THE ZAGROS COLLISION ZONE: WHAT DO WE LEARN FROM INNER AND OUTER ZAGROS OPHIOLITIC BELT?

Much about the evolution of upper Cretaceous Zagros ophiolites – which define the suture between Arabia and Eurasia - remain poorly known. Zagros ophiolites comprise two parallel belts, including Inner Zagros Ophiolitic Belt (IB), along the SW periphery of the Central Iranian block and Outer Zagros Ophiolitic Belt (OB), south of Main Zagros Thrust Fault. IB and OB ophiolites formed at the same time: IB hornblende gabbros yield a K/Ar age of 93 Ma whereas OB diabases and hornblende gabbros yield Ar/Ar ages of 86-93 Ma, similar to ages of other ophiolites of the Mediterranean-Oman belt (Oman ~95 Ma, Cyprus ~90-94Ma). Pelagic limestones resting conformably on IB and OB ophiolites are Turonian-Maastrichtian (93.5-65.5 Ma).  Several lines of evidence from both inner and outer belt Zagros ophiolites indicate formation above a subduction zone, such as high Cr content of harzburgite spinel and flat REE patterns of the lavas with negative Nb-Ta anomalies, also similar to other Tethyan Oman-Mediterranean ophiolites. These similarities in ophiolite ages and tectonic setting suggest that the Oman-Mediterranean ophiolite belt represents a remarkably coherent lithospheric block. Zagros ophiolites lie between the Zagros fold-and-thrust belt, an accretionary prism that formed by off-scraping of Arabian Plate sediments, and the Urumieh-Dokhtar arc, an Andean-type arc that is waning as the region transitions from subduction to collision. The parallel alignment of Zagros ophiolite belts between the Urumieh-Dokhtar arc (to the north and the Zagros fold-and-thrust belt (to the south), further suggests that IB and OB ophiolites represent remnants of a once continuous fore-arc basement, now exposed as limbs of a deformed anticlinoria. In this interpretation, IB and OB represent inner and outer portions of the Late Cretaceous Iranian fore-arc, developed over an incipient N-dipping subduction zone on the southern Eurasian margin.