Open Session - SWISS GEOSCIENCE MEETINGs

6
Symposium 1: Structural Geology, Tectonics and Geodynamics
1. 2
The effects of large sub-marine landslides on the mechanics of thrust
wedges
Smit Jeroen *, Burg Jean-Pierre *, Dolati Asghar *,** and Sokoutis Dimitrios ***
*ETH Zurich, Strukturgeologie, Leonhardstrasse, 19 /LEB, CH-8092 Zurich (smit@erdw.ethz.ch)
**Geological Survey of Iran, Meraj st. Azadi sq., P.O. Box 13185-1494, Tehran, Iran
***Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
Olistostromes cover large portions of active thrust wedges like Makran, Gulf of Cadiz and offshore Borneo. The emplacement
of these olistostromes by submarine mass flows represents an instantaneous and massive mass redistribution. The effect of
olistostrome emplacement on thrust wedge mechanics is discussed after the example of the Iranian Makran wedge and the
results from analogue tectonic experiments.
The Makran accretionary wedge, between the Arabian and Eurasian plates, grows seawards by frontal accretion and underplating
of trench fill sediments since the mid-Miocene, presently at a rate of about 2 cm/yr. The whole wedge is >500 km
wide. This extreme width in combination with a low cross-sectional taper of ca. 3º and little indication for reactivation of
the internal parts suggest low basal friction and, most probably, the presence of one or more weak décollements. Active mud
volcanoes may indicate that such décollements take advantage of overpressured shales.
The Makran includes a giant catastrophic mud-and-debris flow inserted between Eocene to Lower Miocene turbidites and
Upper Miocene deposits. The olistostrome includes blocks of ophiolites and oceanic sediments derived from the mélange to
the north and reworked chunks of the underlying turbidites, on which it rests with an erosional unconformity. Its size and
internal structure make it a fossil equivalent of the large debris flows found along continental margins and unstable volcanic
edifices.
We performed a series of analogue tectonic experiments to test the influence of erosion and deposition on the deformation
style of Makran-type wedges. Sudden erosion and resedimentation were mechanically introduced during shortening to test
the effects of instantaneous mass-redistribution. After erosion/resedimenation, thrusting focused on existing faults in the
emerged internal wedge. Further frontal propagation was prevented by fast redeposition of the erosional products seaward
of the thrust front that locally increased wedge strength. Depending on the thickness and extent of the added load, the
presence of weak décollements in the frontal wedge causes thrusting to jump to the front of the resedimented load, hence
allowing the wedge to quickly grow outward by frontal accretion above the weak mid-level décollement. It seems that the
striking difference in deformation style and intensity we observed in the Iranian Makran may be, at least partially, explained
by the mass-redistribution caused by the Late-Miocene olistostromes.
1. 3
Neotectonics of the Western and Central Alps : geodynamic implications
Sue Christian*, Champagnac Jean-Daniel**, Tricart Pierre***
* IUEM/UMR6538/UEB/UBO, Brest University, France (christian.sue@univ-brest.fr)
** Insitute of Mineralogy, Hannover University, Germany
*** LGCA/UMR5025 Grenoble University, France
The Western Alps’ active tectonics are characterized by ongoing widespread extension in the highest parts of the belt and
transpressive/compressive tectonics along its borders (Sue et al., 1999; Delacou et al., 2004). We examine these contrasting
tectonic regimes, as well as the role of erosional processes, using a multidisciplinary approach including seismotectonics,
numerical modelling, GPS, morphotectonics, fieldwork, and brittle deformation analysis. Extension appears to be the dominant
process in the present-day tectonic activity in the Western Alps, affecting its internal areas all along the arc. Shortening,
in contrast, is limited to small areas located along at the outer borders of the chain. Strike-slip is observed throughout the
Alpine realm and in the foreland. The stress-orientation pattern is radial for σ3 in the inner, extensional zones, and for σ1
in the outer, transcurrent/tranpressional ones. Extensional areas can be correlated with the parts of the belt with the
thickest crust. Quantification of seismic strain in tectonically homogeneous areas shows that only 10 to 20% of the geodesydocumented
deformation can be explained by the Alpine seismicity. We show that Alpine active tectonics are ruled by buo-