HR: 11:05h
AN: T12A-04    [Abstracts]
TI: Subduction and exhumation of continental crust in the Western Alps - A comparison of field-based and dynamic models
AU: * Handy, M R
EM: mhandy@zedat.fu-berlin.de
AF: Dept. of Earth Sciences Freie Universitaet Berlin, Malteserstrasse 74-100, Berlin, D-12249 Germany
AU: Babist, J
EM: babist@zedat.fu-berlin.de
AF: Dept. of Earth Sciences Freie Universitaet Berlin, Malteserstrasse 74-100, Berlin, D-12249 Germany
AU: Konrad, M
EM: mkonrad@zedat.fu-berlin.de
AF: Dept. of Earth Sciences Freie Universitaet Berlin, Malteserstrasse 74-100, Berlin, D-12249 Germany
AB: For more than a century, the Alps have served as a crucible for orogenic models. Most of these models, including all dynamic models so far, have assumed 2D subduction of the European lithosphere beneath the Apulian continental margin. We use the example of the Sesia Zone in the Western Alps to dispel misconceptions about Alpine subduction, and to compare assumptions and results of field-based and dynamic models. The Sesia Zone comprises subducted slivers of Apulian passive continental margin that were initially exhumed by thrusting and oblique-slip shearing during late Cretaceous, dextral transform motion of Apulia with respect to Europe. Deformation of these slivers under eclogite- to blueschist- to greenschist-facies conditions in the presence of excess water enhanced their density contrast with the surrounding mantle. This facilitated buoyancy-driven exhumation from about 60 to 20 km at the base of the upper plate. Further exhumation of these slivers to near the surface involved SE-directed extensional shearing under greenschist-facies conditions and was broadly coeval with Eocene, SE-subduction of the Piemont-Liguria oceanic lithosphere. These kinematics are consistent with the idea that the Sesia basement nappes were wedged upward and laterally thinned above a NW-retreating hinge in the subducting oceanic plate. Oligo-Miocene exhumation of the NE Sesia Zone involved dextral transpressional shearing, backfolding and magmatism in the retro-wedge of the Alpine orogen. The validity of dynamic models is often judged by their ability to (re)produce first-order structures and PTt paths of exhumed crust. Insufficient regard for available 3D kinematic and thermobarometric constraints, especially on the pre-orogenic history of the plates' lithospheres, can lead to models that look realistic, but are misleading from thermal and/or kinematic standpoints. On the other hand, field-based models draw on diverse, sometimes tenuously compatible data sets whose interpretation can be biased by the desire to produce visibly attractive pictures. Nowadays, such pictures are influenced by published dynamic models. This feedback is fruitful if orogenic models are recognized as tests of ideas rather than simulations, and if geologists and modelers spend time understanding each other's Nature.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3909 Elasticity and anelasticity
DE: 8100 TECTONOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005