THREE-DIMENSIONAL STRUCTURAL ANALYSIS OF A POPULATION OF NORMAL FAULTS IN A SCALED PHYSICAL MODEL

We used map-view photographs and serial cross sections spaced 1 mm apart to study the 3D geometry of normal faults in a scaled clay model. In the models, a 4-cm-thick layer of wet clay (consisting of colored sublayers) covered a rectangular latex sheet. Displacing one of the long edges of the latex sheet deformed the sheet and the overlying clay layer. After displacement of 2.5 cm, the clay thinned to ~3.6 cm. The deformation produced a population of normal faults with dip-slip slickenlines; the strike of these faults was subperpendicular to the displacement direction. Photographs of the top surface of the clay taken during deformation showed the nucleation, propagation, and linkage of the faults to produce segmented fault systems in which displacement varied considerably along strike. Offsets of the colored sublayers in cross-sectional view allowed us to recognize faults and to measure their displacements. The faults exhibit conjugate dip directions with dip angles averaging 68 degrees. The number of faults within the model generally increases with depth, and the distance between faults generally decreases with depth. The average displacement of the faults also generally decreases with depth. The base of the clay appears unfaulted, yet probably consists of many very small faults whose offsets are smaller than our detection ability. We analyzed the fault-surface geometry and displacement geometry of two faults in detail. Both fault surfaces are irregular, curving in three dimensions, which is consistent with the complex trace of faults cutting the top of the clay. For the small, relatively isolated fault, maximum displacement occurs near the center of the fault and decreases toward the fault tips; contours of fault displacement are approximately elliptical. The larger, more complex fault has three displacement maxima with intervening minima. Fault-segment linkage likely explains the complex displacement geometry. The areas of higher displacement could be the centers of originally smaller, isolated faults that linked together, and the areas of lower displacement could be fault-segment boundaries.