Discussion and conclusions

Anderson (1951) proposed that failure of homogeneous brittle material at a horizontal free surface should occur by reverse, normal or strike-slip faulting, but not by oblique-slip faulting. At depth, however, the orientation of principal stress axes need not necessarily be vertical or horizontal, and oblique-slip faulting might be possible. In our transpression experiments, initial failure in brittle layers is taken up by either nearly pure strike-slip faults or nearly pure thrust faults. This indicates that principal stresses throughout our model initially lie within sub-horizontal and sub-vertical planes. This implies that the thin layer of PDMS at the base of the model effectively decoupled the sand from the plexiglass bars and reduces the basal drag exerted on the sand by the plexiglass bars. Thus, in our models the basal shear stresses are considered to be negligible. Therefore, Anderson's theory seems appropriate for initial faulting in our experiments, which occurs in response to a stress field in which one principal stress direction is vertical and the two others lie within a sub-horizontal plane. Depending on the strain rate ratio, initial faults in as yet unfaulted granular material will be generated either as pure strike-slip faults or pure thrust faults. Once major faults have formed, however, the sand-glass powder cake consists of competent unfaulted material and incompetent dilatant fault zones. Additional deformation will then mostly be taken up by oblique-slip along favourably oriented pre-existing faults.

The fault pattern at different stages of the experiment provides important information on overall kinematics, stress field modifications and local partial partitioning of fault motion. The early fault style in transpression experiments clearly depends on the imposed ratio of shear strain rate and shortening strain rate. This ratio determines whether initial failure in brittle layers is accommodated by steep strike-slip faults (Riedel shears) or by thrust faults. In those experiments with a relatively high strain rate ratio (Ø3.6) steep strike-slip faults (dipping at 80-90°) formed early. Their en-echelon arrangement can be used as a kinematic indicator for the shear component of transpression (i.e. a left-stepping pattern indicates dextral transpression; whereas a right-stepping pattern would indicate sinistral transpression). The surface strike orientations of the early Riedels (24-37°) are larger than in distributed shear experiments (Schreurs, 2003; 17-24°) and reflect the shortening component of deformation. Obliquity of surface strike of early Riedel shears increases for decreasing strain rate ratio. In low strain rate ratio experiments (ì2.7) pairs of thrust faults (dipping at 30-45°) initially form striking parallel to the longitudinal sidewalls of the model. These faults have opposite vergence and bound pop-up structures.

Older faults determine to a large extent the subsequent fault pattern and evolution, because they are favourably oriented for reactivation. Where strike-slip faults develop initially, further deformation creates several major anastomosing fault zones, consisting of steep oblique-slip faults along which the strike-slip component dominates. Positive flower structures are characteristic of such convergent strike-slip fault zones. The sigmoidal trace of strike-slip faults that laterally become oblique-slip reverse faults can be used as kinematic indicator for the overall sense of shear, i.e. a “lazy” Z-shape for dextral shear and a “lazy” S-shape for sinistral shear component (see also Mandl 1988; Richard et al. 1995). In experiments where gently dipping reverse faults initially accommodate oblique deformation, an increase in strain leads to a fault pattern dominated by oblique-slip reverse faults.

Secondary faults forming in between earlier formed major fault zones reflect local stress field modifications that differ from the far-field stress system. Gently dipping reverse faults, striking very obliquely to the previously formed convergent strike-slip faults (experiment 1764), indicate that the maximum principal stress direction (s1) was locally reoriented sub-parallel to the major strike-slip fault zones. Partial partitioning of fault motion occurs late in experiments 1764, 1770 and 1820). Sub-vertical strike-slip faults generally formed late between or within pop-up structures. These strike-slip faults strike sub-parallel to oblique-slip reverse faults and are simultaneously active. The strike-slip faults usually merge at depth with oblique-slip reverse faults and generally have a curved fault trace (“lazy” Z-shape indicates a dextral shear and a “lazy” S-shape a sinistral shear component) and a dip direction which changes along strike. The close proximity of simultaneously active strike-slip faults and oblique-slip reverse faults indicates rapid lateral changes in the orientation of the principal stress axes.

There is good agreement between our experiments and natural examples of continental transpression (Schreurs & Colletta, 1998). Three-dimensional imaging of analogue models may provide constraints for geometric and kinematic interpretations of complex structures in natural zones of continental transpression.

Acknowledgments

Experiments were carried out at the Division Géologie-Géochimie of the Institut Français du Pétrole (Rueil Malmaison, France). Jean-Marie Mengus is thanked for technical assistance, and the scanner group for its help in obtaining the X-ray CT images. Research was funded by a grant from the Swiss National Science Foundation.

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