Water content and rheology of continental lower crust

Abstract:

The water content and rheology of continental lower crust is crucial for understanding the deformation of continental lithosphere. The rheological stratification strength of the continental lithosphere is traditionally thought to be consistent with the ‘‘jelly sandwich’’ model (a weak lower crust sandwiched by a strong brittle upper crust and a strong plastic lithospheric upper mantle). However, significant seismic activities in some parts of thickened continental lower crust indicate that the rheological strength of continental lower crust can be spatially and temporally heterogeneous. A key question to the controversy is whether the lower crust is dry or wet. Here we report water content and fabric of layered gabbros from the Panzhihua of South China (unaltered Triassic underplating continental lower crust) and granulites from the Manjinggou of North China (Archean continental lower crust). Both rocks show significant amount of water (several hundreds to one thousand ppm) in their constituent nominally anhydrous minerals (clinopyroxene, orthopyroxene and plagioclase). There is a significant difference in water content and partition coefficient between clinopyroxene and plagioclase in lower crusts, which also affects the fabrics and rheological strengths of lower crustal minerals. Our results suggest that both Archean continental lower crust and underplating continental lower crust are both wet and weak rather than being dry and strong.