IS HANGING WALL FOLD GEOMETRY ADEQUATE FOR DETERMINING SUBSURFACE NORMAL FAULT SHAPE?

Since Laubscher (1956) and Hamblin (1965) first proposed a direct relationship between folding of hanging wall strata (reverse drag) and the geometry of underlying normal faults, many geologists have developed and employed area-balancing methods to relate fold and fault geometry in extensional terranes. These “kinematic” methods posit that folding occurs in response to a “space problem” associated with slip on a curved fault plane and thus require a listric fault shape in order to generate reverse-drag folds. Others have noted, however, that similar hanging wall folds are predicted by mechanical models of planar faults that extend to a finite depth in the crust. We employ a boundary element method (BEM) based on the solution of a two-dimensional dislocation discontinuity within an elastic half space to directly compare patterns of displacement, stress, and strain around planar and listric faults and evaluate criteria that may be used to determine subsurface fault geometry from observations of near-surface deformation. Models that incorporate a) a planar fault dipping 60 degrees with constant slip, b) a planar fault dipping 60 degrees with constant slip ending at a free-slipping horizontal detachment, and c) a listric fault dipping 60 degrees at the surface with constant slip ending at a free-slipping detachment all develop hanging wall reverse-drag folds whose width increases only slightly with introduction of the detachment and listric fault shape. All models also predict a region of tension and elevated maximum shear stress in the hanging wall starting at a distance of ~70% of the fault depth from the surface trace of the fault and extending further into the hanging wall. This region expands away from the fault with introduction of a detachment and listric fault geometry. The most apparent difference between the three models is the magnitude of footwall uplift. Footwall uplift decreases slightly with introduction of the detachment and more significantly with the addition of a listric fault shape. Footwall fold width and the ratio of footwall uplift to hanging wall subsidence are thus sensitive to fault geometry. A combination of these observable features may therefore provide a tool to estimate fault geometry rather than simply inferring listric geometry based on the presence of reverse-drag folds.