Using Growth Strata Geometries to Infer the Kinematics of Foreland Thrust Tip Folds

Josep Poblet, Fabrizio Storti, Ken McClay

In many forelands of fold and thrust belts, folds displaying steeply dipping forelimbs and gently dipping backlimbs are interpreted as fault-propagation or detachment folds. Four geometric mechanisms account for the geometry and kinematics of these thrust tip folds. One of these mechanisms was described by previous authors that of a self similar fault-propagation fold, but the rest, involving limb rotation, are presented here for the first time: fault-propagation folding with progressive excess layer-parallel shear, constant limb length detachment folding, and variable limb length detachment folding. Since fault-propagation and detachment folds can adopt the same geometries, determining the type of fold exclusively from final geometry is not easy. However, the nature of f ld-thrust interaction can be inferred by analysing syn-tectonic sediment geometries. The main parameters that control growth strata patterns are: axial surface activity, fold uplift, limb rotation and limb widening rates, and sedimentation rates. Since the characteristics of these parameters depend on folding kinematics, different growth strata geometries are expected for fault-propagation and detachment folds. Understanding the folding mechanism which has operated in a natural fold is crucial for estimating fold amplification and thrust-movement rates. The validity of the kinematic models presented here is demonstrated through their application to some fold examples from the Pyrenees.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995