Mechanics, reaction and fluid flow associated with continental collision along the Alpine Fault, Southern Alps, New Zealand

Upton, Phaedra, 1967.

The mineral chemistry and oxygen isotope signature of a crustal cross-section perpendicular to the Alpine Fault and from the highest uplift region of the western Southern Alps has been examined for the purpose of characterising mechanisms of deformation and material movement in the inboard region of the Australian plate/Pacific plate continental collision. The crosssection consists of Alpine Fault mylonites, curly schist, oligoclase and albite zone Alpine scl1ist. The mineral assemblage of quartzofeldspathic schist and mylonite is quartz + plagioclase + biotite + muscovite + garnet + ilmenite + apatite ± chlorite ± epidote ± graphite ± hornblende ± rutile ± zircon. Excepting albite and oligoclase, all minerals are found throughout the structural section. The mylonites exhibit a larger variation in mineral chemistry than the ·adjacent oligoclase zone schists but there are no sharp changes in mineral compositions. The mylonite mineral chemistry is attributed to incomplete reaction taking place during mylonitisation. All garnets in the Alpine schist are zoned, with Fe and Mg contents increasing toward the rim while Mn and Ca contents decrease. In addition to this zoning pattern which is found in most mylonite garnets, some mylonitic garnets exhibit an outer rim of Ca enrichment. Plagioclase varies from An1s ~o An4o over the oligoclase zone, and by up to 18 mole% An within individual rocks. Plagioclase in both schist and mylonite is complexly zoned, with some grains becoming more albitic toward the rim while other grains in the same rock are more anorthitic on the rim. Mylonite samples are divided into two groups on the basis of stable isotope analyses of whole rock and biotites separates. The whole rock o1Bo values differ by c. 2%o. The diffe~ence in whole rock olBo is attributed to varying lithologies, the two rock packets being brought together along the Alpine Fault. Biotites in both the mylonite rock packets are depleted by c. 1 %o in 18o relative to Alpine schist biotites. Analyses of quartz veins formed beneath the brittle-ductile transition showed variations of up to 2.5 %o in individual veins over distances of c. 2 mm. The quartz has precipitated from water calculated to have had oxygen isotope values ranging from 12 - 6 %o over a temperature range of 600° - 400°C as the rock was transported toward the surface as part of the high uplift region of the orogen. The depletion in the biotites and the isotopic values of the vein-forming fluids are compatible with, although not unique to, the incursion of meteoric water through the brittle-ductile transition and its interaction with ductilely deforming rocks beneath the Southern Alps. Numerical experiments of coupled deformation and fluid flow in a non-associated elastic-plastic material undergoing compression illustrate the effects that the plastic properties of the material and the presence of a pore pressure have upon deformation. Boundary conditions are imposed on the models such that the material representing the overriding plate is pushed into or dragged beneath the material representing a stationary plate. Deformation in a dry material is transient with shear zones moving through the material toward the stationary boundary condition. Plastic deformation in the dry non-dilatant material does not localise into a stable shear pattern. In contrast, in a wet material the deformation localises into a paired synthetic V and antithetic shear of varying intensities, with some deformation occurring between the two shears. Friction- or cohesion-softening capabilities localise deformation further. Fluid flow in a non-dilatant material results from elastic volume changes and is associated with shearing. Experiments of fluid flow in a strain-dependent, dilatant Mohr-Coulomb material showed that the ability of a deforming material to dilate provides a driving force for fluid flow and allows fluid to penetrate into regions of low static permeability. Dilatancy result in spatial and temporal variations in the dynamic permeability which increase fluid flow. The straindependent dilatant flow law creates regions of strain-hardening where dilation allows fluid penetration, and regions of strain-softening from which fluid is expelled. Flow rates are further enhanced by spatial and temporal gradients between lithostatic pore pressures and hydrostatic pore pressure. The flow rates calculated for a region with a hydrostatic pore pressure gradient were of the order of 3 x 10-7 m a-1, those for a lithostatic pore pressure gradient were of the order of 3 x 10-6 m a-1, and those for a pore pressure regime with steps from lithostatic to hydrostatic are on the qrder of 1 x 10-3 m a-1. These correspond to communication-scales of 3 cm, 3 m and 1 km respectively over 1 Ma and are minimum estimates. In the Southern Alps, the advection of rock and fluid leads to a non-equilibrium steady-state isotopic signature for the rock mass. Flow rates of at least 1 m a-1 are required to see a distinct change in the oxygen isotopic signature of the bulk whole rock. Dilatant deformation can provide some but not all of this flow. Reaction rates and flow rates are much more important than diffusion rates in determining this signature. The system is flow-dominated for flow rates >0.001 m a-1. The non-equilibrium distance increases from <1 m for v=0.001 m a-1 to c. 10 m for v=1 m a-1. The model of fluid flow resulting from continental collision is extended to include flow driven by extensive deformation, particularly dilatant, which kneads fluid through the rock mass. Within an orogenic belt, fluid-present metamorphism is a consequence of deformation-driven fluid flow. Without deformation, fluid within rocks of low static permeability}s effectively immobile. Compositionally the minerals of the Alpine schist represent an approach to an equilibrium assemblage developed during metamorphism associated with the Mesozoic Rangitatan orogeny. In contrast, the mylonites and curly schists of the Alpine Fault Zone have a dis-equilibrium signature resulting from their response to the temperature, pressure, fluid and strain rate conditions produced by the current tectonic regime. The dis-equilibrium assemblage represents an arrested movement toward re-equilibrium. Re-equilibration is associated with high strain rates and only occurs in the mylonites and curly schists.

1 v. (various pagings) : ill. (some col.), maps (some col.) ; 30 cm.

1995Upton

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