The Makran, Southeastern Iran: the anatomy of a convergent plate margin active from Cretaceous to Present

Abstract

Summary The inland geology of the Iranian Makran, long neglected by geologists because of its lack of hydrocarbon potential, is now reasonably well known following a regional mapping programme carried out on behalf of the Geological and Mineral Survey of Iran. The mountain range can be divided into seven geotectonic provinces forming an arc round the late Pliocene epeirogenic Jaz Murian Depression at the southern end of the Lut block. From the edge of the Jaz Murian outwards to the S these provinces are: (1) a marginal basin in which well-preserved ophiolites formed and deep-water pelagic sediments were deposited from Jurassic to Palaeocene; (2) a narrow zone of continental crust of Palaeozoic metamorphics capped by shelf limestones of mainly Cretaceous age but including Carboniferous, Permian, Jurassic and Palaeocene developments; (3) a zone of ophiolitic mélange, the renowned Coloured Mélange; (4) a zone of immensely thick Eocene-Oligocene flysch; (5) a similar zone of Oligocene-Miocene flysch; (6) a southern zone of Miocene neritic to molassic sediments and (7) a Miocene-early Pliocene neritic zone W of the Zendan fault. The structure is dominated by steep inward-dipping reverse faults forming schuppen. These developed in a series of events which climaxed in the late middle Miocene with some faults continuing to be active to the present day with continuing uplift. Intense periclinal folds of dominantly chevron style are developed in the flysch, synchronous with the faulting. Over substantial areas in the flysch deformation has been so intense that a mélange has developed, resembling some Franciscan mélanges more closely than the Coloured Mélange. It contains exotic blocks which are tectonically intruded from the Coloured Mélange, which forms the basement to the flysch zones. The most significant discovery of this programme is the continuation into the Makran of the Sanandaj-Sirjan zone in the form of the Bajgan-Dur-Kan complexes (province 2 above), thus forming a sliver of continental crust that stretches from the Bitlis Massif in Turkey to the Makran. This separates the two ophiolitic developments of the Makran: to the south the Coloured Mélange, the trench margin sequence of a north-dipping subduction zone formed mainly in Late Cretaceous-Palaeocene time, and to the north a belt of well-preserved Cretaceous-Palaeocene ophiolites formed in a marginal basin. The latter ophiolites may be the equivalent of the island arc of this subduction system. The Makran was not involved with the third group of ophiolites in this region, those of the Oman, Neyriz and Kermanshah, which were emplaced as a result of collision of the Arabian continental margin with an intra-oceanic NE-dipping subduction zone in the Campanian. Any association of these ophiolites with the Coloured Mélange along the Zagros is due to the Pliocene collision of Arabia and Iran. Following uplift of the Inner Makran in the late Palaeocene northward subduction has continued to the present day with related andesitic volcanism to the north and migration of the trench to the south following tectonic events in the Oligocene and middle Miocene, so that the trench is now 300 km from the present andesitic volcanism. The products of Eocene to Present subduction, including the immense deformed flysch deposits, are thus superimposed on the condensed Cretaceous-Palaeocene subduction system.