Deformation of the Top Basement Unconformity west of the Alpine Fault (South Island, New Zealand): seismotectonic implications

Abstract

Geometry of the Top Basement Unconformity (TBU) west of the Alpine Fault has been reconstructed through a set of cross-sections linking surface and subsurface geology. Onshore, the TBU shows tectonic relief of several kilometres between antiformal pop-ups and synformal depressions in contrast with a smoother topography offshore. This geometry arises from reverse slip on sets of north-south to NNE-SSW faults, mostly dipping 50-66°both west and east, that control folding of the TBU and overlying cover sequence. Some of these faults are inherited Upper Cretaceous-Palaeogene normal faults that displaced the TBU during the extensional phases and were later reactivated as reverse faults under compression, whereas others appear to be newly propagated Neogene reverse faults. The faults that deform the TBU have vertical displacements of 3-5 km and lengths of >150 km, and have the potential of being reactivated in the present stress field. Currently active faults comprise a set of blind reverse faults that propagate upsection from pre-existing extensional fault fabric in the basement, imposing a short-wavelength undulation on the TBU.

Structure, late Quaternary slip rate and earthquake potential of marine reverse faults along the North Westland deformation front, New Zealand

“…These data are widely distributed, and are tied to offshore exploration boreholes (see compilations of Breeze & Brown 1999;Ghisetti et al 2014). The wells pertinent to this study are Cook-1, Toropuihi-1, Kongahu-1 and Haku-1 (Figure 2A).…”

“…These include analogue single-channel airgun seismic data, boomer and 3.5 kHz sub-bottom profiles acquired by the New Zealand Oceanographic Institute (Department of Industrial Research, DSIR) and sparse seismic sections and 3.5 kHz profiles acquired by NIWA. Mineral exploration companies also collected shallow penetration (<200 m) seismic data near shore Litchfield et al 2013;Ghisetti et al 2014). The white arrow shows the relative plate motion vector between the Pacific and Australian plates (Beavan & Haines 2001;Wallace et al 2007 Anderson et al (1993) and Doser et al (1999).…”

“…The faults control uplift of ranges associated with topographic elevation of up to 1900 m. The deformation front is represented principally by the Cape Foulwind and Kahurangi faults, lying up to 30 km offshore beneath the inner continental shelf. These structures have received considerable attention by the petroleum exploration industry ( Figure S1) and appear on regional geological compilations (Rattenbury et al 1998;Nathan et al 2002;Ghisetti et al 2014). However, their potential late Quaternary activity and seismic hazard has been the subject of debate (Table 1; e.g.…”

“…The North Westland region of South Island, New Zealand, west of the Alpine Fault, is a deformed foreland basin shortened by widely distributed, N-S-to NNE-SSW-striking thrust and reverse faults ( Figure 1A) (Nathan et al 1986;Bishop & Buchanan 1995;Ghisetti & Sibson 2006;Stafford et al 2008;Ghisetti et al 2014). These include thrust faults that developed entirely during late Cenozoic time (<15 Ma) and moderately dipping (50-65°) basement faults inherited from systems of Late Cretaceous-Paleogene extensional faults that have been reactivated compressionally.…”

“…However, other workers have interpreted the faults to be active seismic and tsunami hazard sources with slip rates typically of <1.0 mm a -1 , earthquake magnitudes of M w 7.6-7.9 and recurrence intervals of 10 3 -10 4 years (Table 1) (Nathan et al 1996;Stafford et al 2008;Power 2013;Barnes & Ghisetti 2013). This paper builds on the work of Ghisetti et al (2014) and the preliminary work of Barnes & Ghisetti (2013), using marine seismic profiles tied to exploration boreholes and interpreted in terms of sea-level cyclicity to map and evaluate the activity of the North Westland deformation front. We present seismic data and a restored geological section that illustrate late Cenozoic tectonic inversion of the Cape Foulwind and Kahurangi faults and active folding of the cover sequence associated with blind faulting in basement rocks, pointing to activity of the North Westland deformation front in the late Quaternary.…”