3-DIMENSIONAL NUMERICAL EXPERIMENTS OF STRAIN PARTITIONING AT OBLIQUE PLATE BOUNDARIES - IMPLICATIONS FOR CONTRASTING TECTONIC STYLES IN THE SOUTHERN COAST RANGES, CALIFORNIA, AND CENTRAL SOUTH ISLAND, NEW-ZEALAND

A three-dimensional numerical model is used to investigate the nature and distribution of crustal-scale deformation and strain partioning across obliquely convergent, transpressive, plate boundaries. A uniform elastic-plastic crustal layer, obeying Murrell's extension to three dimensions of the Griffith failure criterion, was deformed under basal kinematic boundary conditions that correspond to the oblique convergence between two mantle lithospheres, one of which detaches and subducts beneath the other. The results show deformation concentrated in relatively narrow shear zones. The character (planar versus curved) and dip of these shear zones (or crustal-scale faults) depend on the relative amount of convergence versus transcurrent motion imposed across the plate boundary. In models dominated by transcurrent boundary conditions, two separate sets of coexisting structures develop: (1) a pair of planar, relatively shallow-dipping thrust zones arranged in a ''V'' structure that accommodate convergent motion; and (2) a set of curved, near-vertical strike-slip zones arranged in a ''flower'' structure that accommodates the transcurrent motion. In models dominated by convergence, both contractional and transcurrent deformations are accommodated by oblique slip across a single pair of structures that have the same geometry as the thrust shear zones. There is a gradational behavior between these end-member partitioned and nonpartitioned modes for intermediate boundary conditions. In all cases the dip of the shear zones predicted by the numerical model is in good agreement with the dip of ideal faults predicted by Mohr's incipient faulting hypothesis. The two basic modes are interpreted to underlie the contrasting tectonic styles of the southern Coast Ranges, California, and central South Island, New Zealand. Factors other than obliquity of convergence which play a role in strain partitioning are discussed.