|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
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 ≈ 30Å 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.
fault bend folding
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.
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;
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