A 12,000-YEAR RECORD OF VERTICAL DEFORMATION ACROSS THE YELLOWSTONE CALDERA MARGIN - THE SHORELINES OF YELLOWSTONE LAKE

The 600 ka Yellowstone caldera exhibits several signs of unrest, the m ost evident of which is historic ground deformation including both upl ift and subsidence. We document deformation in the area of the southes tern caldera across similar to 12,000 years using the postglacial shor eline terraces of Yellowstone Lake. Raised shoreline elevations were i nterpreted from 230 leveling profiles surveyed across flights of terra ces, with an accuracy of +/-0.5 m. Of about 11 recognizable terraces, the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns. Shoreline ages are based on minimum- and maximum-limiting radiocarbon and obsidian-hydration date s. Each terrace is interpreted as representing an episode of uplift (s imilar to 1 kyr(-1)) of the caldera interior and subsequent subsidence , with little net volume change. This cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystalliza tion, and/or episodic trapping and release of magmatic fluids (evolved during cooling and crystallization) in a self-sealing reservoir, as h ypothesized by Dzurisin et al. (1990). Early postglacial shoreline def ormation reveals substantial intracaldera subsidence, possibly reflect ing cooling and loss of trapped hydrothermal fluids. Extension along n orth-south trending normal faults and related structures is also appar ent in early to middle postglacial deformation both inside and outside of the caldera margin, including downwarping and local faulting of ho t, weak intracaldera crust. Net deformation over the past similar to 3 kyr has been dominantly up within the caldera interior and slightly d own along the caldera rim, relative to the extracaldera region. This u plift is roughly similar to the historic pattern and may largely repre sent the effects of the most recent inflation episode. Subtraction of the total estimated magnitude of inflation in this episode suggests th at the overall trend of postglacial deformation has been subsidence. T he cause of this trend is undetermined but is most likely related to t he effects of regional extension and long-term cooling within the Yell owstone caldera.