B-type olivine fabric induced by grain boundary sliding

- We document olivine fabric across a mantle shear zone in the Ronda peridotite.
- We highlight a fabric transition from A-type to B-type across the shear zone.
- Our data indicate that B-type fabric results from Grain Boundary Sliding.
- Our findings support experimental data and question the origin of mineral fabric.

Mineral Lattice Preferred Orientation (LPO) in naturally deformed peridotite is typically interpreted to result from the motion of intra-crystalline dislocations during dislocation creep. The degree of mineral alignment (fabric strength) is often assumed to increase with increasing finite strain. Here, we document olivine LPOs in peridotites in a kilometer-scale mantle shear zone in the Ronda massif (Spain) that demonstrate a transition from a flow-parallel [100]-axis LPO (A-type fabric) to a flow-normal [100]-axis LPO (B-type fabric). We conclude that the B-type fabric in the Ronda peridotite results from the enhancement of grain boundary sliding (GBS) with decreasing grain size, rather than a change in the dominant dislocation slip system because: (1) dislocation sub-structures remain consistent with the A-type slip system in all samples; (2) the fabric transition correlates with decreasing fabric strength despite increasing finite strain; (3) the analysis of deformation conditions in Ronda is inconsistent with the experimental conditions, including water content, inferred to promote the dominance of (010)[001] slip in laboratory samples, and (4) our observations are supported by experiments that document B-type fabric in olivine aggregates where deformation involves a component of GBS. Our results have important implications for interpreting the rheological properties of shear zones and upper mantle structures via micro-structural and seismic observations.