Open Session - SWISS GEOSCIENCE MEETINGs

Here, we study the dynamics of two-phase flow by solving the equations directly on grain-size-scale using the Stokes equations
for low-viscosity and high-viscosity regions in two dimensions. We performed systematic studies in order to characterize
the mechanical behaviour of the system as a function of material parameters and melt fraction. Results indicate that for
moderate to large melt fractions, particle interactions are significant, and result in macroscopic Rayleigh-Taylor like instabilities.
This allows us to derive a formula for effective viscosity in a particle-suspension system. Additionally, we quantified
the transition between Rayleigh-Taylor mode (strong interaction between particles) and Stokes-suspension (simple sinking of
particles) mode systems.
To complete the scaling law in the full range of fluid fractions, we currently perform simulations at very low porosities.
REFERENCES
McKenzie, D.P. 1984: The generation and compaction of partially molten rock. Journal of Petrology, 25(3), 713-765.
1.
New results of tectono-sedimentary history of the Makran accretionary
prism
Dolati Asghar*, Müller Carla**, Smit Jeroen*, Bernoulli Daniel*, Spezzaferri Silvia*** & Burg jean-Pierre*
*Earth Sciences, ETH-Zurich, Leonhardstr. 19, CH-8092 Zurich (dolati@erdw.ethz.ch)
**6 bis r Haute 92500 RUEIL MALMAISON, France
*** Department of Geosciences, University of Fribourg, ch du Musee 6, 1700 Fribourg
The Makran Accretionary Prism (MAP) is one of the largest accretionary wedge in the world. It results from the active convergence
between the Arabian and Eurasian plates which began in the late Cretaceous. The MAP grows seawards by frontal
accretion and underplating of trench-fill sediments since the Miocene. Nowadays, the frontal 100–150 km are submarine and
>350 km of the Cenozoic accretionary wedge are exposed on land, in Iran and Pakistan.
New results of stratigraphy and tectono-sedimentary history show that it was a turbidite basin on an active margin between
the late Paleocene – early Eocene and the Serravallian. The oldest, well-dated turbidites are Lower Eocene with typically
rhythmic alternation of brown, usually volcanogenic sandstone, and lighter-coloured shales; this upward-coarsening unit
represents relatively distal slope deposits conformably covering a series of pillow basalts, basaltic flows, pelagic limestones
and shales of early Palaeogene age. New fossil determinations reveal that many turbidite units that were previously mapped
as Eocene are in fact late Oligocene in age. Towards the Lower Miocene the deposits become more proximal and the influx
of turbidites ceased. Carbonate reefs and gypsiferous mudstones indicate a shallow marine and lagoonal environment during
the Burdigalian.
The MAP includes a giant catastrophic mud-and-debris flow inserted between Lower Miocene and Upper Miocene sediments.
We provide evidence for sedimentary gravitational emplacement of the resulting olistostrome between 11.8 and 5.8 Ma or
shortly thereafter. The olistostrome includes blocks of ophiolites and oceanic sediments derived from the ophiolite-bearing,
imbricate thrust zone to the north, and reworked chunks of the turbidites on which it rests with an erosional unconformity.
The chaotic scattering of blocks of any size and lithology and the soft-sediment deformation of the matrix argue against a
tectonic emplacement of the olistostrome. Its size and internal structure make it a fossil equivalent of the large debris flows
found along continental margins and unstable volcanic edifices.
REFERENCES
Burg, J.P., Bernoulli, B., Smit, J., Dolati, A. & Bahroudi, A. 2008 : A giant catastrophic mud-and-debris flow in the Miocene
Makran. Terra Nova, 20, 188-193.
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Symposium 1: Structural Geology, Tectonics and Geodynamics