COMBINING MULTIPLE QUANTITATIVE STRUCTURAL ANALYSIS TECHNIQUES TO CREATE ROBUST STRUCTURAL INTERPRETATIONS

Using high quality seismic reflection data of a fault propagation fold from offshore Nigeria, we combine fault-bend and trishear modeling, area-depth computations, distance-displacement relationships and structural restorations to develop a consistent quantitative structural interpretation. These methods constrain detachment depth, horizontal contraction, internal shear angles and fault propagation history.

Modeling trishear deformation ahead of a propagating fault tip, we reproduce the 2-D forelimb geometry of the structure. The trishear framework confirms a published fault propagation fold model and also accurately models the geometry of both pre-growth and growth horizons.

The variation of bed area above regional with depth (area-depth relationship) definitively confirms that the fault flattens into a horizontal detachment. We model the entire structure using a combination of trishear for the forelimb and fault-bend related shear for the backlimb. Modeling the backlimb geometry with the interpreted detachment depth requires a shear-axis angle of about 40°. In the interpreted data, two back-thrusts rise at a similar shallow angle from the bend where the fault ramps up. The back-thrusts are analogous to antithetic shear axes and accommodate folding related to the main fault bend. Horizontal displacements on the detachment independently calculated from modeling and area-depth agree within 10%, but exceed the shortening based on horizon length measurements. This discrepancy suggests significant sub-seismic layer-parallel shortening within the structure.

Distance-displacement relationships on the main fault show that as folding increases towards the final fault tip, displacement decreases, consistent with our trishear model. The main fault has an initial tip point deep within the pre-growth section and a final tip point near the base of the growth section. The back-thrusts have nearly constant displacements along their traces, indicating that these faults developed rapidly. These relationships are clearly illustrated by restoring the section above a horizon that is at the level of the initial tip point on the main fault.

We illustrate the overall kinematic development of the structure with a quantitatively balanced animation including both the main fault and the back-thrusts.