Cretaceous continental extension was accommodated by the development of the Paparoa Metamorphic Core Complex, resulting in the separation of New Zealand from Gondwana. High grade (Lower Plate) and low grade (Upper Plate) rocks are separated by the Ohika and Pike Detachment Faults. The two detachment faults have distinctly different histories, with greater exhumation along the Pike Detachment Fault. The onset of crustal extension is proposed to have commenced along the Pike Detachment Fault at 116.2 ± 5.9 Ma (Rb/Sr dating). Both geochemical and geochronological approaches are adopted for this thesis, through the in situ analysis of oxygen and hafnium isotope ratios, trace metals and U-Pb content. Chemical changes are tracked during the petrogenesis of the Buckland Granite, with mafic replenishment observed in the later stages of crystallisation. Crystallisation temperatures of the Buckland Granite are calculated using zircon saturation thermometry, with an average Ti-in-zircon temperature of 697℃ (upper-amphibolite facies). Inherited zircons in Lower Plate rocks show distinct age peaks at c. 1000, 600 and 300 Ma, illustrating the incorporation of heterogeneous local crust (Greenland Group and Karamea Batholith). Model ages (TDM) are calculated for inherited zircons of the Lower Plate rocks, which record the time at which magma bodies (zircon host rocks) were extracted from the mantle. Maximum and minimum model ages for the Buckland Granite average at 3410 Ma and 2969 Ma, with the maximum TDM value of 3410 Ma coinciding with the proposed major crustal formation event of the Gondwana supercontinent at c. 3.4-3.5 Ga. Two distinct U-Pb zircon age peaks are observed in the Buckland Granite at 102.4 ± 0.7 and 110.3 ± 0.9 Ma. The 110.3 ± 0.9 Ma age is interpreted as the crystallisation age of the pluton, while the 102.4 ± 0.7 is proposed to represent a younger thermal (magmatic?) event associated with the 101-102 Ma Stitts Tuff.

Potential lacustrine source rocks have been recognised in several Cretaceous syn-rift basins including the producing Taranaki Basin, but have not been officially recognised from drill core and seismic data. The late-Cretaceous Paparoa Coal Measures contain three lacustrine mudstone formations which outcrop in several localities and have been extensively drilled for coal mining. These formations are considered to be an easily accessible analogue for late-Cretaceous lacustrine source rocks in New Zealand and also provide valuable information regarding syn-depositional tectonics and basin formation during the late-Cretaceous. Stratigraphic columns and isopach maps were constructed from field work and drill hole descriptions and results showed variations in lithofacies across the basin. The western side of the basin is characterised by sandy lithofacies, abundant proximal turbidites and debris flows. The transition to a sub-aerial environment is marked by thick conglomerate and meter wide rip-up clasts. The central and eastern sections of the basin show massive mudstone, distal turbidites, low energy fluvial sandstones and thin, discontinuous coal. Isopach maps constructed from drill hole data identified three NNE – SSW oriented lakes with lacustrine sediment of up to 180m thick truncated by the eastern Roa – Mt Buckley Fault Zone. It was determined fault control during deposition was to the west and the basin extended further than its current location. Revisions to isopach models highlighted a lack of change in basin orientation during deposition of the Paparoa sediments. Plate reconstructions combined with direct evidence from the basin indicate formation of the Paparoa Coal Measures could have occurred in either a rift or transtensional basin. The mudstones were geochemically assessed for hydrocarbon potential using a Source Rock Analyser (SRA). Preliminary analysis of the three mudstones has shown TOC values ranging from 1.0 to 4.5 wt.%, HI values ranging from 68 to 552 mHC/gTOC and Tmax results show the mudstones to range in maturity from immature to late – mature. A sample from the Waiomo Formation has excellent potential for oil generation and the low maturity results for the Goldlight Formation make it a potential shale gas resource. These results have shown the potential for hydrocarbon bearing lacustrine source rocks to exist in the Greymouth Coalfield. In addition, revisions have been made to basin formation which should be considered. Due to the availability of data from the Paparoa lacustrine source rocks, they should be used as an accessible analogue for Taranaki and other Late Cretaceous basins.

Cretaceous continental extension was accommodated by the development of the Paparoa Metamorphic Core Complex, resulting in the separation of New Zealand from Gondwana. High grade (Lower Plate) and low grade (Upper Plate) rocks are separated by the Ohika and Pike Detachment Faults. The two detachment faults have distinctly different histories, with greater exhumation along the Pike Detachment Fault. The onset of crustal extension is proposed to have commenced along the Pike Detachment Fault at 116.2 ± 5.9 Ma (Rb/Sr dating). Both geochemical and geochronological approaches are adopted for this thesis, through the in situ analysis of oxygen and hafnium isotope ratios, trace metals and U-Pb content. Chemical changes are tracked during the petrogenesis of the Buckland Granite, with mafic replenishment observed in the later stages of crystallisation. Crystallisation temperatures of the Buckland Granite are calculated using zircon saturation thermometry, with an average Ti-in-zircon temperature of 697℃ (upper-amphibolite facies). Inherited zircons in Lower Plate rocks show distinct age peaks at c. 1000, 600 and 300 Ma, illustrating the incorporation of heterogeneous local crust (Greenland Group and Karamea Batholith). Model ages (TDM) are calculated for inherited zircons of the Lower Plate rocks, which record the time at which magma bodies (zircon host rocks) were extracted from the mantle. Maximum and minimum model ages for the Buckland Granite average at 3410 Ma and 2969 Ma, with the maximum TDM value of 3410 Ma coinciding with the proposed major crustal formation event of the Gondwana supercontinent at c. 3.4-3.5 Ga. Two distinct U-Pb zircon age peaks are observed in the Buckland Granite at 102.4 ± 0.7 and 110.3 ± 0.9 Ma. The 110.3 ± 0.9 Ma age is interpreted as the crystallisation age of the pluton, while the 102.4 ± 0.7 is proposed to represent a younger thermal (magmatic?) event associated with the 101-102 Ma Stitts Tuff.

The Paparoa Metamorphic Core Complex developed in the Mid-Cretaceous due to continental extension conditioning the crust for the eventual breakup of the Gondwana Pacific Margin, which separated Australia and New Zealand. It has two detachment systems: the top-NE-displacing Ohika Detachment at the northern end of the complex and the top-SW-displacing Pike Detachment at the southern end of the complex. The structure is rather unusual for core complexes worldwide, which are commonly characterised by a single detachment system. Few suggestions for the kinematics of the core complex development have been made so far. In this study structural-, micrographic- and fission track analyses were applied to investigate the bivergent character and to constrain the kinematics of the core complex. The new results combined with reinterpretations of previous workers’ observations reveal a detailed sequence of the core complex exhumation and the subsequent development. Knowledge about the influence and the timing of the two respective detachments is critical for understanding the structural evolution of the core complex. The syntectonic Buckland Granite plays a key role in the determination of the importance of the two detachment systems. Structural evidence shows that the Pike Detachment is responsible for most of the exhumation, while the Ohika Detachment is a mere complexity. In contrast to earlier opinions the southwestern normal fault system predates the northeastern one. The Buckland Pluton records the ceasing pervasive influence of the Pike Detachment, while activity on the Ohika Detachment had effect on the surface about ~8 Ma later. Most fission track ages are not related to the core complex stage, but reflect the younger late Cretaceous history. They show post core complex burial and renewed exhumation in two phases, which are regionally linked to the development of the adjacent Paparoa Basin and the Paparoa Coal Measures to the southwest and to the inception of seafloor spreading in the Tasman Sea in a larger context.