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Late Quaternary faulting of the offshore WhakataneGraben, Taupo Volcanic Zone, New ZealandI. C. Wright aa Department of Scientific and Industrial Research , New Zealand Oceanographic Institute ,Private Bag, Kilbirnie, Wellington , New ZealandPublished online: 17 Jan 2012.

To cite this article: I. C. Wright (1990) Late Quaternary faulting of the offshore Whakatane Graben, Taupo Volcanic Zone,New Zealand, New Zealand Journal of Geology and Geophysics, 33:2, 245-256, DOI: 10.1080/00288306.1990.10425682

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New Zealand Journal o/Geology and Geophysics, 1990, Vol. 33: 245-256 0028-8306/90/3302-0245 $2.50/0 © Crown copyright 1990

245

Late Quaternary faulting of the offshore Whakatane Graben, Taupo Volcanic Zone, New Zealand

I. C. WRIGHT

New Zealand Oceanographic Institute Department of Scientific and Industrial Research Private Bag, Kilbirnie Wellington, New Zealand

Abstract The 15-20 km wide zone of active late Quaternary extensional faulting (the Taupo Fault Belt-Whakatane Graben), within the Taupo Volcanic Zone, extends offshore atleast43km towardsWhiteIsland,BayofPlenty. Within the offshore Whakatane Graben, as defined by the White Island Fault and newly named Rurima Fault, some 50 normal faults displacing post-18 ka transgressive sediments are newly mapped from high-resolution 3.5 kHz seismic profiles. The faults strike northeast, appear laterally discontinuous over distances of >10 km, generally dip to the northwest, and bound fault blocks tilted to the southeast. Of these, fournewly named faults-the Ohiwa, Nukuhou, Pukehoko, and Rangitaiki-show large and repeated movements within the last 18 ka.

Post-18 ka seismic stratigraphy, tephrostratigraphy, and 14C dating indicate relative vertical fault displacement rates

vary from 0.2 to 2.8 mm/yr. An extensional block model relating fault geometry, vertical displacements, and exten­sion indicates that absolute subsidence varies within the graben. Subsidence is estimated to average 2-2.5 mm/yr and range locally to a maximum of 3-3.5 mm/yr. Assuming the faults dip at 45 ± 10°, extension is estimated to be at least 3.5± 1.7mm/yr, implying that a significant amount of the 7 mm/yr extension of the Taupo Volcanic Zone, near the Bay of Plenty coast, occurs within the 15-20 km wide Whakatane Graben.

Keywords marine geology; Taupo Volcanic Zone; Whakatane Graben; faulting; subsidence; late Quaternary; tectonics; deformation rates; extension

INTRODUCTION

Late Quaternary faulting of the onshore Taupo Volcanic Zone (TVZ), with concomitant widening and subsidence, is well documented. Various studies within the TVZ, incorporating surface mapping (Nairn 1976), geothermal drilling (Nairn 1986), geodetic triangulation (Sissons 1979), lake levelling (Otway 1986), and microearthquake recording (Robinson 1989), have established a comprehensive account of the position, style, and pattern of onshore faulting. Further, these data, where combined with a well-dated stratigraphy, have allowed the associated rates of tectonic deformation to

G89035 Received 20 July 1989; accepted 2 November 1989

be determined. Although the TVZ is recognised to extend offshore, the structure has been described only on a regional scale (Lewis & Pantin 1984; Wright et al. 1990).

The intent of this paper is to document the offshore continuation of the actively widening and subsiding portion of the TVZ (the Taupo Fault Belt-Whakatane Graben) and associated faults (Fig. I), as determined from new, high­resolution, 3.5 kHz seismic profiles, between the Bay of Plenty coast and White Island.

REGIONAL STRUCTURE AND TECTONICS

Onshore

The regional onshore late Quaternary structure and tectonics of the TauPQ Volcanic Zone and North Island Shear Belt at the Bay of Plenty coast (Fig. 1), although well described by Nairn & Beanland (1989), is summarised here.

Late Quaternary tectonism of the TVZ is dominated by laterally discontinuous, northeast-trending normal faults, which are largely restricted to the 1 5-20 km wide Taupo Fault Belt (Grindley 1960). This zone of intense faulting, with repeated movements in the last 50 ka, extends from Mt Ruapehu to the Bay of Plenty coast, where it is recognised as the Whakatane Graben (Fig. 1). Historic earthquake swarms at Lake Taupo (Grindley & Hull 1986) and the 1987 March Edgecumbe earthquake (Beanland et a1. 1989), with their associated faulting, are the most recent examples of this tectonism. Concomitant widening is clearly demonstrated by both geodetic retriangulation (Sissons 1979; Walcott 1984) and observed fault extension during historic and recent earthquakes (Grindley & Hull 1986; Beanland et a1. 1989). Near the Bay of Plenty coast, extension is estimated to be 12 mm/yr across the full 120 km width of the bay (Walcott 1987), and 7 mm/yr over 40 km across the TVZ (Sissons 1979). Nairn & Beanland (1989) note it is most probable the rate is higher within the Taupo Fault Belt-Whakatane Graben.

At the coast, the Whakatane Graben (MacPherson 1944), infilled with Quaternary alluvium, is actively subsiding at 1-2mm/yr(Nairn & Beanland 1989). The position of normal faults bounding the graben is, generally, equivocal, although faults along the northwestern margin displace Matahina Ignimbrite and appear to be southwestward lateral extensions of the Matata Fault Zone (Fig. I). The southeastern boundary of the graben is defined by the Edgecumbe Fault. Uplift of the graben margins, which comprise mid-late Quaternary marine and nonmarine sediments overlying greywacke basement, is in the order of 0.5-1 mm/yr. Within the graben, the northeast­trending Rotoitipakau, Onepu-Edgecumbe, and Matata Fault Zones (Fig. 1) have all been active within the last 800 years (Ota et a1. 1988; Beanland et al. 1989), with the most recent movements during the 1987 March Edgecumbe earthquake along the Rotoitipakau,Onepu,andEdgecumbeFaults.NNW­trending faults are to date not observed within the graben, although the major strike-slip faults of the North Island Shear

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246 New Zealand Journal of Geology and Geophysics, 1990, Vol. 33

LEGEND

Holocene alluvium

Pleistocene sediments

Pleistocene/Holocene volcanics

Mesozoic greywackes

... Andesite volcanoes

Late Quatemary and Recent faults Shelfbreak R.V. Rapuhla selsmlcproflle

Fig.l Location of high-resolution seismic profiles between Whakatane and White Island (bold lines are the portions of the seismic profiles illustrated in Fig. 2, 4, and 5). Bathymetry after Wright (1989); isobaths at 10 m and 50 m intervals on the continental shelf and slope, respectively. Locations of piston cores 175-1100 previously described by Kohn & Glasby (1978). Onshore geology and late Quaternary faults are from Healy et al. (1964) and N aim & Beanland (1989). Inset: Late Quaternary tectonism and volcanism of the central North Island; I, 2, and 3 represent, respectively, the Okataina, Rotorua and Taupo Volcanic Centres; N.I.S.B., North Island Shear Belt.

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Wright-Offshore Whakatane Graben faulting

Belt (NISB), with a Holocene displacement rate of 14-18 mm/yr across the belt (Lensen 1975), intersect the TVZ in the vicinity of the Bay of Plenty coast. To the southwest, within the Okataina Volcanic Centre, extensions of the northwestern and southeastern boundaries of the graben are defined by the "Haroharo vent zone" and the Tarawera Rift, respectively (Nairn & Beanland 1989).

Offshore Offshore, the TVZ is recognisable as a 45 km wide graben, studded with volcanic knolls, extending to at least the base of the continental slope at 2000 m isobath (Lewis & Pantin 1984), although new long-range side-scan sonar and seismic reflection data indicate the zone continues northward to 36°45'S (Wright et al.1990). On the continental shelf and slope the boundaries of the TVZ are defined by the Motiti Scarp and Tauranga Canyon in the west, and the Motuhora Scarp and headward canyons of the White Island Trough in the east (Lewis & Pantin 1984). Active and late Quaternary andesitic volcanism at White Island and Motuhora Island have been interpreted to be the offshore extension of the line of active onshore andesite volcanoes (Cole 1986).

SEISMIC DATA

Data for this study comprise a suite of shore-normal and shore-parallel, high-resolution 3.5 kHz seismic reflection lines profiled onboard the RV Rapuhia, between Whakatane and White Island, in April 1988 (Fig. 1). Only one line was profiled within the nearshore 13 km of the coast due to the shallow depths surrounding Motuhora and Rurima Islands. Data are of high quality, with subbottom penetrations of up to 55 m (Fig. 2). Three of the shore-parallel lines were also profiled using a 40 in3 airgun and single-channel seismic reflection system.

REFERENCE SURFACES AND DATING

Post-IS ka transgression Reference surfaces recording fault displacements comprise a transgressive sequence of marine sediments associated with the postglacial rise in sea level. On the New Zealand continental margin, this transgression from the c. -113 m isobath since the last glacial maximum at c. 18 ka (Herzer 1981) is known to have been episodic, comprising at least eight stillstands including the -113 m shoreline (Carter et al. 1986). Sedimentation associated with this post-18 ka transgression commonly includes a conformable and onlapping sequence of landward-thinning sediment wedges.

The seismic sequence within the 3.5 kHz profiles presented here is similarly interpreted as recording this post c. 18 ka transgression. Evidence for this interpretation is:

(1) A series oflandward-thinning, conformable and onlapping seismic reflectors form the uppermost 3-55 m thick portion of the seismic sequence;

(2) This conformable and onlapping sequence unconformably overlies an acoustically reflective seismic "basement" (Fig. 2A), which in part includes folded ?late Cenozoic strata (Fig. 2B);

(3) Nearshore, fluvial channels, here interpreted to have formed on the exposed continental shelf during the last

247

glacial, unconformably underlie, and hence predate, the overlying conformable sequence (Fig. 2C);

(4) Two prominent stillstand terraces identifiable to the immediate east of the graben. (i.e., outside the area of subsidence) at depths of -57 m and -44 m, the latter having a landward thinning sediment wedge (Fig. 2D), are correlated to the -56 m and -46 m stillstand shorelines dated at 12 ka and c. 11 ka, respectively, by Carter et al. (1986). A third stillstand shoreline is tentatively identified at-30 m, which would correlate with the -28 m shoreline at c. 9.5 ka (Carter et al. 1986). Correlation of stillstand shorelines dated at between 12 and 9.5 ka with the Bay of Plenty shorelines, which have a conformable and onlapping stratigraphy within the seismic sequence, is consistent with the interpretation of a post-18 ka transgressive sequence.

(5) The extensive stillstand terrace at-l09 m, also recognis­able immediately east of the graben, which is here correlated to the c. 18 ka stillstand -113 m shoreline, conformably underlies a stratigraphically identical, but significantly condensed, seismic sequence to that recognisable within the graben;

(6) The uppermost half to quarter of the transgressive succession consisting of a seismically transparent sequence with no or very few internal seismic reflectors (Fig. 2E), is a seismic unit recognisable over the entire New Zealand continental shelf, and is known to represent the "Holocene" (e.g., Carter & Carter 1986). The base of this seismic unit has been variously dated elsewhere on the New Zealand continental margin between 9.5 and 8.2 ka (Herzer 1981; Carter et al. 1985; Lewis & MildenhalI1985).

Dating and tephrostratigraphy

Directagecontrol,consistentwiththepost-18katransgressive correlation, is provided by the stratigraphy of25 piston cores previously described by Kohn & Glasby (1978),from between the Bay of Plenty coast and White Island (Fig. 1). Core lithologies primarily comprise terrigenous mud, shell, and pebble layers, and rhyolitic and andesitic tephras. Correlation of the rhyolitic tephras by Kohn & Glasby (1978), by way of ferromagnesian mineralogy and titanomagnetite geo­chemistry, and the 14(; dating of shell horizons, establishes a chronology for these 0.61--6.18 m long cores as being < c. 12.5 ka. (Ages for identified tephra are revised 14C ages from Lowe 1988.)

In addition, as recognised here, the base of the acoustically transparent "Holocene" seismic unit occurs, within the immediate vicinity of the bottom of core 179, at a subbottom depth of c. 3 m (Fig. 2E). Kohn & Glasby (1978) identified the Whakatane Ash within core 179, at a depth of 1.85-1.99 m, and tentatively identified the Waiohau Ash at 3.66-3.68 m. These tephras have 14C ages of 4.80 and 12.20 ka, respectively (Lowe 1988). An interpolated age of 9.4 ka for the base of the "Holocene" seismic unit at 3 m, between these dated tephras, agrees reasonably well with the 9.2-8.5 ka ages obtained elsewhere on the New Zealand continental margin, and hence supports the post-18 ka transgressive interpretation described above.

Only two seismic horizons-the base of the acoustically transparent" Holocene" unit (HI) and the base of the post-18 ka sequence (H2)-which have reasonable age control and are ubiquitous within the graben, are used to estimate rates of

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New Zealand Journal of Geology and Geophysics. 1990. Vol. 33

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~I N

' '" W5

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H2:;::>" 150

Fig.2 Selected high-resolution 3.5 kHz seismic profiles from offshore Whakatane Graben (see Fig.l for locations.). 2A, Post-IS ka sediments onlapping over acoustically reflective "basement". (0) Ohiwa Fault, (P) Pukehoko Fault, (RU) Rurima Fault, (WI) White Island Fault, (F) unnamed fault. 2B, Post-I 8 ka sediments unconfonnably overlying tilted and folded ?late Cenozoic sediments (CS). 2C and 2D, Stillstand shorelines S4 (-46 m) and S5 (-56 m) and associated (W5) sediment wedge (nomenclature follows that of Carter et al. 1986) of the postglacial transgression overlying fluvial channels (C) of the ancestral Whakatane River. 2E, Relationship of seismic reflectors HI (9.4-8.5 ka) and H2 (18-16 ka) to core 179 of Kohn & Glasby (1978). (R) Rangitaiki Fault, (F) unnamed fault

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Wright-Offshore Whakatane Graben faulting

defonnation. In cognisance of both the interpretation presented above and that these surfaces are transgressive, HI and H2, as they are identified within the 3.5 kHz profiles, are assigned ages of 9.4-8.5 ka and 16-18 ka, respectively.

OFFSHORE TAUPO FAULT BELT­WHAKATANE GRABEN

Bathymetry

The offshore extension of the Taupo Fault Belt - Whakatane Graben is coincident with a bathymetric trough (Fig. 1) that forms a distinct indent in the otherwise 35-40 km wide continental shelf (Wright 1989). The Motuhora Scarp, being the bathymetric expression of the White Island Fault (Lewis & Pantin 1984), defines the eastern margin ofthe trough. At its inner nearshore end, the scarp is recognisable to within 9kmofthecoast, where it is 12 m high. The scarp progressively increases in height to a maximum of 80 m at the outermost shelf, where, in part, it forms the shelf break. On the outer shelf and uppermost continental slope, the scarp has a plan zig-zag arrangement. Further seaward the scarp is aligned with the White Island Canyon (Lewis & Pantin 1984), which also has a similiar zig -zag arrangement.

The western margin of the graben, although less well defined by the bathymetry (Wright 1989), is recognisable as a4-5 m high scarp aligned along the eastern crest of Rurima Ridge within 3.5 kHz profiles (see Fig. 4C).

Late Quaternary (post-IS ka) faults

Offshore late Quaternary (post-18 ka) faulting is confined to a 15-20 km wide zone, here recognised to be the seaward extension of the Taupo Fault Belt - Whakatane Graben (Fig. 2A, 2E, and 3). Fault displacements as typically recognised within the 3.5 kHz profiles are shown in Fig. 4. Consistent with the structure of the upper 1 km (Fig. 5), the major post-18 ka faults dip northwest, bounding a series of faulted blocks tilted to the southeast. Faults dipping in the opposite direction are generally restricted to the west. Although all faults strike generally northeast, this trend changes along the length of the graben from an average of 045° on the continental shelf to 005-020° on the uppermost continental slope. The bounding Rurima and White Island Faults are clearly recognisable within the 3.5 kHz profiles for 22 km and 41 km, respectively, across the continental shelf and upper continental slope (Fig. 3). In contrast, faults within the graben appear to be laterally discontinuous.

Of these mapped faults, six (the White Island, Rurima, Pukehoko, Nukuhou, Rangitaiki, and Ohiwa Faults; the latter five are newly named here) are considered to be significant because of either their magnitude oflate Quaternary displacement or structural position within the graben (Fig. 3), and they are described further below.

White Island Fault The White Island Fault is the lineament defined by the Motuhora Scarp, the White Island Canyon, and the eastern side of the White Island Trough (Lewis & Pantin 1984), and separates the ?late Cenozoic sedimentary sequence of the Raukumara Plain (Gillies & Davey 1986) from the offshore extension of the TVZ (Lewis & Pantin 1984; Wright et al. 1990). Morphology of the fault is variable, as demonstrated by the presence of a single scarp and a zone comprising a number of normal faults (Fig. 2A and 4A, B). Within airgun

249

profiles, seismic reflectors are clearly displaced by the White Island Fault (Fig. 5). The determination of vertical displacement rates across the fault is problematical, since it cannot be unequivocally demonstrated, within the 3.5 kHz profiles, that displacements postdate various phases of the last glacial transgression. Two lines of evidence, however, although having conflicting results, give some general indication of the rate of vertical displacement across the White Island Fault. Firstly, tenative correlation indicates a 12 m displacement of the -56 m postglacial stillstand terrace across the White Island Fault. This suggests a possible vertical displacement rate of 1 mm/yr. In contrast, a comparison of the thickness of postglacial sediment either side of the fault shows a sequence only 3-4 m thicker on the downthrown block. This suggests that the rate of vertical displacement of less than 0.4 mm/yr.

Rurima Fault The Rurima Fault is a southeast-facing normal fault that defines the western margin of the graben, and extends for some 22 km along the eastern crest of the Rurima Ridge (Fig. 3) as a 4-5 m high scarp (Fig. 4C). The fault can be presently traced landward to within 18 km of the coast. Two fault bifurcations, each 2 km in length, diverge to either side of the main trace (Fig. 3) midway along its length. The fault is not recognised beyond the upper continental slope; however, the projected seaward extension coincides with a zone of gentle warping of ?late Quaternary sediments.

The thinning ofpost-I8 ka sediments on to a seismically opaque basement, within the vicinity of the Rurima Fault, makes estimates of the vertical displacement difficult. Tentative correlation of seismic reflectors across the Rurima Fault gives vertical displacements of 4-6 m and 8-13 m, respectively, for the HI and H2 surfaces, suggesting a postglacial displacement rate of 0.6 ± 0.2 mm/yr. The range of displacements for each of the surfaces reflects the error in measuring reflector vertical displacements from the seismic sections, and the natural variation in fault displacement along the length of the fault. (Quoted errors of the vertical displacement rate are cumulative of both the age and vertical displacement.) Sedimentation rates ranging from 0.4 to 0.7 mm/yr, as derived from isopachs ofpost-18 ka sediment immediately east of the fault (Fig. 6), are consistent with this suggested vertical displacement rate.

Pukehoko Fault The Pukehoko Fault is a northwest facing, near linear, active normal fault (Fig. 2A and 4C) and is marked by a 2-8 m high scarp along its 11 km length (Fig. 3). Extrapolated landward and seaward extensions of the fault do not coincide with any recognisable tectonic structures.

Vertical displacements of 4-6 m and 7-9 m, respectively, for HI and H2 surfaces, gives a postglacial vertical displace­ment rate of 0.5 ± 0.2 mm/yr.

Nukuhou Fault The Nukuhou Fault is a major and active, northwest-facing fault (Fig. 4C), with a 5-10 m high scarp along its 6 km length. At its projected landward extension, the fault merges into a zone of extreme subsidence where at least 50 m of post-I8 ka sediments have accumulated (Fig. 6). Tectonic structures are not presently recognisable at the fault's projected seaward extension.

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Fig. 3 Late Quaternary (post-18 ka) faults of the offshore Whakatane Graben; legend is the same as for Fig. 1.

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Wright-Offshore Whakatane Graben faulting 251

NW SE 50 m

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WI

\ 50 m

H1

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SE om

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I RU

I

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Flg.4 Selected high-resolution 3.5 kHz seismic profiles from the offshore Whakatane Graben (see Fig. 1 for locations.) showing late Quaternary faults: (R) Rangitaiki Fault, (WI) White Island Fault, (0) Ohiwa Fault, (N) Nukuhou Fault, (P) Pukehoko Fault, (RU) Rurima Fault, (F) unnamed faults. Post-IS ka sediments thicken towards the faults.

The progressive increase in the vertical displacement of seismic reflectors with depth demonstrates the repetitive nature of late Quaternary movement on the Nukuhou Fault. Vertical displacements of20--24 m and 36-42 m, respectively, for the HI and H2 surfaces, gives a postglacial vertical displacement rate of 2.4 ± O.4mm/yr.

Rangitaiki Fault TheRangitaikiFault (Fig.4AandB) is an active and essentially linear, northwest-facing fault, and is marked by a 2-4 m high scarp along its 9 km length. Tectonic structures are not presently recognisable at either the fault's projected land­ward or seaward extensions.

A postglacial vertical displacement rate of 2.3 ± 0.5 mml yris derived from measuredoffsetsof18-24 m and34-38m, respectively, for the HI and H2 surfaces. These must be considered to be lower estimates because the thickness of the

post -18 ka sediments on the down thrown block is at places beyond the seismic penetration of the 3.5 kHz system. Accordingly, where this occurred, the depth of the post-IS ka surface on the hanging wall was estimated from downdip extrapolation. Sedimentation rates ranging from 1.9 to 3.0 mm/yr, as derived from isopachs ofpost-18 ka sediment immediately west of the fault (Fig. 6), are broadly consistent with this suggested vertical displacement rate.

OhiwaFault The OhiwaFaultis a major, late Quaternary, active northwest­facing fault, and is marked by a 4-16 m high scarp (Fig. 4C). The fault, as presently interpreted, has a significant change in strike from 0400 to 0650 midway along its 15 km length. The seaward extension of the Ohiwa Fault may continue northeastward to the faults mapped along the eastern side of the Central Arm of the White Island Canyon, although it is

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not presently identifiable within the intervening profiles. At its landward end, the extrapolated extension does not coincide with any recognisable tectonic structures.

A postglacial vertical displacement rate of 0.7 ± 0.3 mm/ yr is derived from measured offsets of 6-8 m and 8-12 m, respectively, for the HI and H2 surfaces. Airgun profiles clearly show vertical displacements increase with depth, being in the order of 40 m at a subbottom depth of 400 m (Fig. 5), assuming a seismic velocity of 1600 m/s for the top 0.5 s (two-way time) of seismic section.

STRUCTURAL MODEL

Extensional fault systems in the upper crust are frequently modelled as a series of subparallel faults bounding tilted blocks or "dominoes" (e.g., Jackson et al. 1988). In such a model, the relative uplift of the hanging wall and subsidence of the footwall (Fig. 7 A) progressively decreases with distance from any given fault. Movements on the bounding faults result in the rotation about a horizontal axis of the intervening block (Fig. 7 A). An important consequence of the model is that syntectonic sediment deposition will thicken towards a fault.

Inspection of seismic reflection data (Fig. 2, 4 and 5) reveals that the structure of the offshore Whakatane Graben, to a t least a depth of 1 km, can be (as a first approxima­tion) modelled in this manner. Accordingly, the major northwest-facing White Island, Ohiwa, Nukuhou, Pukehoko, and Rangitaiki Faults (Fig. 3) are interpreted as bounding a series of blocks, with variable widths, tilted to the southeast (Fig. 7B). The southeast-facing Rurima Fault, which

SE o

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VI "tl c: o u Qj

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VI "tl c: o u Qj

l/)

Fig. 5 Single-channel (40 in3

airgun) seismic reflection profiles across the Whakatane Graben (see Fig. 1 for locations.) showing late Quaternary faults: (R) Rangitaiki Fault, (WI) WhiteIslandFault, (0) Ohiwa Fault, (N) Nukuhou Fault, (P) Pukehoko Fault, (RU) Rurima Fault, (?NlSB) North Island Shear Belt faults, (F) unnamed fault. TWIT is two-way travel time.

defines the western margin of the graben, is interpreted as being antithetic to the northwest-facing faults. Within the onshore portion of the graben, gravity, seismic reflection, and seismological and drillhole data have been inter­preted in essentially the same manner (Nairn & Beanland 1989).

Subsidence

The implication of this model of extension, where any single block is both the footwall of one fauIt.and the hanging wall of another (Fig. 7), is that the rate of subsidence will not be uniform across the zone of widening. Rather, subsidence will vary spatially, in a generally predictable manner, along any transect crossing successive faults. Such variance in the rate of subsidence is seen within the offshore Whakatane Graben, as recorded by both the estimated rates of vertical displacement, and isopachs of post-18 ka sediment (Fig. 6).

Within the immediate area of the continental shelf, the rate of sedimentation, since 18 ka ago, can provide a first approximation for the rate of subsidence. Sedimentation rates within the graben, as derived from isopachs of post-18 ka sediment (Fig. 6), vary considerably between 0.4 and 3.5 mm/yr with an average of c. 2 mm/yr. These rates, however, are here considered to be minima for the subsidence rate because: (1) the thickness of the post-18 ka sediments is, in places, underestimated since the base of the postglacial transgressive sequence is beyond the subbottom seismic penetration of the 3.5 kHz system; and (2) the sediment supply is probably not sufficient to be in equilibrium with the rate of subsidence, as indicated by the existence of a bathymetric depression on the mid to outer shelf (Fig. 1). On

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37°30·

o White Island

/

){ine of section 79

Rurima Island 0 37°50·

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/'i

Fig.6 Isopachs ofpost-18 ka sediment (in metres) within the offshore WhakataneGraben; legend is the same as for Fig. 1.

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254 New Zealand Journal of Geology and Geophysics, 1990, Vol. 33

A

_ extension ------+

B 0.4-0.8

~

the continental shelf, in a similar range of water depths, immediately east and west of the graben, the rate of sediment­ation for post-18 ka sediments is generally < 0.3 mm/yr.

Rates of vertical displacement for the major late Quaternary faults (the White Island,Rurima, Ohiwa,Nukuhou,Pukehoko, and Rangitaiki Faults), as estimated from faulted seismic reflectors, generally record only the relative vertical offset of the hanging wall and footwall, and are not an absolute measure of subsidence from a known datum (e.g., sea level). However, in the absence of quantifiable data within the graben other than the previously discussed sedimentation rates, the estimated rates of vertical displacement of between 0.4 and 2.3 mm/yr for the major late Quaternary faults, provide further data on the maximum rate of relative subsidence on the downthrown block nearest the faults.

Accordingly, in cognisance of the limitations of the sedimentation and vertical displacement rates, the rate of subsidence for the offshore graben is estimated to average 2-2.5 mm/yrandrangelocally, near the major late Quaternary faults, to a maximum of 3-3.5 mm/yr. These estimated rates of offshore subsidence are essentially consistent with estimates of 0.5-3 mm/yr derived from onshore Quaternary data within the TVZ(Pullar 1981; Nairn & Hull 1986; Pillans 1986; Nairn & Beanland 1989).

Extension Assuming for the present that the fault blocks within the graben are rigid (i.e., internal deformation of the blocks is negligible compared with deformation across the bounding faults), estimates of extension for each fault can be derived from the rate of vertical displacement if the fault geometry is known. Within the offshore part of the graben, the geometry of the major White Island, Ohiwa, Nukuhou, Pukehoko, Rangitaiki, and Rurima Faults cannot be reliably established from the existing seismic data,'Onshore, however, a variety of data suggests that the faults within the graben dip at c. 35-55° (Nairn & Beanland 1989; Woodward 1989). Fault-plane solutions (Anderson & Webb 1989) and geodetic data (Darby 1989) for the 1987 Edgecumbe earthquake indicate a fault dip of 45 ± 10° to the northwest, for at least the Edgecumbe Fault. Elsewhere, theory and direct observations (Jackson et al. 1988) similarly indicate that major normal faults within active extensional terrains consistently have dips between c. 30° and 60°.

Within this context the major faults of the offshore part of the graben are argued here to have dips of 45 ± 10°, similar to

Fig. 7 A, Diagramatic model of rotating block ("domino") extension (after Jackson et al. 1988). B, Diagramatic section across the offshore Whakatane Graben (see Fig. 6 for location) showing fault geometry andrelative rates of dip-slip displacement (mm/yr) used to determine the amount of extension.

those recorded from onshore. Similarly, as can be shown from onshore data (Nairn & Beanland 1989; Beanland et al. 1989) it is the assumed here that the dip of the offshore faults (i.e., 45 ± 10°) is essentially constant to at least a depth of 10 km, apart from where the fault plane steepens to approximately vertical within 50-100 m of the sea-floor surface. The implication of this fault model is that the vertical offsets and the associated rates of displacement across the faults, as determined from the shallow subbottom 3.5 kHz profile data, are in fact rates of dip slip and not throw. Horizontal extension has been calculated for each of the major faults using the secular dip-slip rates and fault geometry (Fig. 7B and Table 1). Summation of these fault horizontal displace­ments gives an extension rate of 3.5 ± 1.7 mm/yr, at least during the latest Quaternary, across the full width of the offshore graben. (The error is cumulative of those errors derived from the rate of dip slip and the range of assumed f!,lult dips.) The real rate of extension, however, is greater than the estimate of 3.5 ± 1.7 mm/yr, but not significantly so, if the small displacements on the minor faults were accounted for. In addition, the probable underestimation of the subsidence rate, with the concomitant underestimation of the rate of dip slip on individual faults, would further underestimate the true rate of extension across the graben.

Acceptance of such a rate implies that a significant amount of the total 12 mm/yr extension across the full width of the Bay of Plenty coast (Walcott 1984) and the 7 mm/yr extension for the northern onshore part of the TVZ (Sissons 1979), occurs within the 15-20 km wide Whakatane Graben. These data support the suggestion of Nairn & Beanland (1989) that the extension rate within the Whakatane Graben is higher, and possibly twice the rate outside the graben.

Recentmodel experiments suggest that the total horizontal displacement on faults within extensional terrains may only account for 50-80% of the known extension, the remainder being due to internal block deformation (McClay & Ellis 1987; Kautz & Sclater 1988). If so, the estimate of extension derived here from the summation of horizontal displacements on individual faults will be obviously a minimum for the total extension within the offshore graben. However, internal block deformation is not considered significant here because the extension rate derived from geodetic data (Sissons 1979; Walcott 1984), which records all defonnation, is in acceptable agreement with that estimated here from horizontal displacements on faults, when the differences in length scale of observation are accounted for.

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Wright-Offshore Whakatane Graben faulting

DISCUSSION AND CONCLUSIONS

Whakatane Graben

The apparent seaward continuation of the Whakatane Graben as a zone of active tectonism offshore, for some 43 Ian is significant, and the data presented here provide further evidence for the intensity and rate of late Quaternary deformation of the TVZ. Tectonic structures within the offshore part of the graben, as mapped here, are consistent in orientation and style of deformation with those structures observed onshore (Fig. 3), and some can be directly correlated. Offshore, the RurirnaFault, the northwestern boundary of the graben, is considered to be the seaward extension of the Matata Fault Zone described by Ota et al. (1988) and Nairn & B.e~d(19.89). Likewise, the Rurima Ridge, the bathymetric ~lgh lmmedlately west of the Rurima Fault (Fig. 3), is mterp~eted to record uplift of the graben margin, which on­shore IS some 1 mm/yr for the last 0.30 Ma (Nairn & Beanland 1989). The southeastern boundary of the offshore graben, the White Island Fault, is interpreted to be the seaward extension of the onshore lineament defined by the Te Teko Fault and Tarawera "Rift" (Fig. 3). Further, although possibly coinci­dental,.the consistency in number and spatial relationship of the major and presently active faults, within the offshore and onshore parts of the graben (Fig. 3), is considered important. Offshore, the Ohiwa, Nukuhou, and Pukehoko Faults have similar structural positions within the graben to the onshore Edgecumbe, Omeheu, and Awaiti Faults.

. ~s shown offshore, the intensity oflate Quaternary faulting Wlthm the Whakatane Graben appears to be significantly greater than that presently observed onshore. Largely as a result of widespread alluvial deposition associated with the Taupo Pumice and Kaharoa eruptions 1850 and 850 years ago, respectively, less than 20 late Quaternary faults are recognisable onshore (Nairn & Beanland 1989; Woodward 1989). In contrast, some 50 post -18 ka faults are recognisable offshore. Thus, within the onshore part of the graben, it is likely that the intensity of faulting, and the attendant seismic risk, is greater, and more like that recorded from offshore .

. Alth~ugh many o~ the offshore graben faults are presently actIve (FIg. 2 and 4), It cannot be demonstrated, at this time, that the most recent displacements on these faults are attributable to the large (ML> 5) shocks of the 1977 Matataor 1987 Edgecumbe earthquake sequences (Richardson 1989;

Table 1 Extensional and dip-slip displacement rates of major late Quaternary faults along section Fig. 7B, offshore WhakataneGraben (see Fig. 6 for location).

Fault

White Island Ohiwa Nukuhou Pukehoko Rurima

Dip slip displacement rate

(mm/yr)

?0.2-1.0 0.7±0.2 2.4 ± 0.4 0.5 ±0.2 0.6±0.2

35-55 35-55 35-55 35-55 35-55

Extension* (mm/yr)

?0.8-O.1 0.8-0.2 2.3-1.1 0.6-0.2 0.7-0.2

Total extension 3.5 ± 1.7

*Extensionhas been calculated by taking the maximum and minimum rates of dip-slip displacement and the maximim and minimum angles of fault dip, for each fault, to derive the extreme ranges of extension for individual faults. The ranges of extension for individual faults were then summed to derive a median value of extension across the graben, and a quoted error that covered the summed maximum and minimum ranges of extension.

255

Smith & Oppenheimer 1989). Nevertheless, the offshore faulting within the Whakatane Graben is coseismic in nature ~ome of w~ich possibly occurred during earthquake sequence~ m recent tlmes.

Intersection of the TVZ and North Island Shear Belt

Although the intersection of the North Island Shear Belt (NISB) with the TVZ lies within the vicinity of the Whakatane Graben, the northward continuations of the north-south trending, active, dextral strike-slip faults of the NISB (Lensen 1975) have yet to be recognised within either the onshore (Nairn & Beanland 1989) or the offshore parts of the graben. C?ns?ore, the observed lack of north-south faults may not be slgmficant due to post -1850 year alluvial deposition; however, ~he lack of such faults offshore, although not unequivocal, as mterpreted from the seismic profiles, is here considered to be significant. It indicates that either (1) there has been no NISB faulting within the last 18 ka that has displaced Whakatane Graben sediments, or (2) post-18 ka fault displacements along the northeast-trending faults accommodate both, respectively, strike-slip and normal movements of the NISB and TVZ. The latter of the two is more likely since NISB faults have been active in postglacial times (Berryman & Beanland 1988), and seismological analysis of historical earthquakes located within both the onshore and offshore parts of the Whakatane Graben (e.g., Richardson 1989) have dextral strike-slip components.

There are structures along the southeastern boundary of the graben that may still be a consequence of late Quaternary NISB faulting. Lewis & Pantin (1984) and Wright et al. (1990) interpreted the distinct zig-zag arrangement of the White Island Fault (Fig. 1) as resulting from the intersection of extension of the TVZ and dextral displacement along the NISB. Faults to the southeast of the White Island Fault do occur and may represent the immediate offshore extension of the NISB (Fig. 5A). However, their orientation cannot be presently determined because they are recognisable within only one airgun and 3.5 kHz seismic profile. Onshore, the zig­za~ arrangement of the southeastern boundary of the graben ~lg. I), as defined by TVZ and NISB faults, may be similarly mterpreted.

The components of TVZ extension and NISB dextral displacement result in a complex system of normal and strike­slip faults along the southeastern boundary of the graben, the complexity of which is only partially recognisable from existing data. Within the graben, the two components of movement are accommodated by a single northeast-trending fault system.

ACKNOWLEDGMENTS

Thanks are due to theofficersandcrewoftheR. V.Rapuhiafortheir forebearance while at sea, and to K. Lewis and H. Pantin for stimulating discussions. Critical and perceptive comments on the draft manuscript were provided by S. Beanland, L. Carter, J. Cole, and B. Pillans. The referees, B. Davy and I. Nairn, are also acknowledged for their comments. K. Majorhazi drafted the figures.

REFERENCES

Anderson, H.; Webb, T.1989: The rupture process of the Edgecumbe earthquake, New Zealand. New Zealandjournal of geology and geophysics 32: 43-52.

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Beanland, S.; Berryman, K. R.; Blick, G. H. 1989: Geological investigations of the 1987 Edgecumbe earthquake, New Zealand. New Zealand journal of geology and geophysics 32: 73-9l.

Berryman, K.;BeanIand, S.1988: The rate of tectonic movement in New Zealand from geological evidence. Transactions of the New ZealandlnstituteofProfessionalEngineers 15: 25-35.

----1989: Evaluation of seismic hazard in the Rangitaki Plains, New Zealand. New Zealandjournal of geology and geophysics 32: 185-190.

Carter, L.; Carter, R. M. 1986: Holocene evolution of the nearshore sand wedge, South Otago continental shelf, New Zealand. NewZealandjournalofgeologyandgeophysics29:413-424.

Carter, R. M.; Carter, L.; Williams, J.; Landis, C. A 1985: Modem and relict sedimentation on the Otago continental shelf, New Zealand. New Zealand Oceanographic Institute memoir 93: 43p.

Carter, R.M.; Carter, L.;Johnson, D. P.1986: Submergentshorelines in the SW Pacific: evidence for an episodic post-glacial transgression. Sedimentology 33: 629-649.

Cole, J. W. 1986: Distribution and tectonic setting oflate Cenozoic volcanism in New Zealand. In: Smith, 1. E. M. ed. Late Cenozoic volcanism in New Zealand. Royal Society of New Zealand bulletin 23: 7-20.

Darby, D. J. 1989: Dislocation modelling of the 1987 Edgecumbe earthquake, New Zealand. New Zealand journal of geology and geophysics 32: 115-122.

Gillies, P. N.; Davey, F. J. 1986: Seismic reflection and refraction studies of the Raukumara forearc basin, New Zealand. New Zealand journal of geology and geophysics 29: 391- 403.

Grindley, G. W. 1960: Sheet 8-Taupo. Geological map of New Zealand 1: 250 ODO.Wellington, New Zealand. Department of Scientific and Industrial Research.

Grindley, G. W.; Hull, A. G. 1986: Historical Taupo earthquakes and earth deformation. In: Reilly, W. 1.; Harford, B. E. ed. Recent crustal movements of the Pacific region. Royal

Society of New Zealand bulletin 24: 173-186. Healy, J.; Schofield, J. C.; Thompson, B. N. 1964: Sheet 5-

Rotorua. Geological map of New Zealand 1: 250 000. Wellington, New Zealand. Department of Scientific and Industrial Research.

Herzer, R. H. 1981: Late Quaternary stratigraphy and sedimentation of the Canterbury continental shelf, New Zealand. New Zealand Oceanographic Institute memoir 89: 71 p.

Jackson, J. A.; White, N, J.; Garfunkel, Z.; Anderson, H. 1988: Relations between normal-fault geometry, tilting and vertical motions in extensional terraines: an example from the southern Gulf of Suez. Journal of structural geology 10: 155-170.

Kautz, S. A.; Sclater, J. G. 1988: Internal deformation in clay models of extensional by block faulting. Tectonics 7: 823-832.

Kohn, B. P.; Glasby, G. P. 1978: Tephra distribution and sedimentation rates in the Bay of Plenty, New Zealand. New Zealand journal geology and geophysics 21: 49-70.

Lensen, G. J. 1975: Earth deformation studies in New Zealand. Tectonophysics 29: 541-551.

Lewis, K. B.; Mildenhall, D. C. 1985: The late Quaternary seismic, sedimentary and palynological stratigraphy beneath Evans Bay, Wellington Harbour. New Zealand journal geology and geophysics 28: 129-152.

Lewis, K. B.; Pantin, H. M. 1984: Intersection of a marginal basin with a continent: structure and sediments of the Bay of Plenty, New Zealand. In: Kokelaar,B. P.; Howells,M. F. ed. Marginal basingeology: volcimic and associatedsedimentary and tectonic processes in modem and ancient marginal basins. Special publication of the Geological Society of London 16: 121-135.

Lowe, D. J.1988: Stratigraphy, age, composition, and correlation of late Quaternary tephras interbedded with organic sediments in Waikato lakes, North Island, New Zealand. New Zealand journal geology and geophysics 31: 125-165.

Macpherson, E. O. 1944: Notes on the geology of Whakatane District and Whale Island. New Zealand journal of science and technology 26: 66-76.

McClay, K. R.; Ellis, P. G. 1987: Geometries of extensional fault systems developed in model experiments. Geology 15: 341-344.

Nairn, 1. A 1976: Late Quaternary faulting in the Taupo Volcanic Zone. In: Nathan, S. compo 25th International Geological Congress excursion guide no 55A and 55C: 26-30.

----1986: Geology of Kawerau Geothermal Field-results of drilling, 1977 - present. In: Mongillo, M. ed. DSIR geo­thermal report 10: 23-47.

Nairn, 1. A; Beanland, S. 1989: Geological setting of the 1987 Edgecumbe earthquake, New Zealand.New Zealand journal of geology and geophysics 32: 1-13.

Nairn, 1. A; Hull, A G. 1986: Post-1899 yrs B.P. displacements on the Paeroa Fault Zone, Taupo Volcanic Zone. New Zealand Geological Survey record 8: 135-142.

Ota, Y.; Beanland, S.; Berryman, K. R.; Nairn, 1. A 1988: The Matata Fault: Active faulting at the nortwestem margin of the Whakatane Graben, eastern Bay of Plenty. Research notes 1988. New Zealand Geological Survey record 35: 6-13.

Otway, P.M. 1986: Vertical deformation associated with the Taupo earthquake swarm, June 1983. In: Reilly, W. I.; Harford, B. E. ed. Recent crustal movements of the Pacific region. Royal Society of New Zealand bulletin 24: 187-200.

Pillans, B. 1986: A late Quaternary uplift map for North Island, New Zealand. In: Reilly, W. 1.; Harford, B. E. ed. Recent crustal movements of the Pacific region. Royal Society of New Zealand bulletin 24: 409-417.

Pullar, W. A1981: Recent earth movements in Whakatane Graben suggested by tephra markers and surficial deposits. In: Howorth, R.; Froggatt, P.; Vucetich, C. G.; Collen, J. D. ed. Proceedings of the tephra workshop, June30-July I, Victoria University of Wellington. Victoria University Geology Department publication 20: 110-113.

Richardson, W. P. 1989: The Matataearthquake of 1977 May 31: a recent event near Edgecumbe, Bay of Plenty, New Zealand. New Zealandjournal of geology and geophysics 32: 17-30.

Robinson, R. 1989: Aftershocks of the 1987 Edgecumbe earthquake, New Zealand: seismological and structural studies using portable seismographs in the epicentral region.N ew Zealand journal of geology and geophysics 32: 61-72.

Sissons, B. A 1979: The horizontal kinematics of the North Island of New Zealand. Unpublished Ph.D. thesis, lodged in the Library, Victoria University of Wellington. 117 p.

Smith, E. C. G.; Oppenheimer, C. M. M. 1989: The Edgecumbe earthquake sequence: 1987 February 21 to March 18. New Zealand journal of geology and geophysics 32: 31-42.

Walcott, R. 1. 1984: The kinematics of the plate boundary zone through New Zealand: a comparison of short and long -term deformations. Geophysical journal oftheRoyalAstronomical Society 79: 613-633.

----1987: Geodetic strain and the deformational history of the North Island of New Zealand during the late Cainozoic. Philosophical transactions of the RoyalSociety of London A 321: 163-181.

Woodward, D. J. 1989: Geolo gical structure of the Rangitaiki Plains near Edgecumbe, New Zealand, from seismic data. New Zealandjournal of geology and geophysics 32: 15-16.

Wright, 1. C. 1989: Bay of Plenty bathymetry. 2nd ed. New Zealand Oceanographic Institute chart, coastal series, 1:200 000. Wellington, New Zealand. Department of Scientific and Industrial Research.

Wright, I. C.; Carter, L.; Lewis, K. B.1990: GLORIA survey of the oceanic-continental transition of the Havre-Taupo back­arc basin. Geo-marine letters 10.

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