CONTROLS OF PRE-EXISTING NORMAL FAULTS ON THE GEOMETRY AND LOCATION OF THRUST FAULTS IN FOLD-THRUST BELTS

Abstract

Pre-existing normal faults are commonly cited as influencing both the sequence of faulting and the location of thrust faults in fold-thrust belts. Scaled experimental models for both frictional and ductile detachments were conducted to study the influence of pre-existing normal faults on the subsequent development of thrust faults. Frictional detachment experiments used layers of silica sand directly overlying the detachment, whereas ductile detachment experiments used layers of sand representing sediments overlying silicone gel, representing a ductile layer in 60° angle normal faults. The experiments suggest that the normal faults influence the location and orientation of the frontal thrusts rather than the sequence of fault propagation. Furthermore, the influence is more pronounced for detachment folds and thrusts formed above a ductile detachment than for duplexes above a frictional detachment. The orientation of the normal faults influences the orientation of subsequently formed thrust faults in different ramp settings, so that normal faults with orientations oblique to the direction of contraction result in oblique frontal thrusts. Offsets in the location of the normal faults along lateral ramps or transfer zones result in offsets in the location of the frontal thrusts. The effect of the pre-existing normal faults on the thrust-fold geometry depends on the relative distance between the normal fault and the forethrust positions, detachment type, and the geometry of the normal fault. The results are directly applicable for understanding the locations and orientation of thrust faults, mapping of fold-thrust structures using surface and subsurface data, and for comparison with natural examples.