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

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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.