THREE-DIMENSIONAL GEODYNAMIC MODELING OF OBLIQUE OROGENESIS: APPLICATION TO NEW ZEALAND AND THE ACADIAN OROGEN OF MAINE

Vertical and horizontal strain distribution within oblique convergence reflects imposed tectonic and surficial boundary conditions as transmitted via the rheological structure of the deforming crust. Our knowledge of crustal rheology is poor at best and it is only by combining three-dimensional mechanical modeling with geologic and geophysical observations that we can hope to define the rheological structure of a deforming zone. In order to do so, we begin with a region where the far field driving forces of plate motion and the orogen kinematics are well defined. Results and insights gained from these well-constrained models are then available to be applied to other regions with less well defined far-field tectonic or surficial boundary conditions, such as in the exposed roots of ancient mountain belts, e.g., the Acadian of Maine.

The tectonically active Southern Alps of New Zealand provide an ideal setting in which to study the geodynamic response of a collisional orogen to far-field tectonic boundary conditions and to test the influence of rheological variation on that response. Using a three-dimensional mechanical framework in conjunction with geological and geophysical observations, we define the characteristic strain regimes for an oblique orogen analogous to the Southern Alps of New Zealand. The three-dimensional mechanical models allow us to separate those features in the kinematic field that arise from boundary condition variation from those that result from heterogeneous and transient rheological structure. Using full 3D solutions allow us to compare model predictions of metamorphic fabric with observations of seismic anisotropy and exposed metamorphic fabrics.