Analogue models of caldera collapse in strike-slip tectonic regimes

Abstract

International audienceRegional-scale faulting, particularly in strike-slip tectonic regimes, is a relatively poorly constrained factor in the formation of caldera volcanoes. To examine interactions between structures associated with regional-tectonic strikeslip deformation and volcano-tectonic caldera subsidence, we made scaled analogue models. Tabular (sill-like) inclusions of creamed honey in a sand/gypsum mix replicated shallow-level granitic magma chambers in the brittle upper crust. Lateral motion of a base plate sited below half the sand/gypsum pack allowed simulation of regional strike-slip deformation. Our experiments modelled: (1) strike-slip deformation of a homogeneous brittle medium; (2) strike-slip deformation of a brittle medium containing a passive magma reservoir; (3) caldera collapse into sill-like magma reservoirs without regional strike-slip deformation; and (4) caldera collapse into sill-like magma reservoirs after regional strike-slip deformation. Our results show that whilst the magma chamber shape principally influences the development and geometry of volcanotectonic collapse structures, regional-tectonic strike-slip faults (Riedel shears and Y-shears) may affect a caldera's structural evolution in two main ways. Firstly, regional strike-slip faults above the magma chamber may form a pre-collapse structural grain that is exploited and reactivated during subsidence. Our experiments show that such faults may preferentially reactivate where tangential to the collapse area and coincident with the chamber margins. In this case, volcano-tectonic extension in the caldera periphery tends to localise on regional-tectonic faults that lie just outside the chamber margins. In addition, volcano-tectonic reverse faults may link with and reactivate pre-collapse regional-tectonic faults that lie just inside the chamber margins. Secondly, where regional-tectonic strike-slip faults define corners in the magma chamber margin, they may halt the propagation of volcano-tectonic reverse faults. The experiments also highlight the potential difficulties in assessing the relative contributions of volcano-tectonic and regional-tectonic subsidence processes to the final caldera structure seen in the field. Disruption of the precollapse surface by regional-tectonic faulting was preserved during coherent volcano-tectonic subsidence to produce a caldera floor of differentially-subsided fault blocks. Without definitive evidence for syn-eruptive growth faulting, thickness changes in caldera fill across such regionaltectonic fault blocks in nature could be mistaken as evidence for piecemeal volcano-tectonic collapse

Structural evolution of the El Salvador Fault Zone: an evolving fault system within a volcanic arc.

“…Our model is based on the results of detailed analysis of fractures and faults attitude within the ESFZ, and comparisons between the faults patterns existing in El Salvador and those obtained by analogue and kinematic models. Scaled analogue models have been successfully applied to the analysis of the deformation patterns that are developed in strike-slip regimes (Schreurs, 1994;McClay and Dooley, 1995;Schreurs and Colletta, 1998;Schreurs, 2003;Holohan et al, 2008). The comparison of the results of this work with those from experimental works can shed some light on the level of structural development of the ESFZ.…”

Laboratory Modelling of Volcano Plumbing Systems: A Review

“…23a) (Román-Berdiel 1999;Bonini et al 2001;Corti et al 2003), transtension , strike-slip faulting (Fig. 23b) (Román-Berdiel et al 1997;Román-Berdiel 1999;Corti et al 2005;Holohan et al 2008b), transpression (Benn et al 1998(Benn et al , 2000, to pure compression (Fig. 23c, d) (Musumeci et al 2005;Mazzarini et al 2010;Montanari et al 2010a, b).…”

“…On the other hand, when the reservoir is centred below and is significantly larger than a conical edifice, the topographic load causes steepening of the outward-dipping faults and more focussed subsidence ). Where regional faults formed in pre-collapse extensional or strike-slip tectonic regimes are near to or coincide with the reservoir margins in experiments, they are at least partly reactivated as subsidence-guiding faults, thereby deflecting or even halting the propagation of caldera-related ring faults (Acocella et al 2004;Holohan et al 2005Holohan et al , 2008b. These results provide insight into how regional-tectonic faults may act as conduits and venting sites during caldera-forming eruptions, as inferred at, e.g., Glencoe caldera, Scotland (Moore and Kokelaar 1998).…”

Volcanism in Reverse and Strike-Slip Fault Settings

“…According to Holohan et al (2007), regional-tectonic settings with active strike-slip deformation and associated calderas include:…”

“…Other major volcano-tectonic depressions such as Toba, Ranau (Sumatra), and Atitlan (Guatemala) might have a similar origin. Holohan et al (2007) made scaled analogue models to study the interactions between structures associated with regional-tectonic strike-slip deformation and volcano-tectonic caldera subsidence. Their results show that while the magma chamber shape mostly influences the development and geometry of volcanotectonic collapse structures, regional-tectonic strikeslip faults may have a strong influence on the structural evolution of calderas.…”