The beginning of the 1985–1987 eruptive cycle at Piton de la Fournaise (La Reunion); new insights in the magmatic and volcano-tectonic systems

Abstract

The short eruption of June 14–15 1985 was the beginning of an important cycle of activity of Piton de la Fournaise that lasted more than two years and comprised twenty or so eruptive and intrusive phases. In this article we report and analyze the first events of this cycle.The June 1985 eruption was preceded by inflation and seismicity. A period of aseismic and slow inflation of the summit area was observed in early 1985. Seismic activity began in mid-May, and increased at the end of the month. Simultaneously, the rate of deformation increased. The seismic events clustered at depths ranging from 1 to 2.5 km, beneath the same area as the center of inflation, northwest of the main summit crater (Dolomieu). The shallowness and small lateral extension of the seismicity, and the short wavelength of the deformation showed that the focus of magmatic pressure was shallow and restricted to a small volume. The outbreak was preceded by a less than one hour intrusive crisis characterized by intense seismic activity and by rapid deformation. The intrusion reached the surface along a SW-NE system of fissures in line with the epicentral zone. The detailed analysis of the seismic and deformation patterns during the intrusive sequence, allow us to propose that the near surface path of the intrusion was guided by an existing tensional fissure system, a hypothesis that explains why the eruption did not occur closer to the epicentral zone. The eruption produced about one million cubic meters of transitional aphyric basalt of a composition similar to that of the lavas erupted since 1979.About one month later, an intrusion not followed by an eruption occurred on July 9; it was the first event of this type observed at Piton de la Fournaise since the installation of the Volcanological Observatory in 1980. The seismicity and deformation patterns during this intrusive sequence were similar to those observed before all the outbreaks since 1980. The main seismic zone was beneath the central to south part of Dolomieu crater, but the deformation showed that the intrusion eventually migrated toward the east and stopped at shallow depth, at about 1 km east of Dolomieu. As for the intrusion associated with the June 14 eruption, the path of the July 9 intrusion also seemed to be guided by a preexisting fissure system.As the July 9 summit crisis was ending, a new crisis began on the east flank of the volcano on July 10. It was the first seismic crisis observed in this area. It surpassed the previous summit seismic crises by two orders of magnitude in the number of earthquakes as well as the energy released. Initially, most hypocenters were concentrated at about 3 km ENE of the summit and at depths of 2–5 km beneath sea level. Later, after the climax of the crisis, the hypocenter zone spread over the entire Grandes Pentes area. We suggest that this flank crisis corresponded to the release of compressional stresses accumulated in the central area of the volcano during the repeated intrusions that occurred over the preceding years.The detailed analysis of the events reported in this article brings new insights in the shallow magma reservoir and in the tectonic behaviour of the volcano. The magma reservoir is inferred to be composed of several units rather than a single volume unit. The storage units are distributed over an area that corresponds about to that of Dolomieu crater and at depths between 1 and 2 km or less to at least 2.5–3 km. This reservoir complex has probably not been resupplied since 1977. The asymmetric tectonic behaviour of the volcano, already recognized from surface tectonic features, has been confirmed by the deformation and seismic observations. The contractional stresses accumulated in the central zone to accommodate the inflation of the reservoir and the emplacement of the intrusion, are released by preferential displacement of the free flank (sea-facing flank). A study of this mechanism is important for the recognition of future eruptive and tectonic activity.