MECHANICAL MODELING OF FAULT-PROPAGATION FOLDS AND COMPARISON WITH OBSERVATIONS FROM SEISMIC REFLECTION DATA AND ANALOG MODELS

Fault-propagation folds are an important class of structures that form at the tips of faults as they propagate upwards through sedimentary layers. These structures are common in convergent and passive margin settings throughout the world. Understanding the relationship between fold shape, uplift, and displacement for these structures is critical to seismic hazard studies of blind thrust faults and to the characterization of petroleum systems. While all fault-propagation folds share basic similarities, significant natural variability in structural geometry is observed, which helps to explain the wide range of kinematic models that have been proposed to describe this class of structures.

The availability of a large volume of high quality seismic reflection data provides the opportunity to evaluate the success of existing models and to further quantify empirical relations between fold shape, fault dip, and slip. To this end, we have conducted a thorough study of several examples of fault-propagation folds in seismic reflection data sets in Nigeria, China, Canada, Argentina, and other locales. Evaluation of these data indicate that, in spite of a wide range in natural variability, there are basic relationships between fold shape and fault slip that are common to all fault-propagation fold structures. Specifically, we observe a characteristic linear decrease in displacement with distance up the fault, and a linear relationship between the maximum displacement on the fault and the maximum structural relief in the fold.

Motivated by these and other observations, we have developed a series of fault-propagation fold structures using the Discrete Element Modeling (DEM) approach. The DEM models employ an aggregate of circular, frictional disks that incorporate bonding at particle contacts to represent the numerical stratigraphy. A vertical wall moving at a fixed velocity drives displacement of a mechanically layered medium, which leads to the development of emergent faults and folds to accommodate shortening. By not imposing fault geometry, we are able to make inferences about the relationship between fault shape, fold shape, uplift, and displacement independent of imposed boundary conditions. Comparison with analog models provides further insight into the development of this class of structure.