Geophysical mapping of Yellowstone’s Plumbing System—from Sources to Surface

Abstract:

The pathways along which hydrothermal fluids mix with local meteoric water to produce the geochemical signatures and locations of Yellowstone’s iconic hydrothermal features are almost completely unknown. Helicopter magnetic and time domain electromagnetic (HEM) data over much of Yellowstone reveal the first high resolution synoptic subsurface view of its regional plumbing system. Magnetic contacts, constrained in some places by seismic swarms, delineate potential faults and fractures that may form pathways for fluids from depth. A magnetic model contains deep (~5-6 km depth) low magnetization regions primarily located along the NE caldera boundary, where heat flow is highest and magma is shallowest, that may reflect regions above the Curie temperature. We relate local (1-5 km) wide, low magnetization regions in the upper 1-2 km to hydrothermal clays similar to low resistivity regions in the hydrothermal systems of New Zealand. These clays are the product of water-rock interactions related to the flow of hot fluids. In the upper ~500 m, resistivity models indicate 50-100 m thick near horizontal, low to moderate resistivity (<300 ohm-m) regions that reflect groundwater, both cold meteoric and warm hydrothermal, in permeable intra- and inter- volcanic flow boundaries. Steeply dipping low resistivity zones, often corresponding to magnetic contacts, image near-vertical pathways for fluids. The combination of magnetic, seismicity, HEM and geochemical data, and analogs from hydrothermal systems in New Zealand imaged from deep (3-4 km) wells and regional electromagnetic data, allow us to construct a model for the hydrothermal system of Yellowstone. Hydrothermal and/or magmatic fluids travel from depth along regional faults related to Basin and Range extension, mainly within the caldera and Norris-Mammoth corridor. At ~1-2 km depth, hydrothermal alteration along faults and fracture zones extends into the surrounding rocks, creating zones as much as 5 km wide. In some places, the resistivity models image vertical groundwater flow paths that extend to the near surface (within the ~20 m resolution of the HEM data) such as in Norris, the Geyser Basins, Smoke Jumper Hot Springs, Hot Springs Basin, and Yellowstone Lake. Groundwater beneath impermeable lava flows sometimes intersects hydrothermal regions both near the surface and at depth.