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

加利福尼亚州圣安德烈亚斯断层系压折褶皱旋转的古地磁和地震学研究

• 批准号: 0310355
• 负责人: Craig Jones
• 金额: $ 6.74万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-06-01 至 2005-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0310355&HistoricalAwards=false
• 关键词: Paleomagnetic; Seismological; Investigation; Rotations; Transpressional

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.

摘要 横压褶皱可以是扳手褶皱或滑隔褶皱。两者都可以产生足够相似的结构，可以与圣安德烈亚斯断层相邻或近平行地折叠。两个模型之间的差异要大得多：这是强断层和弱断层之间的差异。哪种模型更好地反映了地质情况，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，这些褶皱造成了仅加利福尼亚州几个超级巨型油田的几何形状。此外，如果这些是滑动分割的折叠，那么它们产生的折叠轴不垂直于最大压缩水平应力，这表明当前的折叠理论是不完整的。在扳手折叠环境中，随着有限缩短应变垂直于走滑断层旋转，折叠（以及任何底层断层）随时间旋转。在距走滑断层相同距离处，古地磁旋转应该是相等的。从焦点机制观察到的微极旋转将会很小，因为小尺度旋转和大规模旋转是相同的。在分区系统上，底层故障不会轮换。然而，在地表褶皱中尚未认识到下伏断层走滑的一个组成部分。最有可能的是，这种剪切应变在与圣安地列斯平行的褶皱背斜轴附近产生垂直轴旋转，正如在物理模型和华盛顿亚基马褶皱带中观察到的那样。旋转的不均匀性和宏观结构旋转的缺失都会产生横向变化的古地磁旋转和比扳手折叠情况更大的微极旋转。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个问题。这些褶皱起源于新近纪活跃的盲冲断层，一直是推断圣安德烈亚斯断层应变分配和低剪切应力的人与推断扳手断层和高应力的人之间争论的焦点。对 Coalinga、Kettleman Hills 和 Wheeler Ridge 背斜的古地磁和地震学调查应确定哪种区域变形运动学模型最适合这些结构。这两种技术相辅相成，工作将并行进行，以便完善的分析策略可以解决另一种方法产生的问题。此外，虽然古地磁分析提供了变形的时间积分视图，但使用这些褶皱中的一些密集余震数据集将使我们能够将微极分析应用于靠近和远离地轴平面的不同深度切片和不同体积。上覆的背斜。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种资源的能力。其次，这些结构具有震源性，并且已经产生了破坏性地震。这项工作直接解决了变形机制，这反过来可能会影响返回时间和地震危险性的研究。这项工作还探讨了圣安德烈亚斯断层的孕震特征，该断层是美国最大的地震灾害之一。 PI还将建设科学和教育基础设施。该项目主要支持女研究生 Joya Tetreault，她是地球科学领域代表性不足的群体。