ANALYSIS OF STRAIN PARTITIONING IN ANALOG OBLIQUE CONVERGENT WEDGES

Oblique plate motion occurs along many convergent margins in the Circum-Pacific and is understood to be a primary factor in determining the style and location of deformation. I have used analog models with quantitative analysis of deformation to gain additional insight into the mechanics of strain partitioning in convergence, in particular, the role of ductile deformation at depth. I have simulated the evolution of oblique double wedges with frictional and layered (frictional over viscous) rheologies. In these experiments I quantify the margin-normal velocity, strain rates, and topography, and the 2D strain, shear and rotation fields. The relative obliquity in these experiments is large, 60º, so significant partitioning of deformation was expected in both wedges, regardless of rheology. The pure frictional wedge is characterized by numerous discrete thrust faults in the pro-wedge and a zone of shear between the pro-wedge and the retro-wedges. The highest rate of contractional deformation is at the thrust front, while the highest rate of shear is isolated on sub-parallel and near-vertical faults at the back of the pro-wedge. The zones of active deformation are narrow compared to the cross-sectional width of the frictional wedge. Overall, the layered model has discrete fault structures on which strain is accommodated but the relative width of the actively deforming zone is substantially wider than that for the purely frictional wedge. And several faults in the pro-wedge are actively slipping at any given time. The layered wedge is better able isolate shear behind the pro-wedge and to partition strain into margin-normal and margin-parallel components. In both experiments the timing of contractional and shear strain were fairly uniform throughout and primarily a function of total back wall displacement and margin obliquity. However, in the layered wedge some coeval margin-normal oriented normal faulting occurred when topography became large and after convergence ceased.