Influence of water on rheology and strain localization in the lower continental crust

Abstract

[1] We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnïya, Norway. Over a distance of $1 m, the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice-preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (a H2O ) during deformation must have been high ($1) in the sheared gabbro compared with the nonhydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promote strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.

High‐temperature fracturing and subsequent grain‐size‐sensitive creep in lower crustal gabbros: Evidence for coseismic loading followed by creep during decaying stress in the lower crust?

“…The estimated stress is comparable with the creep stress (<0.1 MPa) for a long‐term viscous deformation during a seismic cycle using geodetic strain measurements and the viscosity estimates proposed by Rybacki and Dresen []. It is also comparable with the stress estimated during low‐frequency earthquakes at the base of the crust along the San Andreas Fault, where the earthquakes are triggered by a tidally induced shear stress of 1.77 × 10 −4 MPa (differential stress of ~3.5 × 10 −4 MPa) [ Thomas et al , ; Getsinger et al , ]. Such low shear stresses suggest that the microstructures in the Hasvik gabbro shear zones formed during a large local stress drop related to postseismic stress relaxation.…”

Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

“…Therefore, we do not expect a great deal of direct evidence, but consider the AG-fabric to be a good indication of the possibility of the a-c switch in nature. A SPO and a CPO with a [001] maximum aligned subparallel to the shear direction were reported in both experimentally sheared amphibole (Getsinger & Hirth, 2014) and amphibolite-grade natural shear zones (Getsinger et al, 2013). A similar girdled olivine CPO was observed in mantle xenoliths from the Ontong Java Plateau mantle root (Tommasi & Ishikawa, 2014).…”

Strain localization during high temperature creep of marble: The effect of inclusions

“…Strain localization in the brittle regime may be described using damage rheology, continuum models or fracture mechanics-based approaches (Ashby and Sammis, 1990;Lyakhovsky and Ben Zion, 2014;Rudnicki and Rice, 1975). In the ductile field, localization may occur under varying thermodynamic boundary conditions and it may be associated with strain-dependent processes such as shear heating and melting, a change in the constitutive behavior by grain size reduction, enhanced water activity, or by the formation of a layered fabric in polyphase rocks (e.g., Montési, 2013;Getsinger et al, 2013).…”