MODELING LOCALIZATION OF DEFORMATION AND FLUID-FLOW IN A COMPRESSIONAL OROGEN - IMPLICATIONS FOR THE SOUTHERN ALPS OF NEW-ZEALAND

Numerical experiments of coupled deformation and fluid flow, in a non- associated elastic-plastic (Mohr-Coulomb) material undergoing compress ion, illustrate the effect that the plastic properties off the materia l and the presence of a pore pressure can have upon the deformation, B oundary conditions imposed on the models are such that they imitate tw o-dimensional orthogonal continental collision, Deformation in a dry m aterial is transient with shear zones moving through the material towa rd the stationary boundary condition. In contrast deformation in a wet material. localizes into a paired synthetic and antithetic shear, wit h a small amount of deformation occuring between the two shears. Mater ial softening due to a strain-dependent friction angle or cohesion loc alizes the deformation even more,The model results are compared to the continental collision which is currently producing the Southern Alps of New Zealand. The symmetry of the models compared to the asymmetry o f collisional mountain belts is attributed to the conservative mature of the models. The Alpine Fault oft-lie Southern Alps is mimicked by t he synthetic shear of the models while backthrusts off the Alpine Faul t, east of the Main Divide, are mimicked by the single antithetic shea r, A non-zero dilation angle does not affect the deformation of a Mohr -Coulomb material directly but, does have a significant effect on the fluid now regime produced by deformation. Experiments showed that the ability of a deforming material to dilate provides a driving force far fluid flow and allows fluid to penetrate into regions of low static p ermeability, This result is most evident if; the value of the dilation angle varies with plastic strain, The strain-dependent dilatant flow law creates regions of strain-hardening , where dilation allows fluid, penetration, and regions of strain-softening, from which the fluid is expelled, This leads to spatial and temporal variations in the dynami c permeability which. enhances the fluid flow, Flow rates are further enhanced by spatial and temporal -variations in pore pressure, The flo w rates calculated for a region with a hydrostatic pore pressure gradi ent and a static permeability of 10(-18) m(2) were on the order of 6 x 10(-5) ma(-1), those for a lithostatic pore pressure gradient were on the order of 6 x 10(-4) ma(-1), and those for a pore pressure regime with steps from lithostatic to hydrostatic are on the order df 2 x 10( -3) ma(-1), These correspond to communication-scales of 20m, 600m, and 3km respectively over 1 Ma and are minimum estimates, The model descr ibing fluid flow within an active continental collision is extended to include flow driven by extensive deformation, particularly dilatant w hich ''kneads'' fluid through the rock mass, Without deformation, flui d within rocks of low static permeability is effectively immobile.