The Geology of the Flora Saddle - Mount Arthur region, northwest Nelson, New Zealand

Abstract

Detailed mapping in the study area shows the Mount Arthur Group to consist of the Middle to early Late Ordovician Arthur Marble 1, conformably overlain by the pelite, marble and meta-quartzite of the early Late Ordovician Wangapeka Formation, which grades up into the Late Ordovician Arthur Marble 2. A reference section for the Wangapeka Formation is presented. Five major periods of tectonism are recognised in the Flora Saddle - Mount Arthur region. The first period of deformation is characterised by recumbent, east-northeast verging isoclinal folds and thrust faults that formed at a low angle to bedding in the marble units and a slaty cleavage in the pelites of the Eastern Sedimentary Belt of the Takaka Terrane. This deformation was accompanied by prehnite-pumpellyite facies metamorphism and is considered to be the result of latest Silurian to Early Devonian east-directed thrusting in the Eastern Sedimentary Belt. The east-directed thrusting is interpreted as back thrusting associated with the collision of the Buller and Takaka Terranes. This collision is also interpreted as having formed west-directed thrusts and nappes in the Central Sedimentary Belt and upright, isoclinal folds in the Western Sedimentary Belt. The second period of deformation is characterised by metamorphism and coeval displacement on the Devil River Shear Zone and by the intrusion of Riwaka Igneous Complex dikes. Foliated, garnet ? porphyroblasts have formed in pelite hornfels developed on Riwaka Igneous Complex dikes (367 ± 5 Ma). The porphyroblasts were rotated after they grew. Some of the dikes of the Riwaka Igneous Complex were deformed whilst still hot, resulting in the formation of a foliation in some of the dikes. The foliation is straight and is a relict of the slaty cleavage formed during the first tectonic event. The Devil River Shear Zone is a zone of ductile deformation formed adjacent to the Devil River Fault. The Devil River Fault separates the Central and Eastern Sedimentary Belts. Micro- and macroscopic structures indicate that the Eastern Sedimentary Belt is downthrown relative to the Central Sedimentary Belt. The presence of epidote in the shear zone indicates that peak metamorphic conditions reached mid-greenschist facies. Foliated dikes and porphyroblast-bearing rocks with pressure shadows parallel to the mylonitic foliation in the Devil River Shear Zone suggest that deformation in the shear zone was syn- and post-intrusion of the Riwaka Igneous Complex dikes. Together, the Riwaka Igneous Complex dikes and the Devil River Shear Zone are interpreted as having been formed during extension of the Buller and Takaka Terranes. The third period of deformation is characterised by upright, north - south trending, gentle to close folds that deform all Paleozoic rocks in the study area. These folds pre-date the intrusion of the Separation Point Batholith (mid-Cretaceous) and are interpreted as being associated with the collision of the eastern margin of the Takaka terrane with another terrane during the late Paleozoic or Mesozoic. The fourth period of tectonism is a thermal event that is only recorded in the study area by the Onekaka Schist and by metabasites on Mount Arthur. Textural relationships indicate that metamorphism was not dynamic. The Onekaka Schist comprises psammitic schist, pelite and marble in the study area. The presence of biotite, combined with textural relationships indicate that metamorphism reached greenschist facies. The metabasites are rare and probably of Riwaka Igneous Complex parentage. Plagioclase and amphibole chemistry indicate that peak metamorphic temperatures exceeded 510°C and that pressures attending metamorphism were less than 3 kbar. The thermal event associated with the intrusion of the Separation Point Suite granitoids (mid-Cretaceous) is inferred as the heat source for both of the events. Retrograding of the garnet ? porphyroblasts that had formed during the second period of tectonism to chlorite, biotite and quartz probably also occurred at this time. The fifth period of tectonism is a period of dominantly oblique-reverse faulting during the Late Cretaceous to late Cenozoic. Northwest - southeast trending left steps in the northeast - southwest trending dextral strike-slip fault system of the Karamea and Pikikiruna Faults are mapped as northeast dipping thrust faults. Reconstructions of the Northwest Nelson region show that the Karamea and Pikikiruna Faults must have been active during two separate periods since the intrusion of the Separation Point Batholith. The removal of late Cenozoic dextral strike-slip movement along the faults requires 5 km of earlier dextral strike-slip movement of the faults. This 5 km of movement is interpreted as being early Late Cretaceous reverse dextral movement associated with reverse dextral faulting in the Skeet Fault Zone along the western margin of the Separation Point Batholith.