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Analogue Modelling |
1 Shell Research, Postbus 60, 2280 AB Rijswijk, The Netherlands
2 Shell Expro, Shell Mex House, Strand, London WC2 0DX, United Kingdom
Sandbox models of thrust tectonics with multiple detachment levels allow alternative interpretations of fault patterns and evolution, even though they are well controlled experiments. Palinspastic reconstruction resolves these ambiguities and leads to refined interpretations. It reveals that if there are thick viscous interlayers, all non-fault related contraction can be accommodated by folding, without tectonic compaction. It also confirms the complete decoupling of faulting above and below a thick weak viscous layer, and the initial straightness and regular spacing and dips of thrusts above a decollement of low basal friction. A weak elasto-plastic interlayer or decollement merely has a lubricating effect, without complete decoupling.
Many natural thrust systems are underlain by a ductile substratum and have more than one decollement level. These examples almost always exhibit a complex thrust belt geometry and are apparently formed by different thrusting events that can be difficult to interpret and to time relative to one another.
Weak decollement materials and interlayers have a strong effect on the structural style of thrust belts at the moment of detachment. However, other material properties, in particular the viscosity, play a dominant role during the structural development following detachment (the overthrust phase).
The objective of this study was to improve structural interpretation of data from areas of thin-skinned thrust tectonics with weak ductile detachment horizons (i.e. excluding large scale basement involvement). The approach involved:
The result is a set of better constraints on structural interpretation of cross-sections in thrust tectonics. In natural cases, the most common weak decollement lithologies are over-pressured shales and evaporites, in particular rock salt. In our experiments we have simulated rock salt by using silicone putty (‘SP’), a viscous material that can flow slowly as a Newtonian fluid. We modelled overpressured and/or very soft clay with an oil-water emulsion (‘OWE’), which is a ductile, weak elasto-plastic, non-viscous material in which localized shear zones can form, as in any other elasto-plastic solid.
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T. d. Bezemer, H. Kooi, Y. Podladchikov, and S. Cloetingh Numerical modelling of growth strata and grain-size distributions associated with fault-bend folding Geological Society, London, Special Publications, 1998; 134: 381 - 401. [Abstract] [PDF]
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