Impact of Preexisting Faults and Regional Stresses on Collapse Caldera Structures: Insights from Discrete Element Method Modeling

Abstract

Caldera collapses manifest when the roof of a magma chamber caves in during a substantial eruption, stemming from either excessive pressure or the partial drainage of magma from the chamber. A blend of theoretical investigations, field observations, and modeling studies has proposed that collapse calderas take form through the interplay of outward-dipping reverse faults and inward-dipping normal faults. Nevertheless, the influence of preexisting crustal faults and regional stresses on the configuration of a caldera or the alteration of conditions leading to fault initiation remains relatively uncertain. This holds significance as numerous calderas develop within regions characterized by intense crustal faulting, subject to the stresses of tectonic loading. This study employs two-dimensional Distinct Element Method (DEM) models to explore how preexisting structures impact the evolution of collapse calderas and the resulting geometric style. To accomplish this, we simulate a crustal segment containing a shallow magma chamber that undergoes underpressure conditions, effectively mimicking the process of magma withdrawal. The outcomes are juxtaposed between scenarios that replicate a uniform crust and those that emulate a crust containing preexisting faults and/or experiencing tectonic crustal loading. The faults are distributed at intervals ranging from 0.1 to 10 km and encompass diverse fault properties (such as dip, dip-direction, angle of friction, bond strength, etc.), thereby assessing the impact of these variables on the structural progression and eventual configuration of the caldera. Our preliminary results are benchmarked against other well characterized analogue, analytical and DEM model results, deriving similar morphometric evolution and end-member collapse styles. Concerning the innovative elements of our models, numerical findings distinctly demonstrate that preexisting faults and regional stresses do indeed alter both the geometry and manner of collapse, underscoring their significant contribution in the process of caldera formation. Through the exploration of varying fault spacing, fault properties, and tectonic stresses, we establish parameter ranges that delineate when faults play a pivotal role in shaping the overarching collapse geometry and when their influence is less pronounced. These findings hold practical value for the reconstruction of collapse caldera mechanisms based on exhumed cases, as well as for comprehending the necessary conditions for potential future collapses in prospective caldera-forming volcanoes.