Geometries of syntectonic sediments associated with single-layer detachment folds

Abstract

Three kinematic models have been proposed to account for the geometry and kinematics of detachment folds involving a homogeneous competent layer detached over a ductile unit: Model 1 — variable limb dip-constant limb length, Model 2 — constant limb dip-variable limb length, and Model 3 — variable limb dip-variable limb length. Because the same fold shape can be generated by any of the above mechanisms, fold kinematics are best determined by the geometries of syntectonic sediments. In single-layer detachment folds, growth strata patterns are controlled by axial surface activity, limb rotation, limb lengthening, fold uplift rates, sedimentation rates and deformation mechanisms of the syntectonic sediments. Asymmetric kink folds have been modelled and compared with natural examples from the southern Spanish Pyrenees. Under conditions of high sedimentation rates, Model 1 folds produce characteristic fanning growth stratal wedges that initially onlap and then progressively overlap the detachment anticline, whereas at low sedimentation rates, anticlines with fanning growth wedges on both limbs are formed. Model 2 folds, under conditions of high syntectonic sedimentation rates, form anticlines with growth strata largely parallel to the pre-growth units and thinned over the fold crest. In contrast, at low sedimentation rates, Model 2 folds show offlapping growth strata onto both limbs. Model 3 growth folds, under conditions of high syntectonic sedimentation rates, form anticlines with intermediate features from Model 1 and Model 2; at low sedimentation rates, anticlines are formed with offlapping growth structures.