The Vertical, Right‐Lateral, Pure‐Shear‐Dominated Transpressional Transform‐Fault Deformation Associated With the Oceanic Ridge of the Neyriz Ophiolite, Iran

ABSTRACT Structural mapping of 572 km 2 of the Neyriz Ophiolite was conducted to deepen understanding of the internal microstructures of deep transform faults associated with oceanic ridges. We examined the microstructural, structural and kinematic characteristics of an exposed vertical paleo‐oceanic transform fault within the Neyriz Ophiolite. This analysis identified microscale deformation and kinematic mechanisms and inferred the probable controls on oceanic transform faults. The Neyriz transform fault comprises two segments. The upper segment consists of Type I and Type II serpentinite phyllite and phyllonitic mélange. In contrast, the lower segment extends northeast–southwest and consists of high‐temperature, high‐shear‐strain amphibolite mylonite formed as a right‐lateral transpressional or thinning shear zone. The transform fault has undergone significant shear strain, reaching γ = 3.70. This shear strain is decomposed into 47% pure shear and 53% simple (or sub‐simple) shear, based on the estimated kinematic vorticity number ( W m = 0.73 ± 0.07). Amphibole porphyroclasts in the amphibolite mylonite and plagioclase are encased in a fine‐grained amphibole matrix interspersed with slender, elongated ribbons or narrow bands of plagioclase. Kinematic vorticity number measurements of the vertical, high‐shear‐strain amphibolite mylonite along the transform fault indicate pure‐shear‐dominated transpression ( W k < 0.81), with the mylonite oriented vertically and perpendicular (θ = 93°) to the ocean‐spreading ridge. The exposed mantle along the oceanic ridge shows that the harzburgite exhibits a sub‐vertical lineation in the centre and sub‐horizontal lineations on both sides of the mantle diapiric flow.