Abstract
Rheological properties of continental lower crust (CLC) are essential
for understanding many geodynamical processes in the Earth. Here we
performed a series of deformation experiments on synthetic felsic
granulite (65% plagioclase + 15% quartz + 9% clinopyroxene + 6%
orthopyroxene + 5% amphibole) using a 5 GPa modified Griggs-type
deformation apparatus at 827-927 {degree sign}C and 1 GPa. All
experiments reached a steady-state creep at strain rates ranging from
3.0×10-6 s-1 to 1.0×10-4 s-1, yielding a stress exponent of 4.2 {plus
minus} 0.1, a pre-exponential factor of 10-4.4{plus minus}0.2 MPa-4.2
s-1, and an activation energy of 260 {plus minus} 30 kJ/mol.
Microstructural observations show that plagioclase and pyroxene in
deformed samples develop noticeable intracrystalline plasticity and
shape preferred orientation (SPO). Electron backscatter diffraction
(EBSD) measurements demonstrate more significant crystallographic
preferred orientations (CPOs) of pyroxene and plagioclase in deformed
granulites compared to those in hot-pressed ones. By contrast, quartz
develops nearly random fabrics probably due to the simultaneous
activation of multiple slip systems during deformation. These features
indicate that dislocation creep dominates the deformation of these
minerals and felsic granulite, consistent with the obtained stress
exponent. Our data imply that a CLC consisting of felsic granulite is
weaker rheologically compared to the quartz-dominated upper crust and
the olivine-dominated uppermost mantle, which supports the ‘jelly
sandwich’ model for the strength of continental lithosphere.
Additionally, extrapolations of flow laws of our felsic granulite and
formerly-reported mafic granulite strongly favor the hypothesis of
delamination-induced decratonization of the North China Craton.