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

低角度正断层自被发现以来一直是一个谜，因为它们似乎在与最大压应力成高角时滑动，这应该是一个不利的方向。一些走滑断层，如圣安德烈亚斯断层，也具有这种神秘的特征。提出了几种假设来解释这些不利定向断层的明显力学弱点：(1)当接近小角度正断层时，应力场发生旋转；(2)小角度正断层由于断层核心存在固有的软弱物质或发育良好的流动组构而变弱；(3)低角度正断层由于高孔隙流体压力降低了有效正应力而变弱。该项目正在通过对南加州惠普尔和西索尔顿低角度正断层周围形成的断层岩石的构造、岩相学和流体包裹体研究的组合来检验这些假设。构造研究结合了剪切和拉伸断裂露头尺度的数据和拉伸微裂缝和流体包裹体阵列的微观数据，以确定古应力场的方向及其时间和空间变化。定向流体包裹体阵列的显微测温限制了角砾岩和后续压裂过程中流体捕获的压力和温度条件。利用勘探的全岩和稳定同位素数据对可能的流体来源进行了评估。这两条断层都有石英岩下盘，显示出古地震活动的证据，并有良好的滑移和下盘冷却历史。该项目的重点是上底盘，在那里，相对于低角度正断层的宏观构造旋转已知较小。这两条断层具有互补性，因为它们可以研究滑动梯度(有限位移范围为5-50公里)，并可以比较不同地壳水平上发育的断裂带岩石和结构，从惠普尔拆离的孕震带底部到西萨尔顿拆离的上孕震带及其上方。低角度正断层(即与地球成小于30度角的伸展断层？S地表)具有经济和社会意义，因为(A)它们承载着矿藏和石油聚集区，以及(B)它们对犹他州盐湖城等社区构成潜在的地震风险，墨西哥，下加利福尼亚州。由于它们相对于地球中的主要应力的明显反常取向，它们滑动的力学条件还没有被很好地理解。因此，他们的地震危险性也没有得到很好的了解。主要的走滑断层，如圣安德烈亚斯断层，对许多大型人口中心构成了明显的地震威胁，并成为多项努力(如圣安德烈亚斯钻探项目)的目标，以表征断裂带岩石的性质、断裂带内/附近的流体以及附近的应力方向。这项研究将提供来自另一类断层的补充数据集，最终可以与圣安德烈亚斯和其他断层的数据相结合，以(1)记录不同类型主要断层特有的断层岩特征和历史，以及(2)更清楚地了解地壳不同背景下的地震力学。

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.