Structure and mechanics of calderas: reconciling mechanical models of ring-fault nucleation with transcrustal magma plumbing system models

Abstract

The formation of collapse calderas pose a significant geohazard due to their association with large-volume and potentially devasting eruptions. On the flip side, calderas and their underlying structures are often linked to high-potential geothermal systems and valuable mineral deposits. The formation of a new caldera necessitates the initiation of circumferential ring-faults that connect an underlying magma chamber to the Earth’s surface. Subsequently, the roof of the magma chamber collapses inward as a result of either excessive or insufficient pressure within the chamber. However, recent observations suggest that many calderas may form above complex transcrustal magma systems, comprising several individual or interconnected magma chambers situated at various depths within the Earth’s crust. While several attempts have been made to define the mechanical conditions leading to ring-fault nucleation in calderas, these models often assume the presence of a single shallow magma chamber or a combination of a shallow magma chamber and doming of a deep-seated reservoir. Consequently, there have been no endeavors thus far to reconcile the mechanical conditions necessary for ring-fault nucleation and potential caldera formation with the inferred geometries and arrangements of complex transcrustal magma systems. In this work, we address this issue by utilizing Finite Element Methods (FEM) to reconcile the known mechanical conditions for ring-fault nucleation in calderas with the model of transcrustal magma systems by simulating multiple arrangements of magma pockets. Our results indicate that relatively small lateral or vertical differences in the positioning of magma pockets impede the required stress conditions for ring-fault nucleation and propagation in calderas. Out of the various tested arrangements of shallow magma chambers, only a few were found to create the necessary conditions for successful formation of caldera ring-faults, supporting the rarity of caldera formation in nature. Although these findings do not rule out the role of transcrustal magma system dynamics in caldera formation, they suggest that, in most cases, individual magma batches must combine to form a single large source prior to ring-fault nucleation.In this talk, I will also present an overview of our understanding of typical caldera structures and mechanical models used to explain caldera geometries and formation. I will pay attention to the role of fault properties and shallow crustal magma chamber arrangements and discuss knowledge gaps.