The mechanics of first order splay faulting: The strike-slip case

First order splay faults, as defined here, are secondary faults that form at acute angles symmetrically on either side of a primary fault of the same sense of shear. We show that these faults form when the primary fault becomes critically misaligned with the principal stresses such that splay fault formation, on the optimum plane for faulting, is favored. First order splay faults, in distinction from other splay faults, are secondary only in the temporal sense – they are subsequent but not subordinate, in a tectonic sense, to the primary fault. Here we analyze the case of strike-slip faults, and compare it with data for several continental transform fault systems, where we show that the splay faults form in the most favorable direction: parallel to the plate motion vector. We also discuss and speculate on several outstanding problems with regard to first order splay faults: the placement of them in space, means by which primary faults become misoriented in the stress field, and the mechanics of first order splay fault-primary fault junctions, once formed.