HR: 17:30h
AN: T34B-07 INVITED     [Abstracts]
TI: Detachment Faults in Ocean Continent Transitions
AU: * Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: EOST-CGS, 1 rue Blessig, Strasbourg, 67084 France
AU: Peron-Pinvidic, G
EM: gwenn.peron-pinvidic@eost.u-strasbg.fr
AF: EOST-CGS, 1 rue Blessig, Strasbourg, 67084 France
AB: Ancient models of continental break-up conventionally juxtapose normal continental and oceanic crusts. However, deep-sea drilling in the Iberia margin and observations in the Alpine Tethys margins exposed in the Alps provide compelling evidence that these two crusts are separated by continental mantle commonly interpreted to be exhumed at the seafloor by lithospheric-scale detachment faulting. In the Iberia margin, detachment faults were interpreted to coincide with strong seismic reflections (e.g. S and H reflections) and have been drilled at ODP Sites 900, 1067 and 1068. Based on kinematic inversion of seismic sections and drill-hole data, it was shown that the detachment faults formed as a sequence of high-angle faults during a late stage of rifting in a previously thinned, less than 10 km thick crust at rates of 1 to 2 cm/yr. With ongoing extension, the faults rotated and changed from upward to downward concave faults enabling to exhume mantle rocks over tens of kilometres without producing a major seafloor topography. In the Alps, remnants of detachment faults belonging to the former Ocean Continent Transition (OCT) of the Alpine Tethys are spectacularly exposed in several places in SE Switzerland. Like in the Iberia example, these structures show break-aways towards the continent and cut oceanwards into serpentinized mantle peridotites. The detachment faults are covered either by extensional allochthons of continental origin or sediments, further oceanwards also by basalts. Detailed mapping combined with structural and petrological investigations show that these detachment faults were active in the stability field of serpentine. The detachment faults show a complex relationship to high- temperature mantle mylonites (>700°C) and infiltrated mantle peridotites. Further studies are necessary to unravel the complex relationship between shallow and deep lithospheric deformation processes as well as between magmatic and hydration processes interacting with mantle exhumation along detachment faults. The available data favour the hypothesis that the detachment faults did not root into an asthenospheric mantle, but were more likely interacting with a weak subhorizontal decollement in the mantle. Such a weak zone may be related either to a hydration or an infiltration front at temperatures >700°C. The 3D geometry of detachment faults in the OCT is very complex and shows some similarities with those observed at oceanic core complexes. In the Err nappe in the Alps, preserved detachment structures can be mapped over an area of about 30 km2. The mapped fault planes are either corrugated parallel to the transport direction or form lateral ramps reactivating pre-existing structures. Mapping of the reflections interpreted as detachment faults in the Iberia margin shows that on the scale of the margin, these structures form domes and ridges. Moreover, extensional allochthons overlying exhumed mantle can be correlated along strike with a series of fault-bounded blocks overlying strong intra-basement reflections interpreted as detachment faults. These observations suggest that detachment faults in OCT are poly-phase structures that form during a final stage of continental break-up and continue to deform after their exhumation at the seafloor. The scale, 3D geometry and the processes controlling the evolution of detachment faults in the OCT are not yet sufficiently constrained to draw some further conclusions or to compare them with oceanic core complexes.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 8000 STRUCTURAL GEOLOGY
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: Fall Meeting 2005