Zagros Collision Research Articles

Mediterranean extension and the Africa‐Eurasia collision

A number of tectonic events occurred contemporaneously in the Mediterranean region and the Middle East 30–25 Myr ago. These events are contemporaneous to or immediately followed a strong reduction of the northward absolute motion of Africa. Geological observations in the Neogene extensional basins of the Mediterranean region reveal that extension started synchronously from west to east 30–25 Myr ago. In the western Mediterranean it started in the Gulf of Lion, Valencia trough, and Alboran Sea as well as between the Maures massif and Corsica between 33 and 27 Ma ago. It then propagated eastward and southward to form to Liguro‐Provençal basin and the Tyrrhenian Sea. In the eastern Mediterranean, extension started in the Aegean Sea before the deposition of marine sediments onto the collapsed Hellenides in the Aquitanian and before the cooling of high‐temperature metamorphic core complexes between 20 and 25 Ma. Foundering of the inner zones of the Carpathians and extension in the Panonnian basin also started in the late Oligocene‐early Miocene. The body of the Afro‐Arabian plate first collided with Eurasia in the eastern Mediterranean region progressively from the Eocene to the Oligocene. Extensional tectonics was first recorded in the Gulf of Aden, Afar triple junction, and Red Sea region also in the Oligocene. A general magmatic surge occurred above all African hot spots, especially the Afar one. We explore the possibility that these drastic changes in the stress regime of the Mediterranean region and Middle East and the contemporaneous volcanic event were triggerred by the Africa/Arabia‐Eurasia collision, which slowed down the motion of Africa. The present‐day Mediterranean Sea was then locked between two collision zones, and the velocity of retreat of the African slab increased and became larger than the velocity of convergence leading to backarc extension. East of the Caucasus and northern Zagros collision zone the Afro‐Arabian plate was still pulled by the slab pull force in the Zagros subduction zone, which created extensional stresses in the northeast corner of the Afro‐Arabian plate. The Arabian plate was formed by propagation of a crack from the Carlsberg ridge westward toward the weak part of the African lithosphere above the Afar plume.

Petrography and petrochemistry of some zagroside geosynclinal calcarenites from north-eastern Iraq

This paper traces the general chrono-petrographic and -petrochemical trends in the evolution of the sand- and silt-sized carbonate clastic beds of the geosynclinal basins bordering the Zagros orogen in north-eastern Iraq. This orogen was repeatedly uplifted from at least the Late Cretaceous to the present. Clastics in older marginal basins, particularly those near the uplifted orogen, were themselves uplifted and contributed debris to younger basins. All successive basins were therefore filled mostly with carbonate lithoclasts and to a lesser extent by a debris of radiolarian chert. Clastic grains of feldspar, monocrystalline quartz as well as volcaniclastic debris were subordinate. Twenty-two samples collected from calcarenites of the most important Cretaceous to Pliocene Zagros marginal basins were chemically analysed for their major elements and for Cr, Cu, Ga, Ni, Rb, Sr, V, Zn and Zr. In clastics of all these ages, elements such as Al, Ti, Fe, P, Zr, Ga, V and K correlate fairly to crudely with each other as they are particularly concentrated in the detrital non-carbonate particles, viz. ilmenite, magnetite, apatite, zircon, feldspar and clay minerals. On a Ti-Cr-Zr triangular diagram, plots of calcarenites of the same formation tend to cluster together on a loop in an order corresponding to their age. Zr progressively increases in the younger clastics while Ti relatively increases in the older sediments irrespective of whether they were metamorphosed or not. The increase in Ti is suggestive of an earlier basaltic activity. Cr had its maximum relative concentration in the Early to Middle Miocene clastic formations, indicating that the ophiolitic slivers caught up in the initial Zagros collision suture had the maximum exposure to weathering during this interval.

Reactivation of basement faults and crustal shortening in orogenic belts

During the early evolution of some fold mountain belts old listric normal faults in a stretched and thinned basement beneath the sedimentary column may become reactivated as thrust faults. The reversal of motion on these faults allows considerable shortening to occur without subduction or excessive thickening of continental crust. This reactivation hypothesis is supported by the present-day seismicity of the Zagros collision zone and the structure of sedimentary basins.

Finite element modeling of lithosphere deformation: The Zagros collision orogeny

In the Zagros Mountains a formerly stable continental margin is being suddenly deformed. Finite element models of this tectonic flow are used to relate known surface deformations, earthquake locations, and fault plane solutions to unknown rock flow parameters and driving forces, in order to determine the latter. These two-dimensional plane-strain flow models incorporate the effects of nonlinear dislocation creep, frictional faulting, geological inhomogeneity, density anomalies, and varying temperature. All models driven by a subducting slab are unsuccessful because they require subduction of continental crust, which does not match present seismicity. Therefore the former oceanic slab must be detached, and the orogeny must be driven by horizontal compression in the lithosphere. Models also show that the subcrustal lithosphere is not shortened but acts as a stabilizing foundation. These results imply a simple geometry of crustal deformation which can be analytically modeled. The creep strength of the lower crust (75–100 bars) determines the topographic slope of the Zagros. The fact that subduction is not occurring on the old plate boundary places a limit on the shear stress deforming the cold upper crust. This limit is 300 bars if there is limestone at depth in the Crush Zone; otherwise 800 bars. These results are confirmed by a final finite element model. The total driving force of the orogeny associated with these limits is 2.8–5.5 · 10 15 dyne/cm, and the smaller amount could be provided by the gravitational spreading of the Red Sea rift. Shear-strain heating caused by the orogeny to date is less than 20°C. These results imply that even unheated continental crust is considerably weaker than laboratory friction measurements imply, and that it is mechanically decoupled at the Moho from the stronger mantle lithosphere.