Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations

TLDR

Rock‑mechanics experiments, geodetic strain‑transient observations, and structural studies of exhumed ductile shear zones together illuminate the style and rheology of deep crustal and upper‑mantle deformation. Laboratory, geodetic, and field studies collectively show that deformation mechanisms and viscosities in the lower crust and upper mantle vary strongly with stress, grain size, fluids, and local fault‑zone conditions, leading to localized shear zones of reduced grain size and strength that evolve over short spatial and temporal scales.

Abstract

Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro- and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and strength. Deformation mechanisms and rheology may vary over short spatial (shear zone) and temporal (earthquake cycle) scales.