3D ANALYSIS OF SMALL-SCALE FAULTING ASSOCIATED WITH OBLIQUE SHORTENING: A SCALED EXPERIMENTAL MODELING STUDY

Although oblique shortening occurs in many locations throughout the world (e.g., parts of the San Andreas fault, California, USA, and the Alpine fault, New Zealand), the associated deformation, especially the small-scale fault patterns, are not well understood. We have examined the small-scale, 3D deformation patterns produced by oblique shortening using a scaled experimental clay model. The basal boundary condition consisted of a rigid plate moving beneath a discontinuity oriented 20˚ to the movement direction of the plate. The resulting ratio of strike slip to shortening on the discontinuity was 2.7/1. The clay model was ~ 4 cm thick with 11 mechanically identical colored layers. Map-view photographs of the model surface and closely spaced serial sections (~1.4 mm apart) through the model allowed us to map a high-angle main fault system, large- and small-scale folds, and, most importantly, small-scale deformational features.

Our work suggests that Reidel shears, faults splays and fault-bounded lenses formed in both map and cross-sectional views. In map view, the Reidel shears were subtle features with both R shears and R’ shears developing near the main fault system. Fault splays branched off the main fault system and died out along strike. Fault-bounded lenses formed along strike, creating bulges that connected on both ends. In cross-sectional view, fault splays branched off the main fault system and died out up-section into fault-propagation folds. Narrow, fault-bounded lenses were common in the cross sections. They rejoined the main fault system at their top and bottom. In cross-sectional view, the predominant small-scale deformation was the fault-bounded lenses. In conclusion, the experimental model shows that the small-scale deformation associated with oblique shortening is complex with Reidel shears, fault splays and fault-bounded lenses present in both map view and cross-sectional view.