Structure and interpretation of the Caples terrane of the Thomson Mountains, northern Southland, New Zealand

Abstract

In the Thomson Mountains, vokaniclastic submarine fan and trench sediments of the Caples terrane have been deformed and metamorphosed and subsequently extensively faulted. The first of two phases of folding was dominated by large open to tight folds in bedding with subhorizontal axes and vertical axial planes. Few mesoscopic structures were developed in this phase. Metamorphism was coincident with, and may have outlasted, Phase 1 deformation; schistosity is developed in the east of the area and is axial plane to Phase 1 folds in bedding. Phase 2 deformation was limited to warping of metamorphic foliation. Phase 1 folding was accompanied by tectonic sliding. Tectonic slides are manifested as discrete fractures or as wide zones of deformation akin to melanges, intimately associated with major folds. Characteristic mesoscopic structures were developed within tectonic slide zones. Most of the area is within pumpellyite-actinolite metamorphic facies; prehnite occurs in veins and in one small area of prehnite-pumpellyite facies. Lawsonite-albite-chlorite facies assemblages occur and are apparently restricted to tectonic slide zones. A rare occurrence of axinite as a rock-forming metamorphic phase is described. The area is cut by six major north-east-trending faults, among which is Moonlight Fault. Conjugate north-trending faults also occur; on one of these, and on Moonlight Fault, Oligocene sediments of Bobs Cove Beds are preserved. The southern termination of Greenstone Mélange is mapped south of Greenstone River. The Dun Mountain Ophiolite Belt bounds the terrane to the west. Sedimentary facies within the Caples terrane are interpreted as representing deposition in a submarine fan complex on a lower trench slope; possible trench floor, trench-slope basin, pelagic ocean floor sediment, and oceanic crust material is also preserved in the terrane. The sediments are predominantly volcanogenic, with a significant basic-intermediate plutonic component. Derivation from the Brook Street terrane to the west seems most likely. Styles of deformation, paricularly in tectonic slide zones, and metamorphic facies are consistent with the Te Anau Assemblage having been deformed in a subduction zone east of the Hokonui Assemblage.