Thickness-relief methods for the analysis of shortening, with examples of active detachment folds and ramp anticlines

The recent availability of essentially complete seismic images of deformed structures has brought new opportunities for quantitatively deciphering the deformation and history of structures based on analysis of the shapes and thickness variations of the complete set of deformed layers. This thesis develops new thickness-relief methods for determining shortening as a function of height in well-imaged structures based on measurements of area of relief and bed lengths for many layers, which are used to obtain the shortening S(z), mean shortening S¯( H), layer-parallel pure-shear shortening S epsilon (z), layer-parallel simple-shear angle alpha( z), and curvimetric shortening Sc( z), as well as the excess area Ae caused by flow in weak layers and the history of shortening. The primary measurements of the thickness-relief method are made in the thickness domain rather than the depth domain, because it allows a more accurate separation of the stratigraphic thickness variations from the actual deformed shape of the structure.; The analysis is initially applied to active detachment folds in the Nankai trough in Japan, the Cascadia subduction front in Oregon, the Yakeng anticline in China and the Agbami anticline in the Niger delta to gain understanding of their kinematic behavior and shortening history. Results show that [1] three structures are deformed by layer-parallel heterogeneous pure shear plus horizontal compaction, with the exception of Agbami which shows layer-parallel stretching. [2] Shortening rate seems to be constant for a substantial part of the deformation history with some periods of quiescence and increase in deformation. [3] The structures at the front of the deformation complex only consume 1 to 10% of the plate motion, indicating that shortening is not concentrated in these areas. Finally two ramp anticlines at the Niger delta and the Cascadia front offshore Vancouver Island are analyzed to obtain shortening components and distribution of those. Results show for the first time the existence of a basal shear zone formed by a dominant heterogeneous layer-parallel simple shear component and a minor pure-shear component that reaches a maximum of 25% of total shortening. These shear zones are overlaid by sequences of constant shortening that preserve bed length and deform by flexure.