MODEL STRAIN AND MODEL FABRIC DEVELOPMENT IN 3D OBLIQUE OROGENS

The development of metamorphic fabric within model crustal assemblages during orogenesis can be predicted on the basis of the mineralogy and the amount and orientation of strain to which rock packets are subjected as they transit through an orogen. Characteristic strain regimes evolve in oblique orogens and become approximately fixed in Cartesian space for steady state rheologies dominated by quartz and feldspar, thereby serving as foundries through which rock and fluid packets pass. The duration that a packet remains in any of these characteristic strain regimes strongly influences the fabric development and is a function of rheological and boundary conditions including mantle strain and surface processes. Here we define, for a simple three-dimensional oblique orogen analogous to the Southern Alps of New Zealand, the geographical space of these fields of strain for steady state, thermally-dependent rheologies as well as for strain/time-dependent rheologies. Fabric development in the straining regions with steady-state rheology occurs over volumes that extend laterally for distances >20km, are >5km thick, and should produce an anisotropic fabric that can be imaged by shear waves.

Exhumation of deep crustal roots from natural, ultra high-pressure terrains, however, indicates significant kinetically-controlled transient rheological behavior leading to strong strain localization into high strain zones that are thin relative to seismic wavelengths. Reconciliation of model with natural rheological behavior presents a significant challenge that must be met before interpretation of seismic anisotropy in the ductile parts of the crust can become routine.