Sarah Louis , Elco Luijendijk , Istvan Dunkl, Mark Person

We present a 250 ka record of episodic fluid flow along the Malpais fault which hosts the Beowawe hydrothermal system, Nevada, USA. The history of fluid flow was quantified using a novel combination of the apatite (U-Th)/He (AHe) thermochronometer and a model of the thermal effects of fluid flow. Samples show partial resetting of the AHe thermochronometer in a 40 m wide zone around the normal fault. Numerical models indicate that, using current fluid temperatures and discharge rates, fluid flow events lasting 2000 years or more lead to fully reset samples. Episodic fluid pulses lasting 1000 years result in partially reset samples, with 30 to 40 individual fluid pulses required to match the data. Episodic fluid flow is also supported by an overturned geothermal gradient in a borehole that crosses the fault, and by breaks in stable isotope trends in hydrothermal sinter deposits that coincide with two independently dated earthquakes in the last 20 ka. This suggests a system where fluid flow is triggered by repeated seismic activity and that seals itself over ~1000 years due to the formation of clays and silicates in the fault damage zone. Hydrothermal activity is younger than the 6-10 Ma age of the fault, which means that deep (~5 km) fluid flow was initiated only after a large part of the 230 m of fault offset had taken place.