Bulletin of Volcanology

, 77:4 | Cite as

Caldera faults capture and deflect inclined sheets: an alternative mechanism of ring dike formation

  • John BrowningEmail author
  • Agust Gudmundsson
Research Article


The subsurface structures of caldera ring faults are often inferred from numerical and analog models as well as from geophysical studies. All of these inferred structures need to be compared with actual ring faults so as to test the model implications. Here, we present field evidence of magma channeling into a caldera ring fault as exhibited at Hafnarfjall, a deeply eroded and well-exposed 5-Ma extinct volcano in western Iceland. At the time of collapse caldera formation, over 200 m of vertical displacement was accommodated along a ring fault, which is exceptionally well exposed at a depth of approximately 1.2 km below the original surface of the volcano. There are abrupt changes in the ring fault attitude with depth, but its overall dip is steeply inward. Several inclined sheets within the caldera became arrested at the ring fault; other sheets became deflected up along the fault to form a multiple ring dike. We present numerical models showing stress fields that encourage sheet deflection into the subvertical ring fault. Our findings provide an alternative mechanical explanation for magma channeling along caldera ring faults, which is a process likely to be fundamental in controlling the location of post-caldera volcanism.


Calderas Inclined sheets Numerical modeling Ring-dikes Ring-faults 



Fieldwork of JB was partly funded by the geologists’ association Baker-Arber Fund. We would like to thank Adelina Geyer and an anonymous reviewer for their very helpful comments. We also thank the editor Guido Giordano for his helpful suggestions. We are grateful to Hjalti Franzson for providing his geological map of the area and advice on field exposures, Hannah Reynolds and Zoe Barnett for field assistance, and Jonathan Pownall for fruitful discussion.

Supplementary material

445_2014_889_MOESM1_ESM.pdf (466 kb)
ESM 1 As the stresses around a propagating dike tip are dynamic, we include here a model run which simulates the continuation of a dike into the fault zone. Once the sheet has deflected into a vertical dike then the stresses around leading edge of the dike favor vertical propagation (PDF 466 kb)


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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.Department of Earth Sciences, Royal HollowayUniversity of LondonEghamUK

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