Fault Surface Maps: Three-dimensional Structural Reconstructions and Their Utility in Exploration and Mining
Authors
Mineral deposits that have been dismembered and tilted by syn- or post-mineral faults can be structurally reconstructed to achieve scientific and practical objectives. Fault surface maps are plan-view projections of the geology of the footwall and hanging-wall surfaces of a fault, showing rock units, faults, and other features. Here, we describe how to construct fault surface maps, how to present them efficiently in publications, and how to use them to make three-dimensional, stepwise structural reconstructions, restoring the hanging-wall map against the footwall map. Areas of complex geology containing multiple generations of faults with differing slip directions especially benefit from the use of fault surface maps for three-dimensional reconstructions. Where there are adequate geologic markers and three-dimensional control, the restoration results in numerous slip vectors across the fault surface from which the magnitude of slip in the plane of the fault can be calculated for each vector. Additional geologic constraints are required to assess the amount of tilting that occurred concurrent with slip on each generation of faults. To achieve a three-dimensional structural reconstruction of an area, the restoration process is repeated from youngest to oldest faults, working backward in time.
Older faults appear as single lines, commonly known as cutoff lines, on both the footwall and hanging wall surfaces of younger faults. Faults that moved synchronous with the fault surface being mapped (e.g., splays of the same fault) appear as a single branch-line trace in the same position in both footwall and hanging wall maps. Younger, crosscutting faults appear as gaps in the fault surfaces of older faults that are identically positioned on footwall and hanging wall maps, and the gaps widen in the direction of increasing displacement or decreasing dip of the younger fault. Examples from normal faults in the Robinson and Yerington mining districts, Nevada, illustrate the differing appearances of fault surface maps of early generations of faults versus later generations of faults. The types of movement that achieve a restoration differ depending on whether the portion of a normal fault being restored is near or far from either tip where the displacement diminishes to zero. Near-tip regions commonly display an important rotational component pinned near the tip, whereas mid-fault regions are dominated by a down-dip translational component.
Fault surface maps provide an efficient method for portraying the locations and types of three-dimensional geologic controls, thereby allowing readers to see more clearly where geologic interpretations are well constrained versus loosely constrained by the available data. This allows users to evaluate the validity of a proposed reconstruction better, to develop and evaluate alternative hypotheses more easily, and to dispense with unviable alternatives more readily. Structural reconstructions historically have led to discoveries of offset continuations of known orebodies and of new orebodies. Use of fault surface maps for reconstructions facilitates a more three-dimensional approach to mineral exploration in complexly faulted regions, such as certain parts of the Basin and Range province.
Fig. 1. Construction of a geologic fault surface map. (a) Perspective view of block diagram; (b) map view of surface of blocks and exposed fault surface; (c) cross section oriented normal to strike of fault, showing vertical projection of footwall and hanging-wall traces onto horizontal map surface; (d) surface projection of the footwall geology, showing piercing points in footwall; (e) surface projection of the hanging wall geology; piercing points are shown for hanging wall (solid) and footwall (dashed) sides of the fault surface. Vectors pointing toward hanging wall show horizontal projection of slip, i.e., heave component, indicated by offset of piercing points. Reconstruction vectors would be same length but would point in the opposite direction as slip vector, i.e., toward footwall. Elevation difference between head and tail of heave gives throw component. Total slip = sqrt(heave2 + throw2).
