Analogue Models of Calderas In Extensional Settings: Insights For The Development of Elliptic Collapses
Abstract
Collapse calderas are commonly elongated, forming ellipses. In extensional settings, their elongation is often subparallel to the extension direction. Such ellipticity was explained through: (1) the control of pre-rift structures; (2) the presence of an elon- gated magma chamber, (3) the activity of post-caldera normal faults. In order to better understand the role of regional extension on caldera ellipticity, we performed ana- logue models. We used dry sand to simulate the brittle crust and silicone to simulate magma. In all the experiments, regional extension, forming a graben, pre-dated col- lapse; this was obtained withdrawing the silicone within a circular nozzle at the base of the sand pack. The collapsed area was completely contained within the extended area. In map view, the calderas showed a moderate but significant ellipticity (length of major axis/length of minor axis) between 1.05 and 1.30, with the major axis parallel to the extension direction; such ellipticity was never observed in previously published caldera experiments. In section view, the caldera structure consists of an outward dip- ping reverse ring fault nucleated from the circular nozzle, similarly to what observed in previously published caldera experiments. Portions of the outward-dipping caldera fault partially reactivated the regional normal faults. This occurred where the caldera ring fault was tangent (in map view) to the normal faults. Thus, the outward-dipping caldera fault is steeper where this is tangent to the normal faults and less steep else- where. The different dip of the portions of the caldera ring fault is responsible for the observed elliptic collapse at surface. The analogue results are compared with nature, suggesting that part of the elliptic calderas in extensional settings is due to the partial reactivation of the regional normal faults during the collapse.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.5919A