Open Session - SWISS GEOSCIENCE MEETINGs

62
Symposium 1: Structural Geology, Tectonics and Geodynamics
Figure 1. Swiss first and second order levelling network and annual height changes with respect to the reference bench mark in Aarburg
1. 1
Initiation of plate tectonics in the early Earth
Sizova Elena*, Gerya Taras*
* Institut of Geophysics, ETH-Zurich, Schafmattstr. 30, 8093 Zürich (sizova@erdw.ethz.ch)
Plate tectonics is an outstanding example of a self organizing complex system, driven by the negative buoyancy of the thermal
boundary layer resulting in subduction. Although the signature of plate tectonics is recognized with some confidence
in the Phanerozoic geologic record of the continents, its action becomes less certain further back in time. The best way to
improve our understanding of the early Earth is to combine our knowledge from petrological data and facilities of numerical
modeling.
Based on 2D petrological-thermomechanical numerical model of oceanic-continental subduction (using the I2VIS code
(Gerya & Yuen, 2003)) we determined sharp first order transition from modern style of subduction (fig1a) to transitional
“pre-subduction” tectonic regime (fig.1b) at upper mantle temperature rising by around 160 ºC above the present one. This
condition roughly corresponds to upper bound for Neoarchean (≈3Ga) mantle temperature (Davies, 1993; Komiya et al., 1999).
In the “pre-subduction” tectonic regime plates are strongly internally deformable by intense percolation of melts continuously
generated from underlying partially molten mantle. Their convergence results in shallow underthrusting of continental
crust by oceanic plate (fig 1b). Oceanic plate goes quite far (to 200 km) under continental crust moving away and folding
continental lithospheric mantle. In some experiments after the underthrusting oceanic plate starts to sink into the mantle
forming at first stages two-sided subduction. In contrast to present time in the hotter “pre-subduction” regime there is no
formation of backarc basin and new spreading center (the same was assumed by de Wit, 1998 for Archean time). Relatively
shallow and hot tectonic style dominates. Oceanic plates weakened by sub-lithospheric melts are subjected to buckling and
shallow underthrusting (rather then subduction) associated by local melting. This triggers formation of characteristic metamorphic
and magmatic rocks found in Archean (such as, ultra-high-temperature and eclogite-high-pressure granulites, adakites
and granitoids).