Structural evolution, segmentation and activity of the onshore-offshore Waimea-Flaxmore Fault System in south-eastern Tasman Bay, South Island, New Zealand

ABSTRACTThe Waimea-Flaxmore Fault System (W-FFS) from the Nelson-Richmond urban area to D’Urville Island is analysed through six regional transects that join depth-converted seismic lines in Tasman Bay to onshore cross sections. Reverse reactivation of basement faults of the W-FFS by mechanisms of fault-propagation folding at c. 19-10 Ma occurred along the entire length of the fault system. However, the W-FFS remained Quaternary active only from the vicinity of Cape Soucis southwards into onshore Nelson, in contrast with cessation of activity along eastern Tasman Bay since 7 Ma. The inactive western faults of the W-FFS are buried below Plio-Quaternary marine sediments and segmented by the newly identified, c. E-W, Croisilles Fault Zone, Cross Point and D’Urville faults, with cumulative dextral offset of c. 27 km. The c. E-W faults extend from the onshore into Tasman Bay and are interpreted as the northernmost Late Miocene strands of the Marlborough Fault System that have accommodated incipient distributed shear in the crustal block north of the Queen Charlotte Fault Zone. Segmentation and along-strike changes of the W-FFS occur at the transition from the contractional domain of onshore Nelson to the northern Marlborough strike-slip domain, driven by large-scale kinematics of the Australia-Pacific plate boundary.KEYWORDS: Waimea-Flaxmore Fault SystemActive faultsFoldsSeismic linesNelsonTasman BayD’urville IslandMarlboroughSouth IslandNew Zealand AcknowledgementsWe thank I. Hamling for providing data on InSAR measurements in Tasman Bay and J. Ristau for help with locating some earthquakes in Tasman Bay. Comments and suggestions of P. Barnes, G. Browne, D. Eberhart-Phillips and S. Nodder greatly improved an early draft of the paper. We gratefully acknowledge the editorial revisions and constructive criticism provided by J. Scott, H. Seebeck and an anonymous reviewer.Disclosure statementNo potential conflict of interest was reported by the author(s).Data availability statementData that support the findings of this study are provided in Supplemental File 1 (detailed description of the geological setting of the Waimea-Flaxmore Fault System in Nelson City, with accompanying geological map and cross sections of Figure S1-1); Supplemental File 2 (large format tiff files of geological transects T1-T6 all reproduced at the same scale); and Supplemental File 3 (discussion on the role of mechanical weakness and fluid overpressure on reactivation of the inherited faults of the Waimea-Flaxmore Fault System, with accompanying Figure S3-1). The supplementary material is openly available in figshare at https://doi.org/10.6084/m9.figshare.23612340.v1.Additional informationFundingThis work was privately funded by TerraGeologica. Nelson City Council and Tasman District Council funded acquisition of the Map Publisher (Avenza System) software, used for geological mapping.