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

低角度正断层自发现以来一直是一个谜，因为它们在与最大压应力成大角度时出现滑动，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，就具有这种神秘的特征。对于这些不利取向断层的明显力学缺陷，提出了几种假设：(1)当低角度正断层接近时，应力场旋转；(2)低角度正断层因断层核内存在固有的弱物质或发育良好的流动组构而弱；(3)由于高孔隙流体压力降低了有效正应力，低角度正断层较弱。该项目通过对南加州惠普尔和西索尔顿低角正断层周围形成的断层岩石进行结构、岩石学和流体包裹体研究，来验证这些假设。构造研究将露头尺度的剪切和拉伸裂缝数据与微观的拉伸微裂缝和流体包裹体阵列数据结合起来，确定了古应力场的方向及其时空变化。定向流体包裹体阵列的显微测温限制了角化和后续压裂过程中流体包裹体的压力和温度条件。利用探索性全岩和稳定同位素数据评估可能的流体来源。这两条断裂都有石英场下盘，显示出古地震活动的证据，并具有约束良好的滑动和下盘冷却史。该项目的重点是上下盘，那里相对于低角度正断层的宏观结构旋转较小。这两条断层是互补的，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构造，从惠普尔拆离的孕震带底部到西索尔顿拆离的上孕震带内部和上方。低角度正断层（即与地球的角度小于30度的伸展断层）？因为(a)它们拥有矿藏和控制石油积聚的区域，(b)它们对犹他州盐湖城和下加利福尼亚州墨西卡利等社区构成潜在的地震风险，因此具有经济和社会意义。由于相对于地球上的主要应力，它们的方向明显异常，因此它们滑动的力学条件还没有得到很好的理解。因此，它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，对许多人口密集的中心地区构成了明显的地震威胁，并且是一些努力（例如圣安德烈亚斯钻井项目）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。这项研究将提供另一类断层的补充数据集，最终可以与圣安德烈亚斯断层和其他断层的数据相结合，以(1)记录不同类型主要断层的断层岩石特征和历史，(2)更清楚地了解地壳不同环境下的地震力学。

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.