Low‐angle shear zones in Central Otago, New Zealand—their regional extent and economic significance

Abstract

A relation between low-angle shear zones and folds is established in the dynamically deformed Otago Schist. These events occurred during the Cretaceous towards the close of the Rangitata Orogeny (D 3 _ 4). Prominent low-angle north-dipping shears are often subparallel to F 3 axial planes. South-dipping mesoshears are present as an antithetic set. Thrusting was initiated on the shear zones after F 3 folding, accompanying continued regional compression. In western sections, two sequential F 3 fold subphases are distinguished, but to the east the earlier phase is weakly preserved. The existence of two postS 2 folding events, and dominance of north-dipping shears, is important in relation to potential sites of gold mineralisation. Structural factors influencing the development of dilation sites associated with gently dipping macroshears include the F 3 axial plane geometry, variations in fold intensity, and lithology-shear strength control. The lack of widespread low-angle macroshears is believed to be due to the dominantly segregated, coarsely layered nature of the Otago Schist and the localisation of F 3b folds.

Tectonic significance of Cretaceous bivergent extensional shear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand

“…The RSSZ and also the Hyde-Macraes Shear Zone of eastern Otago (Fig. 1B) have previously been regarded as late subduction-related thrusts (e.g., Teagle et al 1990;Winsor 1991). Nonetheless, our data show that the RSSZ and the CGSZ are net normal-sense shear zones.…”

“…Current kinematic interpretations of the Hyde-Macraes Shear Zone emphasise thrusting (Teagle et al 1990;Winsor 1991) and a later normal sense reactivation of the fault (Angus 1992). These 20th century studies did not take differences in metamorphic and textural grade of the hanging wall and footwall into account.…”

“…The best studied low-angle shear zone in the schist is the northeast-dipping, gold-mineralised HydeMacraes Shear Zone in east Otago (Fig. 1B) (Teagle et al 1990;Winsor 1991;Craw et al 1999;De Ronde et al 2000;Forsyth 2001). Kinematic studies of the shear zone have shown that late-to postmetamorphic thrusting was followed by a period of normal faulting (Teagle et al 1990;Angus 1992).…”

Structural and lithological continuity and discontinuity in the Otago Schist, Central Otago, New Zealand

“…Similar overprinting of post-metamorphic fold generations has been described in the Cromwell Gorge, immediately southwest of the Dunstan Range (Fig. 12), by Winsor (1991).…”

“…4). We suggest instead that this apparently curved fold zone represents the intersection of regional-scale fold zones of the two different generations of late metamorphic structures, with overprinting as described above and by Winsor (1991). Manorburn Generation folds occur in a northeast-trending vergence boundary running from Ophir to south of Alexandra (Fig.…”