Neotectonic History of the Eastern California Shear Zone based on U-Pb/U-Th Dating of Syntectonic Precipitates

地震在特定地点和时间发生的可能性取决于许多难以限制的参数。例如，应变率的短期估计超过较长期的断层滑动率是我们认为增加近期累积应变释放风险的一个参数。在许多活动板块的边界上，现代和长期地质条件对地壳变形率的限制大体上是一致的。然而，在南加州的东加利福尼亚剪切带（ECSZ），地质估计仅为大地测量数据的一半。尽管如此，断层滑动率的长期地质记录是基于过去5万年的古地震数据的收集，与ECSZ大约20 Ma的断层活动相比，这些数据只能提供对长期断层过程的非常部分的理解。本研究将结合微观结构分析、同位素和地球化学约束，以及先进的原位绝对（SHRIMP-RG）测年技术，研究东南亚同构造蛋白石沉淀的时间和形成机制，旨在开发一种直接测年断层活动的新方法，并最终建立断层活动的长期记录。具体而言，该项目将寻求确定(1)断层相关蛋白石的流体来源和形成机制；(2)蛋白石沉淀的时间及其与脆性变形事件（如同构造/后构造）的时间关联。该项目的成果包括开发了一种新的方法：(1)利用原位SHRIMP-RG Th/U和U- pb测年技术对断层相关材料进行古地震直接测年；(2)确定断层活动从起始（数百万年）到现在的年代。对ECSZ的莫哈韦沙漠段的这一案例研究的重要预期结果包括对断层活动时间和历史的长期限制，这将有助于解决估计的长期和短期应变积累率之间的差异。鉴于最近发生的破坏性地震，公众越来越担心今天的高震级地震比过去更频繁，同时要求进行更准确的预测。因此，该项目的一个主要目标是向科学界和公众提供最新的古地震学数据。该项目将开发一种新的断层相关材料的高空间分辨率测年技术，然后将通过两个共享的分析设施提供给科学界。该项目的数据将被输入美国地质调查局的古地震数据库。

The likelihood of an earthquake to occur at a specific place and time depends on a number of parameters that are difficult to constrain. For example, short-term estimates of strain rates that exceed longer-term fault slip rates are one parameter that we consider to increase the risk for near-future release of accumulating strain. At many active plate boundaries, there is a general agreement between modern and the long-term geological constraints on crustal deformation rates. However, in the Eastern California Shear Zone (ECSZ), southern California, the geological estimations are half of those suggested by the geodetic data. Notwithstanding, the long-term geological record of fault-slip rates is based on a collection of palaeoseismological data from the past 50,000 years, which compared to the approximately 20 Ma of fault activity in the ECSZ can only offer a very partial understanding of long-term faulting processes. This study will combine microstructural analysis, isotopic and geochemical constraints, and advanced in situ absolute (SHRIMP-RG) dating techniques to study the timing and formation mechanisms of syn-tectonic opal precipitates from the ECSZ, with the aim of developing a new approach for directly dating fault activity, and to ultimately establish a long-term record of fault activity. Specifically, this project will seek to determine (1) the fluid source and formation mechanisms of fault-related opals; (2) the timing of opal precipitation and the temporal association with brittle deformation events (e.g., syn-tectonic/post-tectonic). Outcomes of this project include the development of a new approach for (1) direct dating of paleo-earthquakes using in situ SHRIMP-RG Th/U and U-Pb dating techniques applied to fault-related material; (2) dating fault activity from initiation (millions of years) to present time. Important anticipated outcomes from this case study on the Mojave Desert segment of the ECSZ include long-term constraints on the timing of initiation and history of fault activity that will help to resolve discrepancies between estimated long- and short-term strain accumulation rates.Given the recent and devastating earthquakes, there is a growing public concern that high-magnitude earthquakes are more frequent today than in the past, along with a demand for more accurate prediction. Thus, a major goal of this project is to provide up-to-date palaeoseismological data to both the scientific community and the general public. This project will develop a new high-spatial resolution dating techniques for fault related materials that will then be available to the scientific community through two shared analytical facilities. Data from this project will be contributed to the U.S. Geological Survey paleoseismic database.