Growth pattern of underlithified strata during thrust-related folding

摘要

Asymmetric anticlines with overturned or steeply dipping forelimbs and gently dipping backlimbs are generally interpreted as thrust-related folds. Fold asymmetry occurs as a consequence of forelimb rotation. If deformation takes place in environments dominated by submarine sedimentation, the limbs coincide with the slope (depositional surface) and rotation reflects slope steepening. If folds are nucleated in poorly or unlithified deposits, growth geometry also depends on the properties of the media, such as cohesion and the angle of internal friction. For cohesionless deposits, the tilting of the slope influences the equilibrium of the soft sediments, resulting in gravity-driven flow, re-mobilisation or in situ compaction. The occurrence of mass re-mobilisation is also connected with the limb tilting/lithification ratio. Hence, the presence of non-primary bedding geometries or soft-sediment deformations in folding-related growth strata may provide useful tools for deciphering contractional kinematics. Deformation of underlithified sediments during thrust-related folding is recorded in the outer sector of the Neogene Sicily chain (Central Mediterranean). Deformation occurred during the building of the Pliocene chain. Folding is the driving mechanism of the growth stratal pattern. Fold nucleation and amplification influenced the inclination of the slope of the basin floor where sediments were deposited. Slump and stretching structures in soft sediment occurred during folding and mass accumulation at the base of limbs led to a decrease in slope inclination. Analysis of fault-related fold and gravity-driven geometries enable us to reconstruct the contractional kinematics and the behaviour of syn-tectonic deposits that modify the growth fold pattern in terms of limb-hinge change of length-thickness.