Middle Cretaceous – Tertiary Tectonics and Seismic Interpretation of North Westland and Northwest Nelson, New Zealand

Abstract

Eight seismic units and seven horizons are picked on commercial seismic reflection lines and correlated with onshore geology and exploration well logs. Horizons are of regional extent and are dated as early Motuan (middle Albian), early Haumurian (middle Campanian), Early Eocene, Early Oligocene, Early Miocene, Middle Miocene and near base Pliocene. The formation of these horizons is attributed primarily to regional tectonic events. Computer software by GeoGraphix is used to turn interpreted reflection seismic and well data into 'two say travel time’, depth and isopach maps. Serial transects are produced across the study area. Continental rifting and plutonism started in the study area during the middle Cretaceous, c. 125 Ma. A half-graben in the offshore region is mapped and shows c.20% extension, directed NNE-SSW. A dike swarm with the same trend was intruded during a later extensional phase, at c. 90-80 Ma. A widespread regional unconformity formed at c. 80 Ma contemporaneous with continental break-up and onset of seafloor spreading in the Tasman Sea. Early Haumurian - Paleocene tectonic style is characterized by NNE-SSW trending continental rifts, including the Paparoa and Pakawau basins, and half-grabens now beneath the Grey Valley. Igneous activity continued and dikes were intruded subparallel to the basins. Volcanism and rifting occurred in a transtensional corridor through Taranaki and the West coast. Tectonic quiescence at the end of the Paleocene is contemporaneous with the end of seafloor spreading in the Tasman Sea. A major erosional unconformity formed. Rifting and regional subsidence propogated northwards during the Eo-Oligocene; a half-graben unaffected by Neogene deformation shows c. 5% extension. Subsidence and uplift began in North Westland in the Waitakian (early Miocene), with the initiation of the Alpine Fault, and spread to Northwest Nelson diachronously durinq the Otaian-Lillburnian, Some pre-existing faults were reactivated as major reverse faults, bounding blocks typically 8 km wide. Cretaceous-Paleogene basins were structurally inverted, with intense folding and faulting at block margins, initiation of thrusts and gentle foldinq within blocks, and quasi-ductile deformation of rheologically weak basin sediments. Cretaceous-Paleogene normal faults within blocks commonly escape being reactivated by Neogene shortening. Long-wavelength / low amplitude flexure of the crust is observed on the Western Platform, beyond the western limit of reverse faulting. The Western Platform is buried by a Neogene sedimentary wedge up to c. 2 km thick, which has prograded away from the South Island. The depression of the crust increases with the thickness of the sedimentary overburden. Local sedimentary troughs in the footwalls of reverse faults are attributed to isostatic loading. A study of jointing shows late Neogene shortening to be oriented WNW-ESE. Second-order effects of shortening include strike-slip faulting with offsets of no more than a few hundred metres, and block rotations of a few degrees. Regional transects across the study area show total Neogene shortening to be c. 3-4%. At the southern end of the area, close to the Alpine Fault, vertical thickening greatly exceeds horizontal shortening and deformation of the crust is essentially thick-skinned.