Fault spacing in the El Teniente Mine, central Chile: Evidence for nonfractal fault geometry

We utilize the excellent rock exposure provided by the network of mining tunnels in El Teniente Mine, Central Chile, to test the applicability of fractal fault spacing laws to a ∼2 km × 3 km × 1.3 km body of crystalline rock in an Andean shear zone. Fractal spacing laws can be particularly useful because they allow prediction of fault spacing at unsampled scales. Because of the restrictions of tunnel geometry in the mine we use only “scanline”, one-dimensional, analyses. The large number of parallel tunnels allows us to assess the consistency of one-dimensional scanline samples over a two-dimensional area. We use two methods: 1) an interval counting technique and 2) standard cumulative statistics to test the hypothesis of fractal fault geometry in the shear zone. The interval counting method produces deceptively good fits to a fractal distribution. In fact, by analyzing the residuals and differential slopes of interval counting and cumulative statistical plots and by comparison with synthetic fractal, negative exponential, and log-normal distribution data, we conclude that a negative exponential distribution best describes fault spatial distribution at the mine. We present three possible explanations for the negative exponential spacing distribution: 1) the rock body is at the earliest stage in a deformational path that will produce varying spacing distributions as deformation proceeds, 2) the spacing distribution is inherited through joint reactivation, and 3) the present spacing distribution is the result of a superposition of two spacing distributions each of which is not necessarily negative exponential. Additionally, we attempt to use the deviation of mean spacings in parallel scanlines (after Wu and Pollard (1995)) to further classify deformational maturity. We show that this method cannot be used to compare population development in faults which follow a negative exponential spacing distribution because of the negative exponential's equivalence of mean and standard deviation. Instead we use a relative measure of parallel scanline deviation to compare regions. The distinctly smaller standard deviation of mean spacings in one region (the T3 Isla region) may be a signature of increased local finite strain accumulation.