GROWTH STRATAL ARCHITECTURES ASSOCIATED TO DECOLLEMENT FOLDS AND FAULT-PROPAGATION FOLDS - INFERENCES ON FOLD KINEMATICS

In many thrust and fold belts, asymmetric folds with overturned or ste eply dipping forelimbs and gently dipping backlimbs are commonly inter preted as thrust-related folds. Determining the folding mechanism excl usively from the final fold geometry in the pre-growth units is possib le only for a limited suite of structures. The nature of fold-thrust i nteraction in shallow structures can be inferred by coupling the fold geometric analysis with the study of the syntectonic sediment stratal architectures. Several factors such as axial surface activity, fold up lift, limb rotation and limb widening rates, together with sedimentati on and erosion rates, control growth strata patterns. Because the evol utionary path of most of these parameters depends on the folding kinem atics, different growth stratal architectures are expected for differe nt thrust-related anticlines. In this paper we examine the influence o f the above factors on growth stratal geometries associated to a simpl e kink band, and then we apply the same approach to kinematic models o f decollement folding and fault-propagation folding. Four geometric ap proaches are used to account for the geometry and kinematics of anticl ines located at the tip of a blind thrust: fault-propagation folding w ith no excess layer-parallel shear, fault-propagation folding with pro gressive layer-parallel shear, constant limb length decollement foldin g and variable limb length decollement folding. Whereas fault-propagat ion folding with no excess layer-parallel shear is a self-similar fold ing mechanism, the other three types involve limb rotation. Coherent g eometries between growth and pre-growth sequences develop at high sedi mentation rates in the crest of fault-propagation folds, and incoheren t ones in decollement folds. Minor differences within growth strata pa tterns occur at low sedimentation rates, and when erosion affects the crest of the anticlines.