GEOPHYSICAL AND GEOCHEMICAL CONSTRAINTS ON SUBSURFACE FLUID MOVEMENT BENEATH RABBIT CREEK THERMAL AREA, YSNP

Numerous complex and interrelated processes contribute to the establishment of hydrothermal systems. Three processes– geologic, structural and geochemical, combine to create a hydrothermal system. While processes influencing the chemical evolution of Yellowstone’s hydrothermal systems have been investigated for decades, the subsurface architecture and geologic processes controlling the movement of hydrothermal fluids in the subsurface are poorly known.

To better understand how subsurface processes control the development and evolution of thermal systems we are investigating the subsurface architecture and fluid movement within the South Rabbit Creek thermal area using near-surface geophysical imaging coupled with chemical and isotopic measurements. The South Rabbit Creek thermal area is located along the Rabbit Creek Fault Zone (RCFZ) at the edge of the Mallard Lake resurgent dome on the eastern side of the Midway Geyser Basin. This thermal area was chosen because: 1) the mapped RCFZ provides obvious pathways for deep hydrothermal fluids to be transported to the surface; and 2) the geochemical diversity (ranging from acid-sulfate to neutral-chloride) and highly variable size and activity of its features.

Our imaging shows both the RCFZ and two NW-SE striking reverse faults from the Mallard Lake resurgent dome. Our geophysical surveys further reveal that fluids are following strong fault/fracture pathways to the surface in the southern part of the thermal area and may be laterally transported to the north. Additionally, smaller pathways contribute hydrothermal fluids to the more benign and cooler northern features in the South Rabbit Creek thermal area. From this subsurface imaging we conclude that fault/fracture networks are the dominant pathways for the hydrothermal fluids while the porosity/permeability of the subsurface substrate plays only a secondary role in the movement of hydrothermal fluids.