Tectonostratigraphic architecture and uplift history of the Eastern ...

the related extension structures do not overlap. This is most likely a function of the rheology of theregionally distinct sandstone the deposit occurs in.Step threeIn Step 3, the synthesised structural event histories determined in Step 2 (Fig. 6vi) are correlatedwith one another. At this point our methodology diverges from classical P-T dihedra stressinversion studies (e.g., Angelier and Mechler, 1977) which correlate all structures based on theabsolute direction of stress determined by overlapping individual P-T dihedra. Such an approachassumes that the regional stress is uniform and invariant during any one event over the whole studyarea. While this assumption is safe on the pit scale it is not valid on the regional scale hence theabsolute orientation of stress vectors has not been used to correlate individual events with oneanother. Instead it is the pattern of stress switching which has been used to correlate events withinone another. In conjunction with the pattern of stress switching limited geochronological constraintshave also been used along with regional map patterns. In the case where there were insufficientevents observed in a pit to make a certain correlation as the timing of an observed structure theambiguity in the location of the structure is shown on the Central Eastern Yilgarn DeformationHistory Banner (Appendix 3.1.1).Initially instead of palaeostress switching patterns unique structures such as the penetrative ‘D2’fabric were used to correlate between pits. However through the course of the study it was shownthat this fabric could not be used to correlated between pits since at various deposits this uniformlyNNW striking fabric displayed normal, reverse, strike-slip and pure shear kinematics hence thepenetrative fabric can not be assumed to have necessarily formed in the same event. Based on thisobservation events were correlated based on the pattern of palaeostress switching which isdependant on identification of a unique sequence of palaeostress switches used as a markersequence. The unique pattern of palaeostress switching used in this study is the switch from:extension to NNW-SSE compression followed by NE-SW compression. This pattern has workedwell however it is primarily based on the presence of what is typically a moderate- to low-straindeformation and hence caution should be taken in placing too much emphasis on correlations basedon a very low-strain NNW-SSE compressional event. Furthermore this pattern can be used tocorrelate events in areas where extension is not recorded. In these areas we see a switch from ENE-WSW compression to NNW-SSE compression and back to NE-SW compression. To resolve thisproblem, map patterns were used to determine the relative timing of the early extension andcompression events. The observation that Late Basins overlay a pre-folded sequence yet sit in thehangingwall to extensional shear zones has been used to constrain the timing of D3 extension toafter the D2 compression.13