Mulugeta, G. 2002. Scale Effects and Rheologic Constraints in Ramp-flat Thrust Models. In: Schellart, W. P. and Passchier, C. 2002. Analogue modelling of large-scale tectonic processes. Journal of the Virtual Explorer, 7, 51-59.
Scale effects and rheologic constraints in ramp-flat thrust models

G. Mulugeta

Hans Ramberg Tectonic Laboratory, Department of Earth Sciences
Uppsala University, Villavägen 16, SE-752 36, Uppsala, Sweden.


Dynamic models for the interactive development of ramp-flat thrust styles are studied in centrifuge experiments. Plastilina simulates the competent strain-hardening behaviour of sandstones and carbonates, while viscous or plastic bouncing putties represent incompetent evaporites and shales. The models exhibit various fault-fold geometries; e.g. fault bend folds, footwall synclines, wedge faults as structural expressions of ductile ramp-flat accommodation. Depending on the rheological stratification, the ramps change shape and length as slip accumulates along the fault surfaces in flowing surroundings.
A ductile fault bend folds develops in a single competent layer detached above a rigid base, with or without a soft cover. The load imposed by the overthrusting hangingwall generates a footwall synform in case the yield limit of the material is exceeded. In models where the competent single layer is underlain or sandwiched between stiff ductile strata the initial straight ramps change shape to wedge faults. These exhibit footwall synforms that mirror the hangingwall antiforms. Hangingwall blocks that overthrust soft ductile strata get injected by the substratum material beneath the ramp. The ramp-flat structures simulated in the models may give valuable insight in assessing the rheology of rocks deforming under similar conditions in nature.




The Experimental Models

Experimental Results

The scale-effect of ramp-flat accommodation

Ramp-flat accommodation in a competent layer beneath ductile strata

Ramp-flat accommodation above ductile substrates.

Ramp-flat accommodation in embedded ductile strata


Ductile fault bend folding

Wedge faults