Analogue models of caldera collapse in strike-slip tectonic regimes |
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| Authors: | Eoghan P Holohan Benjamin van Wyk de Vries Valentin R Troll |
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| Institution: | (1) Department of Geology, School of Natural Sciences, Trinity College, Dublin 2, Ireland;(2) Laboratoire Magmas et Volcans, 5 rue Kessler, 63038 Clermont-Ferrand, France |
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| Abstract: | Regional-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 strike-slip 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 volcano-tectonic 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 pre-collapse 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 regional-tectonic
fault blocks in nature could be mistaken as evidence for piecemeal volcano-tectonic collapse. |
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| Keywords: | Caldera volcanoes Subsidence Volcano-tectonics Strike-slip tectonics Analogue models |
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