Deformation of the Assegaai supracrustals and adjoining granitoids,Transvaal, South Africa

The structures in Archaean quartzites of the Assegaai greenstone belt in the Transvaal of South Africa can be interpreted in terms of three major deformation events. Earliest D₁, involved thrusting along brittle ramp and flat crush zones. Rising temperatures then led to the mobilization of SiO₂ and the development of syn-D₁ stylolites and omnidirectional quartz veins in the quartzites. Increasingly ductile behaviour results in some of the early D₁ crush zones being overprinted by late D₁ mylonitic shear zones while eastward-verging recumbent folds formed about N-S axes in the layered quartzites.The Assegaai supracrustals were then separated from a contiguous basement of Ancient Gneiss Complex (AGC) by the intrusion of a granitoid sheet about 3 km thick along their sheared subhorizontal contact either late in D₁ or in the interval between D₁ and D₂. The supracrustals were then refolded about steep NNE-trending axial surfaces by concentric F₂ folds on a variety of scales. Such folds were the second major structures to affect the supracrustals but the first to deform the granitoid sheet.The older of two suites of quartzo-feldspathic veins in the AGC were probably generated penecomtemporaneously with the syn-D₁ quartz veins in ther Assegaai quartzites. This interpretation can be tested by removing the intense D₂ constriction (and 30% volume increase) recorded by a later network of syn-granitoid quartzofeldspathic veins in the AGC from the D₁ + D₂ flattening recorded by the older veins. This procedure can be carried out by geometric vector subtraction on a Hsü diagram. The result reveals that the D₁ strains in the Assegaai supracrustals and the then contiguous AGC were identical penetrative subhorizontal flows prior to the intrusion of the intervening granitoid. The last significant strain in the region is represented by a brittle NNE-trending strike-slip fault and, locally in the quartzites, disharmonic conjugate folds.