Random Crystallographic Preferred Orientation in Quartz Deformed Under Moderate P-T Conditions: an Example From Shear Zones in the Southern Alps, New Zealand
Ruth H Wightman1, David J Prior2, Timothy A Little1
1 School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand
2
Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, U.Kruth.wightmen@paradise.net.nz
Electron backscattered diffraction (EBSD) analyses of quartz veins deformed in shear zones with a ductile shear strain of 5 – 10, revealed the presence of a random crystallographic preferred orientation (CPO). The quartz veins, from the central Southern Alps, New Zealand, contain minor calcite, and are ductily offset by a series of systematically spaced, cm-thick shear zones. Beyond the shear zone boundaries, the quartz veins are undeformed by the Late Cenozoic ductile deformation, and display a well-developed CPO, possibly related to an older, pre-existing foliation. Both the deformed and undeformed regions of the veins have an annealed-looking microstructure, and subgrains and low-angle grain boundaries are rare.
The dextral-oblique shear zones have a NW-up sense of throw, antithetic to the Alpine Fault. They are interpreted as escalator-like sequentially activated backshears related to the uplift of the Pacific Plate onto the Alpine Fault ramp at depth. A kinematical model of shear formation, combined with plate motion rate data, suggest the strain rate of deformation was approximately 1.8 x 10-9 s-1. Geothermometry, combined with fluid inclusion analyses, estimate the conditions of deformation at 450 ± 50° C and 3.1 ± 0.7 kbar. At these deformation conditions, most deformation maps would place quartz within the dislocation creep regime. Experimental studies have shown, however, that quartz deformed by dislocation creep and subsequent recrystallization by either grain boundary migration and/or sub-grain rotation, creates a CPO that overprints any pre-existing fabric at relatively low shear strains (~2). Diffusion creep has often been cited as a deformation mechanism that does not create CPO. Quartz requires a very small grain size to be able to deform by diffusion creep at the estimated deformation conditions, however, and the quartz veins studied here are coarse grained, averaging 100 microns, inside and outside of the shear zones. Another deformation mechanism that is often cited to create random CPO is cataclasis. Absence of any evidence of mixing between the quartz vein and the surrounding wall rock, combined with cathodoluminescence analyses, seems to imply that the veins were not deformed by a cataclastic mechanism.
A possible explanation for the random CPO in the deformed veins is that initial deformation occurred by dislocation creep. Recrystallization resulted in a reduction in grain size, moving the deformation into the diffusion creep regime and destroying the pre-existing CPO. The deformation was then followed by substantial static grain growth. Problems with this model for deformation include the similarity in microstructure and grain size between the veins inside and outside of the shear zones, and the supposed large amount of deformation and annealing that must occur from relatively small shear strains.