JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B01403, doi:10.1029/2007JB005170, 2008 Click Here for Full Article Regional structure and kinematic history of a large subduction back thrust: Taranaki Fault, New Zealand V. Stagpoole1 and A. Nicol1 Received 14 May 2007; revised 8 August 2007; accepted 8 September 2007; published 17 January 2008. [1] The Taranaki Fault is a back thrust antithetic to the Hikurangi margin subduction thrust. Subduction back thrusts, like the Taranaki Fault, accrue displacement transferred from the subducting plate, and growth analyses of these structures contribute to an improved understanding of subduction processes. The Taranaki Fault forms the eastern margin of the Taranaki Basin and is part of a system that extends for at least 600 km in continental crust of western New Zealand. The fault is preserved beneath young sedimentary cover and provides a rare opportunity to investigate the geometry and kinematic history of a large subduction back thrust. Two-dimensional seismic reflection lines (2–5 km spacing), tied to recently drilled wells and outcrop, together with magnetotelluric and gravity models are used to examine the fault. These data indicate that the fault is thick skinned with dips of 25–45° to depths of at least 12 km. The fault accommodated at least 12–15 km of dip-slip displacement since the middle Eocene (circa 40–43 Ma). The northern tip of the active section of the fault stepped southward at least three times between the middle Eocene and early Pliocene, producing a total tip retreat of 400–450 km. The history of displacements on the Taranaki Fault is consistent with initiation of Hikurangi margin subduction during the middle Eocene, up to 20 Ma earlier than some previous estimates. Fault tip retreat may have been generated by clockwise rotation of the subduction margin and associated progressive isolation of the fault from the driving downgoing Pacific Plate. Citation: Stagpoole, V., and A. Nicol (2008), Regional structure and kinematic history of a large subduction back thrust: Taranaki Fault, New Zealand, J. Geophys. Res., 113, B01403, doi:10.1029/2007JB005170. 1. Introduction growth strata provide important constraints on the timing and kinematics of deformation related to subduction [e.g., [2] Fold and thrust belts can form within the overriding Barnes et al., 2002; Nicol et al., 2002, 2007]. plate at subduction margins [e.g., Karig and Sharman, [3] In this paper we examine the regional geometry and 1975; Davis et al., 1983; Moore, 1989; Barnes and de kinematic history of the Taranaki Fault, a large subduction- Le´pinay, 1997; von Huene and Klaeschen, 1999]. These related back thrust in New Zealand’s Taranaki Basin. structures develop in association with plate convergence Analysis of displacements of Late Cretaceous and younger [e.g., Forsyth and Uyeda, 1975; Backus et al., 1981] and strata, up to 8 km thick in the Taranaki Basin, afford a rare may provide information about the kinematics and history opportunity to better understand the circa 43–40 Ma of subduction. Outside of accretionary wedges, back thrusts, kinematic history of the fault and highlight important that form antithetic to the subduction thrust, also accrue relations between fault growth (and death, when the fault displacement transferred from the subducting plate. Analy- becomes permanently inactive) and associated subduction at sis of displacements on these back thrusts and associated the Hikurangi subduction margin. This study provides growth strata provide an opportunity to resolve the timing constraints for the development of the New Zealand plate and magnitude of subduction-related deformation and may boundary, while also demonstrating useful analysis techni- help improve understanding of subduction processes. How- ques applicable to subduction zones in other parts of the ever, many of these contractional structures are poorly world. resolved, because crustal shortening typically results in their [4] The Taranaki Fault is one of the longest (400 km) uplift and erosion (e.g., in the Andes and the Rocky and highest displacement (>10 km) contractional structures Mountains). In contrast to these two regions, synsubduction in New Zealand’s continental crust [King and Thrasher, Tertiary strata are widely preserved on the overriding plate 1996]. The fault forms the boundary of pre-Miocene rocks of New Zealand’s Hikurangi subduction margin. These in the Taranaki Basin and is part of a larger fault system that extends 600 km northward from the South Island’s Alpine 1 Fault in western New Zealand (Figure 1). This fault system GNS Science, Lower Hutt, New Zealand. is contained entirely within continental crust of the over- Copyright 2008 by the American Geophysical Union. riding Australian Plate and includes the Waimea-Flaxmore 0148-0227/08/2007JB005170$09.00 Fault in the northern South Island and the Manaia Fault and B01403 1of19 B01403 STAGPOOLE AND NICOL: TARANAKI FAULT, NEW ZEALAND B01403 Figure 1. Regional map of Taranaki Fault. Map shows the location and geometry of the Taranaki Fault, Manaia Fault, Waimea-Flaxmore Fault, and Tarata Thrust. Contours (2, 4, 6 km) indicate depth below sea level to top basement (data modified from King and Thrasher [1996]). Locations of wells (circles) are shown. Inset shows plate boundary setting. the Tarata Thrust in Taranaki (Figure 1). Displacement convergence [King, 2000; Cande and Stock, 2004; Nicol et within the fault system was driven by subduction of Pacific al., 2007]. Most of this convergence (i.e., >80%) accrued on Plate along the Hikurangi margin and collision of continental the subduction thrust, with the remainder accommodated on crust along the Alpine Fault [Stern et al., 1993]. Currently, upper plate faults, including the Taranaki Fault [Nicol et al., the relative motion between Australian and Pacific plates is 2007]. oblique along the Hikurangi margin [DeMets et al., 1994; [5] Displacement on the Taranaki Fault strongly influ- Beavan and Haines, 2001] (Figure 1); however, during the enced the location and geometry of the Taranaki Basin, New Eocene to Miocene relative plate motion was dominated by Zealand’s only producing oil and gas region. Petroleum 2of19 B01403 STAGPOOLE AND NICOL: TARANAKI FAULT, NEW ZEALAND B01403 Figure 2. Detailed map of Taranaki Fault and cross sections. Cross sections constructed at a high angle to the fault (modified from Thrasher et al. [1995]) show variations in fault geometry and Taranaki Basin architecture from north (A-A0) to south (D-D0). Map indicates locations of cross sections (bold lines), seismic lines (thin lines), wells (circles), and the line of magnetotelluric measurements (bold dashed line). All sections are at true scale. exploration in the basin has supplied a significant body of of the subducting plate, and with subduction commencing subsurface information on the Taranaki Fault, including during the middle Eocene. seismic reflection and well data. The Taranaki Fault is not exposed at the surface, being overlain by up to 3 km of early 2. Data Sources and Analyses Miocene and younger sedimentary rocks (Figure 2). It is, 2.1. Seismic Reflection Data however, visible on many seismic reflection lines, in the regional gravity signature and in petroleum exploration [6] Seismic reflection lines provide information on the wells [e.g., Mills, 1990; King and Thrasher, 1992, 1996; fault to depths of 4–7 km. Forty-three seismic reflection Palmer and Andrews, 1993]. We use two-dimensional (2-D) lines (including Figures 3, 4, and 5), which cross the fault seismic reflection lines tied to petroleum exploration wells and are distributed along 300 km of its length, have been and outcrop to examine the Taranaki Fault where it dis- interpreted (see Figure 2 for locations) with the aid of tie lines places Late Cretaceous and younger strata. These data have (not shown in Figure 2). The total interpreted line length is in been augmented with magnetotelluric and gravity profiles excess of 1000 km, and the data are primarily from open file which provide constraints on the fault geometry at depth petroleum industry sources (available from New Zealand (e.g., 5–12 km). Collectively these data sets indicate that Ministry of Economic Development, http://www.crownminerals. the fault is a thick-skinned thrust which, south of Mount govt.nz/cms/petroleum/technical-data). The seismic lines are Taranaki, probably extends down to the base of the crust. all migrated, while some are prestacked depth-migrated The fault accrued displacement prior to the Oligocene, with sections (e.g., Figure 3). The age and quality of these seismic the northern tip of the fault stepping southward over the last lines are variable, with Figures 3, 4, and 5 providing 30 Ma. This episodic migration is consistent with increas- representative examples of the data. Most seismic lines are ing isolation of the Taranaki Fault from the driving sub- interpretable down to two-way traveltimes of 3–5 s. Up to ducting Pacific Plate, perhaps induced by clockwise rotation eight reflectors were interpreted in each seismic line, includ- ing the top of seismic basement and up to six intra-Tertiary 3of19 B01403 STAGPOOLE AND NICOL: TARANAKI FAULT, NEW ZEALAND B01403 Figure 3. Prestacked depth migration (shown in two-way time) of (a) uninterpreted and (b) interpreted seismic line TRV-434 from the northern end of the fault (see Figure 2 for location). Locations and ages of interpreted horizons are indicated. Interpretations are based on data from Stagpoole [1997]. events. The interpreted reflectors range in age from about 2 nearby exploration wells. Analysis of the MT phase tensor to 80 Ma and have been tied to 20 nearby wells along the [Caldwell et al., 2004] indicates the data show 2-D charac- fault length. Our interpretations are broadly consistent with ter for periods up to 60 s, corresponding to depths of at least previously published data [Thrasher et al., 1995; King and 12 km [Stagpoole et al., 2006].