Collaborative Research: Late Cenozoic Vertical Crustal Motions and Erosional Mass Transfer in the Southern San Andreas Fault Zone

大陆走滑断裂带的地壳垂直运动被广泛认识，但对这些背景下的地下控制和地表对三维应变的响应却知之甚少。观测到的隆升和下沉模式往往与斜应变数值模型的预测不符，这表明现有的走滑断层模型是不完整的。走滑断裂带中沉积盆地发育的构造控制也同样复杂和不完全了解。该项目正在通过对南加州圣安德烈亚斯断裂带的三维应变和相关表面过程的多学科、多研究者研究来解决这些问题。研究小组将使用一套不同的方法来记录地壳垂直运动随时间的速率和几何形状，并检验圣安德烈亚斯断裂演化的两个假设：(1)板块运动倾角对断裂带的三维和时间演化起主要控制作用；(2)断裂带在大约110万至140万年前经历了一次重大变化，以响应板块边界的构造重组。每一种假说都对断裂带内的抬升、侵蚀、下沉和沉积物扩散的时空模式做出了独特的预测，这将使研究小组能够通过实地考察、数据分析和建模的系统程序来测试这些假说。该项目综合了多种研究方法，包括地质填图、地层和构造分析、沉积年龄和块体旋转的古地磁研究、物源分析、碎屑锆石测年、基岩来源的低温(U-Th)/He测年、地貌分析、地震和重力数据研究以及数值模拟。这项研究试图填补南圣安德烈亚斯断裂系统地质演化的巨大空白。圣安德烈亚斯断裂系统是一个复杂的地震活动断裂网络，定义了加利福尼亚州的太平洋-北美板块边界。在地质时间尺度(数百万年)上的形变历史相对鲜为人知，尽管它在塑造地壳结构和控制这种背景下地震的断层几何方面发挥了关键作用。这个项目的方法得益于来自四所大学的学术研究人员和学生与美国地质调查局的地球科学家的独特合作。该团队还与研究索尔顿海下大陆破裂过程的地球物理学家以及研究圣安德烈亚斯断层在较短时间尺度上的古地震学和断层滑移率的科学家合作。这些合作为南加州充满活力的地球科学界提供了接触和贡献新知识的重要途径。在这项研究中学到的经验将被用于开发新的实验和教学练习，这些练习将在项目过程中惠及数千名学生。最终，这项研究的结果将对大陆走滑断裂带内活动板块边界上地壳变形、断裂带复杂性、地形发育、侵蚀和沉积物扩散之间的动态联系提供新的见解。

Vertical crustal motions are widely recognized in continental strike-slip fault zones, yet the underlying controls and surficial response to 3-dimensional strain in these settings are poorly understood. Observed patterns of uplift and subsidence often do no match the predictions of numerical models for oblique strain, suggesting that existing models for strike-slip faults are incomplete. Structural controls on development of sedimentary basins in strike-slip fault zones are similarly complex and incompletely understood. This project is addressing these problems with a multi-disciplinary, multi-investigator study of 3-dimensional strain and related surface processes in the San Andreas fault zone of southern California. The research team will use a diverse suite of methods to document rates and geometries of vertical crustal motions through time, and test two hypotheses for the evolution of the San Andreas fault: (1) that plate-motion obliquity exerts the primary control on the 3-dimensional and temporal evolution of the fault zone; and (2) that the fault zone experienced a major change at approximately 1.1 to 1.4 million years ago in response to tectonic reorganization of the plate boundary. Each hypothesis makes unique predictions about space-time patterns of uplift, erosion, subsidence, and sediment dispersal within the fault zone, that will allow the team to test the hypotheses with a systematic program of fieldwork, data analysis, and modeling. This project integrates diverse research methods including geologic mapping, stratigraphic and structural analysis, paleomagnetic studies of sediment age and block rotations, provenance analysis, detrital zircon dating, low-temperature (U-Th)/He dating of bedrock sources, geomorphic analysis, study of seismic and gravity data, and numerical modeling.This study seeks to fill large gaps in the understanding of the geologic evolution of the southern San Andreas fault system, a complex network of seismically active faults that define the Pacific-North America plate boundary in California. The history of deformation over geologic timescales (millions of years) is relatively poorly known, despite its critical role in shaping the crustal architecture and fault geometries that control earthquakes in this setting. This project's approach benefits from a unique collaboration of academic researchers and students from four universities with earth scientists at the U.S. Geological Survey. The team is also collaborating with geophysicists investigating processes of continental rupture beneath the Salton Sea, and scientists studying paleoseismology and fault slip rates on the San Andreas fault over shorter timescales. These collaborations provide an important avenue for engaging with and contributing new knowledge to the vibrant geoscience community in southern California. Lessons learned in this research will be used to develop new lab and teaching exercises that will reach thousands of students over the course of the project. Ultimately, the results of this study will shed new insights into dynamic linkages between crustal deformation, fault-zone complexity, growth of topography, erosion, and sediment dispersal within continental strike-slip fault zones at active plate boundaries.