Signature of hydrothermal alteration in ground-magnetic surveys at Yellowstone National Park
Abstract
Yellowstone National Park (YNP) hosts a very large hydrothermal system with over 10,000 thermal features. Although hydrothermal alteration in YNP has been extensively studied with field observations, remote-sensing imagery, and core drilling, the volume and geometry of hydrothermal systems at depth remain poorly constrained. Magnetic surveys can help to investigate buried hydrothermal alteration as demonstrated by the high-resolution aeromagnetic survey of YNP (Finn and Morgan, J. Volcanol. Geotherm. Res., 115, 207-231, 2002). Results of this survey show that magnetic lows extend over and beyond areas of hydrothermal activity. This suggests large volumes of buried demagnetized rocks due to hydrothermal alteration of the volcanic substratum. Although the interpretation of magnetic anomalies is non-unique, Finn and Morgan (2002) used these magnetic lows to estimate a minimum volume of buried altered rock assuming complete demagnetization of the substratum. This aeromagnetic survey was of relatively high resolution (flight line spacing < 500 m and flight elevation <350 m above ground), but it was insufficient for detailed mapping of individual thermal areas. In order to obtain a closer look at several areas, we performed ground-based magnetic surveys within YNP using a cesium-vapor magnetometer along 4-5 km long transects crossing four thermal areas (Norris Geyser Basin, Lower Geyser Basin, Lone Star Geyser Basin, and Smoke Jumper Hot-springs). We also performed a detailed survey over an area of about 800 m x 500 m around Lone Star Geyser. We also collected gravity data to help characterize the subsurface geologic structures and performed magnetic susceptibility, magnetic remanence, and density measurements on rock samples collected in the field and from drill cores collected in 1967-1968 to characterize physical properties of fresh and altered geologic units. The long magnetic transects show that magnetic anomalies are damped in altered areas suggesting a significant decrease in the magnetization of shallow volcanic units. The detailed magnetic map of the Lone Star Geyser displays a pronounced negative anomaly centered on the geyser mound and other hot-springs indicating demagnetization directly associated with the hydrothermal plumbing system. These surveys will be used to model the geometry of the demagnetized volcanic substratum associated with hydrothermal alteration.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP41A0980B
- Keywords:
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- 0925 EXPLORATION GEOPHYSICS / Magnetic and electrical methods;
- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 8135 TECTONOPHYSICS / Hydrothermal systems