3-D Geometry and Kinematic Evolution of Deep-Water Toe Thrust Anticlines
Deep water toe thrust anticlines have been interpreted using traditional models derived from observations in orogenic thrust belts and accretionary prisms, such as fault-propagation folds, detachment folds, and both simple- and pure-shear fault bend folds. However, high quality 3D seismic data show that the deep-water anticlines typically display extreme variations in along-strike fold morphology which are not accounted for by any individual one of the above models; multiple conflicting kinematic models would have to be applied to explain different parts of a single structure.
We present a kinematic model of a singular detachment fold within the Niger Delta outer fold and thrust belt. The fold asymmetry and fault vergence change 180 degrees at several locations along the fold, even though the shortening increases monotonically from two minima at the fold terminations, to a single maximum at the fold crest, and no lateral transfer structures are present. Evolution of this fold has been studied using a combination of depth converted seismic data, well data, and structural modeling, including structural restorations, isopach mapping, and area balancing. The results show that the fold originated as a buckle fold and was subsequently cut by several basinward and landward dipping thrusts that penetrated the fold at different depth levels. The kinematic analysis shows that fold grew by limb steepening through progressive limb rotation and magnification of a single crest, around a fixed fold axis. Shortening rates vary from 20 m/Ma (0.2 mm/year) at the low amplitude ends of the fold, to 200 m/Ma (2 mm/year) at the crest.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009