Quantifying the geometry, displacements, and subresolution deformation in thrust-ramp anticlines with growth and erosion: From models to seismic-reflection profile

We quantify the evolution of thrust-ramp anticlines exhibiting growth and/or erosion using the area-depth-strain (ADS) method. We illustrate, validate, and explore the limitations of the ADS method for a series of kinematic models and experimental sand (analog) models. We then apply the method to a seismically imaged thrust-ramp anticline from the deep-water Niger Delta. The ADS method allows determination of the boundary displacements, displacement rates, and layer-parallel strains using the excess areas (areas of marker beds above their regional level) of multiple horizons. The kinematic and experimental models demonstrate that excess areas for pre-deformation units lie on a straight line on an area-depth graph, the inverse slope of which is the boundary displacement and the intercept is the elevation of the basal detachment. For growth strata, the line connecting an area-depth value to the value representing the basal detachment gives the displacement after deposition of that horizon. For partially eroded markers, the calculated displacements are less than the actual displacements; however, originally planar unconformities yield the displacements that accrued after the formation of the unconformities. The experimental sand models show substantial layer-parallel shortening in the anticlines, displacements on the main thrust ramp that are only half the displacement on the basal detachment, and small tectonic dilations. The seismic example is an area-balanced ramp anticline with clearly defined pre-deformation and growth sequences. Fault separation at the center of the ramp in the pre-deformation interval is approximately equal to the total boundary displacement but decreases both upward and downward. The layer-parallel strains have the opposite pattern, with the lowest values at the center of the ramp and increasing values both upward and downward.