Caldera ring-fault intrusion through repeated sheet capture

Abstract:

The subsurface structure of caldera-bounding ring faults remains a contentious issue, with geometries often inferred from numerical and analogue 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 observations of a deeply eroded and uniquely well exposed section of caldera ring fault in a Tertiary central volcano in south-western Iceland. The Hafnarfjall ring fault represents the outermost fault complex of an elliptical caldera with an original diameter of approximately 5 km. Vertical displacement is estimated to be > 200 m on the steeply inward-dipping ring fault. Several thin (< 1 m) dikes occupy an approximately 5 m thick section within the ring fault; this region is interpreted to have once acted mechanically as the fault core with a lower stiffness or Young’s modulus than the surrounding host rock. Many faults which have been active over an extended period develop a damage zone around the fault core; this is a zone of high fracture frequency which is generally stiffer than the core but softer or more compliant than the surrounding host rock. We observe a number of inclined sheets, presumably originating from the part of the shallow magma chamber located the caldera margin, which become either arrested or deflected upon contact with the ring fault. Using the commercial numerical modelling software COMSOL, we offer a mechanical explanation for the deflection of inclined sheets into sub-vertical dikes at a mechanically stratified fault damage zone and core. A model is proposed whereby ring faults can act as a barrier for the propagation of inclined sheets away from a magma chamber within the caldera. Our findings provide an alternative mechanical explanation for magma channelling along caldera ring-faults, which is a process likely to be fundamental in controlling the location of post-caldera volcanism.