Variation in characteristic and texture of quartz veins normal to the Alpine Fault

Reid Lindroos, Zoe.

Vein characteristics and textures, particulary crystallographic preferred orientations (CPO) were investigated using computer integrated polarisation microscopy (CIP) in ~20 quartz veins over the course of this study. Grain size and grain shape preferred orientation (GSPO) were also analysed. Samples were taken from a fault perpendicular section within the hanging-wall mylonites of the Alpine Fault in Cataclasite Creek on the West Coast of the South Island, New Zealand. The quartz veins occur within two different mylonitic lithologies; quartzofeldspathic mylonite and metabasic mylonite (~30-40% total mylonite volume). The proportion of metabasic to quartzofeldspathic mylonite may have implications for localised fault behaviour and amount of dip-slip movement on the Alpine Fault.
The quartz veins mostly lie at low angle to or parallel to the mylonitic foliation. These veins may initially have formed in three different scenarios, each of which should produce a different final CPO; (1) veins inherited from the Alpine Schists which were already deformed before incorporation into the mylonite zone, (2) veins opened parallel to foliation at depth within the mylonite zone, and (3) veins opened at high angle to foliation at depth within the mylonite zone. In all three scenarios we expect an original CPO within the veins.
The samples display CPOs ranging from crossed girdles, representative of low temperature basal <a> glide to concentrated Y-maxima fabrics characteristic of prism <a> glide, and possible constrictional fabrics. CPOs within quartzofeldspathic and metabasic mylonites differ significantly. The metabasic mylonites have more grains favourably oriented for prism <a> glide, and some indicate antithetic sense of shear to adjacent quartzofeldspathic mylonites, and to the overall sense of shear within the Alpine Fault zone. Several veins within both lithologies have a domainal CPO, which may indicate that they have not undergone enough recrystallisation to completely change the initial CPO, or that different starting orientations responded differently to the same deformation. In either case, preservation of these domains suggests these veins were generated within the mylonite zone rather than inherited from the Alpine Schist.
The large variation in CPO between veins hosted in quartzofeldspathic and metabasic lithologies suggests the rheologically distinct metabasic pods may either shield quartz veins from deformation, or focus deformation into them. GSPOs appear to be correlated with dominant quartz slip system; samples displaying prism <a> and rhomb <a> slip have GSPOs at higher angle to the foliation than those exhibiting basal <a> slip.

95 leaves : ill., 1 folded map in pocket ; 30 cm. + 1 CD-ROM (4 3/4 in.)

2010Reid_Lindroos

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