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

We use clay models to simulate how fault population systematics vary as a f unction of rift obliquity. Rift obliquity is related to the acute angle, al pha, between the rift trend and the displacement direction, so that the val ue of alpha is inverse to the degree of obliquity. The range of azimuths in a fault population increases as rift obliquity increases (i.e. as alpha de creases). The length of the longest faults, the sum of fault lengths, and t he width of the deformed zone all increase as rift obliquity decreases (i.e . as alpha increases). The majority of faults in our models are segmented a nd have highly tortuous traces. Tortuosity is maximum when alpha = 30 degre es as segments of widely varying azimuth link during fault growth. Signific ant changes occur in the fault patterns between alpha = 30 degrees and alph a = 45 degrees. At 1-30 degrees two fault populations of equal importance d evelop in the center of the rift zone, one approximately rift-parallel and the other displacement-normal. Between alpha = 30 degrees and 45 degrees. t he number of faults more than doubles, and between alpha = 45 degrees and 6 0 degrees, summed fault length more than doubles. Fault patterns for all mo dels are fractal, with fractal dimensions that increase with increasing alp ha and that are comparable to those found in the field. Fault populations a re not multi-fractal because the cumulative frequency distributions of faul t lengths do not generally follow a power-law relationship. An exponential distribution best describes the data for whole faults, with the characteris tic length increasing with increasing alpha. Segment lengths also follow an exponential distribution with characteristic length varying very little wi th alpha. The fault patterns in our models resemble the spatial pattern of brittle deformation observed at oblique mid-ocean ridge segments and are si milar in geometry to those in oblique continental rift basins. At the rift margins, tensional stresses are modulated and reoriented by a secondary str ess field related to a change in boundary conditions, resulting in the form ation of two distinct sub-populations of faults during oblique rifting. (C) 2000 Elsevier Science Ltd. All rights reserved.