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，其特征是由多个近似平行的断层组成的断层系统，例如加州的圣安德烈亚斯断层系统。为了更好地量化这些断层造成的地震危险，必须确定它们随时间的运动模式。虽然最近在板块边界断层系统附近的大地测量观测方面的进展为相对较短（即，十年）的时间尺度，很可能在更长的时间尺度上，孕震断层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速率可能反映了与最近的大地震有关的瞬态运动，或地壳载荷的其他近期变化。在更长的时间尺度上，像南加州的圣安德烈亚斯断层这样的主要断层的滑动速率被认为在过去的大约10万年里发生了大约两倍的变化。这种变化，如果得到证实，可能是由于地壳结构复杂性的演变，地震在中间时间尺度上的聚集（即，大约10,000年）或其他机制。因此，在1至10万年的时间尺度上量化断层滑动速率对于提高对大陆动力学的理解至关重要，并将满足社会对改善主要断层系统附近地震危险量化的需求。然而，在1至10万年的时间尺度上对断层滑动的可靠测量在数量上相当有限。这部分是因为保存了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，确定长期滑动率的第二个同样苛刻的要求是，必须可靠地确定偏移地层或地貌的年代，现有的地质年轻表面和地层的年代测定方法只能部分满足这一要求。一种新的技术来确定年龄的地貌断层偏移将在本研究项目中开发和应用。具体而言，该项目将通过质谱法扩展和进一步开发土壤碳酸盐的小样品，高精度230 Th/U定年。结果将提供前所未有的洞察力的恒定性埃尔西诺断层是加州南部圣安德烈亚斯断层系统的主要分支，在过去大约10万至20万年的时间间隔内，其滑动速率随时间的变化（或缺乏滑动速率）。该研究项目将进一步开发一种新的方法来确定地质上年轻的地貌，扩大地球科学家确定全世界干旱地区断层滑动速率的能力，并帮助满足对地震危险进行更定量评估的社会需求。该项目还将为博士生提供地质年代学创新技术的综合培训。