Deformation mechanism maps for feldspar rocks Erik Rybacki * , Georg Dresen GeoForschungsZentrum Potsdam, Projektbereich 3.2, Potsdam D-14473, Germany Received 21 July 2003; accepted 19 January 2004 Abstract Deformation mechanism 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 to 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. Plagioclase aggregates containing up to 0.3 wt.% water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks. The strength of water-bearing feldspar rocks is moderately dependent on composition and water fugacity. For a grain size range of about 10–50 Am commonly observed in natural ultramylonites, the deformation maps predict that diffusion-controlled creep is dominant at greenschist to granulite facies conditions. Low viscosity estimates of 10 18 –10 19 Pas from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures >900 jC or, at lower temperatures, diffusion creep of fine-grained rocks possibly localized in abundant high strain shear zones. For similar thermodynamic conditions and grain size, lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks. D 2004 Elsevier B.V. All rights reserved. Keywords: Rheology; Plagioclase deformation maps; Lower crust; Viscosity; Shear zones 1. Introduction Estimates of strength and viscosity of the earth’s lower crust and upper mantle are largely based on the inversion of geophysical and geodetic data and on predictions from rock mechanics laboratory measure- ments. However, the interpretation of existing data on the strength of continental lithosphere is a matter of considerable debate (Burov and Watts, 2003; Podlad- chikov, 2003). In particular, extrapolation of the plastic deformation behavior of major silicate rocks from laboratory to field conditions typically predicts a lower crust that is relatively weak when compared to upper crust and strong underlying mantle lithosphere (Kohlstedt et al., 1995). However, a weak lower crust may conflict with estimates of the effective elastic thickness of the lithosphere and recent observations of the focal depth distribution of earthquakes. At least in some continental areas, the elastic thickness com- 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.01.006 * Corresponding author. Tel.: +49-331-288-1329; fax: +49-331- 288-1328. E-mail address: uddi@gfz-potsdam.de (E. Rybacki). www.elsevier.com/locate/tecto Tectonophysics 382 (2004) 173– 187