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Abstract: Experimental Models of Thrust Belt Fronts

Bernard Colletta, Yann Philippe, Jean Letouzey, Jean Paul Richert, Jean Claude Chermette, Martin Specht

We have used scale-down sand-box experiments analysed by computerized x-ray tomography to study the development and propagation of thrusts.

In purely brittle models (sand and glass powder), thrusts propagate forward in a piggy back sequence. In such a sequence of propagation, only one thrust (the frontal one) is active; the older thrusts are passively transported or poorly reactivated. Vergence of thrusting is controlled by basal friction and major displacements occur along forethrusts. Spacing and thrust sheet length are mainly controlled by basal friction and by depth to decollement. Discontinuities in the thrust sheet poorly disturb the forward propagation.

On the contrary in models composed of one or several layers of viscous material (silicone putty), the normal forward piggy-back sequence of propagation is no more valid and thrust development is strongly controlled by preexisting discontinuities in the thrust sheet and decollement layers. In experiments simulating low viscosity decollements (salt or weak shales), nucleation and thrust sequence are directly related with discontinuities of the viscous layer. Vergence of thrusts can change due to an almost horizontal maximum principal stress. Several thrusts are active simultaneously accommodating shortening over the whole decollement zone. The Jura mountains display several examples of thrust nucleation related to discontinuities in the main Triassic decollement layer.

In the case of two interbedded ductile layers, duplexes can develop from a true passive roof duplex (intracutaneous wedge) to an emergent hinterland dipping duplex. In antiformal structures, decoupling in the uppermost ductile layers induces two opposite verging thrusts producing typical "rabbit ear anticlines" comparable to natural examples from Santa Barbara ranges in Argentina and from Southern Algeria.

In the case of three or more ductile interbedded layers, "fish tails", comparable to the Melville Island structures, can develop. Lateral change of thrust vergence is accommodated by a symmetrical pop-up structure.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France