Subsidence-induced early doming at a large ignimbrite caldera

Uplift at many well-documented resurgent calderas started only after completion of the associated ignimbrite eruption, but arching of the large Bachelor caldera in the Southern Rocky Mountain volcanic field, Colorado (USA), began during the eruption. A well-defined arched or domical structure, initiated within thickly accumulating ignimbrite as the caldera subsided, is documented by growth of keystone faults as the caldera filled, rheomorphism and local diapiric mobilization of early-erupted tuff, decreased dips in upper welding zones, and wedging of a late-erupted dacitic phase against flanks of the growing dome. Early subsidence-induced doming may have been triggered by preferential magma draw-down along ring-fault vents, relatively impermeable vesiculation and differential magma buoyancy centrally within the caldera, peripheral loading at caldera margins by landslides and talus from caldera walls, or some combination of factors. Early inception of caldera doming has implications for models of magma withdrawal and residual compositional gradients in non-erupted magma.