B-type olivine fabric induced by grain boundary sliding

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.