Measurement and correlation of microstructures: the case of foliation intersection axes

Recent studies have used the relative rotation axis of sigmoidal and spiral‐shaped inclusion trails, known as Foliation Inflexion/Intersection Axis (FIA), to investigate geological processes such as fold mechanisms and porphyroblast growth. The geological usefulness of this method depends upon the accurate measurement of FIA orientations and correct correlation of temporally related FIAs. This paper uses new data from the Canton Schist to assess the variation in FIA orientations within and between samples, and evaluates criteria for correlating FIAs. For the first time, an entire data set of FIA measurements is published, and data are presented in a way that reflects the variation in FIA orientations within individual samples and provides an indication of the reliability of the data. Analysis of 61 FIA trends determined from the Canton Schist indicate a minimum intrasample range in FIA orientations of 30°. Three competing models are presented for correlation of these FIAs, and each of the models employ different correlation criteria. Correlation of FIAs in Model 1 is based on relative timing and textural criteria, while Model 2 uses relative timing, orientation and patterns of changing FIA orientations, and Model 3 uses relative timing and FIA orientation as correlation criteria. Importantly, the three models differ in the spread of FIA orientations within individual sets, and the number of sets distinguished in the data. Relative timing is the most reliable criterion for correlation, followed by textural criteria and patterns of changing FIA orientations from core to rim of porphyroblasts. It is proposed that within a set of temporally related FIAs, the typical spread of orientations involves clustering of data in a 60° range, but outliers occur at other orientations including near‐normal to the peak distribution. Consequently, in populations of FIA data that contain a wide range of orientations, correlation on the basis of orientation is unreliable in the absence of additional criteria. The results of this study suggest that FIAs are best used as semiquantitative indicators of bulk trends rather than an exact measurement for the purpose of quantitative analyses.     

Numerical modelling of mechanical controls on coeval steep and shallow dipping auriferous quartz vein formation in a thrust zone,Macraes mine,New Zealand

The Hyde-Macraes Shear Zone (HMSZ) is a regionally continuous, low-angle, NE dipping (~15°) late-metamorphic thrust zone in the Mesozoic Otago Schist. The shear zone, which is host to large volumes of mineralised schist, consists of foliated fissile schist with some massive schist pods. Two sets of quartz veins are found within the HMSZ: thrust-related, shallowly dipping veins that were emplaced parallel or sub-parallel to the shears and swarms of steeply dipping extensional veins, which cut across the metamorphic foliation. The latter are restricted to the massive schist pods. Mutual cross-cutting relationships occur between steep extensional veins and shallow-dipping veins, suggesting that they formed contemporaneously. The co-existence of these two vein types locally implies local rotation of the principal stress axes to produce extensional veins within a regional thrust setting. The steep extensional veins are spatially related to lateral and oblique ramps within the HMSZ. Three-dimensional mechanical models show that these lateral or oblique ramps can produce favourable conditions for extensional vein formation when combined with a high fluid pressure and oblique convergence. Mechanical requirements include a reduced differential stress, a positive volumetric strain and an increase in the horizontal shear stress. Our models show that under certain conditions, it is possible for extension-related structures to form during shortening because of local changes in the stress state without the need for a regional scale switch in the imposed stress field. The convergence direction across the HMSZ during formation of the steep extensional veins was ~WNW.