Deformation around the Creede Caldera: A consequence of isostatic adjustment following Caldera Formation

The pattern of deformation around the Creede caldera (26.5 Ma), southwest Colorado, may provide clues to the physical mechanisms of caldera evolution, particularly resurgent doming. This paper considers the hypothesis that the deformation is due to isostatic relaxation of surface load (topography) following caldera formation, involving uplift of the caldera floor and subsidence of a surrounding tumescent dome. This hypothesis is tested by constructing finite difference models of relaxing topography which have as parameters the mechanical properties of rocks surrounding the caldera. The goal is to find models that predict spatial and temporal patterns of deformation matching those observed around the caldera, as inferred from: (1) the present structural form of the region as indicated by the surface distribution of stratigraphic units and (2) past structural forms as indicated by the locations, orientations, and ages of faults, which are presumed to be due to a combination of regional tension and stresses caused by bending at the surface. Successful models of the deformation have been found using a variable viscosity half‐space overlain by a thin elastic plate. In general, the nature of deformation at the surface is strongly dependent on the radial variation of viscosity at shallow depths beneath the elastic plate and is much less sensitive to variation of viscosity with depth. Models with relatively low viscosities beneath the caldera and relatively high viscosities in the surrounding region accurately predict uplift of the caldera floor which is rapid compared to the rate of marginal subsidence. The results indicate that the rate of deformation is governed by the flow of material with viscosities characteristic of wallrock (roughly 1019–1024 Pa s) rather than magma. A gap in faulting around the caldera at about one caldera radius outside the wall suggests the existence of a relatively low radial viscosity gradient in that position; higher viscosity gradients inside and outside of this low may reflect the margins of two nested magma bodies at depth (a central stock within a larger batholith). In order to match the observed pattern of faults around the caldera the orientation of the regional tension that is superimposed on the bending stresses must rotate clockwise with time. This change in stress direction is consistent in sense but earlier than that proposed by others for the western United States.