Schreurs, G., Hänni, R. and Vock, P.  2002. Analogue modelling of transfer zones in fold and thrust belts: a 4-D analysis. Schellart, W. P. and Passchier, C. 2002. Analogue modelling of large-scale tectonic processes. Journal of the Virtual Explorer, 7, 43-49.
Analogue modelling of transfer zones in fold and thrust belts: a 4-D analysis


The experiments showed a contrast in structural evolution between domains underlain and not underlain by a thin viscous PDMS layer. Thrust faults in the brittle domain were closely spaced and a narrow and high fold and thrust belt formed. The sequence of thrusting propagated forward and the belt had a dominant vergence of thrusts and associated folds. In the brittle-viscous domain, however, spacing between thrusts was greater and the fold and thrust belt was wider and lower. A frontal pop-up structure formed at the frontal sand-PDMS boundary. There was no consistent vergence of thrusts and folds in the brittle-viscous domain. Transfer zones formed in the transition zone between brittle and brittle-viscous domains. Out-of-sequence thrusting and coeval activity of different thrusts occurred in the brittle-viscous domain close to where the transfer zone developed. This resulted from along-strike propagation of forward thrusts from the brittle domain into the adjacent brittle-viscous domain. Location and orientation of transfer zones was directly related to the basal geometry of the viscous layer. Transfer zones rooted in the viscous layer and their strike closely mimicked the orientation of the basal boundary between viscous and brittle material. A lateral ramp formed where this boundary was parallel to the shortening direction, whereas an oblique ramp formed where this boundary was oblique.

Our experiments suggest that salt or other rheologically weak layers at the base of a sedimentary sequence may favour out-of-sequence thrusting and coeval displacement along different forward thrusts in areas closely adjacent to transfer zones. The location and orientation of transfer zones in nature may be controlled by basal rheological changes. Lateral and oblique ramps in transfer zones may have shallow dips (<30°) and dip angles that vary along strike. Backthrusts of frontal pop-up structures in areas underlain by rheologically weak layers may interfere with forward thrusts that formed in purely brittle domains, thus contributing to the complexity of transfer zones in nature.


Funding by Hochschulstiftung Bern and Swiss National Science Foundation Grant 2000-0554.11 98/1 is gratefully acknowledged. Andrea Schneider and Cindy Seiler are thanked for assistance in computerized tomography data acquisition, and Hans-Peter Bärtschi for technical assistance.

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