reinterpretation of seismic reflection data from the moutere depression, nelson region, south...

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Reinterpretation of seismic reflection data from theMoutere Depression, Nelson region, South Island, NewZealandJoanne C. Lihou a ba Department of Geology, Research School of Earth Sciences , Victoria University ofWellington , P.O. Box 600, Wellington, New Zealandb Department of Earth Sciences , University of Oxford , Oxford, OX1 3PR, United KingdomPublished online: 23 Mar 2010.

To cite this article: Joanne C. Lihou (1992) Reinterpretation of seismic reflection data from the Moutere Depression,Nelson region, South Island, New Zealand, New Zealand Journal of Geology and Geophysics, 35:4, 477-490, DOI:10.1080/00288306.1992.9514542

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New Zealand Journal of Geology and Geophysics, 1992, Vol. 35:477-4900028-8306/92/3504-0477 $2.50/0 © The Royal Society of New Zealand 1992

477

Reinterpretation of seismic reflection data from the Moutere Depression,Nelson region, South Island, New Zealand

JOANNE C. LIHOU

Department of GeologyResearch School of Earth SciencesVictoria University of WellingtonP.O. Box 600Wellington, New Zealand*

•Present address: Department of Earth Sciences, University ofOxford, Oxford OX1 3PR, United Kingdom.

Abstract Reinterpretation of an open-file, reconnaissanceseismic reflection survey of the Moutere Depression,integrated with information from two exploration wells,geological maps of the region, and a geophysical survey, hasrevealed three sub-basins beneath post-late Miocene gravels;they are subparallel to Tertiary basins of the West Coast regionand have preserved thicknesses of up to 2500 m. One sub-basin is directly related to late Tertiary outcrop geology, but itsextent and geometry were formerly unknown; the other sub-basins are early Tertiary half-grabens. Their southeasternmargins have been overthrust during mid-Miocene-Pliocene,WNW-directed compression. An axial zone of shortening canbe traced from the coast to the Kawatiri Fault, separatingRotoroa Complex and Separation Point Batholith basementrocks in the Moutere Depression. Shortening across thedepression was previously more widely distributed, but is nowconcentrated along the Waimea Fault System, where tectonicloading since the Pliocene-Pleistocene has caused gravels ofthis age to dip towards the eastern margin.

Keywords Moutere Depression; basins; early Tertiary; half-grabens; compression; crustal shortening; faults; WaimeaFault System; Pliocene-Pleistocene; gravels; seismicreflection survey

INTRODUCTION

The Moutere Depression is an elongate basin which extends65 km SSW from Tasman Bay and is about 25 km wide (Fig.1). It is bounded to the east by the Richmond Range and to thewest by the Hope and Arthur Ranges. Its surface is irregularand dissected, with an elevation of 600 m in the south, whichdecreases gradually northwards towards the coast.

Previous work in the area has mainly been geologicalmapping: early reconnaissance mapping by Bell et al. (1911),Henderson et al. (1959), and Bruce (1962) was elaborated byWalcott (1969), Johnston (1971, 1979, 1981, 1982 a & b,1983, and 1990), Grindley (1980), and Coleman (1981).Anderson (1980) reviewed early geophysical surveys and

G92006Received 5 February 1992; accepted 19 May 1992

produced gravity and magnetic anomaly maps for the area.Earlier, Tasman Petroleum Co. Ltd (1966) had conductedextensive gravity and magnetic surveys in conjunction withsome limited seismic refraction work (reviewed in Anderson1980) and drilled a stratigraphic well, Ruby Bay-1, in thenorth of the area, in order to test a well-defined gravity high.This well confirmed expectations of a shallow basement, butthe geometry and stratigraphy of the rest of the basin remainedunknown. King (1987) reviewed the hydrocarbon pros-pectivity of the Moutere Depression prior to Governmentissuing of an exploration licence.

In 1987, Petrocorp conducted a reconnaissance seismicreflection survey consisting of eight lines along roads in thearea, covering a total distance of about 185 km (Fig. 1)(Petroleum Corporation of New Zealand (Exploration) Ltd1989a). In 1988, Tapawera-1 was drilled by Petrocorp toestablish the hydrocarbon potential of late Eocene coalmeasures, where they pinched out along the western margin ofthe Moutere Depression and were capped by early Oligocenelimestone (Petroleum Corporation of New Zealand(Exploration) Ltd 1989b). However, the seismic eventidentified at 0.362 s as the base of the limestone transpired tobe the base of the Pliocene-Pleistocene Moutere Gravels,which blanket the basin and exceeded previous expectationsas to their thickness. The present study reinterprets the seismicreflection data, in conjunction with drillhole information andoffshore isopach maps in Tasman Bay (Thrasher & Cahill1990), and analyses the deformational history of the basin.Initial results were presented in Lihou (1990).

STRATIGRAPHY

Basement rocks of the Moutere Depression form a series offault-bounded, north- to northeast-trending belts (Fig. 1). Theexposed cover sequence is Late Cretaceous-Pleistocene in age(Fig. 2), but the majority of the depression is covered withPliocene-Pleistocene Moutere Gravels of the Tadmor Group,which obscure older cover rocks, except around Nelson Cityand in the Wangapeka district (Fig. 1). The stratigraphy andpaleoenvironmental interpretation of rocks within thedepression is well documented by Johnston (1971, 1979,1981, 1982 a & b, 1983, and 1990), Grindley (1980), andColeman (1981).

SEISMIC STRATIGRAPHY

For the purposes of this study, four seismic reflectors wereselected, one of which can be correlated right across the basin.They represent unconformities or disconformities and areprobably time-transgressive; their approximate ages and asummary of their seismic character, plus that of interveningunits, is given below, together with an outline of the inferredgeology. Reflection profiles across the basin, illustratingseismic response, are shown in Fig. 3-5.

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478 New Zealand Journal of Geology and Geophysics, 1992, Vol. 35

Fig. 1 Simplified geological mapof the Moutere Depression(adapted from NZGS 1:250 000,1:63 360, and 1:50 000 mapseries), with the location of theMD87 reflection seismic profiles(Petroleum Corporation of NewZealand (Exploration) Ltd 1989b)and drillholes.

— • — • — seismic profilesTERTIARY ROCKS

Moutere GravelsGlenhope/Port Hills FormationJenkins Group sediments

BASEMENT ROCKSTorlesse GreywackeRichmond GroupDrumduan GroupSeparation Point GraniteMaitai GroupDun Mtn Ophiolite BeltBrook Street VolcanicsRotoroa Igneous ComplexLower Paleozoic rocks

0 5 10kmi i

TriassicTriassicJurassic

CretaceousLate Permian

PermianPermian

Early Permian

Unit 5 is the oldest unit, representing pre-Late Cretaceousseismic basement. There are some coherent reflectors visible,but otherwise the unit is seismicalry featureless. It is inferredthat this unit represents mainly igneous and some meta-sedimentary rocks of the Separation Point Batholith, Rotoroa

Igneous Complex, and Brook Street Volcanics Group. Theirinferred distribution from well-log data is given in Wodzicki(1974).

Reflector D represents a pre-Late Cretaceous erosionalbasement surface, chosen as the base of strong reflectors

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Lihou—Moutere Basin seismics 479

Fig. 2 Stratigraphy of Tertiaryrocks in the Moutere Depression,derived from Johnston (1971,1979,1981, 1982 a & b, 1983, and1990) and Coleman (1981).

terrestrial sedimentation,mainly locally derivedgravels

terrestrial, 7fau.lt-controlled deposition

Fig. 3 Seismic reflection profileMD87-OO7. After depth conver-sion, the maximum thickness ofseismic unit 1 is c. 1920 m; thevertical displacement of seismicbasement by the west-vergentSpeargrass Fault (?MesozoicMedian Tectonic Line) is 1230 m,and the fault dips 65°SE.

LINE MD87-007 - SECDP'S 830 - 1400 30 FOLD

STACKED TIME SECTION

500 1000m

characterising the overlying ?coal measure sequence of unit 4,which has high acoustic impedence.

The base of unit 4 has high amplitude, low frequency, andmoderately continuous reflectors, which pass upward into asection of few coherent reflections; an intermediate-leveldisconfonnity is marked by a change upwards to highamplitude, low frequency, continuous reflectors. The unit ispresent in two half-grabens (e.g., Fig. 4). The sequence ispresumed to represent mainly terrestrial coal measures withsome overlying marine mudstones.

Reflector C also represents an erosional surface, formedon basement and older sediments of unit 4. A low-angleunconformity is apparent where the reflector intersects unit 4(Fig. 4), and is identified by terminating reflectors. Where thecontact is with seismic basement, reflector C resemblesreflector D.

At the base of the overlying unit 3, reflections are laterallycontinuous, with high amplitude and low frequency (Fig. 5).

The unit is inferred to be a transgressive sequence of terrestriallate Eocene MotupipiMarsden Coal Measures overlain byOligocene shallow-water limestone. Such sediments incontact with one another tend to be indistinguishable onseismic profiles as they have similar acoustic impedence. Theremainder of the unit has near-reflection-free seismiccharacter and represents marine mudstone of the OligoceneSherry River Formation in the west (Fig. 5) or Wakatu Form-ation in the east (Fig. 4). Sherry River Formation depositswere penetrated by the Tapawera-1 drillhole (Fig. 5) andcorrespond to the interval between 0.362 s and 0.675 s two-way-time (TWT) on seismic profile MD87-OO5 (PetroleumCorporation of New Zealand (Exploration) Ltd 1989a). Theacoustic impedence contrast between high-velocity limestoneand overlying lower velocity calcareous mudstones results in adistinctive, high-amplitude reflection. In the west of thedepression there are also higher amplitude, intermediatefrequency, moderately continuous reflections from the lateral

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••-•-•• '•• • Vr^E^S5

I S ^ ^LINE MD87-001 - SE

CDP's 112 - 700 30 FOLD

STACKED TIME SECTION

Fig. 4 Seismic reflection profileMD87-O01. After depth conver-sion, there is a 2°SE tilt onreflector A; the bounding normalfault dips 55°NW; and the maxi-mum preserved thicknesses ofseismic units 3 and 4 are c. 420 mand c. 1870 m, respectively.

500 1000m

TWTCs)TAPAWERA-1

• • • • • • • • • - . . •

LINE MD87-005 * SECDP's 1 0 1 - 2 1 5 0 30 FOLD

MIGRATED TIME SECTION

2.0

0 500 1000m

Fig. 5 Seismic reflection profile MD87-O05. After depth conversion, the maximum preserved thickness of seismic unit 3 is 1860 m; and abasal thrust dips 30°SE in the zone of overthrusting. Normal faults affecting reflector C have been steepened and overturned during a laterstage of compression. The youngest thrusts are those displacing reflectors A and B.

equivalent of the Sherry River Formation—the Oligocene-early Miocene Burmeister Formation—an alternating sand-stone/mudstone sequence, which was also penetrated by theTapawera-1 drillhole (Fig. 5). It appears to wedge out to thesoutheast, interfingering with more distal, probably deeperwater, Sherry River mudstone.

Reflector B is usually a distinctive, high-amplitudereflection. It represents a regional unconformity, either formedon basement (unit 5) or older sediments of units 3 (Fig. 4) and4; in the latter case, it separates underlying early Miocene andolder rocks from overlying late Miocene sediments of unit 2.

Unit 2 is identified by high amplitude, laterally con-tinuous, moderately high frequency reflections. It probablyconsists of granite-derived terrestrial gravels, quartzose grits,and interbedded carbonaceous mudstone and siltstone of thelate Miocene-Pliocene Glenhope Formation, 30 m of whichwas penetrated by the Ruby Bay-1 drillhole above dioritebasement at 280 m (Tasman Petroleum Co. Ltd 1966).

Reflector A is either developed as an unconformity withseismic basement (unit 5) (Fig. 3), showing onlap onto theerosional basement surface, or as a disconformity with oldersediments of units 2 and 3 (Fig. 5). It is characterised by veryhigh amplitude reflections.

Reflections from the youngest unit, unit 1, show variablecontinuity, intermediate frequency, and high amplitude. Theunit consists of well-stratified, mainly greywacke-derivedclay- or silt-bound Pliocene-Pleistocene Moutere Gravels,penetrated by the Ruby Bay-1 (Tasman Petroleum Co. Ltd1966) and Tapawera-1 (Fig. 5) drillholes, and is exposedacross the width of the depression (Fig. 1).

Conversion of TWT profiles to depth profiles wasachieved using velocity estimates from a number of sources(Table 1). Lateral velocity changes are not resolvable frominterval velocities derived from stacking velocities.

SEISMIC INTERPRETATION

The Moutere Depression is bounded to the east by the WaimeaFault System (see Fig. 1) and possibly by discontinuous faultsalong its western margin which are concealed by post-lateMiocene cover sediments. For example, Thomas (1989)identified the Moutere Fault running from Motueka toWaiwhero from field mapping and aerial photography, andconcluded that it was an active reverse fault. To the south, inthe Wangapeka district, the Matariki Fault has apparent

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normal motion that displaces the base of the Tertiary outcropdown to the east (Johnston 1971; Coleman 1981).

Axial gravity and magnetic anomalies (350 mN/kg and500 nT, respectively; Anderson 1980) were interpreted asuplif ted basement blocks by Garrick (1964 in Anderson 1980)and Stackler & Schoenharting (1971), respectively. The bulkdensity of the Separation Point Batholith is sufficientlydifferent to that of the Rotoroa Complex to produce a gravityanomaly where there are changes in basement type (Anderson1980). Wellman (1973) showed that the magnetic anomalycould be caused by dense, mafic intrusives of the RotoroaComplex. This is supported by the fact that the Ruby Bay-1drillhole, coincident with the magnetic anomaly maximum,penetrated only 280 m of late Miocene-Pliocene TadmorGroup sediments, before reaching diorite of Rotoroa Complexaffinities (Tasman Petroleum Co. Ltd 1966).

Before the reconnaissance work of Petrocorp in 1987(Petroleum Corporation of New Zealand (Exploration) Ltd1989a and b), little more was known about the structure of thebasin beneath the Moutere Gravels, and only seismicreflection techniques were likely to improve the situation.Petrocorp's open-file reports contain one interpreted seismicprofile, MD87-OO5, and a series of TWT maps, which showelongate Tertiary basins separated by a fault-bounded axialbasement high. The sense of displacement appears to changealong the length of the faults, but is predominantly normal dip-slip motion.

The results of my seismic interpretation are shown asdepth sections in Fig. 6 and as simplified TWT reflectionisochrons to mapped horizons in Fig. 7-9 and 11. Correlationof structures between profiles was most difficult in the southand east of the area, owing to the wide spacing of seismic linesand apparent variations in structure between them. Transversestructures imaged by seismic profile MD87-004/6 (Fig. 6)were not intersected by any other seismic lines and thereforeinvolve the most speculation as to their lateral extent. Never-theless, this seismic interpretation has confirmed that the base-ment highs defined by gravity and magnetic anomalies arestructurally controlled, as postulated by King (1987); a zone ofsignificant shortening can be traced from the coast southwestto Glenhope, the uplifted eastern block showing up to anestimated 200-300 m of relief above the base of the MoutereGravels (depth section MD87-005 of Fig. 6). The coastal limitcoincides with the Ruby Bay gravity high, which may extendoffshore into the Southern Taranaki Graben, along a laterallyextensive reverse fault shown on isopach maps produced byThrasher & Cahill (1990) (Fig. 7). It probably also extendssouthwestwards into the Kawatiri-Tutaki Fault Zone (Fig. 1and 7), which separates the main bodies of the Separation Point

Granite and the Rotoroa Complex in the south of the MoutereDepression (and to the east of the Murchison Basin) andincorporates fault-bounded, late Eocene sediments, so is a Ter-tiary feature. It is therefore likely that the axial zone of short-ening in the Moutere Depression marks the same junction.

Other possible basement block boundaries appear onseismic profile MD87-007 (Fig. 3); two reverse faults couldrepresent the northern extensions of the Flaxmore andSpeargrass Faults, which bound the Drumduan terrane whereit is exposed in the St Amaud area (Johnston 1990 and Fig. 1).The Brook Street Volcanics are thrust west over theDrumduan Group in the Lake Rotoiti area, and the junctionbetween these two terranes is regarded as the Median TectonicLine, a late Mesozoic feature (Johnston 1990). On seismicprofile MD87-O07, these faults displace the basement surfaceand the base of the Moutere Gravels (reflector A), suggestingthat the latest movement has occurred since late Pliocene orearly Pleistocene times. Johnston (1990) considered them tobe potentially active faults. If the fault was initiated in theMesozoic, then it has been reactivated with a reverse sense ofmotion (post-late Pliocene/early Pleistocene), displacingseismic basement up to the east by c. 1230 m.

Three sub-basins have been identified beneath the lateMiocene-Pleistocene gravels and are shown on Fig. 8 and 9.They trend NNE-SSW to NE-SW, subparallel to Tertiarybasins of the West Coast region. Two of the sub-basinscoincide with gravity lows centred over Stoke and the Mouterevalley. Anderson (1980) calculated that a minimum sedi-mentary thickness of 1650 m was responsible for thepronounced (220 mN/kg) gravity low centred over Stoke. Theresidual gravity low over the Moutere valley is less pro-nounced and may also be inseparable from the gravityanomaly caused by the Separation Point Batholith (Garrick1964 in Anderson 1980). Any gravity expression of the thirdsub-basin in the Wangapeka district is masked by short-wavelength anomalies caused by lithology changes within thebasement rocks. Hence, a small gravity low at Tadmor cannotbe used to define the extent of this sub-basin, as the anomalycannot be distinguished from the northern extension of a lowcentred over Murchison, which is thought to be related to theSeparation Point Batholith (Anderson 1980).

Two of the sub-basins identified in this study can bedirectly related to outcrop geology around Nelson City and inthe Wangapeka district (Fig. 9), but their extent and geometrywere formerly unknown. One of these sub-basins can betraced from Stanley Brook 30 km southwest to Glenhope (Fig.8), on seismic profiles MD87-005 and -008, and has amaximum preserved thickness of c. 1.2 s TWT, or c. 1860 m(seismic unit 3). The base of the sub-basin is defined by

Table 1 Velocities used in time-depth conversion and sources for this information.

Unit

Unit 5

Unit 4

Unit 3

Unit 2Unitl

Velocity (m/s)

5300330041003400

3100

26003015

Source of information

Garrick (1963) in Anderson (1980)As aboveAs aboveThrasher & Cahill (1990)

Garrick (1963) in Anderson (1980)

Borehole data (Thomas 1989)Interval velocity from

Tapawera-1 time/depth curve

Notes

Rotoroa Complex.Separation Point Granite.Maitai Group.Average from 17 wells in the Taranaki

Basin which penetrated a Paleocene-Eocene sequence of variable lithology.

cf. VINT = 3040 m/s from Tapawera-1time/depth curve.

cf. 3380 m/s (Garrick 1963).

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ACNW)

2000

-2000

-4000

MoutereFault

projected location of"Ruby Bay High"

E'

MD87-001

Uoutere valley >- Dove valley

half-graben

? Rotoroa Complex

A'CSE)-I 2000

WaimeaFault

\ Richmond

\ + Maitai

Terrane

0 m AMSL

-2000

-4000

B CNWJ2000

-2000

MD87-003

F'" WaimeaFault

B'CSEJ032000

half-graben

zone of block-faulting

Fault Zone ? R o t o r o a Complex

Richmond

+ Maitai

Terrane

0 m AMSL

-2000

AMSL

-2000 • -2000

Kawatlri Fault Zone? Rotoroa Complex Terrane

D CNW)

DT - Drumduan Terrane

BSV - Brook Street Volcanics

MT - Maitai Terrane

E CSW)

2000 r

-2000

MD87-008 MD87-007

Speargrass Waimea -Fault Fault

0 m AMSL

2000

-2000Moutere valley

- Dove valley-4000L half-graben

MD87-002

? Separation Point Batholith

E' CNE)-i 2000

0 m AMSL

-2000

-4000

Zone Rotoroa Complex

F CS)2000

-2000

MD87-006

• -2000

F' CN]2000

? channelling

0 m AMSL

-2000

F' CS)2000 r

-2000

MD87-006 F"

:: basal' thrust H^

Wangapeka sub-basin

fcv'l Moutere gravels - Pliocene-Pleistocene

fc--l Glenhope Formation - late Miocene-Pliocene

CNJ CSWJ

MD87-004

M o dextral transform fault

(

[TTTj Jenkins Group - late Eocene-early Miocene

[HI terrestrial syn-rift sediments - ? Paleocene-Eocene

F'" CNEJ- 2000

0 m AMSL

-2000

5 km

Fig. 6 Cross-sections through the Moutere Depression, following MD87 seismic reflection profiles; see Fig. 1 for location of profiles, andTable 1 for depth conversion velocities. Transform faults in profile MD87-004 show a normal dip-slip component for Jenkins Groupsediments, but reverse motion for the Glenhope Formation, so may be reactivated tear faults.

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Fig. 7 Simplified TWT contourmap to seismic basement; offshorecontours are adapted fromThrasher & Cahill (1990). Refer totext for discussion.

seismic profiles

— i — Isochrons , in milliseconds, ofTWT from a sea level datum to themapped horizon; ticks indicatedown-slope direction200 ms contour interval

Limit of outcrop/subcrop

Faults offsetting mapped horizontriangle on upthrown side indicatesdirection of dip of reverse faulttab on downthrown side indicatesdirection of dip of normal fault,cross-hatching shows extent offault planedextral strike-slip component

10km

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WANGAPEKASUB-BASIN

\

Fig. 8 Simplified TWT contourmap of reflector C (late Eocene-Oligocene). Refer to text fordiscussion.

seismic profiles

,__ I sochrons , in milliseconds, ofTWT from a sea level datum to themapped horizon; ticks indicatedown-slope direction200 ms contour interval

Limit of subcrop

Faults offsetting mapped horizontriangle on upthrown side indicatesdirection of dip of reverse fault

••••• tab on downthrown side indicatesdirection of dip of normal fault,

SSS5 cross-hatching shows extent of^ fault plane

dextral strike-slip component

5 10km._ L J

reflector C and is disrupted by minor normal faults (Fig. 5),most of which appear to have been steepened and overturned,or reactivated with reverse motion, during a subsequent phaseof shortening. Its eastern margin is overthrust, and Fig. 5

shows a southeast-dipping thrust with ramp-flat geometry,repeating the strong reflections at the base of seismic unit 3.Younger, lower angle fault splays and associated backthrustshave disrupted reflector A. I have speculated that this profile

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MOUTERE VALLEY . /

RICHMOND-WAKEFIELD

HALF-GRABEN

WANGAPEKASUB-BASIN

\

Fig. 9 Simplified TWT contourmap of reflector D (base of riftsequence); the TWT contour mapof reflector C in the Wangapekadistrict is included (see Fig. 8).Refer to text for discussion.

seismic profiles

, t I sochrons , in milliseconds, ofTWT from a sea level datum to themapped horizon; ticks indicatedown-slope direction200 ms contour interval

Limit of outcrop/subcrop

Faults offsetting mapped horizon— triangle on upthrown side indicates

direction of dip of reverse fault~ tab on downthrown side indicates

direction of dip of normal fault,cross-hatching shows extent offault planedextral strike-slip component

10km

crosses part of a thrust block, which moved towards thenorthwest and was apparently bounded by dextral transformfaults, which were identified on seismic profile MD87-004/6(Fig. 6) and correlate with a mapped north-south-striking faulton map sheet S19 (Coleman 1981).

Between Richmond and Golden Downs, seismic unit 3 hasa reduced thickness of c. 0.28 s TWT, or c. 420 m (Fig. 4;depth sections MD87-O01, -003, and -005 of Fig. 6) and canbe tentatively traced southwest into a narrow zone of complexblock-faulting (Fig. 8), of similar appearance to the minor

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RICHMONDWAKEFIELD HALF-GRABEN

Projected

hinge pointy m o n TMailm A 0 , 2 degrees SE • , , ,• • m i - - - - » • • " - - - - - • • • • _ _ pre-erosional levet

LINE MD87-001 •> SE

Original width of basin - 7.5 km

Maximum preserved thickness 1800 m

Estimated depth before erosion 2000 m

P=1.07

MOUTERE VALLEY- DOVE VALLEY HALF-GRABEN

LINE MD87-002 -> NE

Maximum preserved thickness 2400 m

Fig. 10 True scale cross-sections of half-grabens within theMoutere Depression; the interval velocity used was 3400 m/s, afterthe Paleocene-Eocene seismic sequence of Thrasher & Cahill(1990); P values were derived from the rotating domino model of LePichon & Sibuet(1981), where psinGj = sin80.

normal faulting at the base of the Wangapeka sub-basin.Between Richmond and Wakefield, however, seismic unit 3rests unconformably on seismic unit 4 (Fig. 4), a northeast-southwest-trending half-graben (Fig. 9), which has a maxi-mum preserved thickness of 1.1 s TWT, or c. 1800 m, and isbounded by a normal fault dipping 55°NW after depthconversion (Fig. 10). Extrapolation of semicontinuous internalreflectors within seismic unit 4 produces a hinge point, fromwhich it can be estimated that the original width of the half-graben was in excess of 7.5 km. From this approximate recon-struction, it can also be estimated that a minimum of 200 m oferosion took place prior to the deposition of seismic unit 3, toproduce the terminating reflectors within seismic unit 4.

A NNE-SSW-trending half-graben of similar dimensionsand seismic character is identified beneath the Moutere andDove valleys (depth sections MD87-001, -002, and -003 ofFig. 6; Fig. 9). Seismic profiles MD87-O01 and MD87-O02,which intersect the half-grabens, reveal that there is anunconformity within the preserved upper half of seismic unit 4(e.g., Fig. 4). This break in deposition may have developedduring a period of tectonic quiescence when the sub-basinswere already full, and was followed by renewed extensionalfaulting and basin filling.

The rotating domino model of Le Pichon & Sibuet (1981)was used to calculate ft values of 1.07 and 1.06 for theRichmond-Wakefield and Moutere valley-Dove valley half-grabens, respectively (Fig. 10). Seismic profiles MD87-001and -002 were chosen for the reconstruction because theyafforded the clearest images of the bounding normal faults.However, the calculation assumes that the sections areperpendicular to the original extension direction, which isessentially unknown in this case, although the bounding faultstrend NNE-SSW, suggesting a significant NNW-SSE

component of extension. Hence, seismic profile MD87-001 ismore likely to be in the required orientation and will thereforeprovide a more reliable y3 estimate. In addition, the south-eastern boundaries to the half-grabens appear to have beenoverthrust close to their bounding normal faults, suggestingthat these are zones of crustal weakness which were subjectedto later contractional faulting. The pre-existing normal faultsmay have been steepened during this later stage of over-thrusting, which would effectively decrease the calculated ftvalues for the earlier rift phase.

Reverse faults immediately west of the Waimea Fault,which have no surface expression but are seen on seismicprofiles MD87-OO3 and -005, dip between 55° and 65°SEafter depth conversion, and show up to 1380 m throw onseismic basement (Fig. 6). West-vergent faulting along theWaimea Fault System, loading the eastern margin of the basin,may explain the 2° southeasterly tilt of reflectors from theMoutere Gravels, which is seen on seismic profiles MD87-001, -003, and -005 (e.g., Fig. 4). The present-day dissectedsurface of the gravels apparently dips gendy towards theWaimea Fault System, which can be seen from a vantage pointat Spooners Saddle on State Highway 6. However, fromseismic profile MD87-OO4 (Fig. 6), it can be calculated thatthe base of the Moutere Gravels dips 1.6°NE, which probablyrepresents a depositional component to the true dip of1.9°ENE, since younger gravels progressively onlap thebasement surface to the south. The maximum thickness of theMoutere Gravels is c. 1.15 s TWT, or c. 1920 m, immediatelywest of the Waimea Fault System in the southeast of theMoutere Depression (Fig. 11).

DISCUSSION OF THE DEFORMATION

Timing of basin formationThe Wangapeka sub-basin was penetrated by the Tapawera-1drillhole, which drilled 700 m of Moutere Gravels (seismicunit 1) before sampling late Oligocene (late Duntroonian-Waitakian) sediments of the Sherry River and BurmeisterFormations (seismic unit 3) (Petroleum Corporation of NewZealand Ltd 1989a). The seismic profile MD87-005 (Fig. 5)shows this sequence thickening southeastwards of thedrillhole and overlapping progressively older sediments in thesoutheast. Hence, the earliest sediments of seismic unit 3 in thedeepest part of the sub-basin, with strong, laterally continuousseismic reflections, are older than late Oligocene and areprobably of late Eocene age, equivalent to outcroppingMotupipi Coal Measures and Huia Formation limestone in theWangapeka district (Fig. 2). It is not certain whether this basinshould be extended northeastwards, to connect with theMoutere valley-Dove valley half-graben: there is a similaritybetween the two sub-basins' internal reflectors and positions,immediately west of the axial zone of shortening, but theseismic data are ambiguous (Fig. 9 compares the locations ofthese basins).

If the half-grabens (seismic unit 4, Fig. 4; Fig. 9) have adifferent age and origin from the Wangapeka sub-basin(seismic unit 3, Fig. 5; Fig. 9), they could have formed duringan earlier phase of NW-SE to WNW-ESE oriented rifting inthe Late Cretaceous, associated with the opening of theTasman Sea. However, Thrasher & Cahill (1990) show noLate Cretaceous sediments in the South Taranaki Basin, so it isunlikely that there would be extensive occurrences in theMoutere Depression to the south, although isolated Late

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Fig. 11 Simplified TWT contourmap of reflectors A and B (base ofTadmor Group, late Miocene-Pliocene); the stippled patternindicates the subcrop of seismicunit 2 (Glenhope Formation).

seismic profiles

— -r— I sochrons , in milliseconds, ofTWT from a sea level datum to themapped horizon; ticks indicatedown-slope direction100 ms contour interval

Limit of outcrop

Faults offsetting mapped horizontriangle on upthrown side indicatesdirection of dip of reverse faulttab on downthrown side indicatesdirection of dip of normal fault,cross-hatching shows extent offault plane

- ^ dextral strike-slip component

A i- Monoclinal structure0 5 10kmI I i

Cretaceous terrestrial sediments have been found in the southof the Moutere Depression (Beebys Conglomerate; Johnston1983,1990). The Greville Basin west of D'Urville Island is inthe same structural location as the Richmond-Wakefield half-graben: they are both situated immediately west of the

Waimea-Flaxmore Fault System (Nathan et al. 1986;Thrasher & Cahill 1990), so may have close structuralaffinities. The Greville Basin is inferred to be of Paleocene-Eocene age (Thrasher & Cahill 1990), by comparison withseismic sections further north in the Taranaki Basin which are

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of known Paleocene-Eocene age (G. P. Thrasher pers. comm.1990). Previously, Nathan et al. (1986) inferred a LateCretaceous-Paleocene age for this basin.

By comparison, in response to Anderson's (1980) claimthat the gravity low over Stoke was caused by 1650 m or moreof sediments, Johnston (1982) inferred that these sedimentsconsisted of coal measures overlain by various marinedeposits, similar to those outcropping in Nelson City and westof the Flaxmore Fault. He suggested that these coal measureswere significantly older than those along the Waimea Fault,although no evidence supporting this proposal was cited. Healso added that the coal measures could overlie terrestrialsediments of Late Cretaceous age similar to BeebysConglomerate (Johnston 1983,1990).

In the absence of unequivocal or convincing evidence tothe contrary, I prefer to group deposits associated withextensional faulting (seismic units 3 and 4) into the same timeperiod. However, there is a contrast in geometry between thesub-basin in the Wangapeka district and the half-grabens to thenorth, in that the former is more bowl-shaped, while the latterare almost triangular in cross-section, suggesting differentmodes of formation. Minor syndepositional normal faulting atthe base of each section (Fig. 4, reflector D; Fig. 5, reflectorC), followed by inferred coal measure deposition producingstrong, laterally continuous seismic reflections, suggests thatthe basins initiated under the same tectonic regime. Someminor normal faulting west of the Waimea Fault on seismicprofiles MD87-OO3 and -005 is also interpreted to have takenplace during this phase of extension (Fig. 6). However, rapidexpansion of the northern sub-basins into half-grabens wouldsuggest that extension was more pronounced in the north ofthe Moutere Depression and may have been underway earlierin the Eocene than to the south: the oldest outcropping coalmeasures in the Moutere Depression are in the Nelson Cityarea and as fault slivers along the Waimea Fault, close to theRichmond-Wakefield half-graben (Fig. 9), and are of mid-late Eocene (Bortonian) age. Therefore, I believe that olderEocene, and possibly Paleocene, coal measures are containedwithin the concealed half-grabens.

The depocentre in the Moutere Depression probablyshifted southwards in the late Eocene and may have beenassociated with thermal subsidence of a thinned crustfollowing the earlier rifting phase. This caused the expansionof the sub-basin in the Wangapeka district (Fig. 8), but,apparently, there was little or no further subsidence to thenorth, where only a condensed sequence is preserved onseismic profile MD87-001 (seismic unit 3, Fig. 4). LateEocene-early Miocene sediments in the Wangapeka districtrecord a progressive deepening of the basin, whereas thosearound Nelson City are inferred to have been deposited inrelatively shallow water.

Timing and nature of compressive phase

Basin-wide, WNW-directed compression in the MoutereDepression probably started in the middle Miocene, since lateMiocene sediments (seismic unit 2—Glenhope Formation,and Port Hills Gravel, Fig. 2) unconformably overlie earlyMiocene or older sediments in outcrop and on seismic profiles(e.g., Fig. 5). Thrasher (1989) similarly concluded that aninitial phase of compression took place in Tasman Bay in themiddle Miocene, which he thought represented reactivation ofN-NNE-trending normal faults with reverse motion. LateMiocene terrestrial sediments (seismic unit 2) in the MoutereDepression appear to have been deposited in fault-controlled

paleovalleys (Fig. 11), contemporaneous with shorteningalong the axial zone (Fig. 5), and were gently folded prior todeposition of the Moutere Gravels (Coleman 1981; Johnston1981). Shortening along the axial zone continued into the earlyPliocene, displacing the oldest Moutere Gravels (seismic unit1) up to the southeast, but there is no apparent offset ofyounger Moutere Gravels within the axial zone (e.g., Fig. 5).However, there is much greater displacement of MoutereGravels close to the eastern margin of the basin (Fig. 11),where reverse faulting continues today, shown by activefaulting along part of the Waimea Fault System (Johnston1990). This suggests that since the early Pliocene, deformationhas been focussed away from the axial zone and along theWaimea Fault System. Significant tilting of the gravels inresponse to loading of the eastern margin suggests that upliftof basement blocks by reverse faulting on the Waimea FaultSystem has influenced the development of the depression, as aforeland basin, since at least the Pliocene. Deposition in theMoutere Depression was essentially continuous from the lateMiocene-Pleistocene; during the Pliocene-Pleistocene it wasa major depocentre for material shed off the rising SouthernAlps and channelled northwards into the depression.

Major control on the deformationFrom the northwest-southeast-oriented cross-sections drawnfrom seismic reflection profiles across the MoutereDepression (Fig. 6), shortening estimates for the base of theMoutere Gravels are less than 5% (reflector A). There are noother reflectors which can be traced the full width of the basin,and erosion of the surface below the late Miocene gravels hasremoved an unknown amount of older Tertiary rocks, so it isimpossible to quantify shortening in the sub-basins. Basementblock-faulting is inferred to have been an important control onthe formation and location of the sub-basins within theMoutere Depression. The junction between the RotoroaComplex and the Separation Point Batholith is believed tocoincide with the axial zone of shortening within thedepression, with the Wangapeka and Moutere valley-Dovevalley sub-basins located immediately west of it (Fig. 9); thethird sub-basin, the Richmond-Wakefield half-graben, islocated immediately west of the Waimea Fault System (Fig.9), which is also a basement block junction between the BrookStreet Volcanics Group to the west and the Maitai Group to theeast (Fig. 1). Both extensional and later contractional faulting,by reactivation of normal faults, has been concentrated at thesebasement block junctions, which suggests that they have actedas zones of weakness within the depression.

CONCLUSIONS

Three sub-basins, concealed beneath late Miocene-Pleistocene sediments in the Moutere Depression, have beenidentified by seismic interpretation techniques. They aresubparallel to Tertiary basins of the West Coast region. Thelate Eocene-early Miocene sub-basin in the Wangapekadistrict extends over 30 km and has a maximum preservedthickness of c. 1860 m. The other sub-basins are ?Paleocene-Eocene half-grabens hi the north of the depression, with steep,northwest-dipping, bounding normal faults, and have maxi-mum preserved thicknesses of c. 1800 m and c. 2400 m. TheRichmond-Wakefield half-graben was more than 7.5 km widebefore a minimum estimate of 200 m of sediment was erodedprior to late Eocene deposition. Minor syndepositional

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faulting is present at the base of the Tertiary sequence; synriftsediments are confined to the north of the depression, but thedepocentre shifted south in the late Eocene.

The sub-basins' southeastern boundaries were overthrust,in places, in the middle Miocene during WNW-directedcompression, by the Kawatiri and Waimea Faults, and pre-existing faults were steepened. The Kawatiri Fault Zone istoday concealed beneath late Miocene-Pleistocene gravels inthe Moutere Depression, but can be traced northeastwards tothe coast on seismic reflection profiles, and continues offshorein Tasman Bay. It separates the main bodies of the Rotoroaand Separation Point plutons and represents an axial zone ofshortening, which is possibly broken into NNW-directedthrust segments, bounded by dextral transform faults, andalong which there is an estimated 200-300 m of relief on lateMiocene sediments. Such basement block faults were animportant control on the development of the MoutereDepression, since basement sutures acted as zones ofweakness during extension and compression.

Late Miocene terrestrial sediments were deposited inpaleovalleys, contemporaneous with shortening. Shorteningon the Kawatiri Fault Zone continued into the early Pliocenebut since then has been focussed along the Waimea FaultSystem. Buried reverse faults west of the Waimea Fault dipmoderately southeast and show up to c. 1230 m dip-slipdisplacement on seismic basement. West-vergent faulting andloading of the eastern margin explains the 2°SE tilt of theMoutere Gravels. The Waimea Fault System has been themajor influence on the development of the depression as aforeland basin since at least the Pliocene; however, during thistime, less than 5% shortening has been accommodated acrossthe width of the depression. Thus, during the late Miocene-Pliocene, deformation in the Moutere Depression waslocalised and not very intense. Deposition was essentiallycontinuous from the late Miocene through to the Pleistocenewhen the area was a major depocentre for material shed off therising Southern Alps, covering the depression in a blanket ofgravel, which has obscured the early Tertiary geology.

ACKNOWLEDGMENTS

This work was carried out at Victoria University of Wellington,under the supervision of R. I. Walcott and A. S. Jayko, with fundingfrom the New Zealand Vice Chancellors' Committee and theResearch School of Earth Sciences at Victoria University. I wouldlike to thank Dick Walcott, Dan Bishop, and Glenn Thrasher forreviewing this paper in its initial form. The manuscript was laterimproved by helpful comments from Helen Anderson and ananonymous reviewer, to whom I am also grateful. I would also liketo thank Lynn Ellis of GECO for her efficiency in copying open-filedata.

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reinterpretation of seismic reflection data from the moutere depression, nelson region, south...

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