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, 51-60.
Scale Effects and Rheologic Constraints in Ramp-flat Thrust Models


Geological mapping in overthrust terranes, for over a century (Willis, 1890; Heim, 1921) has shown the intimate association of thrusts/folds. The relative importance of folding-faulting styles are largely controlled by the rheological stratification in a stratigraphic column. At less than crustal scale the modes and kinematics of fold-fault interaction can vary greatly, and their interrelationships has received wide attention. 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). However, the mechanics of thrusting where faulting preceeds folding are not well-understood. In the current literature, these are variously named; fault bend folding (e.g. Suppe, 1983), fault propagation folding (Jamison, 1987; Geiser, 1988) and decollement folding (Jamison, 1987), all based on the initial Rich (1934) concept which suggested that overthrust faulting localises in mechanically weak rocks, parallel to bedding (e.g shales) and steps upwards in mechanically strong rocks (e.g carbonates).

A radically different view was invoked by Gretener (1972) who suggested that a competent layer could act as a stress guide and fail when the strength limit of the material is exceeded and subsequent transfer of movement would take place to weaker horizons. In field examples, Burchfiel et al (1982) have shown localisation of thrust ramps in competent dolostone layers in southern Nevada rather than the underlying weaker rocks. Eisenstadt & De Paor (1987), while accepting the initial nucleation of thrust ramps in comptent members argued that the flats develop in incompetent members as a result of fault tip line migration both up-and down section from multiple source ramps.

Here dynamic models for the interactive development of ramp-flat thrust styles are presented. The models assume initial nucleation of a thrust ramp in a competent brittle member (simply by inducing a ramp in the initial model design) and study the subsequent ramp-flat accommodation after the material has acquired ductile properties, in flowing environments (most probably caused by changes in temperature and pressure conditions or mineral transformations subsequent to faulting). Alternatively, the ramps could be simple old planes of weakness without implying any cause for their origin. In such situations slip along fault surfaces may take place simultaneoulsly as flow and folding accumulates in the surroundings, controlled by the rheological stratification. Such faults in ductile environments are known as stretching faults (e.g. Means, 1989).

Figure 1, a to e shows various ramp/flat accommodation styles encountered in the earth that might reflect ductile accommodation, ranging from fault bend folding to overthrusts emanating from ductile decollements to wedge faults.

Figure 1. Ramp-flat accommodation styles in relatively competent layers: a) Geometric fault bend fold model (e.g. Suppe, 1983). b) and c) in a sandstone layer in the Bloomsburg formation, W Virginia, U.S.A (modified from Cloos, 1961). d) in Kimmeridge Bay (Ramsay, 1992). e) Mt.Terry anticline (Suter, 1981). (Select image for enlargement)

In the sections below a number of dynamic models for the interrelationships between fold and fault geometries are presented and discussed.

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