主要的滑移断层，例如圣安德烈亚斯断层，对许多大人口中心构成了明显的地震威胁，并且是几项努力（例如，圣安德烈亚斯钻探项目）的目标，以表征断层区岩石的性质，内部/附近的断层域内的流体，以及附近的压力方向。这项研究将提供来自另一类故障的互补数据集，这些数据集最终可以与San Andreas和其他故障的数据结合到（1）（1）文档故障岩石特征和历史，这些特征和历史是不同类型的主要故障所独有的，并且（2）在外壳中不同环境中对地震机制的了解更清晰。