Seismic expression and kinematics of a fault-related fold termination: Rosario structure, Maracaibo Basin, Venezuela

Fold terminations are primary features of deformed belts and are critical elements in understanding the 3-D kinematics of fault-related folds. Typically, such terminations form due to loss of displacement on the genetically-related thrust fault and/or to along-strike change in fault attitude, forming a lateral or oblique ramp. To unambiguously determine the mechanism responsible for a given fault-related fold termination, footwall cutoffs and along-strike displacement variation must be constrained. Reflection seismic data can provide the detail necessary to uniquely describe these features. The Rosario structure in the Maracaibo Basin, Venezuela is an example of a natural fault-related fold that possesses a southern termination whose fold/fault geometry and along-strike displacement variation are constrained by industry reflection seismic and well data. We interpret that the fold plunge near the southern termination is due to an along-strike decrease in displacement. The fault geometry associated with the southern termination changes from a flat-ramp-flat at the crest of the structure where displacement is greatest to simply a ramp near the lateral fault tip, without forming a lateral or oblique ramp. We combine these observations regarding displacement and fault geometry with the assumption that along-strike spatial variations reflect temporal variations during development of a fault-related fold to propose a kinematic model for the Rosario structure. Specifically, we suggest that the structure first initiated as an isolated fault ramp within the 'stiff' carbonate and elastic units. With increased shortening, the fault grew to link with upper and lower detachments in the weaker shale units to create a hybridized fault-bend fold. Our model suggests a possible explanation for the evolution of the Rosario structure, and also provides an alternative to existing pseudo-three-dimensional models for fault-related fold growth that relaxes the rigidity in the assumption of along-strike self-similarity. (C) 2002 Elsevier Science Ltd. All rights reserved.