The structure of the lithosphere and upper mantle beneath the Eastern Mediterranean and Middle East

Dan Mckenzie

doi:10.1007/s42990-020-00038-1

Abstract

Surface velocity measurements show that the Middle East is one of the most actively deforming regions of the continents. The structure of the underlying lithosphere and convecting upper mantle can be explored by combining three types of measurement. The gravity field from satellite and surface measurements is supported by the elastic properties of the lithosphere and by the underlying mantle convection. Three dimensional shear wave velocities can be determined by tomographic inversion of surface wave velocities. The shear wave velocities of the mantle are principally controlled by temperature, rather than by composition. The mantle composition can be obtained from that of young magmas. Application of these three types of observation to the Eastern Mediterranean and Middle East shows that the lithosphere thickness in most parts is no more than 50-70 km, and that the elastic thickness is less than 5 km. Because the lithosphere is so thin and weak the pattern of the underlying convection is clearly visible in the topography and gravity, as well as controlling the volcanism. The convection pattern takes the form of spokes: lines of hot upwelling mantle, joining hubs where the upwelling is three dimensional. It is the same as that seen in high Rayleigh number laboratory and numerical experiments. The lithospheric thicknesses beneath the seafloor to the SW of the Hellenic Arc and beneath the NE part of the Arabian Shield are more than 150 km and the elastic thicknesses are 30–40 km.

Highlights

Since its discovery in 1967 it has been clear that plate tectonics only provides a good description of oceanic plate boundaries
The elastic thickness, Te, of much of the oceanic lithosphere is 20–30 km, which is sufficiently large to suppress the surface expression of any mantle convection whose wavelength is less than about 500 km. These properties of the oceanic lithosphere explain why plate tectonics can be so successfully applied to the oceans
Large regions have lithospheric thicknesses of less than 100 km and elastic thicknesses of no more than 5 km. Within such regions the surface elevation, the gravity field and the volcanism reflect any convective motions in underlying mantle

Summary

Introduction

Since its discovery in 1967 it has been clear that plate tectonics only provides a good description of oceanic plate boundaries. Africa and Europe in the Eastern Mediterranean was taken up by the rapid westward motion of Anatolia and by subduction of the Eastern Mediterranean beneath the Hellenic Arc, and not by shortening on east–west striking thrusts This behaviour accounted for the strike slip motion on the North and East Anatolian Faults that had been mapped by Ketin (1948) and Şaroğlu (see Emre et al 2018). Though the westward motion of Anatolia was first discovered from the surface displacements on active faults and the slip vectors of earthquakes, it has been spectacularly confined by GPS and InSAR measurements (e.g. Weiss et al 2020) These have shown that the strain resulting from the motion of Anatolia is confined to a zone centred on the trace of the fault whose width is about 60 km, and that the interior of the Anatolian Plate is scarcely being deformed or elastically strained. D’Agostino et al (2020) have used GPS measurements to show that the present rate of clockwise rotation of fault-bounded blocks in Greece is similar to that obtained from paleomagnetism

Background

Lithospheric structure

Convective structures below the lithosphere

NW Zagros

Findings

Conclusions