JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B03103, doi:10.1029/2006JB004594, 2008 Click Here for Full Article Widespread remagnetizations and a new view of Neogene tectonic rotations within the Australia-Pacific plate boundary zone, New Zealand Christopher J. Rowan1,2 and Andrew P. Roberts1 Received 23 June 2006; revised 14 August 2007; accepted 14 September 2007; published 19 March 2008. [1] Large, clockwise, vertical axis tectonic rotations of the Hikurangi margin, East Coast, New Zealand, have been inferred over both geological and contemporary timescales, from paleomagnetic and geodetic data, respectively. Previous interpretations of paleomagnetic data have laterally divided the margin into independently rotating domains; this is not a feature of the short-term velocity field, and it is also difficult to reconcile with the large-scale boundary forces driving the rotation. New paleomagnetic results, rigorously constrained by field tests, demonstrate that late diagenetic growth of the iron sulfide greigite has remagnetized up to 65% of sampled localities on the Hikurangi margin. When these remagnetizations are accounted for, similar rates, magnitudes, and timings of tectonic rotation can be inferred for the entire Hikurangi margin south of the Raukumara Peninsula in the last 7–10 Ma. Numerous large (50–80°) declination anomalies from magnetizations acquired in the late Miocene require much greater rates of rotation (8– 14° MaÀ1) than the presently observed rate of 3–4° MaÀ1, which is only likely to be characteristic of the tectonic regime established since 1–2 Ma. These new results are consistent with both long- and short-term deformation on the Hikurangi margin being driven by realignment of the subducting Pacific plate, with collision of the Hikurangi Plateau in the late Miocene potentially being key to both the initiation of tectonic rotations and the widespread remagnetization of Neogene sediments. However, accommodating faster, more coherent rotation of the Hikurangi margin in Neogene reconstructions of the New Zealand plate boundary region, particularly in the late Miocene, remains a challenge. Citation: Rowan, C. J., and A. P. Roberts (2008), Widespread remagnetizations and a new view of Neogene tectonic rotations within the Australia-Pacific plate boundary zone, New Zealand, J. Geophys. Res., 113, B03103, doi:10.1029/2006JB004594. 1. Introduction and geological timescales. The short-term velocity field, derived from a combination of geodetic data and Quaternary [2] The Hikurangi margin, on the East Coast of New fault slip rates [e.g., Beanland et al., 1998; Wallace et al., Zealand, structurally links the Tonga-Kermadec subduction 2004], indicates that the entire fore arc is actively rotating zone north of New Zealand, with associated slab roll-back 1 clockwise at 3–4° MaÀ with respect to the Australian and back-arc spreading, to a zone of intracontinental trans- plate (Figure 1a). Paleomagnetic studies of tectonically pression on the South Island of New Zealand, where uplifted marine sediments on the eastern North Island and underthrusting of buoyant continental crust (the Chatham northeastern South Island [Walcott et al., 1981; Walcott and Rise) impedes subduction and transfers interplate motion to Mumme, 1982; Mumme and Walcott, 1985; Wright and the Marlborough and Alpine-Wairau faults (Figure 1a). This Walcott, 1986; Mumme et al., 1989; Roberts, 1992, transition is associated with subduction of anomalously 1995a; Vickery and Lamb, 1995; Thornley, 1996; Little thick oceanic crust of the Hikurangi Plateau [Davy and and Roberts, 1997] also report large clockwise declination Wood, 1994; Wood and Davy, 1994] beneath the East Coast anomalies (Figure 1b), that indicate tectonic rotations have of the North Island. been a long-term feature of deformation on the Hikurangi [3] Vertical axis tectonic rotations are an important fea- margin. Declination anomalies of 30–40° have been ture of crustal deformation in this region, on both decadal reported from late Miocene (6–11 Ma) sediments from the central part of the margin [Wright and Walcott, 1986]; 1National Oceanography Centre, University of Southampton, South- early Miocene sediments from the northeast North Island ampton, UK. (Raukumara Peninsula) indicate no tectonic rotation with 2Now at Department of Geology, University of Johannesburg, respect to the Australian plate [Walcott and Mumme, 1982; Johannesburg, South Africa. Mumme et al., 1989; Thornley, 1996]; and sparse data from the southern North Island have been interpreted to indicate Copyright 2008 by the American Geophysical Union. 0148-0227/08/2006JB004594$09.00 late Miocene rotations that had ceased by 2 Ma [Walcott et B03103 1of22 B03103 ROWAN AND ROBERTS: TECTONIC ROTATIONS, HIKURANGI MARGIN B03103 Figure 1. (a) Contemporary geodynamics of the New Zealand plate boundary region (bathymetry from Smith and Sandwell [1997]). Subduction of thickened oceanic crust (the Hikurangi Plateau) beneath the Hikurangi margin links subduction and back-arc spreading north of New Zealand with intracontinental transpression on the Alpine and Marlborough fault systems; the Hikurangi margin rotates clockwise in response to this transition. The rotating fore arc is divided into a number of separate blocks to account for slip on faults of the North Island Dextral Fault Belt (NIDFB), following Wallace et al. [2004]. SFB, South Fiji Basin; HT, Havre Trough; TVZ, Taupo Volcanic Zone; MFZ, Marlborough Fault Zone. (b) Proposed division of the Hikurangi margin into independently rotating ‘‘domains’’ (shaded) based on inferred differences in tectonic rotations recorded by paleomagnetic data (squares) [cf. Walcott, 1989]. (c) Reconstruction of the Hikurangi margin at 20–23 Ma, following Rait et al. [1991], with early Miocene thrust belts realigned along a NW-SE trending margin that has subsequently rotated up to 90° clockwise. ECA, East Coast Allochthon; ChR, Chatham Rise. 2of22 B03103 ROWAN AND ROBERTS: TECTONIC ROTATIONS, HIKURANGI MARGIN B03103 Figure 2. N-S cross section along the Hikurangi margin, illustrating lateral variations in the structure of the subducting and overriding plates. Vertical dimensions have been exaggerated for clarity. Dotted lines on the Hikurangi Plateau mark the structural divisions of Wood and Davy [1994] (NVR, Northern Volcanic Region; CB, Central Basin; NCT, North Chatham Terrace; NCB, North Chatham Basin). Other abbreviations are as in Figure 1. al., 1981; Lamb, 1988]. In the Marlborough region (north- [4] Prior to the development of key features of the current east South Island), two distinct periods of rotation, in the tectonic regime, including the NIDFB and the Taupo early Miocene and from the early Pliocene (circa 4 Ma) Volcanic Zone (TVZ), since 1–2 Ma [Beanland, 1995; onward, have been observed [Walcott et al., 1981; Lamb, Wilson et al., 1995; Beanland et al., 1998], different 1988; Roberts, 1992, 1995a; Vickery and Lamb, 1995; Little structures must have been involved in accommodating and Roberts, 1997; Hall et al., 2004]. These observations interplate motion. The mismatch between paleomagnetically led to the proposal that the margin is divided into discrete, and geodetically determined rotations may therefore be a fault-bounded ‘‘domains,’’ with independent tectonic histo- consequence of the ongoing evolution of structures in the ries [Lamb, 1988; Walcott, 1989] (Figure 1b). However, New Zealand plate boundary zone. However, reconciling aspects of this model are difficult to reconcile with the paleomagnetic with other geological data has still proven current geodynamics and structure of the Hikurangi margin. difficult, with numerous alternative reconstructions being Wallace et al. [2004] modeled contemporary deformation proposed [King, 2000, and references therein]. At the other using a number of rotating blocks, bounded by major faults extreme from the domain model, the large disparity between of the North Island Dextral Fault Belt (NIDFB) [Beanland, the southwestward transport directions of the Northland and 1995], but these blocks do not correlate to the paleomag- East Coast allochthons [Stoneley, 1968; Spo¨rli, 1982; Rait, netically defined domains, and they all have similar rates 2000], which were obducted in the late Oligocene–early and poles of rotation (Figure 1). Present-day rotation of the Miocene (20–25 Ma) [Rait et al., 1991], and the northeast- margin appears to be linked to a gradual southward increase ward shortening direction recorded by the coeval Wairarapa in the thickness of the subducting Hikurangi Plateau, from thrust belt [Chanier and Fe´rrie`re, 1989], have been cited as 10 to 15 km [Davy and Wood, 1994] (Figures 1a and 2), evidence that the entire Hikurangi margin south of the which gradually increases coupling across the plate inter- Raukumara Peninsula has coherently rotated up to 90° face [Reyners, 1998], and creates a smooth margin-normal clockwise since the early Miocene [Rait et al., 1991], prior shear gradient that is most easily accommodated by bulk to which it shared the northwest-southeast trend of the early clockwise rotation of the entire margin [Walcott, 1989; Miocene Northland arc [Herzer, 1995] (Figure 1c). How- Wallace et al., 2004]. The domain model suggests much ever, this model requires several hundred kilometers of smaller-scale variation of the forces driving rotation, and Neogene shortening in the southern North Island, which structures capable of accommodating large differential rota- appears to be inconsistent with fault displacements in the tions