UNDULATIONS ON NORMAL-FAULT SURFACES: INSIGHT INTO FAULT GROWTH USING SCALED PHYSICAL MODELS OF EXTENSION

We have used scaled experimental models with wet clay to study undulations on normal-fault surfaces; similar undulations or grooves occur on many natural normal-fault surfaces. The models simulate extensional deformation using three common basal boundary conditions: overlapping plates simulate a detached normal fault, a rubber sheet simulates distributed deformation above a thin ductile layer, and a layer of putty simulates distributed deformation above a thick ductile layer. For selected faults within each model, we produced structure-contour maps of fault surfaces using closely spaced serial sections. Fault surfaces in all models have two types of undulations. Large-scale variations in fault strike are likely due to the linkage of originally separate fault segments. Small-scale undulations trend subperpendicular to fault strike, parallel to the slip direction, and are present along the entire extent of fault surfaces. The undulations are not tool-and-groove slickenlines because the length of the undulations exceeds the net slip. We observed the undulations on exposed fault scarps during the model run; thus, they are not artifacts resulting from the construction of the structure-contour maps. Many researchers ascribe the curvature of normal-fault segments in map and cross-section views primarily to lithologic variations. However, lithologic changes cannot be the cause of the undulations in our models because the clay is virtually homogenous. Additionally, fault-segment linkage cannot fully explain the small-scale undulations because a small, relatively isolated fault exhibits the same type of undulations as the larger faults. These undulations may be a result of a fundamental aspect of fault growth: the incorporation of non-coplanar, small-scale fractures onto the expanding fault surface, which has a nearly rectangular geometry (not the commonly assumed elliptical geometry) with lateral tip lines subparallel to the slip direction.