INTERPRETING DISPLACEMENT AND STRAIN IN GROWING THRUST-RAMP ANTICLINES: FROM MODEL TO SEISMIC REFLECTION PROFILE

Thrust-ramp anticlines are the characteristic structures in most foreland fold-thrust belts. Typical interpretation questions include: Is the structure balanced? What is the orogenic shortening? What is the growth history? Can subresolution strain be inferred? We address these issues for growing thrust-ramp anticlines with three examples: a geometric (kinematic) model, an experimental (analogue) model, and a seismic-reflection profile of a thrust-ramp anticline from offshore Nigeria. We show that the area-depth-strain (ADS) method, an area-balance technique, yields accurate quantitative information without assuming a kinematic model and which avoids the problems that may arise in interpretations based on the assumption of constant bed length. The ADS method makes it possible to clearly discriminate between growth and pregrowth units at a change in slope on a graph of excess area versus depth. For pregrowth horizons, the inverse slope of the line through the data points gives the total displacement and the depth to the lower detachment. For a growth horizon, the displacement since deposition is given by the inverse slope of the line connecting its area-depth point to that of the lower detachment. From this information, it is possible to calculate displacement rates, subresolution strain and, for the experimental model with known boundary displacement, the dilation. The experimental model shows layer-parallel shortening of up to ~17% with corresponding layer-normal thickening. The differences between calculated and imposed displacement indicate positive dilation up to +5%, not tectonic compaction as inferred from methods assuming constant bed lengths. The measured displacement on the thrust ramp is ~30% less than the boundary displacement. The strain magnitudes are sensitive to the specific interpretation of bed length vs. fault length but the other values (detachment depth, displacements, dilation) are relatively insensitive to this. The seismic profile is balanced, with layer-parallel shortening up to 23%. The pregrowth interval shows a partitioning between internal strain and fault separation in which the lowest-strain markers have fault separations only 2% less than the calculated total.