Open Session - SWISS GEOSCIENCE MEETINGs

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Symposium 1: Structural Geology, Tectonics and Geodynamics
Starting in early Paleogene, and accelerating in early Miocene, deformation is a strong ongoing process throughout the
whole Caucasus. Located since Paleogene in the paleogeographic realm of the Eastern Paratethys, it remains unclear at
which time the Caucasus emerged above sea level. The zone of highest topography is thought to be the zone of fasted uplift.
Uplift may be linked with N-verging backthrusting, possibly over a ramp system at depth as suggested by seismicity. A possible
candidate obove which the major topography develops could be the Main Caucasus Thrust (MCT). The initial Kura basin
is dissected by an accretion prism into the southern Kura basin and the northern Alasani basin. Deep seated southward
migration of the orogenic front, causing the accretion prism, led to the inversion of the Pliocene to Late Pleistocene sediments,
and the transport of the Alasani basin as a piggy back basin towards the south. The northern foreland basin (Terek)
subsided since the early Pliocene more than 4,000 m, and recently exhibits pitted gravels of Early Pliocene age at 1,600m
elevation. It is however, unclear how strain is partitionned between the active front (to the N and the S) and the zone of fastest
uplift, or the MCT.
Figure: Geological map of the Caucasus area
1. 1
Numerical modelling of subduction initiation at passive margins: Critical
effects of continental mantle strength and density
Nikolaeva Ksenia*, Gerya Taras*, Marques Fernando**
*ETH-Zurich, Institute of Geophysics, Schafmattstrasse 30, 8093 Zurich (nikolaeva@erdw.ethz.ch)
**Univ. Lisboa, Edificio C6, Pisco 2, 1749-016 Lisboa, Portugal
Despite its crucial significance for the plate tectonics theory, subduction initiation remains a largely enigmatic open problem.
It has been showed that at most passive margins elastic and frictional forces exceed gravitational instability and ridgepush
forces, which preclude subduction initiation. From this point of view, the Atlantic Brazilian margin, characterized by
a relatively thin lithosphere, represents a rare case where force balance is favourable for subduction initiation, which is
evidenced by several tectonic/thermal features. In relation to this natural case we studied numerically physical parameters
that control tectonic processes at passive margins characterized by thinned to normal continental lithosphere. The investigated
parameters are thermal age of the oceanic plate, temperature structure (thickness) and density of the continental lithosphere,
and the rheology of crust and mantle. Our experiments show that three different geodynamic regimes can be
discriminated: (1) stable margin, (2) overthrusting and (3) subduction. Both overthrusting and subduction are driven by inherent
gravitational instability of the passive margin due to the strong density contrast between the continental crust and
adjacent oceanic lithosphere. This instability forces continental crust to thrust over the oceanic plate. In the case of over-