PhD Thesis
Abstract
Within many mountain ranges slivers of preserved oceanic lithosphere evidence tectonic processes responsible for their emplacement on top of the continental crust. The first order anomaly inherent to this phenomenon is that dense rocks (ρ>3) end up on top of less dense rocks (ρ≈2.7). The processes responsible for such a tectonic oddity remain uncertain.
The ophiolites of the Lesser Caucasus and NE Anatolia are prime examples of this phenomenon, tectonic transport of fragments of oceanic lithosphere is evidenced on the entire continental marge (>1000 km) around 90 Ma.
The multidisciplinary approach used throughout the study of the ophiolites of the Lesser Caucasus and NE Anatolian regions yielded clues specify the evolution of the Tethys and consequently the obduction of the ophiolites. These results include:
- The existence of a continuous oceanic domain, from East to West, separating Eurasia (to the North) from the South Armenian Block-Taurude-Anatolide Platform (to the South). This ocean was subducting towards the North under the Eurasian margin.
- The creation of oceanic crust in a supra-subduction to back-arc setting, through a slow ocean spreading process between Early Jurassic and Early Cretaceous times (c. 180~150 Ma).
- Nearly simultaneous obduction along the Izmir-Ankara-Erzinncan and Sevan-Akera suture zones of the ophiolites during Turonian-Coniacian-Santonian times (94~85 Ma).
- Directly under the ophiolites, metamorphic units (ophiolitic sole) allow us to constrain the emplacement of the ophiolites.
- Volcanism in the South Armenian Block allows us to propose an unprecedented evolution of the tectonic structures between c. 150~130 Ma.
This dataset strongly suggests common emplacement of the ophiolites of the study area, resembling a thrust sheet. This would be one of the biggest ophiolite nappe complexes in the world (outcropping in a mountain range).
Numerical modeling validates the hypothesis that emplacement of the ophiolitic nappe is due to particular thermal conditions. It suggests that in order to obduct old oceanic lithosphere obduction it needs to have a thermal state close to that of young oceanic lithosphere (0-40 km). Such a thermal rejuvenation is supposed for the ophiolites of the Caucasus s.l. evidenced by alkaline lavas emplaced on the ophiolite prior to the obduction event during the Late Cretaceous. Resulting seamounts and/or oceanic plateaus upon entery of the subduction zone under Eurasia would block it. This is compatible with the observed gap in the volcanism along the Eurasian margin as well as the obduction event on the South Armenian Block-Taurude-Anatolide Platform and Arabia.
(The complete manuscript is available on "Thèses-En-Ligne".)
