Insights into the mechanics of fault-propagation folding styles

Abstract

Fault-propagation folds are common structures that accommodate crustal shortening in various compressional settings worldwide. Motivated by the wide range of geometries observed for fault-propagation folds, we investigated the role played by mechanics in the variations observed for this structural class. Detailed structural measurements of a series of 15 fault-propagation folds from the Niger Delta, Argentina, and southeastern Asia reveal several relationships between aspects of the structural geometries. We found that the decrease in displacement updip along the fault is well approximated by a linear trend that has a relatively consistent slope, and that this gradient remains constant for increasing total displacement. This suggests that the faults propagate self-similarly, consistent with a range of kinematic models that have been used to describe them. Additionally, we observed that uplift has contributions both from rigid translation along a dipping fault and folding, and that the values observed for many natural structures lie between those predicted by the trishear and kink-style models, such as fixed-axis and constant-thickness fault-propagation folding. Finally, we found that fault-propagation folds exhibit a range of fault dips, with many structures having fault dips coincident with those characteristic of fault-bend folds, while another group is characterized by significantly higher fault dips. By developing a series of discreteelement mechanical models, we found that mechanical layering plays a first-order role in the development of different styles of faultpropagation folding. Homogeneous materials produce trishear-like fault-propagation folds, while strongly layered materials produce structures more similar to the kink-style kinematic models. Comparison with the observations from natural structures indicates that these mechanical models reproduce the observed trends, and that most natural structures fall between these two styles of models. This suggests that trishear and the kink-style (fixed-axis and constant-thickness) faultpropagation folding models may be thought of as end members on a continuum of possible fault-propagation folding geometries that are largely dictated by the degree of mechanical layer anisotropy in the stratigraphy. Finally, we suggest that fault steepening in highly anisotropic models may develop due to strain localization in tightly folded structural forelimbs. † Current address: Chevron Energy Technology Company, A Distance along fault Displacement B Figure 1. (A) Schematic depiction of a typical fault-propagation fold (adapted from Shaw et al., 2005), characterized by asymmetric limb widths and dips, a decrease in displacement along the thrust fault toward the fault tip (1) and an active synclinal axial surface pinned to the fault tip (2). (B) A decrease in displacement as a function of distance along the fault toward the fault tip.

How do horizontal, frictional discontinuities affect reverse fault-propagation folding?

“…This occurrence has been observed also in studies based on mechanical models that have analyzed the role of weak layers or mechanically heterogeneous stratigraphy (e.g. Hardy and Finch, 2007;Albertz and Lingrey, 2012;Albertz and Sanz, 2012;Hughes and Shaw, 2015). This suggest that both thin frictional discontinuities and thin weak layers impact on the relationships between faults and folds.…”

“…4 and 7). Hence, the trishear method can be successfully applied to cases displaying a limited variability of the mechanical properties of the rocks hosting the propagating faults, as already suggested by previous studies (Cardozo et al, 2003;Hardy and Finch, 2007;Albertz and Sanz, 2012;Hughes and Shaw, 2015). In the non-isotropic experiments the reactivation of precuts has a deep impact on the displacement pattern (Figs.…”

“…Hence, in layered rocks the kinematic models based on kink-fold models could be successfully applied (e.g. Hughes and Shaw, 2015). Notice that a variation of the displacement vectors is still visible, which is a typical characteristic of the trishear theory.…”

“…Roering et al, 1997), finite element methods (FEM; e.g. Albertz and Lingrey, 2012), and discrete element methods (DEM; Hughes and Shaw, 2015).…”

Response of unconfined turbidity current to deep‐water fold and thrust belt topography: Orthogonal incidence on solitary and segmented folds

“…Among the theories describing the geometrics and kinematics of fault‐propagation folding, the most commonly used are the constant‐thickness and fixed‐axis model (Suppe & Medwedeff, ) and the trishear model (Erslev, ; Allmendinger, ; Cardozo et al ., ; Cardozo, ; Hughes & Shaw, ). The latter model is applied in the present study, because it best replicates curved fold surfaces (Erslev, ).…”