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Geological structure of WairarapaValley, New Zealand, from seismicreflection profilingC.D. Cape a , S.H. Lamb b , P. Vella b , P.E. Wells b & D.J.Woodward aa Geophysics Division , Department of Scientific andIndustrial Research , Wellington , New Zealandb Research School of Earth Sciences , Victoria University ofWellington , New ZealandPublished online: 12 Jan 2012.
To cite this article: C.D. Cape , S.H. Lamb , P. Vella , P.E. Wells & D.J. Woodward(1990) Geological structure of Wairarapa Valley, New Zealand, from seismicreflection profiling, Journal of the Royal Society of New Zealand, 20:1, 85-105, DOI:10.1080/03036758.1990.10426734
To link to this article: http://dx.doi.org/10.1080/03036758.1990.10426734
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© Journal of the Royal Society of New Zealand. Volume 20. Number 1. March 1990. pp 85-105
Geological structure of Wairarapa Valley, New Zealand, from seismic reflection profiling
C.D. Cape*, S.H. Lamb**, P. Vella**, P.E. Wells**, and D.J. Woodward*
Three Mini-Sosie™ seismic reflection profiles across part of southern Wairarapa, North Island, with a combined length of 22 km roughly normal to the main structural trend, reveal shallow crustal structure down to a depth of 1.5 km in upper Cenozoic strata hidden below upper Quaternary gravel deposits. Our stratigraphic correlation of the seismic data is based on the closest mapped outcrops, as there are no deep drill-holes. (TM Trademark of Societe Nationale Elf Aquitane). A profile across the Huangarua Valley, 5 km southeast of Martinborough (Line 101, 3 km long) defines an asymmetrical syncline verging southeastward and formed during the last one million years, as previously described from geological outcrop. Profiles across the Wairarapa Plain, intersecting Highway 2 at a point 3.5 km southwest of Masterton (Lines 201 and 202, combined length c. 19 km) reveal the following structures south-eastward from the Alfredton Fault at the north-western edge of the Wairarapa Plain. 1. Chester Anticline formed in upper Miocene-basal Pliocene strata, buried under the north-west flank of the Taratahi Syncline (see below). 2. Taratahi Syncline, 7 km wide, its north-west flank unconformably overlying Chester Anticline, its bottom flat and nearly level for 5 km, and its south-eastern flank rising in a concentric curve to the adjacent Peter Cooper Anticline. 3. Peter Cooper Anticline and on its south-east side the complementary Fern Hill Syncline, both concentric folds, and strongly developed in Upper Miocene and Pliocene strata, but weakly developed in Quaternary strata. We have mapped in outcrop two growing structures at the south-east side of the Wairarapa Valley (south-east end of Line 201) as the active Gladstone Anticline and Huangarua Syncline, which are well displayed in the profile. Subsidence of the region commenced near the beginning of Late Miocene time, c. 10 million years BP. There was a brief folding event in latest Miocene and/or earliest Pliocene time. There was little tectonic activity other than regional subsidence during most of Pliocene and early Quaternary time. An increase of the folding rate of nearly an order of magnitude commenced in the middle of the Quaternary, c. 1 million years ago, and continues at the present day.
Key words: Wairarapa Valley. seismic rej1ection profiles. geological structure.
INTRODUCTION Southern North Island is the emergent part of the plate boundary between the obliquely
convergent Australian and Pacific Plates (Fig. I). The long-term plate motion (>3 million years) and the short-term rates and styles of deformation are well constrained (Lamb and Vella, 1987). At the plate bOundary zone Mesozoic strata and Cenozoic deposits derived by erosion of Mesozoic strata, with minor volcanic components, rest on the subducted Pacific Plate, the top of which is less than 30 km deep below the Wairarapa Valley. Deformation in this boundary zone is separated into a belt of strike-slip faults located west of the western
* Geophysics Division, Department of Scientific and Industrial Research, Wellington New Zealand. ** Research School of Earth Sciences, Victoria University of Wellington, New Zealand.
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86 Journal a/the Royal Society a/New Zealand, Volume 20,1990
.r;:: '
ii ., o
100
175" 176"
Cape Kidnappers
11 SOmm/year \l re lat ive plate mot ion
177" 178"
.Strike-slipzone-- - Fold & thrust zone----..... ~ .. ----PLATE BOUNDARyZONE --------t __ , A A
SOKilomelres ! , •
Fig. I - Map and cross-section of the plate boundary zone, southern North Island, New Zealand. Map shows positions of seismic reflection lines 10 I, 20 I, and 202 described in this article, and USGS Line 203 (Davey et al., 1986). The cross-section (from Lamb and VeUa, 1987) is drawn to a larger scale than the map, with vertical scale equal to the horizontal scale.
edge of Wairarapa Valley and a fold and thrust belt extending east from the Wairarapa Valley, through the late Cenozoic accretionary wedge to the Hikurangi Trough (Fig. 1).
A seismic reflection profile described by Davey et al., (1986; USGS L203, Fig. 1) has shown fold and thrust structures in the offshore part of the accretionary wedge, from the North Island east coast to the Hikurangi Trough. F. Chanier, Victoria University of Wellington
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Cape et al- Seismic reflectiort profile, Wairarapa Valley 87
Fig. 2 - Map of southern part of Wairarapa Valley and adjacent areas showing folds and some faults that are active now and/or were active during late Quaternary time. Not all the known faults can be shown at this scale. Stipled areas are Torlesse Supergroup, unstipled Upper Miocene to Quaternary deposits. Boxed areas show positions of Fig. 3 (Line 101) and Fig. 8 (Lines 201 and 202). (Manuscript map drawn by R.H. Grapes and P. Vella).
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(unpublished), recently mapped similar structures in Mioceneand older strata in the East Coast ranges adjacent to the coast of Wairarapa. Farther to the west the onshore structures are not well understood.
The following paper presents shallow seismic reflection profiles that define structures in the top 1.5 km of crust underlying the Wairarapa Valley along a total distance of 22 km. The profile lines are located approximately normal to the regional structural trend, and mainly cross flat valley floors covered with very young alluvial gravels that show little or no tectonic deformation. The seismic data show well-developed folds and a few faults in the underlying strata dating from late Miocene to middle Quaternary. We have determined similar folds and faults by surface geological mapping in parts of the Wairarapa Valley that are not covered by very young alluvium, many of which have arched or dislocated middle to late Quaternary formations (Fig. 2).
The profiles described below (Lines 101, 201 and 202, Fig. I), when combined with existing offshore seismic reflection profiles to the east of southern North Island, provide a substantial contribution to the construction of a shallow crustal section across the plate boundary zone.
SEISMIC DATA ACQUISITION AND PROCESSING
We used the Mini-Sosie seismic technique for data acquisition in the Wairarapa Valley. The Mini-Sosie system uses hand-operated ground compactors (wackers) which impact the ground in a random sequence over a "ram" segment. The initial time break of each impact is recorded and the final seismic trace is constructed by time-shifting and stacking the recorded data. This method was designed to record shallow, high resolution land data (Barbier et al., 1976) and has been successfully used in New Zealand for ground water resource evaluation studies (Woodward, 1987).
The first Mini-Sosie line in Wairarapa was Line 101 which we recorded in the Huangarua Valley in February 1986 (Fig. 1), and it produced data of good quality. Therefore, we recorded an additional 19 km transect of the Wairarapa Valley in November 1986 (Lines 20 I and 202, Fig. 1). All three seismic lines were recorded using a 48-channel SERCEL 338-HR seismic recording system. Two wackers were used for the seismic source with 1500 impacts per 20 m ram segment. Geophone arrays, spaced 10 m apart, were used and consisted of six 28 Hz geophones per string, each geophone located 2 m apart in line. Line 101 used an asymmetric split-spread recording geometry with a minimum and maximum offset of 50 m
Table 1 - Basic seismic processing sequence, Wairarapa Lines 101,201 and 202
* * * * * * * * * * * * * * * *
demultiplex debias trace edit common-depth-point sort spherical divergence correction bandpass filtering to remove ground roll deconvolution elevation statics velocity analysis and normal moveout correction automatic gain surface-consistent residual statics final non-surface-consistent residual statics stack final band pass fi Itering automatic gain 2: 1 stack trace sum for final display
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Cape et al- Seismic reflection profile. Wairarapa Valley 89
and 350 m respectively. Lines 201 and 202 were recorded off-end in an attempt to record deeper data with minimum and maximum offsets of 70 m and 540 m respectively. The recording sample rate was I ms with a total record length of 3 s. All three lines have a common-depth-point fold of 1200%.
The basic processing sequence used on the data is listed in Table I. Additional description of the processing and acquisition parameters plus an interpretation of gravity and refraction data acquired along Lines 201 and 202 can be found in Cape (1989). The seismic lines displayed in this paper are unmigrated time sections. However, wave-equation migrations were done on Line 10 I, the east end of Line 20 I, and the west end of Line 202, in order to resolve the structural features and to interpret fault angles.
LINE 101 - HUANGARUA V ALLEY
Location and Background Geology Line 101 is 3 km long and crosses the Huangarua Valley approximately orthogonally to
the known geological structure, south-east to north-west (Fig. 3). Most of it is on nearly flat
, . :~t~'rr ,. , TOf'OOfWIIIC PROFilE LINE 101
o Sea level
Fig. 3 - Map showing position of Line 10 I, Huangarua Valley and topographic profile along the seismic line. Dashed line A-A shows the location of Fig. 4. Numbers 1000 to 4000 along seismic line are selected survey pegs. The NZ national I km grid (NZMS 270, Sheet S27D) is shown.
late Quaternary alluvial terraces. The north-west end extends on to Harris Ridge (Harris Anticline) where late Pliocene and early Quaternary marine strata outcrop along the south-east slope. The objective of the seismic reflection survey was to define the Huangarua Sync line, predicted to underlie the alluvial terraces from geological observations of outcrop on the sides of the valley (Coli en and Vella, 1984). The seismic survey was designed to penetrate down to the upper Pliocene to lower Quaternary Pukenui Limestone (Vella and Briggs, 1971), which is estimated from geological evidence to be about 500 m below the ground surface at the axis of the Huangarua Syncline (Fig. 4). We wanted to find out whether the Pukenui Limestone thickened towards the syncline axis, and whether its dip is parallel to that of the overlying formations.
The outcrop data show that the syncline is asymmetrical (Fig. 4) with the steep flank on the northwest side, containing nearly vertical to slightly overturned late Pliocene and early Quaternary strata. The south-east flank dips gently (5 to 10 degrees) for a distance of about 2.5 km from the axis, then steepens against the
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90 Journal of the Royal Society of New Zealand, Volume 20,1990
SSE NNW
A HUANOARUA
FAULT
Huangarua Syncline A' Windy Peak Anticline
\"
o
Meozolc Basement
1 km
Fig. 4 - Geological cross-section of Huangarua Valley 3.5 km north of Line 101, based on outcrop observations (after Lamb and Vella, 1987).
Alternallno lIuvlal congbmlilrSI, and lal<e slllslOnn and Te Mu"" MIDDLE sand&toMS. wilt'! Il'Iln ligni tes Formation QUATERNARY
Well sorted sand + eol. Hautolara Fm!n LOWER
AtUtrnallng coqulnl Pukenul QUATERNARY
Jimlt&lonl!l and fossil· Limestone to UPPERMOST UJ Herou& sancl$~nt
Formation PLlOCENE w er III .
Greyeli"s IU . Foull.farou!. sands!.
~ anclUntty 5111,t. Formation UPPER PUOCENE IlIa w • ...,._
UPPER PUOCENE laRdy 1111,lon8
Mono_opa,i - - --
Mud,'one Muslve blue-orey MIDDLE PLIOCENE mudslOne Formalion
----I.DIIEA PUOCENE .. ,. M akara Green.d
GtalJCOnlte calclllnlUt Clay C,eek Lst.
MIIIiY, blue-g,ey Bell. Creek UPPER MIOCENE mud$tono Mud,'one
0 Coats. conolonMt'll. SunnY'ide COl Gtoyw ..... """ ,ro"11l8 basement MESOZOIC
Fig. 5 - Composite stratigraphic column determined from outcrop observations at Mangaopari Stream (Vella and Briggs, 1971) and Huangarua River (Collen and Vella, 1984), and predicted to. underlie Line 101. Roman numerals show suggested correlation with numbered seismic reflectors (see Fig. 6).
north-west flank of the Windy Peak Anticline (Fig. 4). Where the seismic line crosses alluvial terraces there is no outcrop of the underlying folded strata. The structure and stratigraphy of the late Pliocene to middle Quaternary strata are well constrained by good exposures immediately south-east of the seismic line, and less well constrained by indifferent exposures at the north-west end of the line. Late Miocene and Pliocene marine strata nearly 1,000 m thick, mostly consisting of si I tstone and muddy sandstone, are predicted to underlie the Pukenui Limestone Formation (Fig. 5).
Seismic interpretation
The unmigrated time section of Line 101 is shown in Fig. 6A. The data quality of this line is very good, with reflection events present to a two-way travel time of 0.7 s. The seismic profile delineates the Huangarua Syncline, as predicted, with gently dipping reflection events on the south-east flank. The
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A: UNE10l
tNI ""- -
C: L1NE201
Cape et al- Seismic rejlection profile, Wairarapa I'alley 91
SE
Fig. 6 - Unmigrated lie i~mic reflection profile~ A: Line 101, Huangaru3 Valley (~ee Fig. 3); B: Line 202 and C; Line 201, Mastcnon (M:(-Fig. K). Refere.nce.datum points 3re: ror Line 101 , 80 m above sealevel; fOJ Line 201. 125 m 3.5.1.. ror Line 202, 165 m a.s.\. Roman numerals show the interpreted strdtigmphic c.:orrelations described in the text (see Figs 5, 9 and 11). Horizontal ~ca le: 100 pegs = I km.
Q """"s.,..,
B:LINE 202 tNI
Q Q .,....".,.,,~'j(fR(IJIQN .. ,...., Q
O6!OIID
-_ ........... - ... .... ____ 101 ___ .. ". ... __ SE
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intersection of the axial plane with the ground surface coincides with that indicated by tilts of late Quaternary alluvial surfaces (Lamb and Vella, 1987).
At the north-west flank of the syncline, the dipping reflectors terminate at depth against a reflection free zone located at the end of the line, near peg 3800. It is not clear if this termination represents a reverse fault associated with the nearby Huangarua Fault, or alternatively, a lack of energy penetration due to very steep dips at the edge of the syncline. Relatively gently dipping strata near the top of the sequence are resolved.
In the gently dipping south-east flank of the syncline strata are resolved to a two-way travel time of 0.7 s near the syncline axis. The seismic reflection events near the axis can be divided into three separate reflection sequences based upon differences in seismic character: an upper sequence of high amplitude, continuous events from 0 to 0.3 s; a middle sequence of low amplitude, continuous events from 0.3 to 0.45 s; and a lower sequence of high amplitude, discontinuous events from 0.45 to 0.6 s. For ease of reference, we designate the base of the upper unit as reflector I, the base of the middle unit as reflector 11, and the base of the bottom unit as reflector III (Fig. 6A).
The reflection events located within the sequence above reflectors I and 11 show a strong divergence towards the syncline axis. This divergence is interpreted as representing true thickening of the units as the seismic velocities are fairly constant across the section (Cape, 1989). The discontinuous events located above reflector III are less divergent, but there also appears to be a slight thickening of the unit towards the basin axis. Weak and discontinuous reflectors are located below reflector III between 0.6 sand 0.8 s.
Geological interpretation
Extrapolation down dip from outcrop in the banks of the Huangarua River, 20 to 50 m east of the south-east end of the seismic line (Fig. 3) suggests that reflector 11 corresponds to the base of the Te Muna Formation (Figs. 4 and 5). The high amplitude events above reflector I appear to represent the conglomerate-dominated part of the Te Muna Formation, while the low amplitude reflectors between I and 11 appear to represent the siltstone-dominated part (Fig. 5). Reflector I or a surface close to it corresponds to an unconformity within the Te Muna which was identified in outcrop (Lamb and Vella, 1987) as an erosion surface cut on steeply dipping lower Te Muna strata as well as older formations on the steep north-west end of the syncline (Fig. 7).
We interpret the thin seismic ally transparent zone immediately below reflector 11 (a; Fig. 6A) as the Hautotara Formation, which is 40 m thick at the Huangarua River immediately
Te MunaRd. <:/
melres
Fig. 7 -Geological cross-section interpreted from Line IDI, Huangarua Valley (Figure 6A). A segment based on surface geology has been added at the north-west end.
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Cape et al - Seismic reflection profile, Wairarapa Valley 93
east of the seismic line (Collen and Vella, 1984). A packet of underlying reflectors down to reflector III (Figs. 6A and 7) would thus represent the Pukenui Limestone Formation (Fig. 5). We presume that the underlying relatively weakly defined reflectors between III and IlIA represent the upper Pliocene Greycliffs Formation (Fig. 5).
The geological cross-section (Fig. 7) drawn from the seismic profile has a segment added at the north-west end based on outcrop observations. It depicts an asymmetric syncline similar to that described from outcrop observations in the Huangarua River banks 3.5 km north-east of the seismic line (Fig. 4). New information provided by the seismic profile is that all the formations identified thicken towards the north-west.
The Te Muna Formation thickens markedly on the south-east (gently dipping) limb but only as far as the axis of the sync line. The seismic profile extends far enough to the northwest to suggest thinning of the Te Muna Formation against the steep north-west limb of the syncline. Reflection events within the Te Muna Formation show on-lap towards the edge of the basin to the east. We interpret these high amplitude reflections as representing contacts between alluvial conglomerate and lacustrine siltstone layers (Fig. 5) which would have large acoustic impedance contrasts.
The formations underlying the Te Muna Formation thicken towards the north-west more gradually than the Te Muna Formation, and possibly continue to thicken beyond the axis of the Huangarua Syncline. The presumed Hautotara Formation, directly underlying reflector 11, thickens from 40 m at the south-east end of the seismic line to c. 100 m at the synclinal axis, in good agreement with surface outcrop to the north-east (Rataul, 1988). The presumed thickening of the pre-Te Muna formations north-westward beyond the axis of the Huangarua Syncline indicates uniform tectonic tilting farther to the north-west, towards the axis of the Wairarapa Valley. The Huangarua Syncline and possibly also the associated Huangarua reverse fault (Figs. 6A and 7) probably started to form at the beginning of Te Muna time, not more than 1 My ago.
LINES 201 AND 202 - MASTERTON
Location and background geology
Lines 201 and 202 overlap to provide a continuous seismic reflection profile running from south-east to north-west across the broadest part of the Wairarapa Plain (Fig. 8). Line 201 is the south-eastern and longer section and follows Wiltons and East Taratahi Roads, crossing Highway 2, 3.5 km south-west of Masterton (Waingawa River). Its south-eastern end extends beyond East Taratahi Road to cross the Ruamahanga River and terminate 0.6 km west of the Tauweru River. Line 202, the north-western section, intersects Line 201 on Wiltons Road, extends across farmland to Chester Road, then runs partly along Mangatarere Valley Road and partly on farmland. The north-western edge of the Wairarapa Plain is crossed on Mangatarere Valley Road 0.5 km north-west of Chester Road (Fig. 8).
All of Line 201 and most of Line 202 are on the plain which is covered by nearly flat late Quaternary alluvial gravel with no outcrop of pre-late Quaternary strata. Locally thin loessic cover beds overlie the gravels. The only drillholes are shallow (maximum depth 30 m) and thus there is no geological information to indicate the structure of pre-late Quaternary strata. Geological data are available from the hill country adjacent to the north-western end of Line 202 and to the south, east and north-east of the south-eastern end of Line 201.
Along Mangatarere Valley the north-west end of Line 202 is flanked by hills of late Miocene, Pliocene and Quaternary sediments (P. Wells, 1989). The late Miocene and Pliocene strata are conglomerate, sandstone, mudstone and limestone, predominantly marine, at least 1,800 m thick (Fig. 9). The Quaternary consists of freshwater strata of unknown but substantial thickness (at least some tens of metres), and is unconformable over the Pliocene. No marine Quaternary strata have been identified.
Known faults crossed by Line 202 in the Mangatarere Valley (Fig. 10) are, from southeast to north-west, the active Alfredton Fault (Vella, 1963b) on the edge of the Wairarapa
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94 Journal of the Royal Society of New Zealand. Volume 20. 1990
... o N W z :::i
N o N W z :::i
i~ ~!~ ~~-le
1
Fig. 8 - Map showing positions of Lines 201 and 202. with topographical profile (vertical scale ID x the horizontal scale).
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Cape et al- Seismic reflection profile, Wairarapa Valley 95
Plain, the active Wairarapa Fault, which exhibited large dextral and vertical displacement at the time of the 1855 Wairarapa earthquake (Ongley, 1943; Adkin, 1954; Lensen and Vella, 1971), and the inactive but major Carrington Fault displacing Miocene and Pliocene strata to the west of the Wairarapa Fault (Wells, 1989). A fourth, the Te Hau Fault (Vella, 1963b), separates the Cenozoic strata from the Torlesse basement rocks of the main Tararua Range front just west of the western end of Line 202. The sense of vertical displacement on the Wairarapa Fault at Carrington in 1855 was down on the north-west side, but elsewhere it was up on the northwest side (e.g. at Waiohine River; Lensen and Vella, 1971). The dextral displacement greatly exceeds the vertical; the ratio is 6: 1 at Waiohine River during the Holocene Epoch (Lensen and Vella, 1971). It is likely that the other three faults at Carrington are predominantly dextral strikeslip faults of the North Island axial shear zone (Walcott, 1978), and have components of vertical displacement up on the northwest side, i.e. the side towards the rising Tararua Range.
The Alfredton, Wairarapa and Carrington Faults all lie close to
2000
50
Gravel
Massive
Massive mudstone
Massive siltstone
Massive sandstone
Interbedded conglomerate and sandstone
QUATERNARY
UPPER PUOCENE
UPPER MIOCENE
-_~io.:I VII Regional - unconformity -t---------t
Greywacke& argillite
MESOZOIC
Fig. 9 - Generalised stratigraphic column determined from outcrop in Carrington district, adjacent to north-west part of Line 202 (after P. Wells, 1989). Roman numerals show suggested correlation with numbered reflectors (see Fig. 6).
the axis of the Carrington Anticline (Fig. 10). On the north-west flank of the anticline limited outcrops show dips towards the Te Hau Fault. They could be interpreted as part of a homoclinal fault angle depression (half graben) or as the south-eastern flank of an asymmetrical syncline with south-eastward vergence (Wells, 1989).
At the south-east end of Line 201 the known pre-late Quaternary geology is more distant from the line than at the north-west end of Line 202, but is better defined. The south-east part of Line 201 (Fig. 8) crosses very young surficial alluvial deposits, including the gravels forming the present bed of the Ruamahanga River. East of the Ruamahanga River, the line crosses an alluvial surface that is still subject to flooding, from both the Ruamahanga River and the Tauweru River 0.6 km east of the end of the line, and is late Holocene in age. Late Pliocene and early Quaternary marine sediments outcrop to the north, east and south-west of the end of the line, and middle Quaternary freshwater strata (Te Muna Formation) outcrop 2.5 km to the east (Shane, 1987).
The Ruamahanga River runs south-west for 5 km from Line 201, then turns westward at Gladstone. Cliffs on the south side opposite Gladstone expose Quaternary strata folded in the
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NW
-500
o
Selected peg numbers projected from Seisnic Line 202
1
..,. C\I C') C\I
CARRINGTO~ ANTICLINE
2
SE
3 kms
Fig. 10 - Schematic geological cross-section of Carrington district adjacent to north-west end of Line 202, based on outcrops (after P. Wells, in prep.).
active Gladstone Anticline (Kennett, 1964), an asymmetric south-eastward verging structure with the active Huangarua Syncline (Collen and Vella, 1984; Lamb and Vella, 1987) on its south-east side. The Gladstone Anticline has been mapped south-south-west from Gladstone for a distance of 17 km (1.D. Collen and P. Vella, unpublished) and continues farther southwest as the Harris Ridge Anticline (Lamb and Vella, 1987; see description of seismic Line 101 above). At Gladstone the anticline plunges gently north-eastward.
In the west bank of Ruamahanga River, 1 km south-west of Line 201, marine muddy sandstone is identified as the late Pliocene Greycliffs Formation (Vella and Briggs, 1971) by the presence of the key gastropod fossil Pelicaria acuminata Marwick (Vella, 1953). The strata dip at 10 degrees to the south-west, striking at right angles to the regional strike, and are considered to be on the crest of the Gladstone Anticline and to show the angle of plunge of the anticline. The south-westward direction of the plunge is opposite to the direction at Gladstone and implies a saddle on the anticlinal crest at some point between 1.0 km and 5.0 km south-west of Line 201 where it crosses the Ruamahanga River. Using the presumed 10 degree angle of plunge the horizon in Greycliffs Formation can be extrapolated to a point where it has now been eroded away, 150 m above Line 201 at the Ruamahanga River. As the Greycliffs Formation may be as much as 100 m thick (Fig. 5) the projected position of the base of the overlying Pukenui Limestone Formation (also now eroded away) would have been between 150 m and 250 m higher, and we have arbitrarily placed it at 200 m above the river.
We predict that the sequence of structures underlying Line 201 is developed in upper Miocene to lower Quaternary marine sediments unconformably overlying Mesozoic Torlesse Supergroup (basement) and overlain by middle Quaternary freshwater strata and minor loess. It is probably similar to the sequence under Line 101 (Fig. 5), except that the middle Pliocene is likely to be represented by calcareous sandstone or coquina limestone, not massive siltstone. The following is a discussion of key strati graphic units which we used as a basis for the seismic interpretation.
Middle Pliocene calcareous blue-grey muddy fine sandstone, informally named Tauweru formation (McSweeney, 1987) containing the Waipipian key bivalve fossil Mesopeplum
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~
A: Te Whanga
700
600
-:-:-:-:-~ Allernating oongl. :,.::.!.:.~.::~ and sandy siltst.
.~~~~~~~~~ 11 Moderately hard coquina 1st.
500- .". III
400- .........
Fossiliferous muddy sst.
Te Muna Formation
Pukenui Limestone
Greycliffs Formation
B: Castlepoint Road
::; 300-Massive mudstone
upper Mangaopari Mudstone
200
100
o
- - - - Massive :-:-:-:-: muddy sinS!.
IV------------g ... ~··:~:·~···d··: Calc.ss! Muddy -~---=-=~ FossilHerous
sandstone T:u:w:e:ru~sa:n:d:st~. ~-~--~:-~:~-:::rt,sandy sinst. _ _ oonglomeratic F-,,;:::.:;.c..rV------? --- - - - glauconHic ss!.
= - : - -I ~~:::e B~~d~~en~k ::~::::: Massive mudst.
-:-:-:-:-:1
MIDDLE QUATERNARY
UPPERMOST PLlOC. to LOWER QUAT.
UPPER PLlOCENE
UPPER PLlOCENE
MIDDLE PLlOCENE
LOWER PLlOCENE
Fig. II - Strati graphic columns of Pliocene to middle Quaternary strata near the south-east end of Line 201. A: determined from outcrop at Te Whanga, 2.5 to 5 km east of the end of the line (after Shane, 1987). B: determined from outcrop on Castlepoint road, 2 km east of Tauweru township, 15 km northeast of the end of the line (after McSweeney, 1987). Roman numerals show correlations with reflectors (see Fig. 6).
crawfordi, outcrops in Raurauhanga Stream (Fig. IIA), 6 km east of the south-east end of Line 20 I, NZMS 260 Sheet T26, grid reference 403164 (Shane, 1987). The formation there is weakly cemented and moderately soft and its contacts with overlying and underlying moderately soft siltstones probably would not provide the seismic velocity contrasts required for strong seismic reflectors.
On the CastJepoint Road, 2 km east of Tauweru Township (Fig. lIB), 15 km north-east of Line 201, the highest 15 m of Tauweru Formation is moderately well sorted cemented sandstone with many strongly cemented lenses, again containing the key Waipipian fossil Mesopep/um cra1l1ordi (McSweeney, 1987). The same horizon is represented by a sandy coquina limestone, at least 5 m thick, containing Mesopep/um cra1l1ordi in the east bank of the Ruamahanga River 7 km north-east of Masterton, Sheet T26 grid reference 344333 (Veil a, unpublished). A small inlier of coquina limestone at Ponatahi, 8 km south of Line 20 I, Sheet S27, grid reference 259073, probably represents the same horizon but has not yielded Mesopep/um crawfordi. Along Line 201 there is likely to be a sandstone or coquina limestone representing the middle Pliocene Mesopeplum crawfordi zone which contrasts with lower Pliocene and upper Pliocene siltstones (Fig. 5). The horizon will be the same or nearly the same as the middle Pliocene limestone mapped by Wells (1989) in Carrington district, and can be expected to be a strong reflector in the Line 202 profile.
At Mangaopari Stream (Fig. 5) the base of the Pliocene is a disconformity marked by the Makara Greensand Formation (Vella and Briggs, 1971). On the northern flank of Aorangi Range the Makara Greensand overlies latest Miocene (Kapitean) coquina limestone, but north of Mangaopari Stream the limestone facies grades laterally to blue-grey siltstone that can not be distinguished from Bells Creek Mudstone (Fig. 5; see Vella and Collen, 1984). The Makara Greensand, however, persists and has been mapped extensively up to 40 km
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98 Journal o/the Royal Society o/New Zealand, Volume 20,1990
north-east of Mangaopari Stream, in Wainuioru district (Crundwell, 1987). It has also been identified at two places in Ponatahi district, 10 km and 13 km south of Line 20 I. Makara Greensand represents an unconformity (usually disconformity) that is present over most of Wairarapa. At the west end of line 202 Makara Greensand is replaced by pebbly, sandy limestone (Wells, 1989). The top of the greensand usually is gradational to overlying lower Pliocene mudstone. The base is a sharp contact, often with burrows in the top of the underlying mudstone which is upper Miocene at places (e.g. Fig. 5) and basal Pliocene at other places. The seismic velocity contrast between the greensand and the mudstone is not known, but the sharp basal contact of the greensand seems likely to be a good seismic reflector. Within the upper Miocene mud stone possible reflectors are rhyolitic tuff beds, some of which are up to 3 metres thick, and occasional packets of turbidites enclosed by mudstone (Ash by , 1987; Crundwell, 1987).
The upper Miocene mudstone ranges up to more than 1000 m thick and is underlain by weakly bedded basal upper Miocene fossiliferous sandstone that varies in thickness up to c. 100 m. Hard carbonate cemented beds in the sandstone might provide good seismic reflectors. Below the sandstone is usually a thin fining-upward conglomerate with pebble-sized clasts at the base and often only c. 1 m thick, resting on angularly unconformable unweathered Torlesse basement. Locally the conglomerate is thicker (tens of metres).
Adjacent to the north-western end of Line 202 (Fig. 8) the lowest 400 m of upper Miocene strata above Torlesse basement, observed in outcrop, consists of alternating layers of conglomerate and sandstone (Wells, 1989; see Fig. 9) and the conglomerate-sandstone contacts might be strong reflectors, like the conglomerate-mudstone contacts in the middle Pleistocene Te Muna Formation (see Line 101 above).
Seismic interpretation The unmigrated seismic sections along Lines 202 and 201 are shown in Figs. 6B and 6C.
The data quality along these lines is on the whole very good, with reflection events to 1.2 s in the middle of Line 202, and events to 1.1 s at the east and west ends of Line 201. Several unresolved seismic artifacts can be seen on the unmigrated time sections, including a classic bow-tie effect from peg 2320 to the west end of Line 202 (Fig. 6B), indicating the presence of a syncline at depth. Diffractions and disruption of the reflection sequence can be seen at peg 1800, Line 202 (a, Fig. 6B) and peg 1660, Line 201 (b, Fig. 6C), associated with the active Masterton Fault (Kingma, 1967).
Geological interpretation of the seismic lines was based on the outcrop geology described in the previous section and the correlation of six main seismic reflection events across the profile (see following section). Interpreted geologic cross-sections of the basin (Figs. 12 to 15) were constructed using smoothed stacking velocities for depth conversion.
Main seismic reflectors in lines 201 and 202
Six main reflectors are labelled consecutively from top to bottom with Roman numerals 11 to VII. Reflectors II and III are thought to represent the same horizons as reflectors 11 and III in Line 101 described above. Reflector I in Line 101 might be represented at the southeastern end of Line 201 (Fig. 6C) but cannot be identified with certainty.
Reflector 11 is generally a very continuous, high-amplitude event and represents a transgressed surface overlain by a packet of strong reflectors (not numbered) which are mostly laterally persistent, and show fanning dips opening down-dip, and on-lap towards anticlines (Figs. 6B, 6C, 12 and 14). The packet of strong reflectors resembles that which represents the Te Muna Formation in the Huangarua Valley profile (Line 10 1). The directly underlying on-lapped surface (reflector II) is presumed to represent the top of partly eroded Pukenui Limestone and Hautotara formations.
Reflector III is the next laterally persistent horizon in the seismic profiles below reflector 11. It is subparallel to reflector 11 in the broad flat bottom of Taratahi Syncline (Figs. 6B, 6C,
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500
1000
Cape et al- Seismic reflection profile. Wairarapa Valley 99
19(0 Peg numbers 19100 18150 -GROUND SURFACf-- --
SE NW 2050 2000 m- fl,!,lIlu statics datum of sleismic profile
vounge a uvium -----
~----I------
lower Pliocene
o 500 1000 ~I~==~~~~I~~~~==~I
metres
~-----? -----Fig. 12 - Geological cross-section interpreted from south-east end of Line 202 (Fig. 6B) showing northwestward onlap of multiple reflectors over Reflector H. The onlapping reflectors are considered to represent contacts of alternating alluvial gravel and silt members within the Te Muna Formation.
13 and 14) and converges gradually towards reflector II on the south-eastern flank of the syncline.
Reflector IV is best defined in the line 202 profile (Fig. 6B). It is subparallel to and below reflector Ill. It cannot be distinguished in the profiles under the flat part of the Taratahi Syncline (Fig. 6B, 6C, 13 and 14), but reappears on the south-eastern flank of the syncline, from where it can be traced as a weakly defined but fairly persistent horizon across the Peter Cooper Anticline, Fern Hill Syncline and north-western flank of the Gladstone Anticline (Figs. 14 and 15). We tentatively identify it in the Huangarua Syncline at the south-eastern end of Line 201 (Fig. 15).
Reflector V is weakly defined in the Line 201 profile, more strongly defined in the 202
0-
500
~ Qj E
1000
1500
NW : , 2200 2100 Peg numbers 2000
500 , I I
metres
Fig. 13 - Geological cross-section interpreted from the south-eastern part of Line 202 (Fig. 6B. southeast from the Alfreton Fault). The late Miocene to basal Pliocene Chester Anticline is buried under Pliocene to Quaternary strata that are now tilted south-eastward on the flank of the Taratahi Syncline.
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100 Journal of the Royal Society of New Zealand, Volume 20,1990
NWI8000N Highway 2 LI~E202 1600 1500 1400 Peg numbers 1300
I GROUND SURF~CE
o young alluvium
I ~ mid Quatemary le Muna Formation ~1A~l\OIIS f. Ol~~'" fO (\e
Perry's Rd. -Q 1200
5OO-L~'----_1I EIIUI&~~ \lJ-1IOIIS ~\iOce ~ uppermosf f>lioc - mid. Qua\'PuY- CLlffSfO
Q · 6.~\l~\'1 ,,- _------Qj nr . e G~E1 fill :...-- _ _,-
SE 1100
E ______ !pper phocen lower_,' . ./ ,,----... 1000 Pllocene 'I upper Miocene ,/' L basal Pliocene?
1000 PETER COOPER t , I ANTICLINE
TARATAHI SYNCLlNE o , . 500 , I, ,
1500 melres
Fig. 14 - Geological cross-section interpreted from the North-western part of Line 201 (Fig. 6C). The flat-bottomed Taratahi Syncline is flanked by the active Peter Cooper Anticline.
profile and is important because it defines an angular unconformity. South-eastward from the Alfredton Fault (Line 202 profile) it obliquely truncates strata between itself and reflector VA and between reflectors V A and VI (Figs. 6B and 13). It cannot be traced under the flat part of the Taratahi Syncline (Figs. 14 and 15). South-east of the Taratahi Syncline we trace it as a weak but fairly persistent horizon, marking an angular unconformity, crossing the Peter Cooper Anticline, Fern Hill Syncline and north-western flank of the Gladstone Anticline (Fig. 15).
Reflector VA is a strong reflector below reflector V in the Line 202 profile (Fig. 6B), extending for a distance of c. 1 km south-east from the Alfredton Fault, and terminating at its south-eastern end obliquely against the angular unconformity defined by reflector V. It possibly reappears for a short distance on the south-east flank of the Chester Anticline (Fig. 6B) but has not been identified anywhere else in the Line 202 and 201 profiles. It is conformable to underlying reflectors in the north-western flank of the Chester Anticline (Fig. 6B and 13).
Reflector VI is a high-amplitude event seen only on Line 202 (Figs. 6B and 13). We consider it to represent an horizon within the upper Miocene, possibly a conglomeratesandstone contact (Fig. 9).
Reflector VII is the deepest event that can be distinguished on Line 202 (Figs. 6B and 13). We tentatively correlate it with the deepest reflector on the north-west flank of the Gladstone Anticline (Figs. 6C and 15). Its depth of approximately 1.5 km directly east of the Alfredton Fault (Fig. 13) suggests that it represents the contact of the upper Miocene with Torlesse basement (Figs. 5 and 9). This depth also correlates fairly well with basement depths
Bristol Rd. NW
500
~ ' -"
~ ~--1000
Fig. 15 - Geological cross-section interpreted from the south-eastern part of Line 201 (Fig. 6C). The axis of Fern Hill Syncline has migrated progressively to north-west. The Gladstone Anticline is a major growing structure. and it is assumed to be separated from the Huangarua Syncline by a reverse fault. The Huangarua Syncline contains thick middle Quaternary Te Muna Fonnation.
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Cape et al- Seismic reflection profile, Wairarapa Valley 101
calculated by Hicks and Woodward (1978) using three-dimensional gravity modelling.
Structural interpretation
From north-west to south-east the profiles along Lines 202 and 201 show the following structures:
1. Carrington Anticline (Wells, 1989) broken by the inactive Carrington Fault on the north-western side, the active Mauriceville (= Wairarapa) Fault near the axis, and the active or recently active Alfredton Fault on the south-eastern side (Figs. 10 and 6B).
2. Taratahi Syncline, here recognised for the first time, a middle to late Quaternary structure, probably still active, extending from the Alfredton Fault (Fig. 13) 7 km southeastward to the Peter Cooper Anticline (Fig. 14). The syncline has no definable axis, being flat-floored, with reflectors 11 and III nearly level below it for a distance of nearly 5 km. Lower reflectors are not resolved in the profiles under the flat part of the syncline.
3. Chester Anticline, here recognised for the first time, is a buried structure under the north-western flank of the Taratahi Syncline. It is developed in the strata that unconformably underlie reflector V (Figs. 6B and 13), and is thought to be an uppermost Miocene to basal Pliocene structure.
4. Peter Cooper Anticline, here recognised for the first time, is on the south-eastern side of Ta rata hi Syncline and is most strongly developed in strata below reflector III (Figs. 6C, 14, and 15). The structure has been growing since late Miocene.
5. Fern Hill Syncline, here recognised for the first time, is complementary to and on the south-east side of Peter Cooper anticline, and is strongly defined below presumed reflector V (Figs. 6C and 15).
6. Gladstone Anticline (Figs. 6C and IS) is an active fold that has been mapped 6 km to the south of Line 201 by Kennett (1964).
7. Huangarua Syncline (Figs. 6C and IS) is also an active fold that has been mapped 20 km to the south-west of the south-eastern end of Line 201 (Lamb and Vella, 1987). It is thought to be separated from the Gladstone Anticline by a reverse fault (Fig. IS).
We discuss the inferred geological history of tectonic events in the Conclusions below.
Stratigraphic interpretation The part of the Line 202 profile (Fig. 6B) north-west of the Alfredton Fault is described by
Wells (1989), and is not repeated here. The proposed strati graphic correlations of numbered seismic reflectors is as follows: Reflector 11: On-lapped surface (Fig. 12) at the base of Te Muna Formation immediately
overlying partly eroded Hautotara and Pukenui Limestone Formations, age c. 1.0 million years (earliest middle Quaternary).
Reflector Ill: Base of Pukenui Limestone Formation, age c. 1.8 million years (very late Pliocene). Correlation is based on stratigraphic position, subparallel relation to reflector 11, and similarity of the seismic character to that of Line 101 (Fig. 6A). Our identification of reflector III in the Line 202 profile is uncertain because Pukenui Limestone and Hautotara Formation are not known to outcrop in the area, but we adopt it as the simplest explanation.
Reflector IV: Base? of middle Pliocene limestone and sandstone, age c. 3.0 to 3.5 million years, locally unconformable over lower Pliocene mudstone.
Reflector V: Angular unconformity, probably base of Makara Greensand and locally base of lower Pliocene limestone (Fig. 9), age c. 4.5 to 5.0 million years.
Reflector V A: Probably uppermost Miocene tephra (Fig. 9) or correlative of Clay Creek Limestone (Vella and Briggs, 1971), age c. 5.5 to 6.5 million years.
Reflector VI: Horizon within the upper Miocene, possibly a conglomerate-sandstone contact, age uncertain (between 6 and 10 million years).
Reflector VII: Possibly the contact between upper Miocene and Torlesse basement, age c. 10 million years.
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102 Journal of the Royal Society of New Zealand, Volume 20,1990
DISCUSSION AND CONCLUSIONS The Huangarua Valley seismic profile (Line 101; Fig. 6A) confinns the geological
interpretation of the Huangarua Syncline by Lamb and Vella (1987) as an asymmetrical fold that fonned rapidly during the last 1.0 million years. It also provides infonnation on the seismic character of the Te Muna, Hautotara and Pukenui Limestone fonnations, which we confidently interpreted in the seismic profile by extrapolation of dipping contacts in outcrop immediately adjacent to the south-eastern end of the profile. The seismic characters that we were able to detennine at Huangarua were a valuable aid to our interpretation of the Masterton profiles, Lines 201 and 202.
The profiles along Lines 201 and 202 overlap on Wiltons Road, 2 km north-west of Highway 2, and provide continuous seismic reflection data across the Wairarapa Plain at nearly its widest part. They show that the plain is a broad structural depression with shallow folds in Pliocene and Quaternary strata along most of its width, and marked uplift at the north-west and south-east sides (Fig 16). Three main tectonic events can be recognised, represented by the basement-upper Miocene unconfonnity, the upper Miocene-Iower Pliocene unconfonnity, and the lower Quaternary-middle Quaternary unconfonnity.
The basement-upper Miocene contact in the profiles is dated as about 10 million years old (Wright and Vella, 1988) and has been mapped at the ground surface to the south, east, north and west of the seismic profiles. It indicates the start of a regional subsidence of Wairarapa at least as far eastward as the present East Coast Range. The subsidence was differential and progressive through late Miocene to early Quaternary time (Vella and Briggs, 1971; Vella, 1968) and led to the deposition of marine sediments locally up to 2.5 km thick. The marine transgression at c. 10 million years BP. may have coincided with the eustatic rise in sea-level that is postulated to have occurred about that time, after a brief low sea-level interval that controlled the stratigraphic boundary between Middle and Upper Miocene (Vella, 1968; Haq et al., 1987).
The lower Pliocene unconfonnity (reflector V) truncates the strong reflector V A which we consider to represent an horizon within the uppennost Miocene (Kapitean). Reflector V also truncates strata above the strong reflector (V A) which are presumed to be very early Pliocene in age. We suggest that reflector V should be correlated with the Makara Greensand and laterally equivalent shallow water limestone which are lower Pliocene (lower Opoitian) c. 4.5 to 5.0 million years in age. At Chester Anticline we estimate that a thickness of at least 600 m of sediment has been stripped by erosion at the lower Pliocene unconfonnity (Fig. 13) and at the Gladstone Anticline at least 500 m (Fig. 15).
The unconfonnity represents a distinct rapid shortening event that occurred in latest Miocene and/or earliest Pliocene time. It is possible that the late Miocene-early Pliocene low sea-level (Vail and Hardenbol, 1979; Haq et al., 1987) accentuated the erosion caused by the folding. and that deposition above reflector V began in response to sea-level rise after the low sea-level interval.
The estimated total shortening below reflector V at Chester Anticline is c. 160 m over a distance of 3 km. i.e. 5 percent. and at Gladstone Anticline at least 100 m over a distance of 2.3 km, i.e. at least 4 percent. The age control is not good enough to allow estimation of shortening rates. The profiles provide no data to detennine the amount of latest Mioceneearliest Pliocene crustal shortening in the area between Chester and Gladstone Anticlines, or between Gladstone Anticline and Huangarua Syncline.
Despite the shortening of Torlesse basement implied by the pre-reflector V folding, regional subsidence continued. leading to deposition of presumed marine strata up to 1000 m thick in the Taratahi and Fern HilI Synclines. ranging from Pliocene to early Quaternary in age. On the north-west flank of the Taratahi Syncline, south-eastward from the Alfredton Fault, reflectors V, IV and III are nearly parallel (Fig. 13) showing that the subsidence was unifonn, with no folding or tilting. even above the Chester Anticline. In the Fern Hill syncline (Fig. 15) there is a local unconfonnity within the Upper Pliocene (between reflectors III and IV) which we tentatively identify as the base of Greycliffs Fonnation (Fig. 5). The
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Cape et al- Seismic reflection profile, Wairarapa Valley 103
/~~
al /::x:~
Q) / zu 0 Q) C / /~~ a: c Q)
~---Q) U
//'~/~ U 0 .Q :.s: [is 0:: =>
a: /w"
8~ "0 ~/ /~g m 0 . wZ a: //tu « ~- "- ~
" '" "'-
N
~ <=-~ '" .c c Q) r- ~ ;; Q) c I~ c Q)
~ Q) U ~::J
" U 0 ~!;1 0 .Q :.s: 0 0:: «>-
,; ::i f-U)
f- :.s: 5 .... ;10 ::;:
Fig. 16 - Generalised geologic cross-section along Lines 20 I and 202 (Figs. 6B, 6C, 12, 13, 14 and 15) taken from Cape (1989). Basement depths were determined from gravity and seismic refraction data. A thin veneer of upper Quaternary gravels has been omitted.
unconfonnity at the base ofthe Greycliffs Fonnation (usually a disconfonnity with no distinct angular discordance) has been attributed to a glacio-eustatic fall of sea-level by Veil a (1963a), but at the Fern Hill Syncline it probably was caused in part by emergence and subaerial erosion on the adjacent north-western flank of the rising Gladstone Anticline.
The Fern Hill Syncline (Fig. 15) differs from the Taratahi Syncline (Figs. 12, 13 and 14) in that it is not flat-bottomed, but is nearly concentric with an axis that has shifted progressively north-westward from late Miocene to Quaternary time. The shift indicates north-westward propagation of the tilt axis of the north-west flank of the Gladstone Anticline and, together with the lower Pliocene unconfonnity, shows that the Gladstone Anticline started to fonn in late Miocene or very early Pliocene time, even though it was folding more rapidly during the Quaternary (Kennett, 1964). By implication the Huangarua Syncline on the south-east side of the Gladstone Anticline may have started to fonn equally early, though the main period of folding was during the last 1 million years (Lamb and Vella, 1987).
Across the 16.5 km of seismic profile southeastward from the Alfredton Fault (Figs. 13, 14, and 15) the folding of reflector 11 shows c. 0.8 km of shortening (5 percent) implying an average horizontal velocity of shortening of c. 0.8 mm/year (0.05 mm/km/year) across the profile. Half of the shortening has occurred between the Gladstone Anticline and Huangarua Syncline, where c. 400 m of shortening is shown over what is now a distance of 600 m from the anticlinal crest to the synclinal trough. The amount of shortening is c. 40 percent, and the rate c. 0.4 mm/year, or for comparison with the average for the profile (0.05 mm/ km/year), 0.67 mm/km/year.
The middle Quaternary unconformity represented by reflector 11 reaches a depth of 500 m below sea-level in the Taratahi Syncline and 600 m below sea-level in the Huangarua Syncline (Figs. 13, 14 and 15). Because the strata above the unconformity have a similar seismic character to that seen in the Huangarua profile (Line IO 1) we infer that they are the same facies, i.e. mainly alternating alluvial conglomerates and lacustrine siItstones. The lake sediments might have been deposited slightly below sealevel, as in Lake Wairarapa at the present day, where the sediment surface in the deepest parts of the lake is about 1.0 m below sea-level. Assuming that the lake sediments represent glacio-eustatic high sea-level phases (Collen and Vella, 1984) average subsidence rates for reflector 11 are 0.5 mm/year in the deep part of the Taratahi Syncline and 0.6 mm/year in the deep part of the Huangarua Syncline. Both synclines were subsiding throughout Te Muna time, and judging from the thickness of alluvium younger than Te Muna, probably are still subsiding.
The seismic profiles (Fig. 6) show structures in a part of
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the Wairarapa depression which was subsiding during most of the time since the beginning of the late Miocene Epoch. A set of asymmetric south-eastward verging folds in the depression (Fig. 16) has been active at varying times since the late Miocene. but especially during the middle and late Quaternary (the last 1.0 miIIion years). Structures of similar size but perhaps different geometry have been determined in offshore seismic profiles between the east coast of North Island and the Hikurangi Trough (Davey et al .. 1986; Lewis and Bennett. 1985).
The Wairarapa depression is bounded to the west by the Rimutaka and Tararua Ranges characterised by active dextral faults (Fig. 2). To the east it is separated from the offshore accretionary prism by the East Coast Range. The Wairarapa depression and the East Coast Range are an emergent part of the plate boundary 50 to 100 km wide. Active folds and faults are restricted to the Wairarapa Valley and immediately adjacent hill country. Further east to the edge of the East Coast Range the hill country displays Upper Miocene to Lower Quaternary strata dipping regularly to the northwest at c. 15 to 20 degrees (Vella and Collen. 1984). and evidently represents a rigid block underlain by basement. back-tilted away from the Hikurangi Trough (Lamb and Vella. 1987).
Most of the shortening within the Wairarapa depression during the last 10 million years (since the start of late Miocene) has occurred within the Wairarapa Valley. and has been most intense during the last I million years. Reverse faults and folds accumulated shortening by an order of magnitude greater than during the previous 9 million years.
ACKNOWLEDGEMENTS The SERCEL seismic recording system used in this study is owned and operated by the Geophysics Division of the New Zealand Department of Scientific and Industrial Research. Processing of the seismic data was done at Geophysics Division using a V AX 11/750 and an in-house seismic processing system. This work was supported by the New Zealand University Grants Committee. Victoria University Internal Research Committee and Research School of Earth Sciences. Geophysics Division. and the Wairarapa Catchment Board. A number of Victoria University Geology students helped during the research programme and we wish to mention particularly Desmond Patterson and David Kelly. Maps and diagrams were drafted by E.F. Hardy and C.N. Hume.
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