Tectonic Inheritance With Dipping Faults and Deformation Fabric in the Brittle and Ductile Southern California Crust

Plate motions in Southern California have undergone a transition from compressional and extensional regimes to a dominantly strike‐slip regime in the Miocene. Strike‐slip motion is most easily accommodated on vertical faults, and major transform fault strands in the region are typically mapped as near vertical on the surface. However, some previous work suggests that these faults have a dipping geometry at depth. We analyze receiver function arrivals that vary harmonically with back azimuth at all available broadband stations in the region. The results show a dominant signal from contrasts in dipping foliation as well as dipping isotropic velocity contrasts from all crustal depths, including from the ductile middle to lower crust. We interpret these receiver function observations as a dipping fault‐parallel structural fabric that is pervasive throughout the region. The strike of these structures and fabrics is parallel to that of nearby fault surface traces. We also plot microseismicity on depth profiles perpendicular to major strike‐slip faults and find consistently NE dipping features in seismicity changing from near vertical (80–85°) on the Elsinore Fault in the Peninsular Ranges to 60–65° slightly further inland on the San Jacinto Fault to 50–55° on the San Andreas Fault. Taken together, the dipping features in seismicity and in rock fabric suggest that preexisting fabrics and faults may have acted as strain guides in the modern slip regime, with reactivation and growth of strike‐slip faults along northeast dipping fabrics both above and below the brittle‐ductile transition.

在中新世，南加利福尼亚的板块运动已经从挤压和伸展体制转变为以走滑体制为主。走滑运动在垂直断层上最容易实现，该地区的主要转换断层带在地表通常被绘制为近乎垂直。然而，一些先前的研究表明这些断层在深部具有倾斜的几何形态。我们分析了该地区所有可用宽带台站的接收函数到时，其随反方位角呈谐波变化。结果显示出来自倾斜叶理对比以及来自所有地壳深度（包括韧性的中地壳到下地壳）的倾斜各向同性速度对比的主导信号。我们将这些接收函数观测结果解释为一种在整个地区普遍存在的与断层平行的倾斜构造组构。这些构造和组构的走向与附近断层地表迹线的走向平行。我们还在垂直于主要走滑断层的深度剖面上绘制了微震活动图，发现在地震活动中始终存在东北向倾斜的特征，从半岛山脉的埃尔西诺断层近乎垂直（80 - 85°），到圣哈辛托断层稍向内陆变为60 - 65°，再到圣安德烈亚斯断层变为50 - 55°。综合来看，地震活动和岩石组构中的倾斜特征表明，预先存在的组构和断层可能在现代滑动体制中起到了应变引导作用，走滑断层在脆性 - 韧性转换带之上和之下沿着东北向倾斜的组构重新活动和生长。