RESEARCH

Hornblende's mechanisms of texture formation

The relation of seismic anisotropy to the deformation and flow of the lower crust relies on the intrinsic properties
of the minerals in the crust and on the formation of crystallographic preferred orientation (CPO). Hornblende is an important constituent of the continental and oceanic lower crust and often exhibits a strong CPO. However, the formation of CPO in hornblende is not well understood, and it is difficult to uniquely relate its texture and anisotropy to conditions of metamorphism-deformation. Petrofabric analysis, was done using electron backscatter diffraction (EBSD), of two hornblende-rich amphibolites from the Mamonia complex (Cyprus) with peak pressure and temperature of 0.6 GPa and 610 ± 40 ◦C. We distinguish between microstructures with varied strains and show that, while the common hornblende CPO—in which the [001] is aligned with lineation—is independent of strain, its symmetry does depend on strain: it is hexagonal (axial-[001]) under low strains but becomes orthorhombic ([001] and (100) parallel to the lineation and foliation normal of the deformation, respectively) under increasing strains. This transition is correlated with a decrease in the shape-preferred orientation and an increase in the intragrain misorientation, whose rotation is consistent with the easy slip system (100)[001] of hornblende. Thus, we interpret the hexagonal CPO to represent a metamorphic fabric (a
texture formed under quasi-static conditions) and the orthorhombic CPO to represent a solid-state deformation
fabric (a texture formed by strain accommodated via dislocation-mediated deformation). We consider the seismic anisotropy of the natural hornblende textures and compare two tectonic scenarios of lower crust deformation - crust thickening and channel flow with marked differences in the expected seismic anisotropy between the two scenarios. Taken together, the dataset presented here can be used as a new framework for understanding and interpreting natural textures and seismic anisotropy from the lower crust.

Topaz et al., Tectonophysics, 2023

Hornblende grains cut by a calcite vein in blue (EBSD map; sample from Aya Varvara, Cyprus).

Hornbelnde grain is surrounded by small grains with similar orientations (left) and intra-grain misorientations at one of its limbs (right). Rotation suggests the activity of plastic deformation mechanism.

Fig. Topaz_lower crust seismic anisotropy.jpg

Implications for seismic anisotropy in the lower crust. Here two end-members of flow in th lower cryst, lateral flow and radial flow, and the expected seismic signature.

Deformation of pre-textured amphibolite

The textural and rheological properties of amphibole are of relevance for assessing the physical properties of different tectonic provinces. Aggregates containing amphibole grains often exhibit a strong texture, i.e., a crystallographic preferred orientation (CPO). Since amphibole possesses inherent anisotropic properties, the CPO will affect the bulk strength and elastic properties. However, amphibole’s rheological behavior is not well understood as its capability to deform purely via plastic deformation remains unresolved, previous studies suggesting numerous deformation mechanisms such as semi-brittle and cataclastic flow, dissolution precipitation, dislocation creep, recrystallization, micro-twinning, and diffusion assisted creep. Here, we use pre-textured natural samples cored at 60° to the foliation and lineation to investigate the deformation mechanism/s activated in a polycrystalline aggregate/rock of well-oriented amphibolite-rich hornblende. Samples from the Mamonia complex (Cyprus) with hornblende as the dominant mineral (> 70 % modal fraction) and strong initial alignment of the [001] axis were deformed using a Griggs-type solid-medium apparatus. Experiments were run at 1 GPa confining pressure, temperatures of 400 to 800 °C, and a strain rate of ~10-5 1/s. Samples show temperature-dependent differential stress that falls below the Goetze criteria (i.e., below the confining pressure, 1 GPa) - ~700, 500, and 200 MPa for samples deformed at 400, 600, and 800 °C, respectively. Microstructural analysis using Electro backscatter diffraction (EBSD) reveals folding and kink bands, accommodated by both plastic mechanisms, via dislocation glide on the hornblende easy slip system, and brittle mechanisms, via micro-fracturing along the crystal cleavage (110). We discuss the implications of the interplay and contribution of different deformation mechanisms for our ability to translate laboratory experiments to flow laws for the lower mantle and subduction zone interfaces.

Boneh, Incel, Renner, EGU, 2022