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CRISES VOLCANO-TECTONIQUES ET DIVERGENCE DE PLAQUES EN ISLANDE : MESURE PAR GPS ET MODELISATION NUMERIQUE

Abstract : The specific geodynamic context of Iceland results from the simultaneous effect of an oceanic rift and a mantellic plume. Many studies realized nowadays about the tectonic activity of Iceland have shown that deformation is expressed by volcanic and seismic crisis. In the northern Iceland, the extensive deformation is expressed by volcano-tectonic crisis which the recurrence is near 250 years in the Northern Volcanic Zone (NVZ). In southern Iceland, the recurrence between each main events evolves between several years in the Eastern Volcanic Zone (EVZ) to 45 to 115 years for the South Iceland Seismic Zone (SISZ). All this studies show that the extensive deformation of Iceland is not continue and is expressed in surface by time variations of displacement.

In this work, we propose to quantify and analyze the time variations of displacement and the steady-state opening in order to better apprehend the mechanisms which control the active deformation of Iceland. The goal of this study is to detect the different types of variations of displacement which occur in the main active structures of Iceland, to quantify the variations on the three component of the displacement, to estimate their duration and their origin, and understand their relation with the steady-state opening of the mid-oceanic ridge.

Firstly, the analysis of the time series of this 17 CGPS stations in Iceland between July 2000 and December 2002 has allowed us to highlight some important displacement variations in the SISZ and the EVZ. For the SISZ, all the stations have presented a displacement oriented between 114°N and 128°N for the second part of 2000 and between 105°N and 115°N until end-2002. The comparison of this time evolution with the progressive decrease of the microseismicity for the same period confirms that this eastward orientation of the displacement highlights a short period of post-seismic deformation (less than 2 years). For the EVZ, the displacements of the CGPS stations show some strong variations mainly detected on the north-south component. The periods of low southward displacements are synchronous with periods of strong seismic activity of the Katla volcano and could correspond with periods of depletion of the volcanic edifice induced by a emptying of the magma chamber by magma injection into sills or dykes located at low depth.

Secondly, from the GPS measurements performed in northern Iceland between 1987 and 2002 and the recent geophysical investigations on the crust and mantle properties from across the rift zone in Iceland, we have realized, with the finite element code ADELI, a 2D then 3D numerical simulation of the Krafla Fissure Swarm crisis (which occurred between 1975 and 1984) and of the displacements evolution from the end of the crisis (1984) to nowadays. In 2D, we have used a simple structure with a rise of several kilometres of the lower crust and the upper mantle. The best result has been obtained with the following rheological parameters : (1) a viscosity of 8.1018 Pa.s for the lower crust and 3.1018 Pa.s for the mantle ; (2) a cohesion of 106 Pa and a friction angle of 15° for the upper crust. In 3D, our models integer the Krafla fissure swarm (simulated as a 3D rheological structure), the Askja fissure swarm and the Husavik-Flatey fault. The results obtained highlight that : (1) this crisis and its post-crisis relaxation had a local impact in the northern Iceland which did not extend beyond 180 to 200 km from the rift axis ; (2) the displacements measured by GPS between 1987 and 1995 can not be simulate without a strong opening (near 1m) in the Askja Fissure Swarm ; (3) the Husavik-Flatey fault does not modify significantly the simulation of the displacements measured between 1987 and 2002, which highlights that this fault have no influence on the syn- and post-crisis deformation of northern Iceland ; (4) the post-crisis response has continued only 10 to 15 years ; (5) the viscosities obtained for the lower crust (LC) and the upper mantle (UM) reach 6.1020 Pa.s for LC and 6.1019 Pa.s for UM which is really stronger than those suggested by previous studies (around 3.1018 Pa.s and 3.1017 Pa.s for LC and UM respectively).
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Antoine Berger. CRISES VOLCANO-TECTONIQUES ET DIVERGENCE DE PLAQUES EN ISLANDE : MESURE PAR GPS ET MODELISATION NUMERIQUE. Géologie appliquée. Université de Savoie, 2004. Français. ⟨tel-00009362⟩

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