Amphibole and lower crustal seismic properties

Abstract

Measurements of seismic anisotropy frequently offer insights of the deformation kinematics in-situ within the Earth. In the continental crust, seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation (LPO) of mica crystals. However, determinations of composition suggest that the deep continental crust contains rather little mica. In this contribution, the role of amphibole, a far more plausible constituent, especially for deep continental crust of basic composition, is assessed. A field analogue approach is adopted, where the LPO are measured, correlated with the strain state of the rocks, and used to calculate the seismic properties. The seismic properties may be up-scaled through the specimen using the modal proportions of the constituent mineral phases. To examine the role of composition in controlling the intensity of seismic anisotropy, seismic properties are computed using an array of modal compositions (so-called ‘rock recipes’). The case history comes from the Central Block of the Lewisian of NW Scotland, where a regional basic dyke swarm (the Scourie dykes) is locally deformed within amphibolite facies shear zones, conditions that are representative of the deep crust. Shear strain (γ) profiles across a 100 m wide shear zone, using deflected banding in the host gneisses, yield values of 0<γ<15 or greater. A Scourie dyke deflected into the shear zone exhibits hornblende with distinct preferred dimensional orientations representative of such strains. In contrast, plagioclase (and quartz) clots are almost totally insensitive to deformation (due to grain-size sensitive creep deformation) and yield approximately constant apparent strains of γ=1–2. Hornblende–plagioclase–quartz LPOs, combined in their modal proportions and modulated by their individual single-crystal elastic properties, define the seismic properties of the shear zone. Rock recipe modelling demonstrates that it is the hornblende that dominates the seismic response and that plagioclase and quartz serve simply to dilute the intensity of anisotropy. The values calculated indicate significant (up to ~13%) seismic anisotropy for strongly sheared amphibolites. Thus amphibole, aligned through deformation, is likely be a major contributor to the seismic anisotropy of the deep continental crust.