Mafic granulite rheology: Implications for a weak continental lower crust

The “jelly sandwich” model of continental rheology, long the consensus model, has recently been challenged. The controversy is based on the rheology of mafic granulite that is thought to make up most of the lower continental crust. However, because of technical issues in experimental rock deformation, there are very few data on this problem. Here we report the rheology and fabrics of a reconstituted, fine-grained nominally dry (i.e., no hydrous mineral) mafic granulite (57% plagioclase+24% clinopyroxene+14% orthopyroxene+5% opaque minerals, 0.16-0.28 wt% H2O) deformed at 1.1-1.2 GPa pressure in a modified Griggs-type deformation apparatus. The rheology of this mafic granulite can be described by the constitutive equation of ε̇=Aσ3.2±0.4exp(−(244±35kJ/mol)/RT) where ε̇ is in s−1, σ in MPa, T in Kelvin and A=10−2.0±1.6 MPa−3.2 s−1. Our results provide new experimental evidence in support of the “jelly sandwich” lithosphere strength model and imply that mafic granulite with a moderate amount of water (>0.05-0.08 wt% H2O) is likely to be a weak layer in the lithosphere. Both pyroxenes and plagioclase develop pronounced fabrics in responding to axial deformation. The deformation mechanism is dislocation creep with (100)[001], and (001)[100]/(010)[100] being the dominant slip systems for pyroxenes and plagioclase, respectively. The low strength of mafic granulite is ascribed largely to the significant weakening effect of dissolved water in pyroxenes and plagioclase. A weak continental lower crust has many important implications for geodynamics of crust-mantle interactions, such as lower crust channel flow and delamination.

► Continental lower crust is likely a weak layer in the lithosphere. ► The weakness of granulite can be attributed to water in pyroxenes and plagioclase. ► The rheology of untreated mafic granulite can be described by ε̇=10−2.0σ3.2exp(−(244kJ/mol)/RT). ► Both pyroxenes and plagioclase are deformed by dislocation creep in granulite.