Paleomagnetic and Seismological Investigation of Rotations in Transpressional Folds, San Andreas Fault System, California

摘要压扭褶皱可以是扭转褶皱，也可以是滑动分隔褶皱。两者可以产生足够相似的结构，邻近和平行于圣安德烈亚斯断层的褶皱可能是其中之一。这两种模型之间的差异要大得多：这是强故障和弱故障之间的差异。哪一种模型更好地反映了地质学，改变了我们对这些褶皱的新构造危险和产生的几何形状的看法，这些几何形状是加州几个超大型油田的原因。此外，如果这些是滑动分隔褶皱，那么它们的褶皱轴不垂直于最大的压缩水平应力，这表明目前的褶皱理论是不完整的。在扭折环境中，褶皱（和任何潜在的断层）随着时间的推移而旋转，因为有限的缩短应变垂直于走滑断层旋转。在距离走滑断层相同距离的任何地方，古地磁旋转都应该相等。从焦点机制观察到的微极旋转将是小的，因为小尺度旋转和大尺度旋转是相同的。在分区系统上，底层故障不会旋转。然而，一个组成部分的走滑对下伏断层尚未被确认在地表褶皱。最有可能的是，这种剪切应变在平行于圣安德烈亚斯的褶皱的背斜轴附近产生垂直轴旋转，正如在物理模型和华盛顿的亚基马褶皱带中所观察到的那样。旋转的不均匀性和宏观尺度结构的旋转的情况下，将产生横向变化的古地磁旋转和更大的微极旋转比扳手折叠的情况下。该项目通过分析圣华金河谷以西圣安德烈亚斯附近褶皱的古地磁和地震学特征来解决这一问题。这些褶皱起源于新近纪的活动性盲逆冲断层之上，一直是圣安德烈亚斯断层上的应变分配和低剪切应力推断者与扳手断层和高应力推断者之间争论的焦点。对科林加、凯特尔曼山和惠勒岭背斜的古地磁学和地震学调查应确定哪种区域变形的运动学模型最适合这些构造。这两种方法相辅相成，工作将平行进行，以便完善的分析战略能够解决另一种方法所产生的问题。此外，虽然古地磁分析提供了一个时间整合的变形视图，在这些褶皱中使用一些密集的余震数据集将允许我们将微极分析应用到不同深度的切片和不同的体积附近和远离轴向平面的覆盖背斜。这将限制应变和微极旋转的三维变化，这将为理解这些褶皱系统中的变形物理提供重要的约束。这项工作的更广泛影响包括研究仍在开采的活跃的超大型油田。更好地了解这些结构的起源可能会提高最有效地管理这一资源的能力。第二，这些结构是地震孕育的，已经产生了破坏性的地震。这项工作直接解决了变形的机制，这反过来又可能影响研究的重现时间和地震危险性。这项工作还解决了圣安德烈亚斯断层，在美国最大的地震灾害之一的地震特征。PI还将建设科学和教育的基础设施。该项目主要支持女研究生Joya Tejulult，这是一个在地球科学中代表性不足的班级。

AbstractTranspressional folds can be either wrench folds or slip-partitioned folds. The two can produce structures similar enough that folds adjacent and subparallel to the San Andreas Fault could be either. The difference between the two models is far more substantial: it is the difference between strong and weak faults. Which model better reflects the geology alters our perception of both the neotectonic hazards of and the geometry produced in these folds, geometry that is responsible for several super giant oilfields in California alone. Furthermore, if these are slip-partitioned folds, then they are produced with a fold axis not normal to the most compressive horizontal stress, indicating that current theories of folding are incomplete. In a wrench-folding environment, the folds (and any underlying faults) rotate with time as the finite shortening strains rotate normal to the strike-slip fault. Paleomagnetic rotation should be equal everywhere at the same distance from the strike-slip fault. Micropolar rotation, observable from focal mechanisms, will be small, because small and large scale rotations are the same. On a partitioned system, the underlying faults are not rotating. However, a component of strike-slip on underlying faults has yet to be recognized in surface folding. Most likely, this shear strain produces vertical-axis rotations near the anticlinal axes of the folds paralleling the San Andreas, as is observed in physical models and in the Yakima Fold Belt in Washington. Both the inhomogeniety of rotation and the absence of rotation of the macroscale structures will produce laterally varying paleomagnetic rotations and larger micropolar rotations than for the wrench folding case. This project addresses this problem through analysis of the paleomagnetic and seismological characteristics of the folds adjacent to the San Andreas west of the San Joaquin Valley. These folds, initiated in the Neogene over active blind thrust faults, have been the focus of debate between those inferring strain partitioning and low shear stress on the San Andreas fault and those inferring wrench faulting and high stresses. Paleomagnetic and seismological investigation of the Coalinga, Kettleman Hills, and the Wheeler Ridge anticlines should determine which kinematic model of regional deformation best fits these structures. These two techniques complement each other, and the work will proceed in parallel so that refined analysis strategies can address questions arising from the other approach. Additionally, while the paleomagnetic analysis provides a time-integrated view of the deformation, use of some of the dense aftershock datasets in these folds will permit us to apply the micropolar analysis to different depth slices and different volumes near and far from the axial plane of the overlying anticline. This will constrain the three-dimensional variations in strain and micropolar rotation, which will provide important constraints for understanding the physics of deformation in these fold systems. Broader impacts of this work include studying active, super giant petroleum fields that continue to be exploited. Improved understanding of the genesis of these structures is likely to improve the ability to manage this resource most efficiently. Second, these structures are seismogenic and have already produced a damaging earthquake. This work directly addresses the mechanism of deformation, which in turn is likely to effect studies of return times and seismic hazard. This work also addresses the seismogenic character of the San Andreas Fault, one of the greatest seismic hazards in the United States. The PI will also be building the infrastructure of science and education. The project largely supports female graduate student Joya Tetreault, an underrepresented class in earth science.