HR: 10:30h
AN: T42A-01 INVITED     [Abstracts]
TI: Remnants of an Ancient Ocean-Continent Transition Exposed in the Alps: A Window for Understanding Break-up Processes
AU: * Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: CGS-EOST, ULP-CNRS, 1 rue Blessig, Strasbourg, 67084 France
AB: In the last two decades, remnants of an ancient Ocean-Continent Transition (OCT) of the Alpine Tethys ocean have been mapped in the Alps in SE Switzerland. The most prominent structure within the reconstructed OCT is a detachment system that can be traced from the thinned continental crust, across a Zone of Exhumed Continental Mantle (ZECM) towards oceanic crust. The detachment system is formed by at least two corrugated detachment faults that are overlain by extensional allochthons, tectono-sedimentary breccias and syn-and post-rift sediments. In the ZECM, basaltic magmas grade from T- to N-MORB oceanwards. Near the continental edge they form isolated volcanic bodies emplaced directly onto exhumed mantle whereas further oceanwards they are more voluminous and associated with syn-magmatic high-angle normal faults. In the OCT, lower crustal rocks are rare and where present related to Permian underplated gabbros preserving primary contacts to mantle rocks. In the ZECM, the mantle rocks change from pyroxenite-rich spinel peridotites close to the continent to pyroxenite-poor peridotites impregnated and intruded by asthenospheric melts further oceanwards. Across the whole OCT, the detachment system can be mapped as a damage zone, up to hundred meters thick, with a core zone formed by gouges. The fault rocks record a complex fluid and reaction assisted retrograde deformation history. Maximum temperatures at which the detachment faults were active are 300C in crustal rocks and 600C in mantle rocks. Enrichment of chromium and nickel along the detachment fault in the continental crust may result from fluids derived from a serpentinizing mantle underlying the extending continental crust. High-temperature granulite facies mylonites are found in the lower crustal and mantle rocks, however, their relation to the detachment is not yet fully understood. In the syn-extensional gabbros microstructures reveal a deformation history ranging from syn-magmatic to seafloor conditions. This deformation was acquired during their intrusion into partially serpentinized mantle rocks and subsequent exhumation. U/Pb on zircon and Ar/Ar on phlogopite ages obtained from these gabbros, interpreted as crystallization and cooling ages respectively, range between 165 and 157 Ma, which corresponds to the age of radiolarian cherts, the first sediments sealing oceanic and continental units in the OCT. The occurrence of isolated allochthons of continental origin stranded onto subcontinental mantle is incompatible with mantle exhumation at a mid-ocean ridge. Therefore, the detachment system preserved in the Alps is more likely related to final rifting leading to break-up and onset of seafloor spreading. All observations reported from the Alpine analogues reveal that the detachment system formed as a downward concave fault in a previously rifted crust that was already thinned to less than 10 km, and that serpentinization and magmatic activity were closely related to detachment faulting. Therefore, the thinning of the crust to 10 km and the localization of rifting within the area of final break-up can not be explained by magmatic and/or serpentinization processes, but may be predetermined by inherited heterogeneities within the pre-rift lithosphere. The interaction of tectonic, magmatic and hydrothermal processes during final break-up may explain the previously enigmatic final stage of continental extension and onset of seafloor spreading in magma-poor systems, which appears to be dominated by downward concave faulting.
DE: 8105 Continental margins and sedimentary basins
DE: 8109 Continental tectonics--extensional (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2005 Joint Assembly