Abstract: Effect of Decollements in Thrust Wedges: Insights from Analog Modeling

MALAVIEILLE JACQUES and MARC JOLIVET

The tectonic evolution of active thrust wedges cannot be observed directly because of the time scales involved. Thus, the structures viewed in depth (from seismic profiles or drilling) or the surface (from topography and geology) represent a snapshot in time. The kinematic history must be reconstructed using structural geological principles and techniques (e.g., section balancing) or can be "forward modeled" through analog modeling. Such processes occurring over several million years require experiments with large convergence (representing 100+ km in nature). The influence of decollement levels on geometry, structure and evolution of thrust wedges was investigated in scaled "large convergence" sandbox experiments. Granular materials presenting brittle Mohr-Coulomb behavior were used in the models as sedimentary layers analogues, while glass microbeads simulated decollement levels.

While experiments with basal decollement produce a classic frontal imbricate fan through repeated failure along frontal thrusts, at high basal frictions, accretionary cycles are observed alternating between frontal accretion of imbricate thrust slices and underthrusting of long, undeformed units.

Interbedded decollement layers induce changes in the surface slope morphology, internal structure and evolution, involving out of sequence thrusting and duplexes development.

Angular relationships between bedding and decollements related to inherited unconformities also modify the geometry of structures.

The results obtained from this study are applied to better constrain a geological transect across the Bolivian Cordillera foreland.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria