IS THE MENDERES MASSIF IN TURKEY ONE BIG NEOGENE SHEAR ZONE?

The Menderes Massif in western Turkey exposes the deepest tectonic units of the Tethyan orogen in western Turkey, a composite nappe stack comprising Precambrian and Phanerozoic tectonic units that were assembled during Eocene to Oligocene crustal shortening. Within the orogen the Cycladic Blueschist unit can be correlated from the Aegean – where it overlies the External Hellenides – into western Turkey, where it overlies the Menderes nappes. This is a significant difference, as the pre-collision tectono-metamorphic history of the External Hellenides and the basement rocks in the Anatolides are very different. This change in structural evolution along strike of the Alpine orogen requires the existence of a structure at depth that separates the Aegean and Anatolia.

Here we propose that a NE-trending lithosphere scale discontinuity exists between the Aegean Sea and western Turkey that has operated as a sinistral wrench zone since at least the Miocene. While this discontinuity does not represent a very obvious feature in the crustal architecture we argue that there are several lines of evidence that support our preposition. To this end we review published and new data regarding the local structure of the mantle as imaged by seismic tomography, the shape and structural evolution of the Menderes Massif, the distribution of Miocene to Recent hydrothermal fluid flow systems, the distribution of earthquakes, and regional patterns in the age and chemistry of igneous rocks.

We argue that – following Alpine crustal shortening – the wrench zone stretched, exhumed, denuded and fragmented the Menderes Massif in a kinematic framework that is consistent with either a tear in the slab, and/or partial delamination of the lithosphere. Our model addresses some of the open questions in the regional geology, but also provides a hypothesis on how lithosphere discontinuities that are not at the scale of plate boundaries can control structural evolution in the crust.