HR: 1340h
AN: S43A-1061    [Abstracts]
TI: Identifying Past Earthquakes on an Active Normal Fault (Magnola, Italy) from the Chemical Analysis of its Exhumed Carbonate Fault Plane
AU: CARCAILLET, J
EM: Julien.Carcaillet@ujf-grenoble.fr
AF: Laboratoire de Geophysique Interne et Tectonophysique, LGIT- CNRS, Universite J. Fourier, BP 53, Grenoble, 38041, France
AU: CARCAILLET, J
EM: Julien.Carcaillet@ujf-grenoble.fr
AF: Laboratoire de Geodynamique des chaines alpines, LGCA, 1381 rue de la piscine, Saint Martin d'Heres, 38400, France
AU: * MANIGHETTI, I
EM: imanighe@obs.ujf-grenoble.fr
AF: Laboratoire de Geophysique Interne et Tectonophysique, LGIT- CNRS, Universite J. Fourier, BP 53, Grenoble, 38041, France
AU: CHAUVEL, C
EM: Catherine.Chauvel@ujf-grenoble.fr
AF: Laboratoire de Geodynamique des chaines alpines, LGCA, 1381 rue de la piscine, Saint Martin d'Heres, 38400, France
AU: BENEDETTI, L
EM: benedetti@cerege.fr
AF: CEREGE, CNRS, Universite de Provence, BP 80, Aix en provence, 13545, France
AU: SCHLAGENHAUF, A
EM: Aloe.Schlagenhauf@obs.ujf-grenoble.fr
AF: Laboratoire de Geophysique Interne et Tectonophysique, LGIT- CNRS, Universite J. Fourier, BP 53, Grenoble, 38041, France
AB: A normal fault scarp formed by repeated strong earthquakes is made of a series of rupture zones having been exposed, thus weathered, over significantly different time spans. We show that such differential weathering can be detected from the chemical analysis of the fault scarp rocks, and its signature used as a base to recover the past earthquake history (in the last 10-15 ka) of the fault. We focus on the Magnola normal fault, Central Italy, whose Holocene seismic slip history has been previously determined from in situ 36Cl surface exposure dating of the limestone fault scarp surface (Palumbo et al., 2004). Five major earthquakes were found to have occurred over the last 12 ka, with slips of 1.5-3 m and recurrence times of 0.7-4.8 ka. We analyze the major and trace element concentrations in 15 carbonate samples collected from base to top of the 10m-high Magnola Holocene scarp, at the emplacement of the Palumbo et al. previous sampling. We find that most element concentrations (70 %) decrease up-dip along the scarp, at a rate averaging 5%/m or 4.5%/ka. That decrease is attributed to both leaching and dissolution/recrystallization of purer calcite, together increasing with exposure time. Superimposed to the up-dip concentration decay, narrow zones of peak-concentrations (concentrations are increased by up to 100%) are found that coincide with the transition zones separating the successive earthquake ruptures. We suggest that those peak-concentrations result from calcite enrichment of the scarp sections which remained stuck in the impurity-doped, acidic upper soil during the quiescence periods that separated the earthquakes. The Rare Earth-Yttrium Elements (REE-Y) are among those most significantly enriched at the earthquake transition zones, which makes them the best chemical markers of the past seismic events. We propose a first-order numerical model that includes the two observed phenomena: a constant concentration decay rate in the exposed scarp rocks, and a constant enrichment concentration in the very upper soil. The model reproduces well the observations. Thus, provided that a limestone fault scarp is densely sampled so that to precisely determine the shape of the concentration-scarp height curves for a few REE-Y elements, the model might be inverted to recover, through iterative adjustments, the best-fitting earthquake slip exhumation history on the fault. Only a few rock exposure dating would then be needed to calibrate that history.
DE: 1039 Alteration and weathering processes (3617)
DE: 1065 Major and trace element geochemistry
DE: 7221 Paleoseismology (8036)
DE: 8010 Fractures and faults
DE: 8109 Continental tectonics: extensional (0905)
SC: Seismology [S]
MN: 2007 Fall Meeting