Influence of tectonic stress and pore-fluid pressure on caldera collapse and resurgence – a 3D analytical solution

Caldera collapse or resurgence is commonly accommodated by slip along a near-cylindrical ring fault system, and is hence often idealized as a rigid piston moving in response to pressure changes in a fluid chamber. Existing piston models explore variations in geometry and mechanical properties of the reservoir and ring fault, but they generally neglect effects of regional tectonic stresses and pore-fluid pressures. Here we present a new analytical piston model that incorporates the regional stress state as a single parameter, the “average earth pressure coefficient,” which is defined as the mean horizontal to vertical effective stress.  The presence of pore-fluids is incorporated by using Terzaghi’s effective stress principle, which governs the effective normal stress acting on the ring fault. Data from 14 active caldera volcanoes that have well-constrained piston dimensions and that span a range of eruptive compositions and collapse magnitudes are used to explore realistic model parameter ranges.

The model results are captured by a dimensionless stability parameter (μK/r̄), combining effective ring fault friction (μ), average earth pressure coefficient (K), and piston radius normalized by its thickness (r̄). This parameter governs piston stability and describes a hysteresis (i.e., a history-dependent lag) between changes in magma reservoir pressure and ring-fault slip. A key finding is that extensional tectonic stresses, low ring-fault friction, and/or elevated pore-fluid pressures are necessary conditions for initiating caldera collapse and resurgence, particularly at calderas with high thickness to diameter (T/D) ratios. Consistent with model predictions, most of the well-constrained calderas examined here occur in extensional or transtensional tectonic settings; collapse or resurgence under a compressional tectonic regime is comparatively rare.