HR: 11:30h
AN: T42A-04    [Abstracts]
TI: Observations From the Alpine Tethys and the Iberia/Newfoundland Margins Pertinent to the Interpretation of Magma-Poor Rifted Margins
AU: * Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: CGS-EOST, CNRS-ULP, 1 rue Blessig, Strasbourg, 67084 France
AU: Lavier, L L
EM: luc@utig.ig.utexas.edu
AF: Jackson School of Geosciences, UTIG, 4412 Spicewood Springs Road, Austin, TX 78759-8500 United States
AB: Although the Iberia/Newfoundland and Alpine Tethys margins are of different age and ultimately had a different fate, they share remarkable similarities. These similarities permit us to compare direct observation and unlimited sampling of the ancient Alpine margins with the drill-hole and geophysical data from the present-day Iberia/Newfoundland margins. This exercise results in new concepts for the near surface response of lithospheric rupturing at magma-poor rifted margins. Rifting at these two pairs of margins can be described by three modes of extension: a stretching mode, a thinning mode and an exhumation mode. Each mode is characterized by a particular isostatic response to extension, its basin architecture and fault geometry, and the bulk rheological evolution of the extending lithosphere. Initial rifting was controlled by the stretching mode. Deformation in the upper crust and upper mantle was decoupled along mylonitic shear zones in the middle crust, rift-shoulder uplift was moderate to weak, and basins were distributed across the whole subsiding margin. During an advanced stage of rifting, extension became localized in the future distal margin and was controlled by the thinning mode. Extension in the upper crust and upper mantle were coupled along major mylonitic shear zones, rift-shoulder uplift was very pronounced and resulted in sub-aerial exposure of parts of the future distal margin. During this stage, the crust in the future distal margin was thinned by more than 15 km although no evidence for upper crustal extension is found in the stratigraphic record. Final rifting was localized in the previously thinned crust and was controlled by the exhumation mode. During this stage, downwards-concave faults exhumed lower crustal and mantle rocks to the seafloor leading to a tens of kilometres wide Zone of Exhumed Continental Mantle (ZECM). Despite of the high extension, these faults did not produce a major fault bounded topography. This final stage of rifting was assisted by serpentinization and magmatism, but in contrast to the following seafloor spreading, the continental lithosphere was not yet broken apart and the asymmetry of the extending system affected the whole distal margin. The rift-evolution observed along the Iberia/Newfoundland and Alpine Tethys margins shows a change in the mode of extension from initially distributed and decoupled (extension mode), to localized, coupled and asymmetric (thinning and exhumation mode), to final localization in a Mid Ocean Ridge. The change in the mode of deformation can be interpreted to reflect an evolution of the bulk rheology of the extending lithosphere. Initial rifting appears to be controlled by inherited heterogeneities and recrystallization processes in mylonitic shear zones (stretching and thinning mode) whereas later rifting and continental rupturing may be controlled by hydration (serpentinization), magmatic and thermal weakening (exhumation mode). Since both serpentinization and magmatism developed during a late stage of rifting within an already thinned crust, they may control the break-up of the continental lithosphere but not the localization of rifting within the area of the future break-up. The localization of rifting within the area of final break-up has therefore to be explained by inherited heterogeneities or another weakening process within the extending lithosphere. At other margins, these modes may interact in a different way depending on the pre-rift conditions and the evolution of the rheology during rifting.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 8100 TECTONOPHYSICS
DE: 8105 Continental margins and sedimentary basins
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2005 Joint Assembly