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

Discussion

Since the early days of geology it has been well-known that thrusts may develop from pre-existing folds, by shearing out of middle limbs of antiform-synform pairs (Heim, 1921; Willis, 1890). Such a folding preceeding faulting sequence has been simulated in centrifuge experiments by Dixon and Liu (1992). In contrast, Buxtorf (1915) showed how low-angle reverse faulting could produce drag folds. Chamberlain and Miller (1918) simulated in laboratory experiments initial nucleation of thrust ramps in plaster models of varying competence, followed by failure parallel to bedding.

Here, centrifuge experiments are used to simulate various ramp-flat accommodation styles which occur in deeper levels of the crust, where flow may progress simultaneously as slip accumulates along fault surfaces. In this environment flow and faulting are independent and interactive and not sequential processes, where the one results from the other. The conception is that of discrete faults in flowing environments which change length while slip accummulates in the slip direction, in the manner discussed by Means (1989). It is this aspect of ramp-flat accommodation which the models simulate.

It is well known from previous experiments that a rock subjected to compressive loading in the laboratory, at low temperature and pressure fails by shear failure when the strength limit is exceeded. Invariably, these faults are oriented at angles of ≈ 30 with respect to the maxium compression direction (e.g. Paterson, 1978). However, as pressure and/or temperature are increased or pore pressure or strain rate are lowered strain localisation by shear failure may give way to stretching faults (Means 1989), and ultimately to ductile flow (Heard, 1976). This means ramp-flat accommodation in changing environments, spanning from the brittle to the ductile field.

However, most existing geometric models assume parallel behaviour where faults do not change shape and where folds in the hangingwall are inferred to form by layer parallel slip and angular kinking (e.g. Suppe, 1983). These assumptions may not be valid in particular situations where flow is involved with faulting. Evidence from structures formed in experiments and nature invariably suggest that aspects of the interactive development of ramps-flat geometries may involve ductile material response when and if enviromental conditions change.

In the tests under discussion, ramp-flat accommodation exhibited different styles, ranging from ductile fault bend folding in single competent layers (Fig. 4c) to wedge faults in rheologically stratified strata (Figs. 6c and 7c). In these models ductile deformation progressed simultaneously as slip accumulated along the thrust surfaces.

Ductile fault bend folding
In the models fault bend folds with smooth fold shapes developed only in cases where decoupling and overthrusting of the plastilina hangingwall block progressed above surfaces of reduced frictional resistance. However, this geometry developed when there was a near balance between the yield stress and gravity stress (s/rgh≈1, cf. Fig.4 a & b). Under particular boundary conditions, the geometry of ductile fault bend folding (Figs.4c and 5c) is very similar to the classic fault bend fold model (e.g. Suppe, 1983) except for the accumulation of ductile strain and change of fault shape during forward transport. In the classic fault bend fold model the range of fold geometries which can be produced is a simple function of fault shape and finite displacement. By comparison, in the material models both the hangingwall and footwall pairs, as well as the surroundings accumulated ductile strain (by layer thickening or thinning) in the range 10-25 %, simultaneously as slip accumulated along the fault surfaces. Moreover, the shapes of the hangingwall anticlines have rounded forms which confirm better to a contact strain model than a kink model (Ramsay, 1992). In the models footwall synclines also developed in case the load imposed by the overthrusting hangingwall blocks exceeded the yield limit of materials, even in situations where deflection of the footwall into the substratum was constrained by a rigid base (e.g. Fig. 4 ). However, the form of footwall synclines differ depending on the strength contrast between the layers. In the classic fault bend fold model, folds are only developed in the hangingwall of the structure, with the footwall remaining inert.

Wedge faults
The experimental results discussed above suggest that the observed sigmoidal shape of wedge faults not only require brittle failure and subsequent movement on that ramp (Cloos, 1961) but also a ductile accommodation response. The geometry of wedge faults observed in experiments is in very good accord with that observed at kimmeridge (Ramsay, 1983).

The experiments addressed the geometry of ductile ramp-flat accomodation in flowing environments. However, it remains a challenge to identify and distinguish the whole sequence of ramp-flat accommodation, spanning from the brittle to the ductile field, in changing environments, controlled by the rheological stratification.

Conclusions

This paper attempted to discuss some of the geometric features of ductile ramp-flat accommodation in terms of mechanical behviour of model materials. The models show that the geometry of ramp-flat thrust accommodation in ductile environments clearly differ from the classical fault bend fold model (Suppe, 1983). Some of these differences include involvement of layer shortening, formation of wedge faults and footwall synforms. Based on the results of the model experiments, the following conclusions seem warranted.

1) Ramp-flat accommodation in a single competent plastilina layer detached above a rigid base was by ductile fault bend folding;
2) When the competent plastilina layer was underlain or sandwiched between stiff ductile strata (exhibiting an order of magnitude contrast in strength and viscosity) the initial ramp adjusted to a wedge fault where the footwall synforms mirrored that of the hangingwall antiforms. Such models are analogous to the kimmeridge model of thrust accommodation,
3) Ramp-flat accommodation above soft viscous substrata resulted in the weak material being injected into the ramp during overthrusting.

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