Post-Glacial Inflation-Deflation Cycles, Tilting, and Faulting in the Yellowstone Caldera Based on Yellowstone Lake Shorelines

Pierce, Kenneth; Cannon, Kenneth; Meyer, Grant; Trebesch, Matthew; Watts, Raymond

The Yellowstone caldera, like many other later Quaternary calderas of the world, exhibits dramatic unrest. Between 1923 and 1985, the center of the Yellowstone caldera rose nearly one meter along an axis between its two resurgent domes (Pelton and Smith, 1979, Dzurisin and Yamashita, 1987). From 1985 until 1995-6, it subsided at about two cm/yr (Dzurisin and others, 1990). More recent radar interferometry studies show renewed inflation of the northeastern resurgent dome between 1995 and 1996; this inflation migrated to the southwestern resurgent dome from 1996 to 1997 (Wicks and others, 1998). We extend this record back in time using dated geomorphic evidence of postglacial Yellowstone Lake shorelines around the northern shore, and Yellowstone River levels in the outlet area. We date these shorelines using carbon isotopic and archeological methods. Following Meyer and Locke (1986) and Locke and Meyer (1994), we identify the modern shoreline as S1 (1.9 ± 0.3 m above the lake gage datum), map paleoshoreline terraces S2 to S6, and infer that the prominent shorelines were cut during intracaldera uplift episodes that produced rising water levels. Doming along the caldera axis reduces the gradient of the Yellowstone River from Le Hardys Rapids to the Yellowstone Lake outlet and ultimately causes an increase in lake level. The 1923-1985 doming is part of a longer uplift episode that has reduced the Yellowstone River gradient to a “pool” with a drop of only 0.25 m over most of this 5 km reach. We also present new evidence that doming has caused submergence of some Holocene lake and river levels. Major cycles of inflation and deflation are thousands of years long. Le Hardys Rapids has twice been uplifted ~8 m relative to the lake outlet. These two locations span only the central 25% of the historic caldera doming, so that if we use historic doming as a model, total projected uplift would be ~32 m. This “heavy breathing” of the central part of the Yellowstone caldera may reflect a combination of several possible processes: magmatic inflation, tectonic stretching and deflation, and hydrothermal fluid sealing and inflation followed by cracking of the seal, pressure release, and deflation. Over the entire postglacial period, subsidence has balanced or slightly exceeded uplift as shown by older shorelines that descend towards the caldera axis. We favor a hydrothermal mechanism for inflation and deflation because it provides for both inflation and deflation with little overall change. Other mechanisms such as inflation by magma intrusion and deflation by extensional stretching require two separate processes to alternate and yet result in no net elevation change. In addition to inflation and deflation, new LIDAR data demonstrates previously unrecognized local deformation along the north shore of Yellowstone Lake. The newly recognized Fishing Bridge fault shows a progressive increase in offset from 0.5 m for the ~8 ka S2 to perhaps 5 m for the ~12.6 ka S5. Uplift of the Storm Point hydrothermal center tilts shorelines westward as much as 6 m over one kilometer. A local anticline has as much as 3 m relief in 0.5 km. LIDAR data also shows the Mary Bay II hydrothermal explosion debris has a surface relief of about 1 m over 100 m, and that it overlies S5.5 and S6 shorelines, but not S. Although the postglacial deformation record does not indicate voluminous magma accumulation or other large-scale eruption precursors, strong local deformation associated with hydrothermal centers does suggest the possibility of future hydrothermal explosions and associated hazards.

Post-Glacial Inflation-Deflation Cycles, Tilting, and Faulting in the Yellowstone Caldera Based on Yellowstone Lake Shorelines

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