Changes in fault displacement populations correlated to linkage between faults

Abstract

In Carboniferous strata exposed at the western margin of the southern Appalachian (U.S.A.) fold—thrust belt, mesoscopic faults accommodated strains as the Cumberland Plateau thrust sheet moved ≤ 1 km WNW on a sub-horizontal detachment. These faults are confined to a layer ∼ 75 m thick above the detachment. In the upper part of this ∼ 75 m thick layer, a laterally persistent coal seam separates two stratigraphic units subjected to different amounts of sub-horizontal elongation. Distinct sets of mesoscopic faults that cut bedding at high angles and have normal offsets accommodate the strains in each unit. Below the coal seam, a sample line ∼ 500 m long intersects 504 faults. A plot of log N (size rank) vs log d (dip separation in mm) for these faults is linear, suggesting that a scaling law N = ad −c holds for this population. Least-squares regression gives c ≈ 0.8 and log 10 a ≈ 3.1. Above the coal seam, a sample line ∼250 m long intersects 745 faults. A plot of log N vs log d for these faults has two distinct linear trends. Faults with separations ≤ 1.25 m yield a scaling law N = ad −c with c ≈ 0.5 and log 10 a ≈ 2.9. Seventeen faults with separations d ≥ 1.25 m define a second linear segment N = a′ d −c′ , with c′ ≈ 1.8 and log 10 a′ ≈ 6.9. The break in slope at d ≈ 1.25 m corresponds to faults with heights of ∼30 m, roughly equal to the thickness of the faulted strata. Faults here with d ≤ 1.25 m and d ≥ 1.25 m, which exhibit different scaling characteristics, are, respectively, analogous to faults within and those that extend across the brittle crust in regional studies. Comparing the two displacement populations described here, small-offset faults make a proportionally smaller contribution to the total strain in the larger strain setting. This pattern, which is observed elsewhere, suggests that fault systems within individual layers exhibit different scaling characteristics at different stages in their history. This contribution argues that fault linkage is the key to such changes in fault scaling behavior; it is a mesoscopic to macroscopic structural change that alters the subsequent development of faults. Fault linkage, like fault size or sampling dimension, should be considered in examinations of fault scaling laws.