Marine Forearc Extension in the Hikurangi Margin: New Insights From High‐Resolution 3‐D Seismic Data

Abstract

Upper‐plate normal faults are a widespread structural element in erosive plate margins. Increasing coverage of marine geophysical data has proven that similar features also exist in accretionary margins where horizontal compression usually results in folding and thrust faulting. There is a general lack of understanding of the role and importance of normal faulting for the structural and tectonic evolution of accretionary margins. Here we use high‐resolution 2‐D and 3‐D seismic reflection data and derived seismic attributes to map and analyze upper‐plate normal faulting in the marine forearc of the accretionary Hikurangi margin, New Zealand. We document extension of the marine forearc over a wide area along the upper continental slope. The seismically imaged normal faults show low vertical displacements, high dip angles, a preference for landward dip, and often en echelon patterns. We evaluate different processes, which may cause the observed extension, including (1) stress change during the earthquake cycle, (2) regional or local uplift and decoupling of shallow strata from compression at depth, and (3) rotation of crustal blocks and resulting differential stresses at the block boundaries. The results suggest that normal faults play an important role in the structural and tectonic evolution of accretionary margins, including the northern Hikurangi forearc.

Three‐Dimensional P Wave Velocity Structure of the Northern Hikurangi Margin From the NZ3D Experiment: Evidence for Fault‐Bound Anisotropy

“…This feature is consistent with the low seismic reflectivity at the top of the seamount while still accounting for the location of a high-relief magnetic source on the subducting plate (Barker et al, 2018). (Böttner et al, 2018; a gray shade). We calculated the slope gradient from the bathymetric grid data using GMT (Wessel & Smith, 1998).…”

“…The other area is the most landward part of the 3-D box where the fast axes of Vp rapidly change to the trench-parallel direction (Figures 5, 9, and 12). This change may be related to the Tuaheni Ridge and the edge of the North Tuaheni basin, in an area where forearc extension has been previously mapped (Böttner et al, 2018; Figures 9c and 12a). The variation in seismic anisotropy is consistent with the results of shear wave splitting analysis (Zal et al, 2020; Figure 9b).…”

Pockmarks in the Witch Ground Basin, Central North Sea

“…We have used seismic attributes to enhance seismic interpretation including the Kingdom Suite Symmetry attribute, which is a post‐stack, post migration structural feature detection tool (e.g., fracture detection) based on a 3‐D log‐Gabor filter array (Yu et al, ). This attribute is highly sensitive to seismic amplitude variations and therefore correlates with curvatures and discontinuities associated with geological structures, for example, faults, fractures, and discontinuous events (Böttner et al, ). In addition, we use the root‐mean‐square (RMS) amplitude calculated over a time window of ±50 ms around the picked horizon (see horizon in Figure b, dashed blue line).…”

Focused fluid seepage related to variations in accretionary wedge structure, Hikurangi margin, New Zealand

“…2A-2C and 4A-4C). In the north, where the convergence rate is ∼5 cm/yr, the Tuaheni Seep Field is spatially correlated with localized and possibly shallow extensional faulting (Böttner et al, 2018), immediately east of major thrust faults 1 GSA Data Repository item 2020017, methodologies, equipment specifications, individual presentation of seepage features, grid sizes, and observation thresholds, and an MSExcel spreadsheet containing all fluid expulsion features presented in this study, with coordinates, depth and voyage/dataset reference, is available online at http://www.geosociety. org/datarepository/2020/, or on request from editing@ geosociety.org.…”