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| Titel |
Evidence of flank failure deposit reactivation in a shield volcano. A favorable context for deep-seated landslide activation (La Réunion Island) |
| VerfasserIn |
Pierre Belle, Bertrand Aunay, Vincent Famin, Jean-Lambert Join |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094414
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| Publikation (Nr.) |
 EGU/EGU2014-9822.pdf |
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| Zusammenfassung |
| Giant flank failures are recurrent features of shield volcanoes, and their deposits (i.e. breccia),
constitute a significant volume in a volcanic edifice. On La Réunion Island, the growth and
development of Piton des Neiges volcano has been punctuated by several flank failure
episodes. One of these failures is a deep-seated landslide (>200 Mm3) occurring nowadays
in Grand Ilet, a plateau inhabited by 1 000 people in the cirque of Salazie, on the northern
flank of Piton des Neiges.
Here we present the results of a multidisciplinary study (structural geology and field
mapping, GNSS monitoring, borehole logging) performed to characterize the geological
structure the Grand Ilet landslide, and identify the instability factors that control this category
of destabilization.
Basic breccia deposits, up to 160 meters thick, constitute the main geological formation
of the unstable mass. This breccia are cut by the headwall scar of the landslide, and covered
by lava flows, indicating a minimum age of 200 kyr for the destabilization that produced the
deposits. The breccia is consolidated out of the landslide area. The NE toe of the landslide is
evidenced by an important compressional deformation of the base of the breccia, and striated
surfaces in this deformed volume indicate a NE-direction of transport. In this deformed
bulge, a clay-rich layer at the base of the breccia has been identified as the main slip
plane. Using a video inspection of drill casings on three exploration boreholes, we
reconstructed the 3D geometry of the slip plane at the base of the breccia. This
reconstruction shows that the landslide plane has an average dip of 6° toward the
NE.
The displacement monitoring network shows that the unstable mass has a 5.5 km2
extension, with a variable azimuth of movement direction (N140° for the SW sector, and
N45° for the NE sector). The planimetric displacements velocities range between 2 cm/year
in the inner part of the unstable mass to 52 cm/year at the landslide toe. The dip of
displacement vectors vary from 34° ± 9 uphill to 7° ± 2 downhill near the landslide
toe. This displacement field, the topography and the drill casings inspection show
that secondary shear zones are located inside the landslide mass, characterized
by a lower deformation rate than the basal shear zone. However heterogeneous is
the deformation, it more important at the base of the breccia (locally in the clay
layer).
Ultimately, our study suggests that the main slip plane has localized at the base of the
breccia despite its induration. Thus we conclude that the Grand Ilet landslide is in fact a
present-day reactivation of an old destabilization. |
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