THE ANKARA MELANGE, TURKEY: TECTONIC IMPLICATIONS FOR CLOSURE OF THE NEO-TETHYS

The Ankara mélange defines the suture zone between the Sakarya and Kirsehir continental blocks in north-central Turkey, and consists of metamorphic, limestone-block, and ophiolitic mélange units (in descending structural order). It represents a series of broken thrust sheets emplaced during the Late Cretaceous closure of a branch of the Neo-Tethyan Ocean. Remnants of this Neo-Tethyan seaway (Izmir-Ankara-Erzincan Sea) occur within the ophiolitic mélange unit and include serpentinized peridotites, cumulate to isotropic gabbros, quartz diorite, doleritic and plagiogranitic dikes, and volcanic rocks in km-size mega-blocks. The matrix of the ophiolitic mélange is made of sheared and locally brecciated serpentinite and/or a flysch unit. The minimum igneous age of the ophiolitic components of the Ankara mélange has been constrained as the early Middle Jurassic based on the zircon concordia age of 179±15 Ma from a plagiogranite dike. The geochemistry of these plagiogranite and coeval dolerite dikes suggests a depleted asthenospheric mantle source modified by subduction-derived hydrous components in a backarc environment (Dilek & Thy 2006).

Matrix and block relations in the Ankara mélange reveal an original tectonic setting similar to modern closure in Indonesia of backarc basins formed originally by slab rollback and later closed by collision. Although the age and paleogeography of the Izmir-Ankara-Erzincan backarc basin are still poorly constrained, we propose a new geodynamic model involving two phases of stratal disruption. The first was associated with fragmentation due to supra-subduction zone extension. The second phase initiated as the north-facing subduction zone was clogged by collision and strain was partitioned into the backarc that consumed most of the composite backarc basin floor. The Ankara mélange records the make-up of this backarc basin and provides new constraints for paleogeographic reconstructions of the northern margin of the Neo-Tethys.