STRUCTURAL AND MICRO-STRUCTURAL ANALYSIS OF A PALEO-TRANSFORM FAULT ZONE IN THE NEYRIZ OPHIOLITE, IRAN

On-land exposures of fracture zone related ophiolites provide an excellent opportunity to compare rocks and structures in these paleo-transform fault systems with the observations from the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). This comparison is essential to obtain new information on the kinematics and mechanisms of deformation within oceanic fracture zones. An ENE-trending and steeply dipping possible fossil oceanic fracture zone, consisting mainly of harzburgite and gabbros, occurs in the Neyriz ophiolite in SW Iran. Scattered exposures of pillow lavas, sheeted dykes and mafic plutonic rocks crop out in the eastern side of the fracture zone. Mafic rocks within the fossil fracture zone display C-, C'- and S-bands foliations consisting of amphibole- and feldspar-rich bands. Deformed hornblende porphyroclasts are observed to show symmetrical tails or s -type porphyroclast systems in XY thin sections. Some mantled porphyroclasts also show anticlockwise stair-stepping rotation and oblique foliation with asymmetrical complex stripes and wings, forming d -type porphyroclasts systems. Hornblende c-axis of high-grade S-C mylonites exhibit strong lattice preferred orientation (LPO) with M-and G-type origin. The mean orientation of harzburgite tectonite foliations in the peridotites and the measured mean trends of the LPO patterns of hornblende c-axis in mylonitic mafic rocks are generally ENE. The LPO patterns of hornblende c-axis show weakly to moderately developed asymmetrical girdles with respect to foliation. These microstructural observations collectively indicate a sinistral sense of shearing that affected the mafic rocks within the fracture zone. Regional tectonics of the Neyriz ophiolite suggests that this fracture zone acted as a left-lateral transform fault, which was part of a slow-spreading (4.25 cm/y) system. The active spreading center segment was located to the NW of the current exposure of the ENE-trending fossil oceanic fracture zone. The trend of the inferred fossil oceanic fracture zone in the Neyriz ophiolite mimics the general orientation of the fracture zones of the Carlsberg Ridge system in the Indian Ocean.