Reverse drag revisited: Why footwall deformation may be the key to inferring listric fault geometry

Abstract

Although reverse drag, the down warping of hanging wall strata toward a normal fault, is widely accepted as an indicator of listric fault geometry, previous studies have shown that similar folding may form in response to slip on faults of finite vertical extent with listric or planar geometry. In this study we therefore seek more general criteria for inferring subsurface fault geometry from observations of near-surface deformation by directly comparing patterns of displacement, stress, and strain around planar and listric faults, as predicted by elastic boundary element models. In agreement with previous work, we find that models with finite planar, planar-detached, and listric-detached faults all develop hanging wall reverse-drag folds. All of these model geometries also predict a region of tension and elevated maximum Coulomb stress in the hanging wall suggesting that the distribution and orientation of near-surface joints and secondary faults may also be of limited utility in predicting subsurface fault geometry. The most notable difference between the three models, however, is the magnitude of footwall uplift. Footwall uplift decreases slightly with introduction of a detachment and more significantly with the addition of a listric fault shape. A parametric investigation of faults with constant slip ranging from nearly planar to strongly listric over depths from 1 to 15 km reveals that footwall fold width is sensitive to fault geometry while hanging wall fold width largely reflects fault depth. Application of a graphical approach based on these results as well as more complete inverse modeling illustrates how patterns of combined hanging wall and footwall deformation may be used to constrain subsurface fault geometry.