Caldera subsidence in areas of variable topographic relief: results from analogue modeling

Calderas form in volcanic areas commonly associated with topographic relief. Pre-existing topography plays an important role in the style of the caldera subsidence; topography increases the load and affects the principle stress trajectories located between the roof of the magma chamber and the surface. The morphology, internal structure, and temporal evolution of calderas are therefore sensitive to the local topography. We carried out scaled analogue experiments to investigate the effect of pre-existing topographic relief on caldera subsidence by modeling the presence of stratocones and plateaus, of variable mass, diameter and positions, prior to collapse. We induced collapse in sandbox experiments by withdrawing water from a rubber bladder to simulate caldera collapse into a large, shallow reservoir. Deformation was first manifested by sagging of the sand at the surface, followed by the upward propagation of a set of subsidence-controlling faults from the bladder. In experiments with no topography, these faults usually reached the surface near the centre of the cylinder. As evacuation and incremental growth progressed, a second outer set of subsidence-controlling faults developed; this outer set dominated the subsidence. By increasing the topographic load by 6%, the subsidence efficiency increased, producing calderas up to approximately 20% deeper. The inner set of subsidence-controlling faults steepened and controlled most of the collapse, by contrast to the experiments without topography. Adding load also reduced outward growth of peripheral sagging and development of extensional faults; leading to a diameter of sagging 20% smaller than with no topography. A comparative analysis of our results with Mount Mazama at Crater Lake, Oregon, and Glass Mountain at Long Valley, California, supports the interpretation that the addition of topographic load modifies the evolution of the major caldera faults.