A 12,000‐year record of vertical deformation across the Yellowstone caldera margin: The shorelines of Yellowstone Lake

Abstract

The 600 ka Yellowstone caldera exhibits several signs of unrest, the most evident of which is historic ground deformation including both uplift and subsidence. We document deformation in the area of the southeastern caldera across ∼12,000 years using the postglacial shoreline terraces of Yellowstone Lake. Raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces, 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 dates. Each terrace is interpreted as representing an episode of uplift (∼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 crystallization, and/or episodic trapping and release of magmatic fluids (evolved during cooling and crystallization) in a self‐sealing reservoir, as hypothesized by Dzurisin et al. (1990). Early postglacial shoreline deformation reveals substantial intracaldera subsidence, possibly reflecting cooling and loss of trapped hydrothermal fluids. Extension along north‐south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin, including downwarping and local faulting of hot, weak intracaldera crust. Net deformation over the past ∼3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim, relative to the extracaldera region. This uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode. Subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence. The cause of this trend is undetermined but is most likely related to the effects of regional extension and long‐term cooling within the Yellowstone caldera.