HR: 16:45h
AN: S22E-04    [PDF]
TI: Deformation Mechanism Maps for Feldspar Rocks
AU: * Dresen, G H
EM: dre@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg D425, Potsdam, 14473 Germany
AB: Feldspar is the most abundant mineral in the Earth's crust. The deformation behavior of feldspar-rich rocks like granitoids and gneisses largely controls the strength of the continental crust. Deformation mechanisms maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates. The maps display constant temperature contours in stress-grain size space for strain rates ranging from 10$^{-16}$ s$^{-1}$ - 10$^{-12}$ s$^{-1}$. Two fields of dominance of grain boundary diffusion-controlled creep and dislocation creep are separated by a strongly grain size-sensitive transition zone. For wet rocks diffusion-controlled creep dominates below a grain size of about 0.1-1 mm, depending on temperature, stress, strain rate, and feldspar composition. The strength of feldspar rocks is moderately dependent on composition and water fugacity, but is strongly affected by water trace content. Dry feldspar rocks are stronger than water-bearing aggregates by more than two orders of magnitude in stress. For a grain size range of about 10-50 $\mu$m commonly observed in rocks from natural shear zones the deformation maps predict that diffusion-controlled creep is dominant at pressure and temperature conditions typical for the lower crust. Low-viscosity estimates of 10$^{18}$-10$^{19}$ Pa.s from modelling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures $>$ $900deg$ C or, at lower temperatures, diffusion creep of fine-grained rocks possibly localized in abundant high-strain shear zones. Viscosity of the lower crust is predicted to be less than one order of magnitude smaller than that of the upper mantle.
DE: 3902 Creep and deformation
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2003 Fall Meeting