Open Session - SWISS GEOSCIENCE MEETINGs

20
Symposium 1: Structural Geology, Tectonics and Geodynamics
1.10
Structural evolution and fission track ages of the Makran accretionary
wedge in SE Iran
Dolati Asghar*, Seward Diane*, Smit Jeroen*, Burg Jean-Pierre*
* Earth Sciences, ETH-Zurich, Leonhardstr. 19, CH-8092 Zurich
(dolati@erdw.ethz.ch)
The Makran Accretionary Prism (MAP) is formed by active convergence between the Eurasian and the Arabian plates. The
oceanic crust of the Arabian plate has subducted to the north under the Eurasian continent, thereby building the MAP. This
wedge is characterized by a shallow subduction angle (500 km, >300 km of which are exposed onshore.
New mapping results and structural sections document the structural development of the accretionary wedge. Shortening
in the internal Makran is accommodated along large E-W trending thrusts, and folds, which are sealed by a giant catastrophic
mud-and-debris flow (11.8 - 5.8 Ma). To the south of inner Makran, sediments younger than middle Miocene are affected
by open synclines with long wavelength (>20 km) and low amplitude alternating with tighter anticlines. Normal faults in
the south of MAP, are further imaged offshore by seismic data. Field observation and seismic data indicate that most of
normal faults cut Plio-Pleistocene sediments.
Low temperature geochronology (apatite and zircon fission track ages) provides new time constraints. Apatite has undergone
partial annealing since sedimentation. However, analysis of the radial plots suggests that cooling began in the late Miocene,
after burial. Zircon fission track ages span 60-180 Ma with no evidence for annealing or resetting. We conclude that the burial
thickness of the MAP as always been shallower than annealing zone (≈210°C).
1.11
Stability of thermal boundary layers for convection in spherical shell:
Application to the dynamics of planetary mantles
Duchoiselle Lionel*, Deschamps Frédéric*, Tackley Paul *
*Institut für Gepophysik, ETH Hönggerberg (HPP) CH-8093 Zürich (duchoiselle@erdw.ethz.ch)
Improving the knowledge of convection into mantle of terrestrial planets required a better understanding of its physical and
chemical state. Recently, with the help of massive computational resources, significant progresses were achieved in the numerical
modeling of planetary mantles convection. Models with a high degree of complexity (including realistic viscosity
laws, mixed mode of heating, spherical geometry, thermo-chemical convection, …) are now available. Among the parameters
that recently became accessible, spherical geometry is a key ingredient because it affects the relative strength of the top and
bottom thermal boundary layers. Despite these progresses, many details of planetary mantles convection remain unclear
and so far, no model of Earth’s mantle convection fits all available geophysical, geochemical, and geological constraints.
Using STAGYY, which solve the usual conservative equations of mass, energy and momentum on a yin-yang grid, we explored
the influence of various parameters on convection in spherical geometry. First, we have performed several numerical
experiments varying important parameters including the Rayleigh number, the curvature (ratio between radius of the core
and the planet one), the mode of heating (only from below or with an internal heating component), rheology (isoviscous or
temperature dependence). In particular, we studied the evolution of the style of convection, average temperature, heat flux
and critical Rayleigh number depending on these parameters. We have then built scaling laws between the parameters and
observables, for instance between the Nusselt and Rayleigh number, and between the temperature and curvature factor. Our
results suggest that extrapolations previously made from Cartesian models may not be valid in spherical geometry. In particular,
the dependence of temperature on curvature differs significantly from that expected by Cartesian scaling laws. In
addition, it also depends on the Rayleigh number. A possible explanation for these discrepancies is the asymmetry between
the top and bottom thermal boundary layers, which may alter their relative stability. The new scaling laws we obtained enable
to reconsider some aspects of thermal evolution and physical states of terrestrial planets like Earth, Mars, Mercury or
some giant planets satellites.