THE INFLUENCE OF PREEXISTING THRUST FAULTS ON NORMAL-FAULT GEOMETRY IN NATURE AND IN EXPERIMENTS

Relations between normal faults and pre-existing thrust faults are cla ssically described in terms of three basic situations: normal faults c an cross-cut thrust faults; they can branch out from thrust faults at depth on a decollement level, or they can entirely reactivate thrust p lanes. The mechanical aspects of these types of interaction were studi ed by analogue modelling in which sand simulates the 'brittle' rocks a nd silicone putty an interlayered decollement. The models underwent co mpression, producing thrust faults with variable dips, followed by ext ension. Three possible ways of interaction are described here: (a) no interaction occurs in the case of low-dip thrust faults (<32 degrees /- 1 degrees) and normal faults are developed independently, displayin g a listric geometry; (b) branching at depth on the decollement level occurs when dip of the thrust faults reaches 32 degrees +/- 1 degrees. In this case, the dip of the normal faults, whose geometry becomes pl anar, decreases with increasing thrust dip. We suggest that this chang e in dip of normal faults depends upon the rotation of stress tenser a xes along the pre-existing fault zone, where a drop in the friction co efficient is likely to occur; (c) reactivation occurs in brittle mater ial when dip of the pre-existing fault exceeds 41 degrees +/- 1 degree s.