From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, Oct 8, 2010
Use of the thickness-relief method to constraint shortening and shortening history in detachment folds and a ramp anticline. Examples from Japan, US and Nigeria.
Link to streaming video TBA: available 10.08.2010 at 8:55am
Dr. Ramon Gonzalez-Mieres
Interpreter Geophysicist - Chevron, Houston TX
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. Moving away from the typical kinematic analysis we develop a thickness-relief method 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. Using those measurements we obtain the shortening, mean shortening, layer-parallel pure-shear shortening, curvimetric shortening, and in some cases layer-parallel simple-shear angle, as well as the excess area 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. Here we are going to show the results for the Nankai Trough, the Cascadia fold and the Agbami anticline.
Our results show that  the structures are deformed by layer-parallel heterogeneous pure shear plus some horizontal compaction, with the exception of Agbami which shows layer-parallel stretching.  Shortening rate seems to be constant for a substantial part of the deformation history with some periods of quiescence and increase in deformation. Finally on more regional sense  the structures at the front of the deformation complex (Nankai and Cascadia) only consume 1 to 10% of the plate motion, indicating that shortening is not concentrated in these areas.
The last example of the methodology is on a ramp anticline at the Niger delta to obtain shortening components and distribution of this. Our results show 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 zone is overlaid by a sequence of constant shortening that preserve bed length and deform by flexure.