Discussion

 

Area-Depth Strain analysis¹ is an extremely useful, model-agnostic method for detemining the depth of a horizontal detachment using only the shape of the deformed layers above the detachment (see this video for an example of area-depth strain analysis of a Niger Delta section).  However, many folded structures form above faults that dip to considerable depth.  In petroleum exploration and development, locating the controlling fault is a matter of practical importance. Seismic and well data may constrain the upper fold geometry but  the standard Area-Depth Strain analysis method is not sufficient to determine the dip and location of the controlling fault.  

StructureSolver structural geologists have generalized the Area-Depth Graph method to handle dipping faults². We refer to this generalized method as the Fault Trajectory method and we have implemented it in an intuitive and fully interactive manner in StructureSolver.  In this video, we demonstrate the application of the Fault Trajectory method to synthetic examples of dipping contractional and extensional faults, and then show analysis of the major fault propagation fold structure beneath the Bermejo Basin, adjacent to the Sierras Pampeanas, Argentina.

The Bermejo Basin structure is a fault propagation fold that deforms Paleozoic and Cenozoic sediments in the upper section as well as the Precambrian metamorphic and Cenozoic plutonic rocks that make up the basement.  The seismic data resolves much of the structural details in the upper sedimentary section, but becomes less well resolved below the basement-sediment interface.

As you can see, the pre-growth horizons on the distal hangingwall are elevated above the footwall but nearly flat. This is consistent with a planar but dipping fault at depth.  So, we use the Fault Trajectory method of the Area-Depth Strain Analysis tool in StructureSolver to estimate the dip and depth of the fault.  The fault location and dip calculated from the Fault Trajectory method aligns nicely with the deep event within the basement.  

In addition, the layer parallel strain profile shows increasing layer parallel shortening towards the top of the fault. This is consistent with kinematic expectations for fault propagation fold behavior where fault displacement is progressively consumed by folding towards the final fault tip point.  

¹ Epard and Groshong 1993, Excess Area and Depth to Detachment, AAPG Bulletin, V. 77, No. 8

² Eichelberger, Nunns Groshong, and Hughes, 2015, Predicting the DIp and Location of Master Faults Beneath Forced Folds, Geological Society of America Abstracts with Programs, v. 47, n. 7, p. 511