Transtensional faulting patterns ranging from pull-apart basins to transform continental margins: an experimental investigation

Seventeen small-scale experiments were performed to study the deformation induced in brittle-ductile models by a releasing stepover between two transform faults, and by intersection between transform and divergent plate boundaries. In both cases, faulting depends on the rheological layering (presence and strength of a ductile layer at depth) and the width of the basal stepover. Successive types of pull-apart basins are observed in the first set of experiments, which are compared with natural examples. Firstly, a lazy Z-shaped basin appears, which is bounded by Y faults above transform boundaries and R faults propagating from each corner of the stepover. Then, R' Faults replace R faults, leading to a rhomb-shaped graben. At every stage, the basin length-to-width ratio remains between 2.2 and 3.8, suggesting that scale independence of pull-apart basins is related to the geometrical shape of bounding faults. In a second set of experiments, deformation at the end of a transform boundary is characterized by a horsetail splay bounding a divergent basin. Within the horsetail splay, block tilting and block rotation about vertical axes lead to a surface slope perpendicular to the slope of the divergent basin, a feature that can be compared with marginal ridges at transform margins. (C) 1998 Elsevier Science Ltd. All rights reserved.