Interaction between normal faults and pre-existing thrust systems in analogue models

The influence of pre-existing thrusts on the development of later normal faults was investigated using scaled laboratory analogue models. Experiments consisted of a phase of shortening followed by extension at variable angles of obliquity (a) to the shortening direction. Results suggest that the angle alpha has a major influence on the surface fault pattern and on the interaction between shortening-related structures and later extensional structures. Three different modes of interactions were identified depending upon the extension kinematics. (1) For orthogonal extension (alpha = 0 degrees), shortening-related fold and thrust structures strongly influence the development of normal faults: graben structures nucleate within anticlines and the normal faults reactivate thrusts at depth (branching at depth mode of interaction). (2) For highly oblique extension (alpha >= 45 degrees), shortening-related structures exert no influence on normal faults as extension-related steeply-dipping faults (characterized by an oblique component of movement) displace early thrusts (no interaction mode). (3) For intermediate obliquity angles (alpha = 15 degrees, 30 degrees), an intermediate mode of interaction characterizes the experiments, where the no interaction and branching at depth modes coexist in different regions of models. Modelling results can be used to infer regional extension directions as is shown for the Northern Appenines (Italy).