HR: 10:50h
AN: T11E-03 INVITED     [PDF]
TI: On-land Exposures of Ocean-Continent Transitions: A Window for Understanding Rifting and Slow Sea-Floor Spreading Processes
AU: * M\"{u}ntener, O
EM: othmar.muentener@unine.ch
AF: Geology Institute, University of Neuchatel, Rue Emile Argand 11, Neuchatel, 2007 Switzerland
AU: Manatschal, G
EM: manatschal@illite.u-strassbg.fr
AF: EOST, Universite Louis Pasteur, 1 rue Blessig, Stassbourg, 67084 France
AB: Direct observations and nearly unlimited sampling in ancient margins and ocean-continent transitions exposed in the Alps combined with drill-hole and geophysical data from the present-day Iberia margin result in new concepts of how magma-poor passive continental margins evolve towards (ultra?-) slow seafloor spreading systems. We review data from both the shallow and deep structure of magma-poor passive margins and propose that three temporal and spacially different fault systems are sufficient to explain the overall evolution from continental extension to early seafloor spreading. (1) During an initial stage of rifting the architecture and rift structures are controlled by inherited heterogeneities in the intermediate and lower crust which lead to a different response of the future distal and proximal margins. Future proximal margins are characterized by frequent normal faulting soling out at approximately 10km, while future distal margins show little evidence for early rift-related normal faults. However, the deep structure underneath the future distal margin is controlled by crustal-scale faults which thinned the crust to about 10km by excising up to 20 km of intermediate to lower crust. Thus we prefer the hypothesis of a 'within crust decoupling' of deformation and suggesting that boudinage of the lower crust is an important process during early rifting. A second important aspect is that after the initial stage of rifting mantle rocks are accessible for serpentinization. (2) Late-stage rifting is controlled by detachment faults. The most compelling evidence are extensional allochthons of upper continental crust emplaced on exhumed mantle rocks and tectono-sedimentary breccias covering detachment faults. Strain is extremely localized and leads to the formation of characteristic black fault gouges which are documented in ocean-continent transitions. Low angle detachment faults cut accross the entire, previously thinned continental crust and provide a mechanism for exposure of subcontinental mantle and rarely lower continental crust at the seafloor. The exhumed mantle rocks and the associated mafic crust show mineralogical and geochemical characteristics that are spatially corrrelated with their distance to the continental margin. (3) The transition from late rifting to the onset of seafloor spreading is most probably controlled by the interactions of rising asthenospheric mantle with concave downward faults. Such faults exhume deeper (and hotter) mantle oceanwards and are temporaly and spatially juxtaposed with the onset of magmatic activity. Evidence from Alpine ophiolites and from Iberia suggest regional-scale melt infiltration and melt/rock reaction which is most obviously expressed by the widespread formation of plagioclase peridotites in zones of exhumed continental mantle. Whether or not rift-related melt infiltration and heating is recorded by exhumed mantle rocks depends on the relative position to the underlying upwelling asthenosphere. Thus during final breakup the rheology of the extending lithosphere seems to be controlled by the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling and hydrothermal alteration by exhumation.
DE: 8010 Fractures and faults
DE: 8105 Continental margins and sedimentary basins
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting