Abstract
This study considers microstructures developed during creep deformation of metachert in the mylonite zone of the Alpine Fault, a major crustal boundary that runs through the South Island of New Zealand. The shape and crystallographic characteristics of quartz grains were investigated within samples from Stony Creek, near Franz Joseph, on the West Coast of the South Island of New Zealand.
Quartz grain boundary characteristics and microstructures were indicative of recrystallisation at temperatures >300° C but <500 °C at the estimated steady state rates of 10-11 and 10-12 s-1. Optically observed grain boundary characteristics appear to depend on the size and distribution of second phase minerals such as garnets and micas. The presence of chlorite within S-C’ shear bands indicates temperatures <500°C which corresponds to depths <25 km within the Alpine Fault zone.
Crystallographic preferred orientation (CPO) analyses suggest that simple shear strains of γ > 7 have been accommodated within the rocks sampled. Grain boundary sliding on basal mica planes is thought to account for high simple shear strain accumulation. Shear sense indicators are generally synthetic to the overall sense of motion on the Alpine Fault although some antithetic structures are observed. The rotation sense of CPOs is also consistent with the overall sense of shear on the Alpine Fault. Three types of c-axis patterns are observed within X-Z sections; these are Y maxima, single girdles and one cross girdle. These patterns indicate dominant rhomb <a> and prism <a> slip with minor basal <a> slip. These slip systems have been observed elsewhere to develop to the same temperature range indicated by recrystallisation microstructure observations. A number of the CPOs display a strength asymmetry; no mineral lineation is observed within the samples but the greatest quartz elongation is parallel to the Y-axis rather than the X-axis; this could explain the asymmetry of some CPO patterns.
Grain shape analyses show that the rotation of both particle (PAROR) and grain boundary (SURFOR) orientation distribution functions (ODFs) is consistent with the overall sense of shear within the Alpine Fault zone. Grain size analyses have an inhomogeneous distribution across the quartz layer; grains are smaller near the edges of the layer, which may indicate simple shear strain partitioning into the edges. The greatest differential stress estimated by quartz paleopiezometry is 40 MPa.