Rates and mechanics of rapid frontal accretion along the very obliquely convergent southern Hikurangi margin, New Zealand

Abstract

Abstract. Analysis of seismic reflection profiles, swath bathymetry, side-scan sonar imagery, and sediment samples reveal the three-dimensional structure, morphology, and stratigraphic evolution of the central to southern Hikurangi margin accretionary wedge, which is developing in response to thick trench fill sediment and oblique convergence between the Australian and Pacific plates. A seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby, by coccolith and foraminifera biostratigraphy of three core and dredge samples, and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment. Structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds, on the growth and mechanics of frontal accretion under variable convergence obliquity, and on the amounts and rates of horizontal shortening. The data place constraints on the partitioning of geological strain across the entire southern Hikurangi margin. The principal deformation front at the toe of the wedge is discontinuous and represented by right-stepping thrust faulted and folded ridges up to 1 km high, which develop initially from discontinuous protothrusts. In the central part of the margin near 41 øS, where the convergence obliquity is 50 ø, orthogonal convergence rate is slow (27 mm/yr), and about 75% of the total 4 km of sediment on the Pacific Plate is accreted frontally, the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening. At least 80% of this shortening has occurred within the last 0.4 __. 0.1 m.y. at an average rate of 12 __. 3 mm/yr. This rate indicates that the frontal 30 km of the wedge accounts for about 33-55% of the predicted orthogonal contraction across the entire plate boundary zone. Despite plate convergence obliquity of 50 ø , rapid frontal accretion has occurred during the late Quaternary with the principal deformation front migrating seaward up to 50 km within the last 0.5 m.y. (i.e., at a rate of 100 km/m.y.). The structural response to this accretion rate has been a reduction in wedge taper and, consequently, internal deformation behind the present deformation front. Near the southwestern termination of the wedge, where there is an along-the-margin transition to continental transpressional tectonics, the convergence obliquity increases to >56 ø, and the orthogonal convergence rate decreases to 22 mm/yr, the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward-verging folds. These structures have accommodated not more than 0.5 km of horizontal shortening at a rate of < 1 mm/yr, which represents < 5% of the predicted orthogonal shortening across the entire plate boundary in southern North Island. The landward-vergent structural domain may represent a transition zone from rapid frontal accretion associ...

Structure and deformational history of the inner forearc region, Hikurangi subduction margin, New Zealand

“…For example, a number of aclhe structures occur in profile 203 (Davey et al 1986a, b) (s.e Fig. 15 for location) landward of the shortening rates data i > f Barnes & Mercier de Lepinay (1997). Therefore, we sugge -,t that structures within the region labelled "no rates data" n Fig.…”

“…The amount of slip observed decreases markedly towards the north, being c. 2 mm/yr in the northern Hikurangi forearc (Beanland 1995). Several workers (e.g., Collot et al 1996;Barnes & Mercier de Lepinay 1997) have described changes in structure along the accretionary part of the forearc and related these to the change in obliquity (i.e., the angle between a margin-normal line and the plate motion vector at that location) from north to south along the Hikurangi margin.…”

“…From the relative plate motion vector, components of c. 27-35 mm/yr and c. 25-29 mmA r orthogonal and parallel to the margin, respectively, aie derived. In the upper plate, margin-normal contraction f -r regions including part of the Wanganui Basin (derived from Anderton 1981), axial ranges, forearc basin, and part of the coastal ranges (this study), and the frontal area of the accretionary wedge (Barnes & Mercier de Lepinay 1997), account for 12-22 mm/yr over the last 0.3-0.5 m.y., that: .-,, 34-81% of the margin-normal plate motion (27-35 mm/yr) at the latitude (c. 40^4-1 °S) of the geological strain daia (Fig. 15).…”

Investigating subduction earthquake geology along the southern Hikurangi margin using palaeoenvironmental histories of intertidal inlets

“…1A). The offshore accretionary wedge is !100 km wide adjacent to the lower North Island and narrows rapidly towards the south (Barnes & Mercier de Lepinay 1997;Barnes et al 1998). The plate interface dips westward beneath the region and lies at approximately 25 km depth beneath Wellington and Blenheim (Fig.…”

Quaternary faulting in the offshore Flaxbourne and Wairarapa Basins, southern Cook Strait, New Zealand

“…To the east of Cook Strait, the thrust front at the toe of the Wairarapa margin advanced seaward rapidly and continued to rotate clockwise within the last 1 m.y. (Barnes & Mercier de Lepinay 1997). Similarly, new contractional structures have developed during the Quaternary at the toe of the eastern Marlborough continental slope advancing the deformation front toward the southeast (Fig.…”