HR: 14:10h
AN: T43E-03    [Abstracts]
TI: Eclogitic Pseudotachylytes in Subducted Oceanic Crust: are Intermediate-Depth Earthquakes Producing Pathways for Fluid Flow?
AU: * John, T
EM: tj@min.uni-kiel.de
AF: Institut fuer Geowissenschaften and SFB 574, Universitaet Kiel, Olshausenstr. 40, Kiel, 24098 Germany
AU: Schenk, V
EM: vs@min.uni-kiel.de
AF: Institut fuer Geowissenschaften and SFB 574, Universitaet Kiel, Olshausenstr. 40, Kiel, 24098 Germany
AB: Subduction of oceanic crust and the underlying lithospheric mantle leads to densification and dehydration of the downgoing slab. Associated processes are slab pull which forces the subduction, intermediate-depth (70-300 km) seismicity, and arc magmatism. These processes are somehow linked because intermediate-depth earthquakes usually occur where dehydration reactions are expected to take place and furthermore released slab fluids trigger arc magmatism. Hence, it is suggested that high fluid pressures can cause dehydration embrittlement which may result in earthquakes. Another possibility to generate intermediate-depth earthquakes in slabs is shear instability. Such a process requires that very narrow shear zones and high shear strain-rates exist in slabs. However, up to now most of the knowledge concerning intermediate-depth earthquakes and their relations to intraslab fluid flow is based on theoretical approaches, experiments and interpretations of seismic data. Field evidences, such as pseudotachylytes (the only doubtless geological evidence for paleo-earthquakes), confirming the suggested hypotheses are rare. For the first time we now can present evidence that imply that first frictional failure with melting occurred. In eclogites of a paleo-subduction zone textural features occur, which are usually found in quenched melts of active volcanoes. We interpret them as representing former frictional melts and thus pseudotachylytes. The seismic failure was followed nearly contemporaneously by infiltration of an external fluid. Long-standing fluid flow produced hydraulic fracturing and continuous vein formation in the eclogites during ongoing burial. Furthermore, during their passage through the rocks the external fluids leached most of those trace elements from the eclogites which are required to produce the characteristic slab component of arc magmas.
DE: 8045 Role of fluids
DE: 7209 Earthquake dynamics and mechanics
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
DE: 1030 Geochemical cycles (0330)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting