HR: 1340h
AN: T33C-0575    [Abstracts]
TI: Looking Down a Plate Boundary - an Exhumed Fossil Subduction Channel
AU: * Bachmann, R
EM: raik@gfz-potsdam.de
AF: GFZ Potsdam Section 3.1, Telegrafenberg, Potsdam, 14473 Germany
AU: Oncken, O
EM: oncken@gfz-potsdam.de
AF: GFZ Potsdam Section 3.1, Telegrafenberg, Potsdam, 14473 Germany
AB: The subduction channel of active convergent plate margins cannot be directly accessed, only geophysical methods, numerical modeling or sandbox simulations throw light on the shallow parts of currently active zones. We additionally use direct investigation of exposed ancient subduction zones to understand their internal geometry and processes operating. Therefore, we studied an up to 1000 m thick m‚lange within the Central Alps of Europe, which trace a former plate boundary. This zone is sandwiched between the overlying Austroalpine nappes (African plate) and the underlying Penninic/Helvetic nappes (European plate) during Alpine convergent plate motion. The analysis of transects crossing the former plate boundary contributes to the identification of changes within the fossil subduction channel. Overprint of the upper plate basement by Alpine deformation increases towards the south of the working area. The matrix of the subduction channel is composed of metasediments with increasing metamorphic grade from north to south. Deformation of the metasediments increases similarly, which is expressed by the increasing dominance of foliation planes, and more pronounced stretching lineation. The matrix contains clasts of different size consisting of upper plate basement and metasediments as well as slivers of the oceanic lower plate and its sedimentary cover. The clast size varies from a few millimeters to hundreds of meters. With increasing metamorphism and deformation both upper plate basement and metasedimentary clasts are strongly mylonitized along their rims - internally they still retain their original texture. Mylonitic shear zones cut into the clasts and enforce disintegration. Pseudotachylytes as evidences for unstable slip have been found at a limited depth range (app. 3-6 kbar, <350°C) at the base of the upper plate. We never found them within metasedimentary rocks, either matrix or clasts. However, metasedimentary clasts and metabasics reveal abundant hydraulically fractured veins exhibiting chaotic mineralization. Additionally, a vast number of foliation parallel mineralized veins invade the matrix of the fossil subduction channel, pointing to transient changes in fluid pressure and stress system. The relationship of these coarse grained mineralized veins to seismic faulting has yet to be evaluated. They may mirror dehydration processes during prograde metamorphism within the subduction zone. Since the exposed ancient subduction channel has experienced flow and fracturing over several cycles (including some overprint during collision and exhumation) it still may bear resemblance to cumulative active margins that have been active over 10s of Myrs.
DE: 7209 Earthquake dynamics (1242)
DE: 8104 Continental margins: convergent
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