Pits,rifts and slumps: the summit structure of Piton de la Fournaise |
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| Authors: | Adam Carter Benjamin van Wyk de Vries Karim Kelfoun Patrick Bachèlery Pierre Briole |
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| Institution: | (1) Department of Geology and Planetary Science, University of Pittsburgh, 200 SRCC Building, 4107 O’Hara Street, Pittsburgh, PA 15260-3332, USA;(2) LMV, OPGC, Université Blaise Pascal, 5 Rue Kessler, 63000 Clermont-Ferrand, France;(3) LSTUR, 15 Avenue René Cassin, BP 7151, 97715 Saint Denis Cedex 9, Réunion, France;(4) CNRS, IPGP, 4 Place Jussieu, 75005 Paris, France |
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| Abstract: | A clear model of structures and associated stress fields of a volcano can provide a framework in which to study and monitor
activity. We propose a volcano-tectonic model for the dynamics of the summit of Piton de la Fournaise (La Reunion Island,
Indian Ocean). The summit contains two main pit crater structures (Dolomieu and Bory), two active rift zones, and a slumping
eastern sector, all of which contribute to the actual fracture system. Dolomieu has developed over 100 years by sudden large
collapse events and subsequent smaller drops that include terrace formation. Small intra-pit collapse scars and eruptive fissures
are located along the southern floor of Dolomieu. The western pit wall of Dolomieu has a superficial inward dipping normal
fault boundary connected to a deeper ring fault system. Outside Dolomieu, an oval extension zone containing sub-parallel pit-related
fractures extends to a maximum distance of 225 m from the pit. At the summit the main trend for eruptive fissures is N80°,
normal to the north–south rift zone. The terraced structure of Dolomieu has been reproduced by analogue models with a roof
to width ratio of approximately 1, suggesting an original magma chamber depth of about 1 km. Such a chamber may continue to
act as a storage location today. The east flank has a convex–concave profile and is bounded by strike-slip fractures that
define a gravity slump. This zone is bound to the north by strike-slip fractures that may delineate a shear zone. The southern
reciprocal shear zone is probably marked by an alignment of large scoria cones and is hidden by recent aa lavas. The slump
head intersects Dolomieu pit and may slide on a hydrothermally altered layer known to be located at a depth of around 300 m.
Our model has the summit activity controlled by the pit crater collapse structure, not the rifts. The rifts become important
on the mid-flanks of the cone, away from pit-related fractures. On the east flank the superficial structures are controlled
by the slump. We suggest that during pit subsidence intra-pit eruptions may occur. During tumescence, however, the pit system
may become blocked and a flank eruption is more likely. Intrusions along the rift may cause deformation that subsequently
increases the slump’s potential to deform. Conversely, slumping may influence the east flank stress distribution and locally
control intrusion direction. These predictions can be tested with monitoring data to validate the model and, eventually, improve
monitoring. |
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| Keywords: | Fracture Pit crater Rifting Slumping Shield volcano Piton de la Fournaise Digital elevation model |
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