Open Session - SWISS GEOSCIENCE MEETINGs

Further increase in the mantle temperature (by around 250 o C above present) causes transition from “pre-subduction” regime
to “horizontal-tectonics” regime (fig 1c). At this stage horizontal movements of small deformable plate fragments are predominant
and even shallow underthrusts do not form under imposed convergence.
Our experiments also show that apart of mantle temperature another crucial parameter controlling the tectonic regime is
degree of lithospheric weakening induced by upward movement of extracted sub-lithospheric melts. At the high Archean
mantle temperatures when melts are always present in the upper mantle lithospheric weakening by melts should be low to
preserve coherency of the plates and to allow stable subduction. Neither increasing radiogenic heat production nor lithospheric
weakening by slab derived fluids has notable effects for the identified transitions in the early Earth.
REFERENCES
Davies G.F. 1993: Cooling the core and mantle by plume and plate flows. Geophys. J. Int. 115, 132–146.
de Wit, M. J. 1998: On Archean granites, greenstones, cratons and tectonics: does the evidence demand a verdict?
Precambrian Research 91(1-2): 181-226.
Gerya, T.V. & Yuen, D.A. 2003: Characteristics-based marker-in-cell method with conservative finite-differences schemes for
modeling geological flows with strongly variable transport properties. Phys. Earth Planet. Interiors 140, 295-320.
Komiya, T., S. Maruyama., T. Masuda, S. Nohda, M. Hayashi, K. Okamoto 1999: Plate tectonics at 3.8-3.7 Ga: Field evidence
from the Isua Accretionary Complex, southern West Greenland. Journal of Geology 107(5): 515-554.
Figure 1. Illustrations of different tectonic regimes: a – “normal subduction” regime with arc and backarc basin formation
– T’ m (present mantle temperature at 70 km)=1360 °C; radiogenic heating (H’) – present day; b – “pre-subduction regime”:
underthrusting of continental crust by oceanic plate – T m =T’ m +175 °C; H=H’x2.5; c – “horizontal-tectonics” regime: no subduction,
no underthrusting, only horizontal movements - T m =T’ m +250 °C; H=H’x3. Left parts of pictures – oceanic lithosphere,
right – continental lithosphere; dark grey – lithospheric mantle; below – mantle (grey), a little darker – partial molten
mantle.
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Symposium 1: Structural Geology, Tectonics and Geodynamics