Using 230Th/U Dating of Pedogenic Carbonate to Provide a Time-Axis for Slip on the Elsinore Fault, Southern California

利用成土碳酸盐的 230Th/U 测年为南加州埃尔西诺断层上的滑动提供时间轴

• 批准号: 0636053
• 负责人: Warren Sharp
• 金额: $ 12.17万
• 依托单位: Berkeley Geochronology Center
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-02-01 至 2009-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0636053&HistoricalAwards=false
• 关键词: Using; 230Th; Dating; Pedogenic; Carbonate

Continental plate boundaries commonly consist of broad regions of deformation characterized by fault systems consisting of multiple, approximately parallel faults, such as the San Andreas fault system of California. To better quantify the seismic hazard posed by such faults, it is essential to determine their pattern of motion through time. While recent advances in geodetic observations in the vicinity of plate boundary fault systems provide unprecedented insight into patterns of crustal deformation on the relatively short (i.e., decadal) timescale, it is likely that over longer timescales seismogenic faults exhibit variable displacement rates. That is, slip rates derived from geodetic data may reflect transient motions associated with large earthquakes in the recent past, or other recent changes in crustal loading. On a longer timescale, slip rates on major faults such as the San Andreas of southern California are thought to have varied by about a factor of two within the last approximately100,000 years. Such variability, if confirmed, could perhaps result from redistribution of slip due to evolving crustal structural complexities, clustering of earthquakes on intermediate timescales (i.e., approximately 10,000 years), or other mechanisms. Quantifying fault slip rates on timescales of 1 to 100,000 years is therefore fundamental to an improved understanding of continental dynamics and will serve societal needs for improved quantification of seismic hazards near major fault systems. Reliable measurements of fault slip on the timescale of 1 to 100,000 years, however, are quite limited in number. This is in part because of the relative rarity of geological situations that preserve the appropriate stratigraphic or geomorphologic information needed to precisely determine offsets on active faults. A second and equally demanding requirement for determining long-term slip rates, however, is that offset strata or landforms must be reliably dated, and existing methods for dating geologically young surfaces and strata only partially meet this demand. A novel technique for determining the age of landforms offset by faulting will be developed and applied in this research project. Specifically, the project will extend and further develop small-sample, high precision 230Th/U dating of soil carbonate via mass spectrometry. The results will provide unprecedented insight into the constancy (or lack thereof) of slip rates over time on the Elsinore fault, a principal strand of the San Andreas fault system in southern California, over a range of intervals in the past approximately 100,000 to 200,000 years.The research project will further develop a novel means of dating geologically youthful landforms, extend the capacity and capability of earth scientists to determine fault slip rates in arid regions worldwide, and help to address societal needs for more quantitative assessment of seismic hazards. The project will also provide comprehensive training in innovative techniques of geochronology for doctoral candidate.

大陆板块边界通常由广泛的变形区域组成，其特征是断层系统由多个近似平行的断层组成，例如加利福尼亚州的圣安德烈亚斯断层系统。为了更好地量化此类断层造成的地震危害，有必要确定它们随时间的运动模式。虽然板块边界断层系统附近大地测量观测的最新进展为相对较短（即十年）时间尺度上的地壳变形模式提供了前所未有的见解，但在较长时间尺度上，震源断层可能表现出可变的位移速率。也就是说，从大地测量数据得出的滑动率可以反映与最近的大地震相关的瞬态运动，或地壳载荷的其他近期变化。在较长的时间尺度上，诸如南加州圣安德烈亚斯断层等主要断层的滑移率被认为在过去大约 10 万年内变化了约两倍。如果得到证实，这种变化可能是由于不断变化的地壳结构复杂性、中等时间尺度（即大约 10,000 年）的地震聚集或其他机制导致的滑移重新分布造成的。因此，量化 1 至 100,000 年时间尺度上的断层滑移率对于加深对大陆动力学的了解至关重要，并将满足改善主要断层系统附近地震灾害量化的社会需求。然而，在 1 至 100,000 年的时间尺度上对断层滑移的可靠测量数量非常有限。部分原因是，保存精确确定活动断层偏移所需的适当地层或地貌信息的地质情况相对罕见。然而，确定长期滑移率的第二个同样严格的要求是，必须对偏移地层或地貌进行可靠的测年，而现有的地质年轻表面和地层测年方法只能部分满足这一要求。本研究项目将开发并应用一种确定断层偏移地貌年龄的新技术。具体来说，该项目将通过质谱法扩展并进一步开发小样本、高精度的土壤碳酸盐 230Th/U 定年法。这些结果将提供前所未有的见解，让我们了解埃尔西诺断层（南加州圣安德烈亚斯断层系统的一条主线）在过去约 10 万至 20 万年的一系列时间间隔内滑动率随时间的恒定性（或缺乏恒定性）该研究项目将进一步开发一种测定年轻地质地貌年代的新方法，扩大地球科学家确定全球干旱地区断层滑移率的能力，并帮助解决社会需要对地震危害进行更定量的评估。该项目还将为博士生提供地质年代学创新技术的全面培训。