HR: 1340h
AN: T53C-1453    [Abstracts]
TI: Fracturing and Transformation Into Veins Beneath the Crustal Scale Brittle Ductile Transition - a Record of Co-seismic Loading and Post-seismic Relaxation
AU: * Nüchter, J A
EM: jens.nuechter@ruhr-uni-bochum.de
AF: Sonderforschungsbereich 526, Institute for Geology, Mineralogy und Geophysics, Ruhr-University Bochum, Universitätsstrasse 150, Bochum, 44780 Germany
AU: Stöckhert, B
EM: bernhard.stoeckhert@ruhr-uni-bochum.de
AF: Sonderforschungsbereich 526, Institute for Geology, Mineralogy und Geophysics, Ruhr-University Bochum, Universitätsstrasse 150, Bochum, 44780 Germany
AB: Metamorphic rocks approaching the crustal scale brittle-ductile transition (BDT) during exhumation are expected to become increasingly affected by short term stress fluctuations related to seismic activity in the overlying seismogenic layer (schizosphere), while still residing in a long-term viscous environment (plastosphere). The structural and microstructural record of quartz veins in low grade - high pressure metamorphic rocks from southern Evia, Greece, yields insight into the processes and conditions just beneath the long-term BDT at temperatures of about 300 to 350°C, which switches between brittle failure and viscous flow as a function of imposed stress or strain rate. The following features are characteristic: (1) The veins have formed from tensile fractures, with a typical length on the order of 10^-1 to 10¹ m; (2) The veins are discordant with respect to foliation and all pre-existing structures, with a uniform orientation over more than 500 km²; (3) The veins show a low aspect ratio of about 10 to 100 and an irregular or characteristic flame shape, which requires distributed ductile deformation of the host rock; (4) Fabrics of the sealing vein quartz indicate that - at a time - the veins were wide open cavities; (5) The sealing quartz crystals reveal a broad spectrum of microstructural features indicative of crystal plastic deformation at high stress and temperatures of about 300 to 350°C. These features indicate that opening and sealing of the fractures commenced immediately after brittle failure, controlled by ductile deformation of the host rock. Vein-parallel shortening was generally less than about 2%. Crystals formed early during sealing were plastically deformed upon progressive deformation and opening of the vein. The structural and microstructural record is interpreted as follows: Brittle failure is proposed to be a consequence of short term co-seismic loading. Subsequent opening of the fracture and sealing to become a vein is interpreted to reflect the slower (but still very short term on geological time scales) deformation during post-seismic stress relaxation, with precipitation of minerals from the pore fluid percolating into the evolving cavity. The record provides insight into earthquake-related damage in the uppermost plastosphere and the transient crustal properties during post-seismic creep and stress relaxation.
DE: 3653 Fluid flow
DE: 5104 Fracture and flow
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: Fall Meeting 2005