Regional Structure and Kinematic History of a Large Subduction Back Thrust: Taranaki Fault, New Zealand V

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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

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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

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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, including the top of seismic basement and up to six intra-Tertiary

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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]. Thrasher, 1996; Stagpoole, 1997]. 2.3. Gravity Data 2.2. Magnetotelluric Data [8] There are over 10,000 onshore and offshore (marine [7] A magnetotelluric (MT) survey was undertaken track line) gravity observations in the region (Figure 6a). All across the Taranaki Fault in 2003 [Stagpoole et al., 2006]. onshore data (available at http://maps.gns.cri.nz/website/ The resistivity contrast between Cretaceous and Cenozoic gravity/) have been reduced to anomalies as described by strata (3–30 ohm m) and the older Mesozoic basement Reilly [1972]. Marine track line data are available from the rocks (50–1000 ohm m) make the fault a suitable target for U.S. National Geophysical Data Center (http:// investigation using the MT technique. The survey com- www.ngdc.noaa.gov/mgg/geodas/trackline.html). A map of prised 13 broadband soundings, measured using 3 Phoenix the combined onshore Bouguer anomaly and offshore free- MTU-2000 instruments and a single four component re- air anomaly data (Figure 6a) is dominated by a large NE- corder, along a 25 km west-east profile on the Taranaki SW minimum, related to the Pacific Plate subducting Peninsula (Figure 2). Data were recorded for about 40 h at beneath the North Island [Bannister, 1989; Stern et al., each site and modeled using the 2-D code of Rodi and 1993], and positive anomalies in the north and west that Mackie [2001]. Modeling constraints were provided by relate to lithospheric structure [Stern et al., 1987] and the seismic reflection data and borehole resistivity logs from lack of subsidence following deposition of recently depos-

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Figure 4. Migrated (a) uninterpreted and (b) interpreted seismic line tp-09 (see Figure 2 for location on the peninsula). Locations and ages of interpreted horizons are indicated and based on data from the Toko-1 and Huinga-1 wells. ited sediments offshore in the Taranaki Basin [Holt and over 12 km grid cells and smoothed to generate the regional Stern, 1991]. In the vicinity of the Taranaki Fault, these gravity map (Figure 6b). The residual gravity field, repre- regional long-wavelength effects (>50 km) tend to mask the senting the gravity effect of upper crustal structure, was shorter wavelength anomalies associated with upper crustal compiled by subtracting the regional gravity from the structure, and have been separated by computation of observed gravity. regional and residual gravity fields (Figures 6b and 6c) to [10] Bouguer and free-air gravity anomaly data, with no reveal the Taranaki Fault. regional-residual separation, were used for two-dimensional [9] The regional gravity field was computed using gravity (2-D) models across the fault. The 2-D gravity models are data more than 25 km from the Taranaki Fault (Figure 6b) relative to a standard density model with 28 km thick crust that were either observed on basement rocks or where the and 5 km of sediment (Figure 7), similar to the structure thickness of the sedimentary succession was known from interpreted to occur at the western end of each line where seismic [Thrasher et al., 1995] and well data. A correction the gravity anomaly is close to zero. The models are for the density deficiency of the sedimentary succession constrained by seismic, magnetotelluric and earthquake (based on the compaction curve for mudstone of Funnell et data. Information for the general sedimentary structure in al. [1996] assuming a grain density of 2.7 Mg/m3) was the Taranaki, Wanganui and East Coast basins are from applied to gravity values over the sedimentary basins on seismic reflection lines [Anderton, 1981; King and both sides of the fault (see Figure 6b for location of data Thrasher, 1996; Field et al., 1997]. For the southern profile, points). Additional corrections have been made for the deeper crustal structure is based on interpreted seismic data water depth of offshore data and for departures from the from southern Taranaki [Stern and Davey, 1990; Holt and standard compaction curve due to uplift and erosion Stern, 1994]. For the northern profile, crustal structure is [Armstrong et al., 1998]. These data were then averaged

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Figure 5. Migrated (a) uninterpreted and (b) interpreted seismic lines P81-23 and P95-509 spliced together (see Figure 2 for location south of the peninsula). Locations and ages of interpreted horizons are indicated and based on data from the Kupe South-4 and Kupe-1 wells. based on the seismic interpretation of Harrison and White change in strike of 25° to north-northwest. The fault can [2006] and Stratford and Stern [2006]. On both sides of the be traced on seismic reflection lines to at least latitude 37°S Taranaki Fault, the model is locally isostatically compen- (a further 125 km). Northward from the Te Ranga-1 well sated (compensation depth of 80 km) with respect to the fault displacements decrease, the fault is progressively standard density model. There is some uncertainty in the buried by the sedimentary succession and can be masked crustal structure beneath the Taupo Volcanic Zone [Stratford by submarine volcanic cones. Locally the fault may change and Stern, 2006; Harrison and White, 2006] and this region in strike by as much as 15–25°. Notable changes in fault has not been modeled in detail. Earthquake hypocenters, strike occur immediately west of Rotokare-1 and about 15 km relocated as part of the 3-D tomographic inversion [Reyners south of Awakino-1 wells (Figure 1). These changes in et al., 2006], which occur within 30 km of the lines, are strike coincide with locations where a thrust splays into the shown on the models and correlate with the position of the footwall of the main fault; the Tarata Thrust is one of these subducting Pacific Plate beneath the North Island. Models footwall splays. are constructed in a similar manner to Bannister [1989] and [12] Westward displacement on the fault in the Cenozoic Holt and Stern [1994], incorporating both the overriding has resulted in uplift and emplacement of a wedge of and subducted plate structure beneath the North Island. Mesozoic basement rocks over Late Cretaceous and Tertiary strata of the Taranaki Basin (e.g., Figures 3–5). The 3. Fault Geometry basement wedge comprises greywacke and argillite of the Murihiku Terrane, which is Late Permian to Late Jurassic in [11] The Taranaki Fault strikes north-south from 40°30’S age [Raine et al., 2004]. Outcrop, exploration wells and to 38°S a distance of about 250 km (Figure 1). At latitude seismic reflection lines indicate that the Taranaki Fault is 38°S (about 25 km north of the Te Ranga-1 well) there is a approximately parallel to the strike and dip of bedding in the

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Figure 6. (a) Contours (10 mGal interval) of the onshore Bouguer and offshore free-air gravity anomalies and the data locations (dots), (b) contours of the calculated regional field and data points (small crosses) used for its compilation, and (c) shaded residual gravity field. See text for method used to create regional and residual gravity fields. B01403 B01403 STAGPOOLE AND NICOL: TARANAKI FAULT, NEW ZEALAND B01403

Murihiku Terrane. West of the fault, basement rocks are 1997; Stagpoole et al., 2006]. The location and geometry of inferred to be Permian age Brook Street Terrane [Spandler the Taranaki Fault could therefore be strongly influenced in et al., 2005], and the boundary between terranes, which the upper crust by the location and orientation of the formed during Mesozoic accretion and faulting [Thrasher, preexisting Murihiku-Brook Street terrane boundary. 1990], probably lies at the Taranaki Fault [Mortimer et al.,

Figure 7

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[13] The density contrast between basement rocks on the due to uncertainties in the measurements, including those up-thrown eastern side of the fault and lower density arising from the seismic velocity model, which are collec- Cretaceous and Cenozoic sediments in the west means the tively estimated to be ±7°. These uncertainties are, however, Taranaki Fault is marked by a strong residual gravity insufficient to account for all of the variability in fault dip gradient that extends along the west coast of the North (up to 20–30°) over strike distances of 5–80 km. Therefore Island to latitude of 37° 300S (Figure 6c). North of 37° 300S the data also capture local and regional variations in dip the fault does not have a strong gravity expression, because along strike (Figure 8) but do not include up-fault changes of the greater sediment thickness over the fault and the in dip along individual sections, such as the apparent smaller vertical throw seen on seismic data. Thicker sedi- shallowing of dip at the upper fault tip (e.g., Figure 3). ment cover is also indicated by the lower gravity gradient There is a spatial coincidence between low fault dips and between the negative anomalies associated with thick (3– low altitudes of the western tip of the thrust wedge 7 km) low-density sediments in the Wanganui and Taranaki (Figure 8), which is consistent with the suggestion that Basin. The fault relay zone between the Manaia and some of the along-strike variability in dip may be limited to Taranaki faults is clearly visible on the residual gravity the upper crust (Figure 8 inset diagram). For example, using map (Figure 6c). 2 km for the variation in the altitude of the tip of the thrust [14] In cross section the fault defines the base of the wedge and a general change in fault dip from 30° on the basement wedge, while the top of the basement wedge structural lows to 45° on the structural highs, we estimate comprises a ridge (variously referred to as the Patea- that corrugations on the fault surface could extend to depths Tongaparutu High and Herangi High). Seismic reflectors of 6–7 km. and beds dip away from the crest of the basement high or [16] Fault dips in the upper crust are consistent with those ridge and indicate that it forms an anticline with a hinge estimated for depths of 5–12 km using MT and gravity 15–20 km east of the upper tip of the Taranaki Fault models. Two dimensional models of MT data [Stagpoole et (Figure 2). Seismic reflection lines and well information al., 2006] show a zone of low resistivity (3–30 ohm m), indicate that, in many cases, the Taranaki Fault comprises corresponding to Late Cretaceous and Tertiary sediments multiple slip surfaces which may splay from the main fault overlying higher resistively (20–300 ohm m) basement surface within 2–6 km vertical distance of the upper tip. rocks (Figure 7). A large change in sediment thickness These thrust splays can occur entirely within Tertiary strata (5 km) between western and eastern sides of the model (e.g., Tarata Thrust cross section C-C0 in Figure 2 and occurs in the region of the Taranaki Fault. Models also the thrust at the Awakino-1 well in cross section B-B0 in predict a discontinuity in basement resistivity at the fault. Figure 2), produce interfingering basement and Cretaceous- The discontinuity extends at a dip of about 45° to the base Tertiary strata (e.g., Te Ranga-1 well cross section A-A0 in of the models (12 km), well beyond the penetration depth of Figure 2) or are confined to the basement wedge. In the seismic reflection data (Figure 7). This resistivity contrast Pukearuhe-1 well, for example, four thrust-bound blocks of between basement rocks on eastern and western sides of the basement occur in mixed stratigraphic order separated by 5 fault indicates a difference in electrical properties, possibly thrusts [Raine et al., 2004]. Such fault zone complexities are relating to basement lithology, and supports the interpreta- typical of thrusts or thrust systems with significant displace- tion of Mortimer et al. [1997] that the Taranaki Fault is ment (e.g., >10 km) [e.g., Boyer and Elliott, 1982] and developed on the Murihiku-Brook Street terrane boundary. appear to occur widely along the length of the Taranaki [17] Two 2-D gravity models across the North Island Fault. However, individual splays tend not to extend further (Figure 7) are also consistent with the interpretation of an than a few 10s of kilometers along strike; for example, the eastward dipping fault. The prominent (100 to 150 mGal) Tarata Thrust comprises segments ranging from about 10 to NE-SW minimum that dominates the gravity anomaly 40 km in length. distribution over the North Island [Robertson and Reilly, [15] The average dip of the principal Taranaki Fault 1958; Bannister, 1989] occurs at the eastern end of each surface to depths of 3–7 km ranges from 20 to 50°. These gravity profile (Figure 7) and follows the trend of seismicity dips were estimated along approximately 300 km of the in the subducting plate. Gravity modeling indicates that no fault length using depth-converted seismic lines crossing the more than about 60 mGal of this negative anomaly can be fault (Figure 8). They take account of compaction of the Late attributed to the low-density sediments in the East Coast and Cretaceous-Tertiary succession beneath the fault, which Wanganui basins (Figures 6 and 8), with the remaining resulted in an average reduction in fault dip of about 4°, 50 to 100 mGal regional gravity anomaly inferred to be and of the occasional obliquity between seismic line and related to mass deficiencies beneath the sedimentary sec- fault strike. The resulting range of fault dips may be partly tion. Uncertainties in the subsediment density distribution

Figure 7. Two-dimensional gravity (top and center) and magnetotelluric (bottom left) models of the Taranaki Fault. In the gravity models Figure 7 (top) shows the Bouguer and free-air gravity data (crosses) and the predicted gravity (line) for the structural cross section in Figure 7 (bottom). The gravity anomaly of the cross section is calculated relative to the crustal model at lower right (densities are in Mg/m3). The location map shows the lines of data used for modeling. Gravity models are constrained by seismic reflection data (in the Taranaki Basin) and earthquake hypocentres [Reyners et al., 2006] within 30 km of each profile (crosses) that mark the position of the subducting slab. The northern gravity profile is also constrained by wide-angle seismic data [Harrison and White, 2006; Stratford and Stern, 2006] that have been used to define the base of the crust east and west of the Taupo Volcanic Zone (bold lines). The magnetotelluric model is overlaid on southern gravity profile in alignment with the position of the fault.

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Figure 8. Variation in dip of the Taranaki Fault from 36.8°S (zero distance) southward to about 40.4°S (400 km distance). Dips are average decompacted values calculated for the upper 5–7 km of the crust using depth-converted seismic sections. Velocity information for depth conversion was obtained from seismic stacking velocity and well check-shot data (Awakino-1, Te Ranga-1, Ariki-1, Te Kumi-1) and have an estimated error of ±5%. Decompaction analysis [Sclater and Christie, 1980] on the depth- converted data indicates that the burial depth of tip of the fault has increased by between 400 and 900 m due to compaction, resulting in an average reduction in fault dip of 4°. Errors are estimated to be less than ±7°. The locations of seismic lines in Figures 3, 4, and 5, the Tarata Thrust and Manaia Fault, and four wells are indicated. Grey shaded line approximates the elevation of the western tip of the thrust wedge (see Figure 9) and shows the spatial coincidence between low fault dips and low elevations. These variations in dip may be limited to the upper crust (inset diagram). mean that gravity models cannot be used to accurately 2005] and seismic tomography [Reyners et al., 2006] predict the fault dip or conclusively identify offsets at the indicate that it occurs on the eastern side of the fault at base of the crust (as shown in Figure 7, our preferred about the latitude of Pukearuhe-1 well (Figure 1). The model). However, these data are consistent with the mod- northward decrease in thickening across the fault is consis- erate to low dips measured from the seismic lines and the tent with a decrease in fault displacement in the same MT model. direction (see next section); however, we cannot presently [18] Modeled changes in crustal thickness across the fault discount the possibility that it was also influenced by are consistent with the view that the fault is thick skinned and processes unrelated to Taranaki Fault displacements. Stern extends to the base of the crust at its southern end. From west et al. [2006] argue, for example, that crustal thinning in the to east across the fault, the crustal thickness increases by central and western North Island was induced by the post about 11 km (27–38 km) in the southern profile (Figure 7). Miocene convective removal of mantle lithosphere that was This change in crustal thickness across the fault is in thickened in the early Miocene. agreement with deep crustal seismic reflection data south of the peninsula from which Stern and Davey [1990] infer a 4. Fault Displacements thick skinned fault model in southern Taranaki. By contrast gravity modeling predicts only about 3 km (22–25 km) [19] Measurement of fault displacements requires the change in crustal thickness across the fault on the northern reliable correlation of horizons across a fault [e.g., Muraoka profile (Figure 7). The crust west of the fault has more-or- and Kamata, 1983; Chapman and Williams, 1984; Barnett less uniform thickness between northern and southern et al., 1987; Ellis and Dunlap, 1988]. Displacement on profiles, whereas there is a marked difference in crustal thrust faults is typically accompanied by folding, particu- thickness on the eastern side of the fault (25–38 km). This larly close to the fault tip [e.g., Boyer, 1986; Chester and north-south change in crustal thickness was noted by Stern Chester, 1990; Suppe and Medwedeff, 1990]. Contraction et al. [1987], and earthquake data [Sherburn and White, across the Taranaki Fault in the upper 5–6 km of crust is

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Figure 9. Vertical separation diagram showing the elevation (with respect to sea level) of the top basement and base Oligocene horizons along the crest of the basement high in the fault hanging wall and in the footwall west of the fault together with the tip of the thrust wedge (see Figure 10 for details of measurements). The elevation of the tip of the basement wedge is from this study, while data for the top basement and base Oligocene are principally from structure contour maps of Thrasher et al. [1995] and King and Thrasher [1996]. accommodated by a combination of fault displacement and depth (as would be expected for a strike-slip flower struc- folding. For the purposes of this study we measure the ture) and bends in the fault do not appear to be associated vertical separation of horizons deformed by the fault. These with local pop-up or basin structures, we infer that the fault vertical separations incorporate displacement on the princi- did not accommodate a significant component of strike slip. pal slip surface, displacement on subsidiary faults in the [20] The vertical separation profiles in Figure 11 confirm zone around the Taranaki Fault and fault-related folding the large fault displacements of King and Thrasher [1996]. (Figure 9). Vertical separations for the top basement, top The vertical separation profile for top basement horizon, for Cretaceous, base Oligocene, top early Miocene and top example, reaches a maximum of at least 7 km. Given the Miocene have been estimated by measuring the difference observed fault dips, a vertical separation of 7 km requires in elevation of these horizons in the footwall and hanging that at least 10 km of shortening and 12–15 km of dip-slip wall of the fault (Figure 10). Our estimates of separation displacement accrued on the fault since the mid-Cretaceous assume that each horizon was approximately horizontal (i.e., the age of the oldest sedimentary rocks resting on top prior to faulting, which is consistent with paleoenvironmen- basement horizon). tal interpretations of the Taranaki Basin succession [King [21] Vertical separations show a progressive increase in and Thrasher, 1996]. Because of uplift, material has been displacement with horizon age from which it can be inferred eroded from the top of the basement high in the fault that the fault was active from prior to the Oligocene until hanging wall. The amount of hanging wall erosion is immediately after the end of the Miocene. This interpreta- unknown, in all cases except for the base Oligocene from tion may in part arise because the top Cretaceous, the the Herangi High north (210 km distance in Figure 9) and southern part of the base Oligocene and the top early the top Miocene, and vertical separations are minimum Miocene are not present on the hanging wall, and therefore values for the eroded horizons (for further discussion, see vertical separations for these horizons are minimums and Figure 11 caption). The fault dips to the east with the could be larger. However, the vertical separations for the hanging wall block displaced upward relative to the foot- base Oligocene from the Herangi High northward and all of wall block, indicating that the fault accommodated a com- the top Miocene are well constrained, because these hori- ponent of reverse dip slip. As the fault does not steepen with zons are present on both the hanging wall and footwall.

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Figure 10. Schematic cross section showing the general thrust geometry and measurement of horizon elevations for Figure 9.

From the vertical separation profiles, the minimum pre- of 300 km distance along the displacement profiles the Oligocene vertical separation across the fault from the minimum base Oligocene separation is 1–3 km greater than Herangi High north is the difference in separation between the top Miocene. Comparison of the two profiles (i.e., top the top basement horizons and base Oligocene horizons and Miocene and base Oligocene) suggests that 70% of the ranges from 2 to 5 km (Figure 11). Along this northern Oligocene and younger separation on the fault is pre- section of the fault (<200 km distance) 70% of the total Pliocene. separation accrued on the fault prior to the Oligocene. North

Figure 11. Vertical separation profiles for top basement, top Cretaceous, base Oligocene, top early Miocene, and top Miocene. Separation profiles derived in part from the data in Figure 9. The vertical separation on the base Oligocene north of the 210 km distance (solid line) is constrained by Oligocene rocks exposed onshore and mapped in seismic reflection profiles offshore on both sides of the fault. South of this point (dashed line) the vertical separation on the base Oligocene horizon is a minimum, derived by calculating the difference in altitude for the base Oligocene in the footwall from the top basement in the hanging wall. The impact of mid-Miocene and younger normal faults striking oblique to the Taranaki Fault (which locally offset the fault) and post-Miocene regional tilting to the west have been removed from the separation profiles.

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[22] The separation profiles in Figure 11 also change responsible for uplift of the fission track samples. This shape with decreasing age. In particular, the younger inference is supported by our interpretation of a petroleum displacement profiles shift to the right and appear to become exploration seismic data west of Auckland that shows more asymmetric. The top basement profile, for example, thickening of Paleocene and Eocene strata toward an extends from 50 km to 400 km and is flat topped, while eastward dipping fault (Figure 12), which we map as the base Oligocene profile extends for 50–100 km to 370 km northward continuation of the Taranaki Fault. The seismic and is strongly asymmetric with higher separations in the interpretation indicates that the eastward dipping fault dis- south. Comparison of these two profiles suggests maximum places Eocene and older sedimentary rocks, but does not pre-Oligocene displacement on the northern half of the fault offset Oligocene strata. This confirms the pre-Oligocene age with the greatest post-Oligocene displacement at the south- of the fault in this region, but the precise timing of this ern end of the fault. Taken collectively the separation displacement cannot be determined from the seismic line profiles indicate that the northern tip of the active section (Figure 12). On the basis of their fission track results, Kamp of the fault migrated southward throughout the Tertiary. By and Liddell [2000] infer uplift between 45 and 80 Ma. As contrast the southern fault tip appears to have been approx- normal faulting dominated in the Taranaki Basin during the imately stationary (Figure 11), with displacement being Late Cretaceous to early Paleocene (e.g., 80–55 Ma) [King transferred onto the Manaia Fault across a large relay zone and Thrasher, 1996], pre-Oligocene contraction and reverse (Figure 1). Therefore migration of the northern tip occurred displacement on the fault may postdate the early Paleocene in conjunction with both a southward shift in the locus of (i.e., occurred between 55 and 34 Ma). fault activity and a shortening of the active fault length. The [26] We suggest that pre-Oligocene displacement on the total southward retreat of the northern tip was 400– Taranaki Fault commenced during the middle Eocene at 450 km, with 250 km during the time interval between circa 40–43 Ma. The evidence for this conclusion is base Oligocene and top Miocene. threefold. First, the Manaia and Taranaki faults are separated by 20–30 km wide relay ramp (Figure 1), the 5. Fault Kinematic History presence of which suggests that these structures were active synchronously with displacement transferred between faults [23] Vertical separations in Figure 11, together with [cf. Childs et al., 1995]. Growth strata across the Manaia faulting and folding of stratigraphy adjacent to the Taranaki Anticline (and within the relay ramp) indicate displacement Fault, indicate a history of movement which is both older on the Manaia Fault from the middle-late Eocene (circa 43– and younger than 34 Ma. For the purposes of this paper 35 Ma) [Voggenreiter, 1993; Holstege and Bishop, 1998]. we discuss separately the pre- and post-Oligocene deforma- Therefore we infer that the Taranaki Fault was also active tion across the fault. during the middle-late Eocene. Second, seismic lines on, 5.1. Pre-Oligocene and south of, the Taranaki Peninsula which resolve pre- Oligocene strata typically show the Eocene sequence to [24] The difference in vertical separation across the Tar- thicken by 20–50% toward the Taranaki Fault (e.g., Figure anaki Fault between the top basement and base Oligocene 2 cross sections C-C0 and D-D0, Figure 5), consistent with horizons provides strong evidence for deformation on the Eocene movement on the Taranaki Fault. Third, during the fault prior to the Oligocene (Figure 11). The increase in middle Eocene and north of Taranaki Peninsula, the fault vertical separation with horizon age is accompanied by a separated a shelfal to bathyal marine environment in the westward thickening of the Cretaceous-Tertiary sequence west from terrestrial peat deposition in the hanging wall of across the fault [King and Thrasher, 1996; this study]. From the fault [King and Thrasher, 1996]. These relations suggest the Herangi High northward, late Eocene-Oligocene coal at least several hundred meters of relief across the fault and measures and marginal marine limestone rest directly on provide a local source for the influx of sand-dominated basement in the fault hanging wall [Nelson et al., 1994; sediments into the basin during the middle-late Eocene (i.e., Edbrooke, 2005]. Immediately to the west of the fault, in the 43–35 Ma) [Palmer and Andrews, 1993]. The undulating Taranaki Basin, Late Cretaceous (80 Ma) to late Eocene morphology of the basement-cover contact, which occurs as strata are 4–5 km thick [King and Thrasher, 1996]. These far east as the Waikato Coalfield [Edbrooke et al., 1994], is thickness variations between the successions on each side of consistent with basement in the upthrown hanging wall of the fault, which are implicit in the changes in vertical the Taranaki Fault being eroded prior to circa 37–32 Ma separations (Figure 11), indicate differential subsidence (i.e., the age of strata resting on basement [Nelson et al., across the fault and support the view of Palmer and 1994; Edbrooke, 2005]). Late Eocene erosion of the hang- Andrews [1993] that it was active prior to the Oligocene. ing wall block of the fault is further supported by an influx [25] Apatite fission track data from Murihiku Terrane of reworked Mesozoic pollen and basement fragments in the basement rocks, which crop out 10–20 km east of the Awakino-1, Mokau-1, and Te Ranga-1 wells [Palmer and Taranaki Fault in the Port Waikato region (see Figure 1 for Andrews, 1993; King and Thrasher, 1996]. We concur with location), indicate up to 3–4 km of uplift and erosion Palmer and Andrews [1993] and infer that this erosion was [Kamp and Liddell, 2000]. Uplift was accompanied by induced by the onset of uplift on the Taranaki Fault at about eastward tilting (5°) of Murihiku basement and is inferred 43–40 Ma, the start of which was coincident with the to have been due to normal displacement on the Taranaki formation of the Porangan unconformity and a basinward Fault or a fault close to the present coastline [Mills, 1990; facies shift identified in eastern Taranaki Basin exploration Kamp and Liddell, 2000]. However, we have been unable to wells [King and Thrasher, 1996]. identify such a fault in the seismic data and suggest that pre- Oligocene reverse displacement on the Taranaki Fault was

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Figure 12. Migrated (a) uninterpreted and (b) interpreted seismic line CNL95a-54 at that the northern end of the Taranaki Fault west of Auckland (see inset map for location). At this location the fault offsets pre-Oligocene strata, with little or no offset visible in Oligocene and younger strata. Interpreted horizons are tied to wells in the northern Taranaki Basin. Locations and ages of interpreted horizons are indicated and based on data from the Northern Taranaki Basin wells.

5.2. Oligocene and Younger Basin and thickening of early Miocene footwall strata [27] Growth strata on the limbs of the Manaia and adjacent to the fault by up to 200% [King and Thrasher, 0 0 0 0 Herangi High anticlines indicate that these structures con- 1996, enclosure 1 cross sections A-A ,B-B,C-C,D-D]. tinued to develop during the Oligocene [Voggenreiter, 1993; Thickening of strata of this age appears to be most pro- Palmer and Andrews, 1993; Nelson et al., 1994]. In the nounced on Taranaki Peninsula and further south. This early region of the Herangi High, for example, calcareous sand- Miocene event is thought by some to signify regional uplift stone and siltstone, and limestone units of Oligocene age and erosion of the parts of New Zealand continental crust indicate gentle eastward tectonic tilting of 1–1.5°/Ma from caused by onset of subduction along the Hikurangi margin 32–28 Ma to the start of the Miocene [Nelson et al., 1994]. and the inception of the Alpine Fault [e.g., Kamp, 1986; Growth strata indicate that both the Manaia and Taranaki Rait et al., 1991; King, 2000]. We propose, however, that faults probably remained active throughout the Oligocene. pre-Miocene displacements on the Taranaki Fault signify This conclusion is supported by a thickening of Oligocene that subduction commenced prior to the Miocene. strata in the footwalls of, and toward, the Taranaki and 5.3. Fault Tip Retreat Manaia faults on Taranaki Peninsula and further to the south [King and Thrasher, 1996, enclosure 1 cross sections B-B0, [29] To understand better how the age of the fault changes C-C0,D-D0]. along strike, we have plotted in Figure 13 the location of the northern fault tip (as inferred from the separation profiles in [28] Early Miocene displacement on the fault has been widely recognized [e.g., King and Thrasher, 1992, 1996; Figure 11) and the age of oldest unfaulted strata which Palmer and Andrews, 1993; Nelson et al., 1994; Holt and overlie the upper fault tip. These two sets of data are Stern, 1994]. It was accompanied by a change from calcar- complementary and confirm that the northern tip of the eous to terrigenous sediment deposition in the Taranaki active section of the Taranaki Fault migrated southward during the Tertiary. North of Port Waikato (distances 50–

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Figure 13. Plot showing variations in the age of strata immediately overlying the upper tip of the Taranaki Fault (open squares) and the age of the northern fault tip (inferred from Figure 11) versus distance along the fault from an arbitrary point west of Auckland. The locations and timing of formation of the Waitemata, Otunui, and Wanganui basins in the hanging wall of the Taranaki Fault are from Kamp and Furlong [2006]. The plots show southward migration of basins and the northern fault tip during the Tertiary. Fault lengths for 34–43, 20–34, 16–20, and 4–16 Ma are indicated.

70 km) Oligocene and younger strata are not faulted, constant rate southward we would expect to see a more suggesting that in this region fault movement was Eocene gradual southward reduction in the age of strata overlying and older (Figure 13). Between Port Waikato and Awakino the upper fault tip, which appears not to be the case. (distances of 50–200 km) fault displacements predate 18–19 Ma, with uplift and folding resulting in erosion 6. Implications for Sedimentary Basin and Plate of Eocene and Oligocene strata from the west limb of the Boundary Evolution hanging wall anticline. Further south, the oldest unfaulted strata appear to be about 14–16 Ma in age from Awakino to [31] A southward stepping of fault activity appears to be Hawera (200–300 km) and 4–5 Ma in age from about consistent with episodic migration in the same direction of Hawera south (Figure 13). Although the Taranaki Fault is progressively younger depocenters in the hanging wall of presently inactive, the Waimea-Flaxmore Fault (which is as the fault [Stern et al., 1993, 2006; Kamp and Furlong, little as 50 km south of Figure 13 and is part of the Taranaki 2006] (Figure 14). We suggest that the onset of sediment Fault System) comprises active traces for at least 150 km deposition into these depocenters was linked to death of the north of the Alpine Fault [Rattenbury et al., 1998; Fraser et Taranaki Fault and the manner in which it’s northern tip al., 2006]. retreated southward. This inference is supported by the [30] As the southern tip of the Taranaki Fault appears to observation that at any given point along the fault, the main have been approximately fixed during the Tertiary, south- period of basin development in its hanging wall appears to ward retreat of the northern tip resulted in a reduction in the post date fault death. In the Wanganui Basin, for example, active fault length with decreasing time from the present sedimentation commenced at 4–5 Ma, when uplift, base- (Figure 13). We estimate that the total active length of the ment erosion and displacements on the Taranaki and Manaia fault in the middle Eocene (40–43 Ma) was at least 450 km faults ceased [Anderton, 1981; Lamarche et al., 2005]. and that the fault was abandoned completely at 4–5 Ma. These relationships are consistent with the view that locally Given these values the minimum average rate of fault tip fault displacements either produced uplift or significantly retreat was about 12 km/Ma. However, retreat of the reduced subsidence. The net effect of this was to reduce the northern fault tip does not appear to have occurred at a accommodation space in the hanging wall of the fault uniform rate. The stepped decreases in the age of strata during its growth. With the cessation of faulting subsidence overlying the upper fault tip from 34 to 18–20 to 15–16 may have increased, enhancing the likelihood of basin to 5 Ma suggest that fault death may have been achieved, in formation. part, by geologically instantaneous (i.e., over 1–2 Ma or [32] The timing and kinematics of displacement of the less) abandonment of long (i.e., 80–150 km) sections of the Taranaki Fault may have important implications for the fault. If the northern lateral fault tip had migrated at a near development of the New Zealand plate boundary during the Cenozoic. The fault is a back thrust which is antithetic to

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Figure 14. Map and regional schematic cross sections of the Hiukrangi subduction margin showing the location and geometry of the Taranaki Fault System relative to the subducting plate and the Waitemata, Otunui, and Wanganui basins. Locations of the northern fault tip at 34–43, 20–34, 16–20, and 4–16 Ma are from Figure 11. Active fault tip from Rattenbury et al. [1998] and Fraser et al. [2006]. The locations and timing of formation of the Waitemata, Otunui, and Wanganui basins in the hanging wall of the Taranaki Fault are from Kamp and Furlong [2006]. Cross section A modified from Litchfield et al. [2007], cross section B modified from Nicol et al. [2007], and cross section C from Ansell and Bannister [1996] and this study. Arrows and rates indicate relative plate motion vectors from Beavan et al. [2002]. the Hikurangi subduction thrust and may displace the entire Hikurangi subduction margin. We infer that this shortening overriding plate (Figure 14 cross sections A and B). We was driven by westward directed subduction along the follow Holt and Stern [1994] in suggesting that the fault Hikurangi margin which commenced during the middle accrued displacement transferred from the subducting Pa- Eocene (40–43 Ma) rather than during the early Miocene cific Plate into the overriding Australian Plate. If this is the (i.e., 20–24 Ma) as previously inferred from geological case, then displacement on the Taranaki Fault provides data [e.g., Ballance, 1976; Rait et al., 1991; Kamp, 1999]. information about the timing and location of subduction Our preferred age for the onset of Hikurangi margin along the Hikurangi margin during the Cenozoic. Displace- subduction is consistent with seafloor spreading data [e.g., ments on the Taranaki Fault (together with growth of the Stock and Molnar, 1982; Sutherland, 1995]. Herangi High and Manaia Anticline) indicate that during the [33] Our inferred timing for the inception of Cenozoic middle Eocene to early Pliocene the crust in the Taranaki subduction can be rationalized with the early Miocene Basin was being shortened in a direction approximately acceleration of upper plate contraction [e.g., Ballance, normal to the strike of the fault and to the trend of the 1976; Rait et al., 1991] and onset of subduction-related

16 of 19 B01403 STAGPOOLE AND NICOL: TARANAKI FAULT, NEW ZEALAND B01403 volcanism [Hayward et al., 2001], because southward regional structure and kinematic history of the Taranaki migration of the Pacific-Australia Euler pole away from Fault. The fault is part of a larger fault system with a New Zealand [Sutherland, 1995] increased plate conver- minimum known length of about 600 km, and includes the gence along the Hikurangi subduction margin from about Waimea, Flaxmore and Manaia faults and the Tarata Thrust. 30 Ma. In the middle Eocene to early Oligocene (circa 43– The Taranaki Fault typically dips to the east at 20–45° and 30 Ma) the Euler pole was close to the North Island [King, has accommodated a maximum dip-slip displacement of at 2000] and was associated with the rate of plate convergence least 12–15 km in the last circa 80 Ma. Analysis of vertical along the Hikurangi margin was approximately 5–15 mm/a separations indicates that reverse displacement and folding (calculated from Sutherland [1995]). Between circa 30 and accrued before and after deposition of the base Oligocene 20 Ma the Euler pole appears to have migrated southward (i.e., 34 Ma). Tertiary displacement may have commenced about 1000 km away from New Zealand [King, 2000] result- during the middle Eocene at about 40–43 Ma and continued ing in an increase of relative plate motion of 30 mm/a at until 4–5 Ma. Middle Eocene displacement on the Taranaki the Hikurangi margin. This increase in the rate of plate Fault may indicate that subduction commenced along the convergence is consistent with the observed increase in Hikurangi margin at this time. The northern tip of the active upper plate shortening, including displacement on the section of the Taranaki Fault stepped southward a total of Taranaki Fault, during the early Miocene. The increase in 400–450 km during the Tertiary. Southward retreat of the convergence rate may have also promoted the onset of fault tip is consistent with clockwise rotation of the sub- subduction-related volcanism in northern New Zealand. ducting Pacific Plate, resulting in progressive isolation of The 15–20 Ma delay between commencement of subduc- the fault from the downgoing plate, the driving force for tion and the onset of volcanism may partly reflect the low deformation in the overriding plate. rates (5–15 mm/a) of subduction prior to 30 Ma. [34] Decreases in the active length of the Taranaki Fault [36] Acknowledgments. The authors thank Austral-Pacific Energy throughout the Tertiary may suggest that the subducting and Shell Todd Oil Services for providing some of the well and seismic data used for this study. We thank Rob Funnell, Brad Ilg, and Beate plate changed both location and orientation relative to the Leitner for constructive reviews of the manuscript. The project has been fault. Currently, the fault system strikes oblique to the funded by the Foundation for Research, Science and Technology con- subducting Pacific Plate at the Hikurangi trough, east of tracts CO5X0302 and CO5X0703. New Zealand, and to the Alpine Fault in the northern South Island (Figure 14). At its northern end, the Taranaki Fault References lies more than 500 km west of the subduction trench, while Anderton, P. W. (1981), Structure and evolution of the South Wanganui Basin, New Zealand, N. Z. J. Geol. Geophys., 24(1), 39–63. the fault system terminates against the plate boundary (i.e., Ansell, J. H., and S. C. Bannister (1996), Shallow morphology of the Alpine Fault and possibly also the subduction thrust, see subducted Pacific Plate along the Hikurangi margin, New Zealand, Phys. Figure 14 cross section C) in the south. Given the present Earth Planet. Inter., 93, 3–20. Armstrong, P. A., R. G. Allis, R. H. Funnell, and D. S. Chapman (1998), location and geometry of active (Waimea-Flaxmore Fault) Late Neogene exhumation patterns in Taranaki Basin (New Zealand): and the inactive (e.g., Taranaki Fault) parts of the fault Evidence from offset porosity-depth trends, J. Geophys. Res., system, it is inferred that the Taranaki Fault was active when 103(B12), 30,269–30,282. Backus, G., J. Park, and D. Garbasz (1981), On the relative importance of it intersected the subduction thrust or was contained within the driving forces of plate motion, Geophys. J. R. Astron. Soc., 67(2), crust immediately above the subduction thrust. If this model 415–435. is correct, then the subducting plate was once much closer Ballance, P. F. (1976), Evolution of the upper Cenozoic magmatic arc and to the Taranaki Fault than it is today. The southward plate boundary in northern New Zealand, Earth Planet. Sci. Lett., 28(3), 356–370. episodic retreat of the northern fault tip may suggest that Bannister, S. C. (1989), Gravity interpretation profile across Hikurangi the subducting plate rotated clockwise away from the subduction zone using seismic constraints: Hawke’s Bay to Hikurangi Taranaki Fault and western New Zealand. Rotation of the Trench, J. R. Soc. N. Z., 19(4), 385–397. Barnes, P. M., and B. M. de Le´pinay (1997), Rates and mechanics of rapid subducting plate is consistent with the suggestion of slab frontal accretion along the very obliquely convergent southern Hikurangi roll-back north of New Zealand [e.g., Walcott, 1987] with margin, New Zealand, J. Geophys. Res., 102(B11), 24,931–24,952. the subducting plate migrating from a north-northwest trend Barnes, P. M., A. Nicol, and A. J. Harrison (2002), Late Cenozoic evolution in the Eocene to early Miocene to a more northeasterly trend and earthquake potential of an active listric thrust complex above the Hikurangi subduction zone, New Zealand, Geol.Soc.Am.Bull., today [see also Nicol et al., 2007, and references therein]. 114(11), 1379–1405. Such rotation would increase the distance between the fault Barnett, J. A. M., J. Mortimer, J. H. Rippon, J. J. Walsh, and J. Watterson and the downgoing Pacific Plate more in the north than the (1987), Displacement geometry in the volume containing a single normal fault, AAPG Bull., 71, 925–937. south. With this increase in distance from the Pacific Plate, Beavan, R. J., and J. Haines (2001), Contemporary horizontal velocity and displacement rates on the fault decreased until it became strain rate fields of the Pacific-Australian plate boundary zone through inactive, followed by a period of subsidence and basin New Zealand, J. Geophys. Res., 106(B1), 741–770. Beavan, J., P. Tregoning, M. Bevis, T. Kato, and C. Meertens (2002), development in the hanging wall, first in the north and then Motion and rigidity of the Pacific Plate and implications for plate bound- progressively further south. The stepwise north to south ary deformation, J. Geophys. Res., 107(B10), 2261, doi:10.1029/ abandonment of the Taranaki Fault (Figure 13) suggests that 2001JB000282. the clockwise rotation of the subduction zone may have Boyer, S. E. (1986), Styles of folding within thrust sheets: Examples from the Appalachian and Rocky Mountains of the and Canada, U.S.A., been episodic rather than continuous through time. J. Struct. Geol., 8, 325–339. Boyer, S. E., and D. Elliott (1982), Thrust systems, AAPG Bull, 66, 1196– 1230. 7. Conclusions Bradshaw, J. D. (1989), Cretaceous geotectonic patterns in the New Zealand region, Tectonics, 8, 803–820. [35] The new detailed interpretation of seismic, gravity Caldwell, T. G., H. M. Bibby, and C. Brown (2004), The magnetotelluric and MT data has improved our understanding of the phase tensor, Geophys. J. Int., 158, 457–469.

17 of 19 B01403 STAGPOOLE AND NICOL: TARANAKI FAULT, NEW ZEALAND B01403

Cande, S. C., and J. M. Stock (2004), Pacific-Antarctic-Australia motion Litchfield, N., S. Ellis, K. Berryman, and A. Nicol (2007), Insights into and the formation of the Macquarie Plate, Geophys. J. Int., 157, 399– subduction-related uplift along the Hikurangi Margin, New Zealand, 414. using numerical modeling, J. Geophys. Res., 112, F02021, doi:10.1029/ Chapman, T. J., and G. D. Williams (1984), Displacement-distance methods 2006JF000535. in the analysis of fold-thrust structures and linked fault systems, J. Geol. Mills, C. (1990), Gravity expression of the Patea-Tongaporutu High and Soc. London, 141, 121–128. subsequent model for the Taranaki Basin margin, in New Zealand Pet- Chester, J. S., and F. M. Chester (1990), Fault-propagation folds above roleum Exploration Conference (1989: Queenstown, NZ), pp. 191–200, thrusts with constant dip, J. Struct. Geol., 12, 903–910. Minist. of Econ. Dev., Wellington. Childs, C., J. Watterson, and J. J. Walsh (1995), Fault overlap zones within Moore, J. C. (1989), Tectonics and hydrogeology of accretionary prisms: developing normal fault systems, J. Geol. Soc. London, 152, 535–549. Role of the decollement zone, J. Struct. Geol., 11, 95–106. Davis, D. M., J. Suppe, and F. A. Dahlen (1983), Mechanics of thrust-and- Mortimer, N., A. J. Tulloch, and T. R. Ireland (1997), Basement geology of fold belts and accretionary wedges, J. Geophys. Res., 88, 1153–1172. Taranaki and Wanganui basins, New Zealand, N. Z. J. Geol. Geophys., DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein (1994), Effect of 40(2), 223–236. recent revisions to the geomagnetic reversal time-scale on estimates of Muraoka, H., and H. Kamata (1983), Displacement distribution along min- current plate motions, Geophys. Res. Lett., 21, 2191–2194. or fault traces, J. Struct. Geol., 5, 483–495. Edbrooke, S. W. (2005), Geology of the Waikato area, scale 1:250,000, Nelson, C. S., P. J. J. Kamp, and H. R. Young (1994), Sedimentology and map, 68 pp., 1 folded map, Inst. of Geol. and Nucl. Sci., Lower Hutt. petrography of mass - emplaced limestone (Orahiri Limestone) on a late Edbrooke, S. W., R. Sykes, and D. T. Pocknall (1994), Geology of the Oligocene shelf, western North Island, and tectonic implications for east- Waikato Coal Measures, Waikato Coal Region, New Zealand, Inst. Geol. ern margin development of Taranaki Basin, N. Z. J. Geol. Geophys., Nucl. Sci. Monogr., vol. 6, 236 pp., Inst. of Geol. and Nucl. Sci., Lower 37(3), 269–285. Hutt. Nicol, A., R. J. Van Dissen, P. Vella, B. V. Alloway, and A. Melhuish Ellis, M. A., and W. J. Dunlap (1988), Displacement variation along thrust (2002), Growth of contractional structures during the last 10 m.y. at the faults: Implications for the development of large faults, J. Struct. Geol., southern end of the emergent Hikurangi forearc basin, New Zealand, 10, 183–192. N. Z. J. Geol. Geophys., 45(3), 365–385. Field, B. D., et al. (1997), Cretaceous-Cenozoic geology and petroleum Nicol, A., C. Mazengarb, F. Chanier, G. Rait, C. Uruski, and L. Wallace systems of the East Coast Region, New Zealand, Inst. Geol. Nucl. Sci. (2007), Tectonic evolution of the active Hikurangi subduction margin, Monogr., vol. 19, 301 pp., Inst. of Geol. and Nucl. Sci., Lower Hutt. New Zealand, since the Oligocene, Tectonics, 26, TC4002, doi:10.1029/ Forsyth, D. W., and S. Uyeda (1975), On the relative importance of the 2006TC002090. driving forces of plate motion, Geophys. J. R. Astron. Soc., 43(1), 163– Palmer, J. A., and P. B. Andrews (1993), Cretaceous-Tertiary sedimentation 200. and implied tectonic controls on the structural evolution of Taranaki Fraser, J. G., A. Nicol, J. R. Pettinga, and M. R. Johnston (2006), Basin, New Zealand, in Sedimentary Basins of the World, vol. 2, South Paleoearthquake investigation of the Waimea-Flaxmore fault system, Pacific Sedimentary Basins, edited by P. F. Ballance, pp. 309–328, Else- Nelson, New Zealand, in paper presented at New Zealand Geotechnical vier, Amsterdam. Society 2006 Symposium, Nelson, New Zealand, Feb. 2006. Raine, I., R. A. Cook, and P. Strong (2004), Eastern Taranaki boundary Funnell, R. H., D. Chapman, R. G. Allis, and P. Armstrong (1996), Thermal fault paleotology and timing of thrusting, GNS science report, 63 pp, state of the Taranaki Basin, New Zealand, J. Geophys. Res., 101(B11), GNS Sci., Lower Hutt. 25,197–25,215. Rait, G., F. Chanier, and D. W. Waters (1991), Landward- and seaward- Harrison, A. J., and R. S. White (2006), Lithospheric structure of an active directed thrusting accompanying the onset of subduction beneath New backarc basin: The Taupo Volcanic Zone, New Zealand, Geophysical Zealand, Geology, 19(3), 230–233. J. Int., 167(2), 968–990, doi:910.1111/j.1365-1246X.2006.03166.x. Rattenbury, M. S., R. A. Cooper, and M. R. Johnston (1998), Geology of Hayward, B. W., et al. (2001), K-Ar ages of Early Miocene arc-type vol- the Nelson area, scale 1:250000, map, 67 pp. 1 folded map, Inst. of Geol. canoes in northern New Zealand, N. Z. J. Geol. Geophys., 44, 285–311. and Nucl. Sci., Lower Hutt. Holstege, G. C. J., and D. J. Bishop (1998), Structural evolution of Reilly, W. I. (1972), New Zealand gravity map series, N. Z. J. Geol. Geo- the Kapuni Field, New Zealand, paper presented at New Zealand phys., 15(1), 3–15. Petroleum Exploration Conference, Minist. of Econ. Dev., Queenstown, Reyners, M. E., D. Eberhart-Phillips, G. Stuart, and Y. Nishimura (2006), New Zealand. Imaging subduction from the trench to 300 km depth beneath the central Holt, W. E., and T. A. Stern (1991), Sediment loading on the Western North Island, New Zealand, with Vp and Vp/Vs, Geophys. J. Int., 165, Platform of the New Zealand continent; implications for the strength of 565–583. a continental margin, Earth Planet. Sci. Lett., 107, 523–538. Robertson, E. I., and W. I. Reilly (1958), Bouguer anomaly map of New Holt, W. E., and T. A. Stern (1994), Subduction, platform subsidence, and Zealand, N. Z. J. Geol. Geophys., 1(3), 560–564. foreland thrust loading: The late Tertiary development of Taranaki Basin Rodi, W., and R. L. Mackie (2001), Nonlinear conjugate gradients algo- New Zealand, Tectonics, 13, 1068–1092. rithm for 2-D magnetotelluric inversion, Geophysics, 66, 174–187. Kamp, P. J. J. (1986), The mid-Cenozoic Challenger rift system of western Sclater, J. G., and P. A. F. Christie (1980), Continental stretching; an ex- New Zealand and its implications for the age of Alpine Fault inception, planation of the post-Mid-Cretaceous subsidence of the central North Sea Geol. Soc. Am. Bull., 97, 255–281. basin, J. Geophys. Res., 85, 3711–3739. Kamp, P. J. J. (1999), Tracking crustal processes by FT thermochronology Sherburn, S., and R. S. White (2005), Crustal seismicity in Taranaki, New in a forearc high (Hikurangi margin, New Zealand) involving Cretaceous Zealand using accurate hypocentres from a dense network, Geophys. subduction termination and mid-Cenozoic subduction initiation, Tectono- J. Int., 162, 494–506. physics, 307, 313–343. Spandler, C., K. Worden, R. Arculus, and S. Eggins (2005), Igneous rocks Kamp, P. J. J., and K. P. Furlong (2006), Neogene plate tectonic reconstruc- of the Brook Street Terrane, New Zealand: Implications for Permian tions and geodynamics of North Island sedimentary basins: Implications tectonics of eastern Gondwana and magma genesis in modern intra-ocea- for the petroleum systems, paper presented at the 2006 New Zealand nic volcanic arcs, N. Z. J. Geol. Geophys., 48(1), 167–183. Petroleum Conference, Minist. of Econ. Dev., Wellington. Stagpoole, V. M. (1997), A geophysical study of the northern Taranaki Kamp, P. J. J., and I. J. Liddell (2000), Thermochronology of northern Basin, New Zealand, Ph.D. thesis, 244 pp., Victoria Univ. of Wellington, Murihiku Terrane, New Zealand, derived from apatite FT analysis, Wellington. J. Geol. Soc., 157(2), 345–354. Stagpoole, V. M., S. L. Bennie, H. M. Bibby, M. R. Ingham, and Karig, D. E., and G. F. Sharman (1975), Subduction and accretion in S. Dravitzki (2006), Upper crustal structure of the Taranaki Fault imaged trenches, Geol. Soc. Am. Bull., 86, 377–389. by a magnetotelluric study, in Geological Society of New Zealand/New King, P. R. (2000), Tectonic reconstructions of New Zealand 40 Ma to the Zealand Geophysical Society Miscellaneous Publication 122A (2006: present, N. Z. J. Geol. Geophys., 43(4), 611–638. Massey University), pp. 80–81, Geol. Soc. of N.Z., Wellington. King, P. R., and G. P. Thrasher (1992), Post-Eocene development of the Stern, T. A., and F. J. Davey (1990), Deep seismic expression of a foreland Taranaki Basin, New Zealand: Convergent overprint of a passive margin, basin: Taranaki Basin, New Zealand, Geology, 18(10), 979–982. in Geology and Geophysics of Continental Margins,editedbyJ.S. Stern, T. A., E. G. C. Smith, F. J. Davey, and K. J. Muirhead (1987), Crustal Watkins et al., AAPG Mem., 53, 93–118. and upper mantle structure of the northwestern North Island, New Zeal- King, P. R., and G. P. Thrasher (1996), Cretaceous-Cenozoic geology and and, from seismic refraction data, Geophys. J. R. Astron. Soc., 91, 913– petroleum Systems of the Taranaki Basin, New Zealand, Inst. Geol. Nucl. 936. Sci. Monogr., vol. 13, 243 pp., Inst. of Geol. and Nucl. Sci., Lower Hutt. Stern, T. A., G. M. Quinlan, and W. E. Holt (1993), Crustal dynamics Lamarche, G., J.-N. Proust, and S. D. Nodder (2005), Long-term slip rates associated with the formation of Wanganui Basin, New Zealand, in Se- and fault interactions under low contractional strain, Wanganui Basin, dimentary Basins of the World, vol. 2, South Pacific Sedimentary Basins, New Zealand, Tectonics, 24, TC4004, doi:10.1029/2004TC001699. edited by P. F. Ballance, pp. 213–223, Elsevier, Amsterdam.

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Stern, T. A., W. R. Stratford, and M. L. Salmon (2006), Subduction evolu- Thrasher, G. P., P. R. King, and R. A. Cook (1995), Taranaki Basin Pet- tion and mantle dynamics at a continental margin: Central North Island, roleum Atlas, Inst. of Geol. and Nucl. Sci., Lower Hutt. New Zealand, Rev. Geophys., 44, RG4002, doi:10.1029/2005RG000171. Voggenreiter, W. R. (1993), Structure and evolution of the Kapuni Anti- Stock, J., and P. Molnar (1982), Uncertainties in the relative positions of cline, Taranaki Basin, New Zealand: Evidence from the Kapuni 3D seis- the Australia, Antarctica, Lord Howe, and Pacific plates since the Late mic survey, N. Z. J. Geol. Geophys., 36(1), 77–94. Cretaceous, J. Geophys. Res., 87, 4697–4714. von Huene, R., and D. Klaeschen (1999), Opposing gradients of permanent Stratford, W. R., and T. A. Stern (2006), Crust and upper mantle structure of strain in the aseismic zone and elastic strain across the seismogenic zone a continental backarc: Central North Island, New Zealand, Geophys. of the Kodiak shelf and slope, Alaska, Tectonics, 18, 248–262. J. Int., 166, 469–484. Walcott, R. I. (1987), Geodetic strain and the deformational history of the Suppe, J., and D. A. Medwedeff (1990), Geometry and kinematics of fault- North Island of New Zealand during the late Cainozoic, Philos. Trans. propagation folding, Eclogae Geol. Helv., 83, 409–454. R. Soc. London, Ser. A, 321, 163–181. Sutherland, R. (1995), The Australia-Pacific boundary and Cenozoic plate motions in the SW Pacific: Some constraints from Geosat data, Tectonics, 14, 819–831. A. Nicol and V. Stagpoole, GNS Science, P.O. Box 30-368, Lower Hutt, Thrasher, G. P. (1990), Tectonics of the Taranaki Rift, in New Zealand New Zealand. ([email protected]) Petroleum Conference (1989: Queenstown, NZ), pp. 124–133, Minist. of Econ. Dev., Wellington.

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