FOLD-AND-THRUST BELTS ON SALT: 3D GEOMETRY OF THRUST FRONTS IN NATURAL SETTINGS AND IN PHYSICAL MODELS

3D GEOMETRY AND KINEMATICS OF FTB DETACHING ON SALT

Some accretionary and orogenic wedges detaching on salt show very peculiar geometry and kinematics. They deform by Short, Curvilinear and Anastomizing Thrust Traces (SCATT geometry) in map view. Good examples of such a 3D thrusting geometry are clearly represented by the Mediterranean Ridge, the topography of which was recently emphasized by full coverage multibeam surveys (Lobrieu et al., 2000), and the southern Zagros "simply folded belt”, as clearly shown by the Landsat TM image.

PHYSICAL MODELING

For this study we designed some models using dry quartz sand and silicone to reproduce respectively the frictional cover and the viscous salt. The models had different Length versus Width ratios (L/W) and variable Width versus thickness of the Brittle cover ratios (W/B). They were deformed at velocities ranging from 3mm/h to 15 mm/h and have different boundary conditions.

The experiments performed for this study seem to demonstrate that the 3D geometry of models reproducing FTB in salt-bearing basins strongly depends on shortening velocity, shape of salt basins, thickness of the brittle cover overlying the viscous decollement and backstop geometry.

Applying the results from modeling to natural prototypes must be done with some caution. Nevertheless the striking similarities between both the Landsat TM image of the southern Zagros belt and the topography of the eastern Mediterranean Ridge and the map view of some of our models seems to demonstrate that the results from the models could apply to natural FTB detaching on salt.

Finally it is noteworthy to observe that the SCATT geometry could strongly influence hydrocarbon migration and trapping.

References Lobrieu, B., Satra, C., and Cagna, R., 2000. Cartography by multibeam echo-sounder of the Mediterranean Ridge and surrounding areas. Scale 1:1,500,000, Ifremer-Ciesm, Ifremer Editions.