Propagation of a strike slip plate boundary within an extensional environment: the westward propagation of the North Anatolian Fault

Abstract

The Sea of Marmara marks a key point in the propagation of the North Anatolian Fault (NAF) toward the northern extremity of the Aegean subduction during the last 12-11 Myr. There is no indication that a localized plate boundary existed to the west of it, north of the Aegean portion of the Anatolia plate, before 2 Ma. Prior to 2 Ma, the shear produced by the motion of Anatolia-Aegea with respect to Eurasia was distributed over the whole width of the Aegean-West Anatolian western portion. This was most probably related to the presence there of a N/S component of extension produced by the southward migration of the Aegean-West Anatolian subduction. Fast subduction of the oceanic Ionian lithosphere had been initiated 15 Ma ago, a few million years before the Anatolia westward motion began 11 Ma ago. These two processes are obviously tightly linked. We document the establishment of the Aegea-Anatolia/Eurasia plate boundary in Plio-Pleistocene time in this geodynamic context. We show that the beginning of the formation of a localized plate boundary occurred between 4.5 and 3.5 Ma ago at the location of the present Sea of Marmara by the initiation of a shear zone comparable to the Gulf of Corinth one in Central Greece. Thus the first part of the formation of the Sea of Marmara was purely extensional. We discuss the transition from the initial extensional basins to the present strike-slip system, that today cuts across the whole Sea of Marmara and that is called Main Marmara Fault (MMF). We show that the beginning of its development is not earlier than 2.5 Ma ago. Shortly after, the plate boundary migrated west of the Sea of Marmara along the northern border of Aegea from the North Aegean Trough, to the Gulf of Corinth area and to the Kefalonia fault. There, it finally linked with the northern tip of the Aegean subduction zone, completing the system of plate boundaries delimiting the Anatolia-Aegea plate. We have related the remarkable change in the distribution of shear over the whole Aegea from Miocene to Pliocene to the formation of a relatively undeforming block in Pliocene that forced the shear to be distributed over a narrow plate boundary to the north of it. We attribute the formation of this relatively undeforming Aegean block to the northeastward progression of the cold oceanic Ionian slab. We propose that the slab cuts the overlying lithosphere from asthenospheric sources and induces a shortening environment over it.