Abstract

The active dextral-reverse Wairarapa fault in the North Island, New Zealand, cuts ca. 12 ka, post–Last Glacial Maximum (post-LGM) fluvial gravels with a highly segmented fault trace. This <350-m-wide fault zone displaces the gravels laterally by ∼130 m and warps them into elongate deformational bulges in the left-stepover regions between overlapping synthetic fault strands. Aerial photography, field observations, and global positioning system surveying were used to map the structural geology of a central section of this major oblique-slip fault zone. Topographic data collected at two particularly well-expressed and exposed pressure bulges of contrasting size were used to undertake a kinematic-volumetric analysis and to calculate the likely vertical extent of the bulge-related distributed deformation. In the near-surface, the Wairarapa fault zone is characterized by a two-order hierarchy of en echelon fault segments. Type A segments are the longest (2–7 km) and are separated by the largest stepover widths (250–350 m). Volumetric calculations suggest that these splays converge downward into a master fault plane at depths of ∼100–260 m, well below the base of the post-LGM gravels. Accordingly, we infer that type A faults and the bulges that form in their stepovers had already developed in the lithified basement rock prior to deposition of the surficial gravels. Type B segments are shorter (500 m to ∼4 km long) and are separated by 30–150-m-wide stepovers. We calculate that these splays converge at shallow depths of <20 m near the base of the poorly consolidated gravels, where they developed in response to distributed deformation of that surficial cover during the past ∼12 k.y. The segmented surface trace of the Wairarapa fault in some ways resembles that produced in analogue models of deformation above a strike-slip fault in basement rock; however, the synthetically slipping fault segments of the Wairarapa fault zone strike at a smaller angle (2°–18°) to the overall strike of the fault than do most analogue models (typically, 10°–30°), raising questions about the angles at which “R-shears” may develop along natural strike-slip faults that slip with a convergent component. Also, the natural fault zone is narrower (350 m wide) than equivalent scaled widths of most analogue models (typically 1–2 km). These differences are interpreted to reflect the shallow depth of splaying of the natural Wairarapa fault zone in comparison to thickly sedimented analogue models. Based on our field data, we present a general model for (1) the relationship between an oblique-slip fault and the asymmetry of its near-surface synthetic fault splays and pressure bulges that deform gravel, and (2) the along-strike partitioning of strike- and dip-slip components of slip between adjacent splays in the near-surface in relation to the oblique slip accumulating on the main fault at depth.

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