Radiogenic isotopes record a ‘drop in a bucket’ – A fingerprint of multi-kilometer-scale fluid pathways inferred to drive fault-valve behavior

Abstract

Minerals precipitated from fluids carry a chemical signature of fluid origin and transport history. We use this concept to test the hypothesis that a subset of veins precipitated in the Loma Blanca fault zone, Rio Grande rift, NM, USA, record fault-valve behavior. Previous studies of this site show syntectonic calcite veins formed in response to post-seismic fluid discharge following repeated ground-rupturing earthquakes. U-Th ages on veins constrain the timing of fault slip over a period of >400,000 years. The radiogenic isotope (Sr, Pb, and U) data we report here demonstrate that a cluster of earthquakes that occurred at ∼430 ka, when fault-slip frequency increased by nearly one order of magnitude, corresponds with a brief period of particularly large and rapid variations in post-seismic fluid chemistry. Isotopic variations are consistent with fluid input from the Socorro magma body, a large, active, mid-crustal basalt intrusion 19 km below the surface. The short duration of the earthquake cluster (<50,000 years) precludes a role of thermal weakening of the crust in increased extension. We suggest that volatile exsolution and heating of ambient fluids during magma crystallization increased fluid volume and thereby pore-fluid pressure at depth, weakening the fault and driving more frequent failure through fault-valve action. This conclusion could not have been reached using traditional approaches in structural geology, stable isotope geochemistry, or geochronology alone, illustrating the utility of integrated data sets including radiogenic isotope analyses in studies of fluid-fault interactions.