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

大陆板块边界通常由广泛的形变区域组成，其特征是由多条近似平行的断层组成的断层系统，如加利福尼亚州的圣安德烈亚斯断层系统。为了更好地量化这些断层造成的地震危险，确定它们随时间的运动模式是至关重要的。虽然在板块边界断层系统附近的大地测量方面的最新进展提供了对相对较短(即十年制)时间尺度上地壳形变模式的前所未有的洞察，但在较长时间尺度上，孕震断层很可能表现出不同的位移速率。也就是说，从大地测量数据得出的滑移率可能反映了与最近发生的大地震有关的瞬时运动，或地壳负荷的其他最近变化。在更长的时间尺度上，主要断层的滑移率，如南加州的圣安德烈亚斯，被认为在过去大约10万年内变化了大约两倍。如果得到证实，这种变异性可能是由于地壳结构复杂性的演变、地震在中间时间尺度(即大约10000年)上的聚集或其他机制造成的滑移的重新分布。因此，在1至100000年的时间尺度上量化断层滑移率对于更好地了解大陆动力学是基本的，并将满足改进对主要断层系统附近地震危险的量化的社会需要。然而，在1到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。这在一定程度上是因为保存了准确确定活动断层偏移量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苛刻的要求是，必须可靠地确定偏移地层或地貌的年代，而现有的测定地质年轻表面和地层的方法只能部分满足这一要求。这项研究项目将开发一种新的确定断层错动地貌年龄的技术并将其应用于该项目。具体地说，该项目将扩大和进一步发展小样本、高精度的土壤碳酸盐质谱仪230 TH/U测年。这一结果将提供前所未有的洞察力，了解南加州圣安德烈亚斯断层系统的主要断层埃尔西诺断层在过去大约10万至20万年的一段时间内滑动速率的稳定性(或缺乏稳定性)。该研究项目将进一步开发一种新的方法来测定地质年轻的地貌，扩大地球科学家确定全球干旱地区断层滑动速率的能力，并有助于满足社会对地震灾害更定量评估的需求。该项目还将为博士生提供地质年代学创新技术方面的全面培训。

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.