HR: 1330h
AN: V32A-1005    [PDF]
TI: Link Between Volcanism and Mantle Exhumation at the Axial Zone of a Fossil Slow Spreading Ocean
AU: * Chalot-Prat, F E
EM: chalot@crpg.cnrs-nancy.fr
AF: CRPG-CNRS - NANCY University, BP20, Vandoeuvre les Nancy, 54501 France
AB: Within an axial zone of a slow spreading ocean, the mechanisms checking together volcano emplacement, mantle exhumation and ocean enlargement are poorly known. In order to better assess how they are linked, a detailed study of a fossil ocean-floor structure, preserved from alpine tectonic and metamorphism, was performed in the Chenaillet unit (Franco-Italian Alps)(Chalot-Prat et al, submit.). The 3D geometry of the ophiolite evidences that from its dimensions, topography, morphology and the architecture of the basaltic cover, the Chenaillet unit is a witness of an axial zone of Atlantic type. The basement (serpentinized peridotites; gabbros), below and on both sides of the volcanic cover, is capped by cataclasite horizons underlining detachment faults responsible for its exhumation at the seafloor. Clasts of basalt found within the cataclasite indicate an active exhumation during and after volcano emplacement. Stair- and Comb-type volcanic systems check the distribution of individual volcanoes. The higher the edifice, the younger it is relative to the others. In a S-type, each step is formed with a pillow and tube tongue stacking fed from fissures located at the root of each step. The system formed by uplift, step by step fracturation of an already exhumed basement, and magma injection along the fissures. A C-type consists in alignments of pillow and tube conic edifices. Their central feeder dykes are located at the crossing of fractures oblique and parallel (line) to the main branch of the comb. Along a same line, eruptions are coeval. The comb formation needed initial basement fracturation, then uplift and exhumation of a new basement along a detachment fault which also controlled magma injection. Once formed, volcanoes were dragged away and down on the travelator to give place to new volcanoes. The pseudo-symmetry of the C-type evidences that the exhumation process occurred synchronously, but not at the same rate, in opposite directions, as observed at any mid-oceanic ridge axis. In both systems, fast alternation between partial melts and their differentiates suggest that the main magma conduits were rooted within frequently fed small reservoirs. The regular recurrence of partial basaltic melts also suggests that the main conduits were rooted within the mantle source. Lithospheric stretching could control both magma ascent and mantle melting.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 5480 Volcanism (8450)
DE: 8159 Rheology--crust and lithosphere
DE: 9335 Europe
DE: 9609 Mesozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting