Influence of exhumation on the structural evolution of transpressional plate boundaries: an example from the Southern Alps, New Zealand

Concentration of erosional activity along transpressional plate boundaries can significantly alter the pattern of mechanical behavior through the influence of exhumation on crustal strength. Three-dimensional numerical modeling of an obliquely convergent orogen shows that a single oblique plate-bounding structure is stable if asymmetric erosion patterns, such as those observed in orographic mountain belts, pertain, and if Earth's crust has a strong-on-weak rheology. In early stages of oblique convergence of an initially laterally homogeneous material, lateral (boundary-parallel) strain is accommodated along a near vertical structure and convergent (boundary-normal) strain is concentrated on structures dipping at moderate angles into the orogen. Exhumation of deep crustal material along the convergent structure results in thermal weakening along this dipping structure. When the upper crust beneath the orogen is significantly weakened by exhumation, lateral strain abandons the vertical structure and shifts to the dipping structure, combining with the convergent strain to form a single oblique fault that accommodates the plate motion in the upper crust, as is the case along the Alpine fault, New Zealand. The process of thermal thinning is controlled by advection and occurs on time frames of ~1-2 m.y. The two components of strain remain separate in the lower crust. During active convergence, exhumation of lower crustal material occurs only along those structures accommodating convergent strain. Consequently, material exhumed from lower regions of ductile deformation, as is the case along the Alpine fault, contains lineations that indicate a greater component of convergence than predicted from the total plate motion. Postorogenic exhumation of the roots of an oblique plate boundary will expose two parallel shear zones, one dominantly convergent and one dominantly strike slip. Widely reported orogen-parallel transport in the late stages of ancient oblique convergence may represent not a change in plate vector, but the exhumation of the lateral transport zone. Keywords: numerical models, transpression, erosion, lineations.