Influence of rift obliquity on fault-population systematics: results of experimental clay models

Abstract

We use clay models to simulate how fault population systematics vary as a function of rift obliquity. Rift obliquity is related to the acute angle, α, between the rift trend and the displacement direction, so that the value of α is inverse to the degree of obliquity. The range of azimuths in a fault population increases as rift obliquity increases (i.e. as α decreases). The length of the longest faults, the sum of fault lengths, and the width of the deformed zone all increase as rift obliquity decreases (i.e. as α increases). The majority of faults in our models are segmented and have highly tortuous traces. Tortuosity is maximum when α=30° as segments of widely varying azimuth link during fault growth. Significant changes occur in the fault patterns between α=30° and α=45°. At α=30° two fault populations of equal importance develop in the center of the rift zone, one approximately rift-parallel and the other displacement-normal. Between α=30° and 45°, the number of faults more than doubles, and between α=45° and 60°, summed fault length more than doubles. Fault patterns for all models are fractal, with fractal dimensions that increase with increasing α and that are comparable to those found in the field. Fault populations are not multi-fractal because the cumulative frequency distributions of fault lengths do not generally follow a power-law relationship. An exponential distribution best describes the data for whole faults, with the characteristic length increasing with increasing α. Segment lengths also follow an exponential distribution with characteristic length varying very little with α. The fault patterns in our models resemble the spatial pattern of brittle deformation observed at oblique mid-ocean ridge segments and are similar in geometry to those in oblique continental rift basins. At the rift margins, tensional stresses are modulated and reoriented by a secondary stress field related to a change in boundary conditions, resulting in the formation of two distinct sub-populations of faults during oblique rifting.