HR: 1340h
AN: T43B-1394    [Abstracts]
TI: Mechanisms of Continental Extension at Magma-Poor Rifted Margins: Constraints from the Iberia/Newfoundland and Alpine Tethys Margins
AU: * Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: EOST-CGS, 1 rue Blessig, Strasbourg, 67084 France
AU: Lavier, L
EM: luc@utig.ig.utexas.edu
AF: University of Texas, 4412 Spicewood Springs Road #600, Austin, TX 78759 United States
AU: Peron-Pinvidic, G
EM: gwenn.peron-pinvidic@eost.u-strasbg.fr
AF: EOST-CGS, 1 rue Blessig, Strasbourg, 67084 France
AB: While classical models for rifting are successful at explaining the basic link between subsidence and deformation at low stretching factors in proximal margins, they are ineffective at predicting the same relationship at highly thinned crust in more distal parts of magma-poor margins. Based on geological/geophysical observations from the Alpine Tethys and Iberia/Newfoundland margins we propose some new conceptual ideas for the evolution of magma-poor rifted margins and compare them with the results of numerical modelling. A comparison of the reconstructed Alpine Tethys margins and present-day Iberia/Newfoundland margins shows that although the two pairs of margins are of different age and had a different fate, their architecture and tectonic evolution is very similar. At both pairs of margins the pre-rift crust was ~30 to 35 km thick, dominated by quartzo-felsdpathic upper and middle crust and a gabbroic lower crust and the pre-rift geotherm was about 20°C/km, compatible with the lack of syn-rift volcanic activity. The early rift evolution was characterized by broadly distributed and symmetric extension and subsidence across the whole future margin. During late rifting, extension was localized in the distal margin and accommodated by crustal-scale detachment systems that thinned the crust and exhumed subcontinental mantle over tens of kilometres to the seafloor leading eventually to continental breakup. One key observation that applies to both pairs of margins is that the architecture and rift history of the distal margin is very different from that of the proximal margins, and that there is little evidence of `observable' crustal extension in the region of final continental breakup. The fact that both pairs of margins show a very similar tectonic evolution suggests that the underlying physical processes are the same. In order to link observations with physical processes, we define modes of extension characterized by the isostatic response, the basin architecture and the fault geometry. Three distinct and successive modes were defined: (1) a stretching mode during which fault bounded rift basins form over the whole margin; (2) a thinning mode where extension is more localized and controlled by a system of superimposed concave-downward faults that simultaneously exhume middle crust and upper mantle; and (3) an exhumation mode during which serpentinized mantle is exhumed to the seafloor along downward-concave faults. In order to understand why the style of rifting changes during extension, we compared the observations with numerical modelling experiments that were able to reproduce the key observations determined for the Alpine Tethys and Iberia/Newfoundland margins. The numerical experiments suggest that the observed changing style of rifting reflects a change in the bulk rheological evolution of the extending lithosphere. This leads to the conclusion that locale softening/hardening of the extending lithosphere due to phase transitions, cooling/heating or magmatic processes may be the key processes that control the evolution and architecture
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 8000 STRUCTURAL GEOLOGY
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: Fall Meeting 2005