DATING PALEO-SEISMIC/TECTONIC EVENTS BASED ON U-SERIES DISEQUILIBRIUM IN EVAPORATES FROM THE FAULT ZONES

Evaporite minerals (e.g.. gypsum and halite) are formed in active fault zones when deep saline groundwater is episodically forced up by an seismic/tectonic event. Precise dating of such deposits bears important information on past changes in regional tectonics. In this study, we investigated the potential of ²³⁸U-²³⁴U-²³⁰Th radioactive disequilibria in evaporites as a tool for such dating. Samples were collected from the hyper-arid Atacama Desert, northern Chile and each sample was separated into several sub-samples via suspension method. Isotopes of U (²³⁸U and ²³⁴U) and Th (²³²Th and ²³⁰Th) were analyzed using a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS). Results show that significant ²³⁸U-²³⁴U-²³⁰Th disequilibria occured in the evaporites and may be associated with the authigenic phases in clay minerals. Uranium and thorium are well preserved in the evaporites due to the cementation or locking-in of clay minerals by gypsum and/or halite minerals. With precise MC-ICPMS analytical technique, this study shows that an isochron method can be established based on the observed ²³⁸U-²³⁴U-²³⁰Th disequilibria in evaporites to date the paleo-seismic/tectonic events.