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Aggradation surfaces and implications for displacement rates along the Wairarapa Fault, southern North Island, New Zealand
CATENA
vol. 18, p. 453 469
Cremlingen 1991 ]
A G G R A D A T I O N SURFACES A N D IMPLICATIONS FOR D I S P L A C E M E N T RATES A L O N G THE WAIRARAPA FAULT, S O U T H E R N N O R T H ISLAND, NEW ZEALAND R.H. Grapes, Wellington Summary Three Last Glaciation aggradation surfaces, Waiohine (11_+1 ka), Rata (c30 ka) and Porewa (c60 ka), are displaced by the Wairarapa Fault, Wairarapa Valley, southern part of the North Island of New Zealand. Dextral displacements of 125+5 m (Waiohine), 385+5 m (Rata) and c770-800 m (Porewa) indicate an average uniform dextral slip rate along the Wairarapa Fault of 11.5 mm/yr during at least the last 60 ka. This rate accounts for a third of the total oblique convergence rate of 33 mm/yr for the Australian and Pacific Plates along the Hikurangi Trough to the east of the North Island. Vertical displacement rates along the Wairarapa Fault are variable and range from 0.1 to 1.9 mm/yr. Where the Wairarapa Fault cuts aggradation gravels on greywacke basement this variation is controlled by growing bulges developed on the upthrown N W side as a result of left sidesteps of the fault trace. Where the fault cuts gravels overlying Tertiary rocks it is sometimes upthrown to the SE and at one locality the aggradation surfaces indicate the possibility ISSN 0341-8162 ~)1991 by CATENA VERLAG, W-3302 Cremlingen-Destedt,Germany 034t 8162/91/5011851/US$2.00 + 0.25 ( ' A I h N A An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
of two reversals of vertical movement within the last 60 ka. Reversals in throw may be related to differential movement on fault clusters at depth that are located near or on growing anticlines in Tertiary strata above upbulged greywacke undermass. The pattern of long term (<100 ka) uplift is a general northeasterly decrease along the Wairarapa Fault. This is consistent with the uplift along the fault during the last earthquake in 1855 and, on a larger scale, the NE divergence of the Wairarapa Fault from the axes of greatest uplift of the Rimutaka and Tararua Ranges to the west.
1
Introduction and geologic setting
The southern part of the North Island of New Zealand is a c200 km wide actively forming plate boundary between the obliquely converging Australian and Pacific Plates coming together at a relative velocity of c50 mm/yr. Seismic data (KAYAL 1984, ROBINSON 1986) indicate that the region is underlain by the gently dipping Pacific Plate at depths of between 15 and 25 km and that this plate is being subducted along the Hikurangi Trough located some 100 km to the east GEOMORPHDLOGY
Grapes
454
(fig. 1). Deformation of the Australian Plate boundary zone can be separated into a belt of NNE-striking dextral faults located to the west of the Wairarapa Valley and a fold-thrust belt extending east from the Wairarapa Valley through a late Cenozoic accretionary wedge to the Hikurangi Trough (VAN D E R LING E N 1982, DAVEY et al. 1986), (fig. 1). Within the Wairarapa Valley and further eastwards, deformation of Tertiary and Quaternary sediments by folding and faulting has been documented by G H A N I (1978), LAMB & VELLA (1987) and CAPE et al. (1990). Shortening rates determined from growing fold structures (at least for the last 100 ka, LAMB & VELLA (1987)) are in good agreement with the geodetic shortening strain rate observed over the last 50 years (WALCOTT 1984) of c38 mm/yr normal to the NW-SE trend of the fold structures, The shear component of the observed geodetic strain rate in the southern part of the North Island is c33 mm/yr. This is taken up by the dextral strike-slip zone of which the Wairarapa Fault is the most easterly and the most active (fig. 1). In this paper surface deformation of Last Glacial fluvial aggradation surfaces along the western edge of the Wairarapa Valley is described in terms of slip rates along the Wairarapa Fault over the last 60 ka or so.
2
The Wairarapa Fault
A map showing the extent of the Wairarapa Fault is given in fig. 2. For 13 km from the coast at Palliser Bay to the southern end of Lake Wairarapa, the fault lies mainly within greywacke of the Rimutaka Range. The fault is not
('AIENA
a well defined trace but a 100 300 m wide shear zone of highly pulverized greywacke. Offshore the position of the fault is marked by an 11 km long, 360 mhigh submarine scarp along the west side of Palliser Bay. Seismic reflection data (CARTER et al. 1988) indicate that the fault can be traced across Cook Strait for at least another 16 km SW from Turakirae Head. For 32 km N E of the southern end of Lake Wairarapa the Wairarapa Fault forms the boundary between greywacke of the Rimutaka Range and the Holocene and latest Pleistocene river gravels of the Wairarapa Valley. This is by far the best defined part of the fault as shown in pholo 1. Although mapable as a continuous line on scales of 1:250,000 or smaller the fault trace is actually a series of straight segments separated by left-sidesteps or stepovers of up to 250 m. Because the sense of strike-slip movement on the fault is dextral each left-stepped offset is marked by a bulge or push-up on the upthrown side of the fault as schematically illustrated in fig. 3A. Five bulges occur along this section of the fault. Their locations are shown in fig. 2 and their respective morphologies are shown in fig. 3B. For the remaining 25 km to Mauriceville in northern Wairarapa (fig. 2), the Wairarapa Fault cuts young Cenozoic strata with late Pliocene coquina limestone at places on the NW side and late Miocene to early Pliocene mudstone on the SE side. Along this section there are several places where the fault is upthrown on the SE side, not on the NW side as is normal further south (fig. 2). Beyond Mauriceville the line of the fault is uncertain but there are several recently active, subparallel fault traces within late Miocene mudstone (LENSEN 1968,
AnlnlerdisciplinaryJournalot'S()ll
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HYDR()LOG~
(itl)M()Rt'H()tOG'I
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
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ISLAND
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4
i '1 '
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Fig. 1: Map showing the boundary zone between the Pacific and Australian plates, southern North Island, New Zealand. 1970) that may join the Wairarapa Fault to the Mangatoro Fault mapped by ONG L E Y (1943).
3
Fluvial aggradation surfaces: Distribution and age
Three main gravel surfaces (Waiohine, Rata and Porewa in order of increasing age) are recognized in the Wairarapa Valley and represent periods of fluvial aggradation during the Last Glaciation (VELLA 1963, PALMER 1984). The periods of aggradation and coeval loess deposition followed by periods of degradation are correlated with climatic cooling and warming respectively (e.g. COWIE & M I L N E 1973, M I L N E & SMALLEY 1979, P A L M E R 1984, P A L M E R & V U C E T I C H 1989) and the eustatic sea level curve over the last 100 ka (CHAP-
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PEL & S H A C K L E T O N 1986). The relationship is summarized in fig. 4 along with a suggested flood plain displacement curve for the three aggradation phases in the Wairarapa Valley.
3.1
Waiohine surface
This is the youngest and most extensive aggradation surface in the Wairarapa Valley (VELLA 1963). It is characteristically loess-free (fig. 5A) and is formed by coalescing fans mainly centered on the Tauwharenikau, Waiohine, Waingawa and Ruamahanga Rivers (fig. 5B). Towards the east and south the surface is covered by Holocene silt. The surface also slopes more steeply to the east and south than the present day river flood plains and the size of surface boulders decrease more rapidly away from the Rimutaka Range than they do in the
HYI2,ROLO(;~ G E O M O R P H O L O G Y
456
Grapes
NORTH ISLAND -aO °
Area of Map~
LMc
,/
SOUTI-I
#c
"v
B Bulge upthrown aid downthrown
~ / L a k e Onoke
/ Turakir~e
/ Palliser
F-Featherston G-Greytown C-Carterton M-Masterton Mc-Mauriceville
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/
/
///
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0 I
I7~o 15'
I
10 i
175* 30' I
i
20kin
175°[45
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Fig. 2: Map showing the extent of the Wairarapa Fault. The double dashed lines near the southern on-land extension of the fault indicate a wide crush zone. C r o s s h a t c h e d a r e a = L a t e Triassic - J u r a s s i c g r e y w a c k e . D o t t e d a r e a = L a t e M i o c e n e - Pleistocene c o v e r beds. T h e n a m e s , R i m u t a k a R a n g e a n d T a r a r u a R a n g e , are a l i g n e d a l o n g the axes o f m a x i m u m uplift o f the respective r a n g e s (after G H A N I 1978).
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(_;EOMORPHOLOGY
A
/
Wa
~,~E"~'TO.
%
o
,l°,l,llL,~,liJi
ugn.,.~
--
Spot height
Trench
-- Porewa aggradatlon surface
0
500
1000 R B Mira
Hill slope (hatched] against aggradation gravels.
Terrace riser or fault scarp, with "u" a n d ' d " fault, without - terrace riser
Marshy or s w a m p y and a b a n d o n e d river channel.
Steep and high
Holocene fan.
Po
Fla -- Rata aggradation surface
W a -- Waiohine aggradation surface
,L 9 4 m
Tr
'~'...~.~-- G r o u n d r e n t s
N
t
Fig. 3: A. Diagrams illustrating the./brmation o/'a bulge by a l~/t step-over along a dextral fauh. B. Detailed maps of bulges developed on the upthrown side o/" the Wairarapa Fault. Location o[" bulges (numbered 1 to 5) is given in fig. 2.
,,
.//
204m •
Subsided wedge
B
i-.-a
4~
i
i
I
i
i
Grapes
458
Inferred age of Waiohine Surface
Year
Author(s)
Comments
1952
TE P U N G A
3 ka
14C date of log within youngest aggradation terrace, Rangitikei Basin
t955 1956
WELLMAN LENSEN et al.
10 ka 10 ka
End of Last Glacial and age of youngest aggradation surface throughout New Zealand
1962
COWIE & WELLMAN
>10 ka
14C dated logs o f T E P U N G A deposited by flash flood. Age estimated from soil profiles and loess accumulation rates in Rangitikei Basin
1971
LENSEN & VELLA
a. 35 ka b. 20 ka or younger
Waiohine Surface at Waiohine River a. LENSEN's age represents last stadial of Last Glacial because second to last Stadial of Last Stadial had only "weak" effects in North Island b. V E L L N s age correlation with Last Stadial of Last Glacial as Waiohine Surface is overlain by Holocene sediments around Lake Wairarapa and lacks loess cover
t972
WELLMAN
9.6 ka 11.5 ka
Ages given from correlation between dated uplifted beach ridges at Turakirae Head and five youngest displaced degradation terraces at Waiohine River. Average rate of vertical and dextral faulting assumed to be uniform
1973
SUGGATE & LENSEN
18 ka
Criticism of W E L L M A N ' s 1972 correlation. Age based on 14C ages of "correlative" aggradation surfaces in South Island
1973
MILNE
12 ka
Age of youngest (Ohakean 3) terrace (same surface dated by TE P U N G A and C O W I E & W E L L M A N ) based on rate of river downcutting, Rangitikei Basin
1979
MILNE & SMALLEY
9.5+0.1 ka
14C age for top of loess that blew from youngest aggradation surface. Base of loess dated at 26.3_+0.8 ka, Rangitikei Basin
1980
HUBBARD & NEALL
11.8_+0.15ka
14C age for alluvial fan, southern Ruahine Range
1986
M A R D E N et al.
10.3_+0.1ka 12.9_+0.2 ka
Various 14C ages of wood from a few metres below aggradation surface, 2 k m N W of Woodville. Gravels derived from southern Ruahine Range
1987
TOMPKINS
8.5_+0.12 ka
14C age of peaty material near base of peat overlying Waiohine Surface south of Greytown
1988
GRAPES & WELLMAN (unpub. data)
12.5 ka
14C age of tree roots in growth position at top of loess that is covered by fan gravels
1990
SUGGATE
16 ka
Age inferred from constant rate of loess accumulation from 35.5 ka with respect to position of Kawakawa tephra layer dated at 22.5 ka
Data for the age of the Waiohine or equivalent aggradation surface, southern North Island. T a b . 1:
CA]'ENA
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Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
459
Photo 1: Oblique aerial photo looking southwest across the trace of the Wairarapa
Fault fi'om Featherston (top left corner) to Waiohine River (bottom). The extensive .Hat surface displaced by the .fault is the Waiohine aggradation surface. Rimutaka Range in background.
present day rivers (fig. 5C). These features reflect a graded profile adjusted to low eustatic sea level that was over 100 m lower than present (fig. 4). The Waiohine Surface is correlated with similar loess-free surfaces elsewhere in the southern part of the North Island (e.g. M I L N E 1973, K A E W Y A N A 1980). Because of it's wide extent the Waiohine and equivalent surfaces provide an important time plane for determining Holocene deformation rates. De-
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bate over the age of the surface has gone on for the last 30 years (tab. 1). Based on the available evidence an age of I l ±1 ka for the Waiohine Surface is adopted in this paper. 3.2
Rata surface
The Rata Surface is best developed in the northern and eastern parts of the Wairarapa Valley (VELLA 1963, P A L M E R 1984). The surface is covered by 1-2 m of loess that was blown off the
GEOMORPHOLOGY
Grapes
460
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/3
Fig. 4: Ages o1" Wairarapa aggradation surfaces, respective loesses and flood plain displacement curve for aggradation periods and degradation periods related to sea-level curve (after CHAPPELL & SHA CKELE 7DN 1986).
"7 k
Sea l e v e l ( u r v u > ,![I -
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Waiohine Surface. Of particular significance is the occurrence of the Kawakawa Tephra (dated at 22.5 ka by WILSON et al. 1988) about a third of the way up from the top of the aggradation gravels. This indicates that the Rata Surface represents the culmination of the second to last fluvial aggradation c50 ka to c30 ka ago (fig. 4).
3.3
P o r e w a surface
The Porewa Surface (VELLA 1963, PALMER 1984) is also best preserved in northern and eastern Wairarapa. It CA]ENA
Wa = Waiohine; Ra= Porewa.
Rata:
Po =
is typically dissected by streams and is mantled by two loess units derived from both the Rata and Waiohine surfaces respectively and typically separated by a paleosol. Porewa aggradation gravels overlap the 80 ka marine bench (PALMER 1984) and loess derived from the Porewa surface is found on marine benches of 80 ka and older ( G H A N I 1978) indicating that the aggradation phase is younger than 80 ka. The top of the Porewa loess is weathered to a paleosol that contains residues of andesitic tephras erupted about 60 ka ago ( M I L N E 1973,
A n Interdisciplinary Journal of S O I L S C I E N C E
HYDROJ
OGY
(_?FOMORPHOLO(~Y
,
2 ~ ,
~
~ ~
,
s ,
,
s ,
Wa~hir~ R~vm Bed
,
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~0k surface. N u m b e r s in s q , a ~ = stone diameters in the ~ t - - d a y ~ r~ bed. Ra = Rata surface. Cross hatched area = gceFv~acke~
soo ....
river
7ig. 5: Data for the stony appearance (A), morphology and distribution (B), and boulder sizes (C) of the ;Vaiohine surface, Wairarapa Valley.
[~l
-:~w. ~ o
~aximun Ve~'tlCMd t s p t ~ t atm,tg ~ ~tttt$ are:~ n Fault = 5.Zm: M a ~ P ~ o n Fault ~ 8~Om: ~okonui Fault = 6.8m {LENSEN 1968). Ra = Rata surface, 'rcns h a t c h e d area = greywa~.k~ of tim Rimutaka Range.
G - Greylown: = R u a m a h a n S a Riv~x, F = ~ m ~ '= Car~rtor~ M = Masr~ton. Fa~t~ I ~ b A n $ b c x a ~e Wairar~pa Fault are normal faults ~ are m o l l y Ownthrown tO t l ~ s o u t h , F r o m s o t # ~ to t ~ r t h h ~ al~-
t a p ~ t~e w ~ e ~ce wi~ 11~ ¢ ~ m u r s . w ~ Tauwharmnllcau R i v e . W h ~ tegaiolg~ Rive~;
462
Grapes
17 130
24. - - -
_E × ~ ¢ o
120
B5
110
B2
B~
4_1 I
2
B4
R 25
/t [2627 R
5
FEATHERSTON
•
Fig. 6: Plot o f dextral and vertical displacement o f the Waiohine surfi~ce along a 57 km section o f the Wairarapa Fault. Numbered localities together with displacement values are listed in tab. 2. B1, 2 etc. refer to bulges developed on the upthrown side o f the fault. R = reversal in upthrown side, i.e. to the southeast, otherwise upthrow is to the northwest. P A L M E R & V U C E T I C H 1989). The Porewa agradation is therefore correlated with the sea level low between c80 and c60 ka (fig. 4).
4
Displacement of aggradation surfaces along the Wairarapa Fault
Vertical and dextral displacements of the 11_1 ka Waiohine surface along a 57 km length of the Wairarapa Fault are listed in tab. 2 and the displacements plotted in fig. 6. Displacement data for the c30 ka Rata and c60 ka Porewa surfaces are listed in tab. 3.
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4.1
Dextral displacement
Dextral displacement of the three aggradation surfaces is recorded from five localities. The Waiohine surface is displaced by 125+5 m as determined from offset channels (Locality 11), an offset riser (Locality 24a) and offset hill slopes at the back of the surface (Localities 6, 17, 19), (tab. 2). GRAPES & WELLMAN (1988) estimated that the total cumulative Holocene dextral displacement of 125-t-5 m is the result of at least ten large (cM8) earthquakes with about 12 m of dextral movement for each event, the last occurring in 1855. At Waingawa River the .riser of the Rata surface has been dextrally offset
An Interdisciplinary Journal of SOIL SCIENCE
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GEOMORPHOLO(]Y
R 31
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
Locality
Displaced
No.
feature
Upthrown side
Vertical
Displacement Dextral
1
Surface
NW
11.5
--
463
Dextral/ Vertical
Comments Minimum
value.
Downthrown
side has
H o l o c e n e silt c o v e r 2
Surface
NW
t4.0
--
Stepped scarp. Minimum
value.
c. 1 m silt c o v e r on d o w n t h r o w n 3
Surface
NW
11.0
--
-
4
Surface
NW
17.0
-
5
Surface
NW
12.5
.
6
Hill edge
7
Surface
NW
12.0
--
8
Surface
NW
11.0
--
9
Surface
NW
13.0
--
10
Surface
NW
7.0
--
11
4 channels
NW
18.5
c. 125_+5
12
Surface
NW
19.0
--
.
.
.
c. 120
side
-.
.
--
Protruding
halfscarp
---7.0
--
13
Surface
NW
20.5
--
14
Surface
NW
7.5
--
--
--
15
Surface
NW
10.5
--
16
Surface
NW
11.5
--
--
17
Hill edge
126_+6
--
Protruding halfscarp
18
Surface
NW
3.0
--
--
--
--
19
Hill edge
. . . . . .
20
Surface
SE
2.0
21
Surface
NW
6.0
--
--
--
22
Surface
NW
5.0
--
--
--
23
Surface
SE
0.1
--
--
--
24a
Riser
SE
1.0
25a
Surface
SE
6.5
--
26
Surface
SE
3.0
.
27
Surface
SE
3.0
--
--
28
Surface
NW
1.3
--
--
--
29
Surface
0
--
--
--
30
Surface
SE
1.0
--
--
--
31
Surface
NW
2.6
--
--
-
Tab.
Tab.
(AIENA
2:
110±5
--
.
.
124_+5
Minimum
124.0
--
-.
.
value
.
.
-. --
Displacement data for the Waiohine Surface along the Wairarapa Fault.
Locality
Displaced
No.
feature
24b
Surface riser
24a
Surface
25b
Hill slope
?SE
--
3"
-
.
Upthrown side
Displacement
Dextral/
Vertical
Dextral
Vertical
SE
7.0
385_+5
55.0
NW
18.0
-c. 8 0 0
Comments Rata surface Porewa surface ? Porewa surface
Displacement data for Rata and Porewa surfaces along the Wairarapa Fault.
An Imcrdisciplinar 3 Journal of SOIL SCIENCE
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Grapes
464 soo-~
/
600 t
q.~x / 4D~\X)
400
/Y
-
E 200 L2
÷ Wa
0
~1855 EQ 1=0
210
3r0 410 Age (x103yrs)
510
6~0
7~0
Fig. 7: Dextral slip rate on the Wairarapa Fault Ji'om displacements of Waiohine,
Rata and Porewa aggradation surfaces. Surface ages are from fig. 4, by 385+5 m (Locality 24b, tab. 3) but there are no features that record dextral displacement of the Porewa surface. By extrapolating rates derived from younger surfaces, the c60 ka Porewa surface should be displaced about 770 m. About 7 km N E of Waingawa River a loess mantled hill that may be of Porewa age has been displaced about 800 m (Locality 25b, tab. 3). Dextral displacements for the three surfaces are plotted against their inferred ages in fig. 7 and indicate an average uniform dextral slip rate for the Wairarapa Fault of 11.5 mm/yr over the last 60 ka.
4.2
Vertical displacement
South-east of the position where there is a 10 ° change in the strike of the Wairarapa Fault (fig. 2), variation in the amount of vertical displacement of the
Waiohine surface (fig. 6) appears to be related to the distribution of growing bulges along the upthrown side of the fault. From fig. 6 it can be seen that the amount of throw increases towards the SW end of each bulge. Three of the bulges (Nos. 1, 4 and 5) provide information for determining rates of deformation of the Waiohine and Porewa surfaces. Bulge 4 immediately south of the Waiohine River (fig. 3B) is composed of the Waiohine aggradation gravels and is the result of a 110 m left-sidestep on the Wairarapa Fault. The top of the bulge is now 20 m above the surrounding undeformed surface indicating an average uplift rate of 1.8 mm/yr. This value is similar to the maximum uplift rate of the undeformed Waiohine surface on the NE side of the Waiohine River (Localities 11 13; tab. 2) with an uplift rate of 1.7-1.9 mm/yr. Minimum vertical dis-
CAPENA An Interdisciplinary Journal of SOII SCIENCE HYDROLOG~t (]EOIMORPHIJIO(iY
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand placements of the Waiohine Surface are 7.0 m and 7.5 m. Both localities are at the NE ends of bulges 4 and 5 respectively (Localities 10 and 14, tab. 2) and indicate average uplift rates of about 0.6 mm/yr. Bulge 1, south of Featherston, has developed from a 180 m left-sidestep on the fault (fig. 3B). The surface of the bulge is composed of fluvial gravel of inferred Waiohine surface age and slope washed solifluxion debris that mantles a core of sheared greywacke exposed along two stream gorges that cut through the bulge. The maximum height of the bulge is about 25 m above the same surface to the west of the bulge and about 45 m above the silt-covered downthrown side of the Waiohine surface to the east. The 25 m maximum height difference implies an uplift rate of 2.3 mm/yr, twice the uplift rate of 1.(~1.3 mm/yr for the undeformed surface further to the SE (Localities 1, 2, tab. 2). The two streams that cut the bulge are older and antecedent. They are dextrally displaced by c260 m. Assuming a uniform dextral slip rate of 11.5 mm/yr the 260 m indicates 22 ka of cumulative offset. If the bulge began to grow at this time the uplift rate would reduce to 1.1 mm/yr, in agreement with the lower rate for the undeformed Waiohine surface. Vertical deformation of the Porewa surface occurs in the vicinity of Bulge 5 fig. 3B, NE of Waiohine River, that is the result of a 190 m left-sidestep on the fault. The bulge is composed of a core of highly sheared greywacke covered by loess and solifluxion material that can be traced SW to merge with an undeformed but dissected Porewa surface preserved of the upthrown side of the Wairarapa Fault NE of Waiohine River. The top of the bulge is 50 60 m above this surface indicating that it could have formed with
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an average uplift rate of about 1 mm/yr over the last 60 ka. Where the trace of the Wairarapa Fault cuts late Cenozoic rocks that underlie aggradation gravels in river valleys, the direction of throw changes from NW to SE at several places (tab. 2, 3). At Waingawa River the Waiohine and Rata surfaces are upthrown to the SE and the Porewa surface to the NW (photo 2). This indicates at least one reversal in the sense of vertical movement over the last 60 ka along the Wairarapa Fault. The very small throw of only 1 m for the Waiohine surface compared with the 7 m throw for the Rata surface suggests the possibility that another reversal has occurred sometime in the last 11 ka and that successive Holocene earthquakes have diminished a previously greater vertical displacement of both surfaces. Where the upthrow of the Waiohine surface is to the NW (Localieties 21, 22, 28, 31 ; tab. 2), uplift rates are low and range from 0.1-0.5 mm/yr.
4.3
Dextral/vertical ratio
At Waiohine River the average dextral/vertical ratio for the Waiohine surface is about 6:1. At Waingawa River the Waiohine surface ratio is 1124:1; for the Rata surface the ratio is 55:1 and for the Porewa surface (assuming 770 m dextral displacement) the ratio is 39:1. The apparent increase in the dextral/vertical ratio since c60 ka BP at Waingawa River has been caused by the reversal of the upthrown side of the fault.
5
Discussion
Displacement of aggradation surfaces extending back to 60 ka along a 57 km length of the Wairarapa Fault indicate; GI~OMORPHOLOGY
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Photo 2: Oblique aerial photo looking northwest across the Wairarapa Fault showing dextral and vertical offsets of the Waiohine ( Wa), Rata ( Ra) and Porewa ( Po ) surfaces at Waingawa River. The Waiohine and Rata surfaces are upthrown to the southeast and the Porewa surface is upthrown to the northwest. Arrows indicate the trace of the Wairarapa Fault. Foothills of the Tararua Range in background. Photo: Lloyd Homer, New Zealand Geological Survey.
1. a uniform dextral 11.5 mm/yr, and
slip
rate
of
2. highly variable vertical uplift rates ranging from 0.6-1.9 mm/yr due to the development of surface and possibly subsurface bulges along the fault.
CAI'ENA
The uniform dextral rate is consistent with a uniform rate of plate convergence at least over the last 100 ka or so. The Wairarapa Fault dextral rate is high and accounts for about one third of the 33 mm/yr oblique convergence component of the Australian and Pacific Plates.
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Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
The uplift pattern in the time <100 ka has been complicated by surface bulges along the fault. Stepovers of the type that produce bulges appear to be fundamental features of strike-slip fault systems (e.g. A Y D I N & N U R 1985) but their genesis remains an unsolved problem. Data from Bulge 4 at Waiohine River indicate that the bulges may be of shallow origin. If the amount of gravel and underlying greywacke being upbulged is calculated from the offset and the dextral rate and then compared with the volume of the 20 m high bulge at Waiohine River a thickness of about 60 m is all that is required to form the bulge. If the effect of the growing bulges on vertical displacement is ignored then there appears to be a general decrease in the amount of uplift, or conversely an increase in the dextral/vertical ratio, northeastward along the Wairarapa Fault, at least for the Waiohine surface which provides sufficient data. This is consistent with a N E decrease in the amount of uplift along the fault that occurred during the 1855 earthquake (and probably previous earthquakes), i.e. from 2.9 m at Turakirae Head, Palliser Bay, to c0.3 m near Mauriceville a distance of about 100 kin. On a larger and longer time scale the NE decrease in vertical displcement might be related to the N E divergence of the Wairarapa Fault from the axes of greatest uplift for the Rimutaka and Tararua Ranges (fig. 2). Where the axis of the Rimutaka Anticline intersects the coast near Turakirae Head the uplift rate along the submarine extension of the fault determined from the 6.5 ka Holocene bench is 4 mm/yr (WELLM A N 1967). This is twice the rate at Waiohine River (cl.8 mm/yr), 60 km inland, and about eight times the rate for the Waiohine Surface 17 km further to
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the N E on the SW side of Waingawa River (0.5 mm/yr, Locality 22; tab. 21. An increase in the amount of vertical displacement of the Waiohine Surface towards the SW end of the bulges suggests propagation of successive earthquake uplifts along the Wairarapa Fault proceeded in the N E direction, as was apparently the case for the last earthquake in 1855. Reversals in throw of aggradation surfaces along the Wairarapa Fault may be related to bulges developing in basement rocks and are restricted to those areas where the fault cuts gravels that overlie relatively soft Tertiary rocks. The trace of the Wairarapa Fault is generally less clear where it cuts Tertiary rocks than where it cuts gravel surfaces on greywacke bedrock and it often appears as multiple, discontinuous subparallel traces that have variable senses of throw. A seismic reflection profile across the Wairarapa Fault (near Locality 20) by CAPE et al. (1990) shows that the fault is one of several subsurface faults arranged in a high angle "flower" structure (CAPE & WELLS 1988) associated with a Tertiary anticline above updomed greywacke basement. Reversals in throw might therefore be caused by differential movement on one or more faults in such a cluster during growth of anticlinal structures adjacent to the Wairarapa Fault.
Acknowledgements Many thanks are due to Professor Paul Veila and Dr. Mike Crozier of the Research School of Earth Sciences, Victoria University of Wellington, for helpful comments that clarified several parts of the manuscript. During the field work Professor Harold Wellman pro-
GEOMORPHOLO(kY
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v i d e d s t i m u l a t i n g c o n v e r s a t i o n w i t h res p e c t to t h e f o r m a t i o n a n d a g e o f a g g r a dation surfaces and aspects of faulting along the W a i r a r a p a grams were draughted
Fault.
The dia-
by R o b i n
Mita
a n d h e r e x p e r t i s e is g r a t e f u l l y a c k n o w l edged. References
GRAPES, R.H. & WELLMAN, H.W. (1988): The Wairarapa Fault. Victoria University of Wellington, Geology Board of Studies Publication 4, 55 pp. HUBBARD, C.B. & NEALL, V.E. (1980): A reconstruction of Late Quaternary erosional events in the West Tamaki River catchment, southern Ruahine Range, North Island, New Zealand. New Zealand Journal of Geology and Geophysics 23, 587 -593.
ADYIN, A. & NUR, A. (1985): The types and rates of stepovers in strike-slip tectonics. Society of Economic Paleontologists and Mineralogists, Special Publication 37, 35~44.
KAEWYANA, W. (1980): Late Quaternary alluvial terraces and their cover bed stratigraphy. Eketahuna and Pahiatua Districts, New Zealand. Unpublished MSc thesis, Victoria University of Wellington.
CAPE, C., LAMB, S.H., VELLA, P., WELLS, P.E. & WOODWARD, D.J. (1990): Geological structure at Wairarapa Valley, New Zealand, from seismic reflection profiling. Journal of the Royal Society of New Zealand 20, 85 105.
KAYAL, J.R. (1984): Microseismicity and tectonics at the Indian/Pacific plate boundary: southeast Wellington province, New Zealand. Geophysical Journal of the Royal Astronomical Society 77, 567-592.
CAPE, C., WELLS, P.E. (1988): Seismic expression of the Wairarapa Fault system.Abs. Workshop on the active deformation of the Wellington region, 19 20 September 1988, Wellington, Geophysics Division, Department of Scientific and Industrial Research.
LAMB, S.II. & VELLA, P. (1987): The last million years of deformation in part of the New Zealand plate-boundary zone. Journal of Structural Geology 9. 877 891.
CARTER, L., LEWIS, K.B. & DAVEY, F. (1988): Faults in Cook Strait and their bearing on the structure of central New Zealand. New Zealand Journal of Geology and Geophysics 31, 431-446. CHAPPEL, J. & SHACKLETON, N.J. (1986): Oxygen isotopes and sea level. Nature 324, 137 140. COW1E, J.D. & MILNE, J.D.G. (1973): Maps and sections showing the distribution and stratigraphy of North Island loess and associated cover deposits, New Zealand. 1:1,000,000. New Zealand Soil Survey Report 6. COWIE, J.D. & WELLMAN, H.W. (1962): Age of the Ohakea terrace, Rangitikei River. New Zealand Journal of Geology and Geophysics 5, 617 619 DAVEY, F.J., MAPTON, M., CHILDS, J., FISHER, M.A., LEWIS, K.B. & PETTINGA, J.R. (1986): Structure of a growing accretionary prism, Hikurangi Margin, New Zealand. Geology 14, 663 666. GHAN1, M.A. (1978): Late Cenozoic vertical crustal movement in the southern North Island, New Zealand. New Zealand Journal of Geology and Geophysics 22, 117 126.
CAIENA
LENSEN, G.J. (1968): Sheet N 158 Masterton. Late Quaternary map of New Zealand, 1:63,360. New Zealand Department of Scientific and Industrial Research, Wellington, New Zealand. LENSEN, G.J. (1970): Sheet N 153 Eketahuna. Late Quaternary map of New Zealand, t:63,360. New Zealand Department of Scientific and Industrial Research, Wellington. New Zealand. LENSEN, G.J., STEVENS, G.R. & WELLMAN, H.W. (1956): The earthquake risk in the Wellington district. New Zealand Science Review 14, 131 135. LENSEN, G.J. & VELLA, P. (1971): The Waiohine River faulted terrace sequence. Royal Society of New Zealand Bulletin 9, 117 119. MARDEN, M., PAINTIN, I.K., LEES, C.M. & NEALL, V.E. (1986): Woodville neotectonics and Quaternary stratigraphy field trip. Geological Society of New Zealand 16th Annual Conference Field Trip Quides. Massey University, Palmerston North, B3-1, B3-30. MILNE, J.D.G. (1973): River terraces in the Rangitikei Basin. New Zealand Soil Bureau maps 142/1, 142/2, 142/3, 142/4. Wellington. New Zealand Department of Scientific and Industrial Research.
A n Interdisciplinary lournal of S O I L S C I E N ( E
tIYDROLO(JY
(i[IIMI)RPHOLO(]Y
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
M1LNE, J.D.G. & SMALLEY, l.J. (1979): Loess deposits in the southern part of the North Island of New Zealand : an outline stratigraphy. Acta Geologica Aeademica Scientarium Hungaricae 22, 192-2-4. ONGLEY, M. (1943): The trace of the 1855 earthquake. Transactions of the Royal Society of New Zealand 85, 205 212.
WELLMAN, H.W. (1972): Rate of horizontal fault displacement in New Zealand. Nature 23% 275 277. WILSON, C.J.N., SWITSUR, V.R. & WARD, A.P. (1988): A new 14C age for the Oruanui (Wairakei) eruption, New Zealand. Geological Magazine 125, 297 300.
PALMER, A.S. (1984): Quaternary geology of Wairarapa. Geological Society of New Zealand Miscellaneous Publication 31 B, 20..59. PALMER, A.S. & VUCETICH, C.G. (1989): Late Glacial loess and early Latc Glacial vegetation history of Wairarapa Valley, New Zealand. New Zealand Journal of Geology and Geophysics 32, 499 514. ROBINSON, R. (1986): Seismicity, structure and tectonics of the Wellington region, New Zealand. Geophysical Journal of the Royal Astronomical Society 87, 379-409. SUGGATE, R.P. (1990): Kawakawa tephra and Ohakea loess. Geological Society of New Zealand Newsletter 88, 36-37. SUGGATE, r.P. & LENSEN, G.J. (1973): Rate of horizontal fault displacement in New Zealand. Nature 242, 518 519. TE PUNGA, M.T. (1952): Radiocarbon dating of a Rangitikei river terrace. New Zealand Journal of Science and Technology B35, 45~48. TOMPK1NS, J. (1987): Late Quaternary pollen stratigraphy, geology and soils of an area south of Greytown, Wairarapa. Geological Society of New Zealand Newsletter 77, 5 6. VAN DER LINGEN, G.J. (1982): Development of the North Island subduction system, New Zealand. In: Trench-Forearc Geology. Special Publication of the Geological Society of London 10, 259 273. VELLA, P. (1963): Upper Pleistocene succession in the inland part of the Wairarapa Valley, New Zealand. Transactions of the Royal Society of New Zealand 4, 63-78. WALCOTT, R.I. (1984): The kinematics of the plate boundary zone through New Zealand: a comparison of short- and long-term deformations. Geophysical Journal of the Royal Astronomical Society 79, 613 633. WELLMAN, H.W. (1955): New Zealand Quaternary tectonics. Geologische Rundschau 43, 248 257. WELLMAN, H.W. (1967): Tilted marine beach ridges at Cape Turakirae, New Zealand. Journal of Geosciences, Osaka City University 10, 1-6.
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Address of author: R.H. Grapes Research School of Earth Sciences Victoria University of Wellington RO. Box 600 Wellington New Zealand
(3EOMORPtIOLOGY
vol. 18, p. 453 469
Cremlingen 1991 ]
A G G R A D A T I O N SURFACES A N D IMPLICATIONS FOR D I S P L A C E M E N T RATES A L O N G THE WAIRARAPA FAULT, S O U T H E R N N O R T H ISLAND, NEW ZEALAND R.H. Grapes, Wellington Summary Three Last Glaciation aggradation surfaces, Waiohine (11_+1 ka), Rata (c30 ka) and Porewa (c60 ka), are displaced by the Wairarapa Fault, Wairarapa Valley, southern part of the North Island of New Zealand. Dextral displacements of 125+5 m (Waiohine), 385+5 m (Rata) and c770-800 m (Porewa) indicate an average uniform dextral slip rate along the Wairarapa Fault of 11.5 mm/yr during at least the last 60 ka. This rate accounts for a third of the total oblique convergence rate of 33 mm/yr for the Australian and Pacific Plates along the Hikurangi Trough to the east of the North Island. Vertical displacement rates along the Wairarapa Fault are variable and range from 0.1 to 1.9 mm/yr. Where the Wairarapa Fault cuts aggradation gravels on greywacke basement this variation is controlled by growing bulges developed on the upthrown N W side as a result of left sidesteps of the fault trace. Where the fault cuts gravels overlying Tertiary rocks it is sometimes upthrown to the SE and at one locality the aggradation surfaces indicate the possibility ISSN 0341-8162 ~)1991 by CATENA VERLAG, W-3302 Cremlingen-Destedt,Germany 034t 8162/91/5011851/US$2.00 + 0.25 ( ' A I h N A An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
of two reversals of vertical movement within the last 60 ka. Reversals in throw may be related to differential movement on fault clusters at depth that are located near or on growing anticlines in Tertiary strata above upbulged greywacke undermass. The pattern of long term (<100 ka) uplift is a general northeasterly decrease along the Wairarapa Fault. This is consistent with the uplift along the fault during the last earthquake in 1855 and, on a larger scale, the NE divergence of the Wairarapa Fault from the axes of greatest uplift of the Rimutaka and Tararua Ranges to the west.
1
Introduction and geologic setting
The southern part of the North Island of New Zealand is a c200 km wide actively forming plate boundary between the obliquely converging Australian and Pacific Plates coming together at a relative velocity of c50 mm/yr. Seismic data (KAYAL 1984, ROBINSON 1986) indicate that the region is underlain by the gently dipping Pacific Plate at depths of between 15 and 25 km and that this plate is being subducted along the Hikurangi Trough located some 100 km to the east GEOMORPHDLOGY
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(fig. 1). Deformation of the Australian Plate boundary zone can be separated into a belt of NNE-striking dextral faults located to the west of the Wairarapa Valley and a fold-thrust belt extending east from the Wairarapa Valley through a late Cenozoic accretionary wedge to the Hikurangi Trough (VAN D E R LING E N 1982, DAVEY et al. 1986), (fig. 1). Within the Wairarapa Valley and further eastwards, deformation of Tertiary and Quaternary sediments by folding and faulting has been documented by G H A N I (1978), LAMB & VELLA (1987) and CAPE et al. (1990). Shortening rates determined from growing fold structures (at least for the last 100 ka, LAMB & VELLA (1987)) are in good agreement with the geodetic shortening strain rate observed over the last 50 years (WALCOTT 1984) of c38 mm/yr normal to the NW-SE trend of the fold structures, The shear component of the observed geodetic strain rate in the southern part of the North Island is c33 mm/yr. This is taken up by the dextral strike-slip zone of which the Wairarapa Fault is the most easterly and the most active (fig. 1). In this paper surface deformation of Last Glacial fluvial aggradation surfaces along the western edge of the Wairarapa Valley is described in terms of slip rates along the Wairarapa Fault over the last 60 ka or so.
2
The Wairarapa Fault
A map showing the extent of the Wairarapa Fault is given in fig. 2. For 13 km from the coast at Palliser Bay to the southern end of Lake Wairarapa, the fault lies mainly within greywacke of the Rimutaka Range. The fault is not
('AIENA
a well defined trace but a 100 300 m wide shear zone of highly pulverized greywacke. Offshore the position of the fault is marked by an 11 km long, 360 mhigh submarine scarp along the west side of Palliser Bay. Seismic reflection data (CARTER et al. 1988) indicate that the fault can be traced across Cook Strait for at least another 16 km SW from Turakirae Head. For 32 km N E of the southern end of Lake Wairarapa the Wairarapa Fault forms the boundary between greywacke of the Rimutaka Range and the Holocene and latest Pleistocene river gravels of the Wairarapa Valley. This is by far the best defined part of the fault as shown in pholo 1. Although mapable as a continuous line on scales of 1:250,000 or smaller the fault trace is actually a series of straight segments separated by left-sidesteps or stepovers of up to 250 m. Because the sense of strike-slip movement on the fault is dextral each left-stepped offset is marked by a bulge or push-up on the upthrown side of the fault as schematically illustrated in fig. 3A. Five bulges occur along this section of the fault. Their locations are shown in fig. 2 and their respective morphologies are shown in fig. 3B. For the remaining 25 km to Mauriceville in northern Wairarapa (fig. 2), the Wairarapa Fault cuts young Cenozoic strata with late Pliocene coquina limestone at places on the NW side and late Miocene to early Pliocene mudstone on the SE side. Along this section there are several places where the fault is upthrown on the SE side, not on the NW side as is normal further south (fig. 2). Beyond Mauriceville the line of the fault is uncertain but there are several recently active, subparallel fault traces within late Miocene mudstone (LENSEN 1968,
AnlnlerdisciplinaryJournalot'S()ll
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HYDR()LOG~
(itl)M()Rt'H()tOG'I
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
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Fig. 1: Map showing the boundary zone between the Pacific and Australian plates, southern North Island, New Zealand. 1970) that may join the Wairarapa Fault to the Mangatoro Fault mapped by ONG L E Y (1943).
3
Fluvial aggradation surfaces: Distribution and age
Three main gravel surfaces (Waiohine, Rata and Porewa in order of increasing age) are recognized in the Wairarapa Valley and represent periods of fluvial aggradation during the Last Glaciation (VELLA 1963, PALMER 1984). The periods of aggradation and coeval loess deposition followed by periods of degradation are correlated with climatic cooling and warming respectively (e.g. COWIE & M I L N E 1973, M I L N E & SMALLEY 1979, P A L M E R 1984, P A L M E R & V U C E T I C H 1989) and the eustatic sea level curve over the last 100 ka (CHAP-
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An lmerdisciplinary
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PEL & S H A C K L E T O N 1986). The relationship is summarized in fig. 4 along with a suggested flood plain displacement curve for the three aggradation phases in the Wairarapa Valley.
3.1
Waiohine surface
This is the youngest and most extensive aggradation surface in the Wairarapa Valley (VELLA 1963). It is characteristically loess-free (fig. 5A) and is formed by coalescing fans mainly centered on the Tauwharenikau, Waiohine, Waingawa and Ruamahanga Rivers (fig. 5B). Towards the east and south the surface is covered by Holocene silt. The surface also slopes more steeply to the east and south than the present day river flood plains and the size of surface boulders decrease more rapidly away from the Rimutaka Range than they do in the
HYI2,ROLO(;~ G E O M O R P H O L O G Y
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Fig. 2: Map showing the extent of the Wairarapa Fault. The double dashed lines near the southern on-land extension of the fault indicate a wide crush zone. C r o s s h a t c h e d a r e a = L a t e Triassic - J u r a s s i c g r e y w a c k e . D o t t e d a r e a = L a t e M i o c e n e - Pleistocene c o v e r beds. T h e n a m e s , R i m u t a k a R a n g e a n d T a r a r u a R a n g e , are a l i g n e d a l o n g the axes o f m a x i m u m uplift o f the respective r a n g e s (after G H A N I 1978).
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(_;EOMORPHOLOGY
A
/
Wa
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-- Porewa aggradatlon surface
0
500
1000 R B Mira
Hill slope (hatched] against aggradation gravels.
Terrace riser or fault scarp, with "u" a n d ' d " fault, without - terrace riser
Marshy or s w a m p y and a b a n d o n e d river channel.
Steep and high
Holocene fan.
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Fla -- Rata aggradation surface
W a -- Waiohine aggradation surface
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Fig. 3: A. Diagrams illustrating the./brmation o/'a bulge by a l~/t step-over along a dextral fauh. B. Detailed maps of bulges developed on the upthrown side o/" the Wairarapa Fault. Location o[" bulges (numbered 1 to 5) is given in fig. 2.
,,
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458
Inferred age of Waiohine Surface
Year
Author(s)
Comments
1952
TE P U N G A
3 ka
14C date of log within youngest aggradation terrace, Rangitikei Basin
t955 1956
WELLMAN LENSEN et al.
10 ka 10 ka
End of Last Glacial and age of youngest aggradation surface throughout New Zealand
1962
COWIE & WELLMAN
>10 ka
14C dated logs o f T E P U N G A deposited by flash flood. Age estimated from soil profiles and loess accumulation rates in Rangitikei Basin
1971
LENSEN & VELLA
a. 35 ka b. 20 ka or younger
Waiohine Surface at Waiohine River a. LENSEN's age represents last stadial of Last Glacial because second to last Stadial of Last Stadial had only "weak" effects in North Island b. V E L L N s age correlation with Last Stadial of Last Glacial as Waiohine Surface is overlain by Holocene sediments around Lake Wairarapa and lacks loess cover
t972
WELLMAN
9.6 ka 11.5 ka
Ages given from correlation between dated uplifted beach ridges at Turakirae Head and five youngest displaced degradation terraces at Waiohine River. Average rate of vertical and dextral faulting assumed to be uniform
1973
SUGGATE & LENSEN
18 ka
Criticism of W E L L M A N ' s 1972 correlation. Age based on 14C ages of "correlative" aggradation surfaces in South Island
1973
MILNE
12 ka
Age of youngest (Ohakean 3) terrace (same surface dated by TE P U N G A and C O W I E & W E L L M A N ) based on rate of river downcutting, Rangitikei Basin
1979
MILNE & SMALLEY
9.5+0.1 ka
14C age for top of loess that blew from youngest aggradation surface. Base of loess dated at 26.3_+0.8 ka, Rangitikei Basin
1980
HUBBARD & NEALL
11.8_+0.15ka
14C age for alluvial fan, southern Ruahine Range
1986
M A R D E N et al.
10.3_+0.1ka 12.9_+0.2 ka
Various 14C ages of wood from a few metres below aggradation surface, 2 k m N W of Woodville. Gravels derived from southern Ruahine Range
1987
TOMPKINS
8.5_+0.12 ka
14C age of peaty material near base of peat overlying Waiohine Surface south of Greytown
1988
GRAPES & WELLMAN (unpub. data)
12.5 ka
14C age of tree roots in growth position at top of loess that is covered by fan gravels
1990
SUGGATE
16 ka
Age inferred from constant rate of loess accumulation from 35.5 ka with respect to position of Kawakawa tephra layer dated at 22.5 ka
Data for the age of the Waiohine or equivalent aggradation surface, southern North Island. T a b . 1:
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GI~OMORPHOL()(}'~
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
459
Photo 1: Oblique aerial photo looking southwest across the trace of the Wairarapa
Fault fi'om Featherston (top left corner) to Waiohine River (bottom). The extensive .Hat surface displaced by the .fault is the Waiohine aggradation surface. Rimutaka Range in background.
present day rivers (fig. 5C). These features reflect a graded profile adjusted to low eustatic sea level that was over 100 m lower than present (fig. 4). The Waiohine Surface is correlated with similar loess-free surfaces elsewhere in the southern part of the North Island (e.g. M I L N E 1973, K A E W Y A N A 1980). Because of it's wide extent the Waiohine and equivalent surfaces provide an important time plane for determining Holocene deformation rates. De-
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bate over the age of the surface has gone on for the last 30 years (tab. 1). Based on the available evidence an age of I l ±1 ka for the Waiohine Surface is adopted in this paper. 3.2
Rata surface
The Rata Surface is best developed in the northern and eastern parts of the Wairarapa Valley (VELLA 1963, P A L M E R 1984). The surface is covered by 1-2 m of loess that was blown off the
GEOMORPHOLOGY
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r \kgrat[dliorl / T - I iI
&
loess
"
--
I
/5
/3
Fig. 4: Ages o1" Wairarapa aggradation surfaces, respective loesses and flood plain displacement curve for aggradation periods and degradation periods related to sea-level curve (after CHAPPELL & SHA CKELE 7DN 1986).
"7 k
Sea l e v e l ( u r v u > ,![I -
2
[
b
"" 120~
!
[:;I)
l
r
I
1
I)
I 51)
I
I
t
i~jo Age (xl avrs.J
Waiohine Surface. Of particular significance is the occurrence of the Kawakawa Tephra (dated at 22.5 ka by WILSON et al. 1988) about a third of the way up from the top of the aggradation gravels. This indicates that the Rata Surface represents the culmination of the second to last fluvial aggradation c50 ka to c30 ka ago (fig. 4).
3.3
P o r e w a surface
The Porewa Surface (VELLA 1963, PALMER 1984) is also best preserved in northern and eastern Wairarapa. It CA]ENA
Wa = Waiohine; Ra= Porewa.
Rata:
Po =
is typically dissected by streams and is mantled by two loess units derived from both the Rata and Waiohine surfaces respectively and typically separated by a paleosol. Porewa aggradation gravels overlap the 80 ka marine bench (PALMER 1984) and loess derived from the Porewa surface is found on marine benches of 80 ka and older ( G H A N I 1978) indicating that the aggradation phase is younger than 80 ka. The top of the Porewa loess is weathered to a paleosol that contains residues of andesitic tephras erupted about 60 ka ago ( M I L N E 1973,
A n Interdisciplinary Journal of S O I L S C I E N C E
HYDROJ
OGY
(_?FOMORPHOLO(~Y
,
2 ~ ,
~
~ ~
,
s ,
,
s ,
Wa~hir~ R~vm Bed
,
Map~
~0k surface. N u m b e r s in s q , a ~ = stone diameters in the ~ t - - d a y ~ r~ bed. Ra = Rata surface. Cross hatched area = gceFv~acke~
soo ....
river
7ig. 5: Data for the stony appearance (A), morphology and distribution (B), and boulder sizes (C) of the ;Vaiohine surface, Wairarapa Valley.
[~l
-:~w. ~ o
~aximun Ve~'tlCMd t s p t ~ t atm,tg ~ ~tttt$ are:~ n Fault = 5.Zm: M a ~ P ~ o n Fault ~ 8~Om: ~okonui Fault = 6.8m {LENSEN 1968). Ra = Rata surface, 'rcns h a t c h e d area = greywa~.k~ of tim Rimutaka Range.
G - Greylown: = R u a m a h a n S a Riv~x, F = ~ m ~ '= Car~rtor~ M = Masr~ton. Fa~t~ I ~ b A n $ b c x a ~e Wairar~pa Fault are normal faults ~ are m o l l y Ownthrown tO t l ~ s o u t h , F r o m s o t # ~ to t ~ r t h h ~ al~-
t a p ~ t~e w ~ e ~ce wi~ 11~ ¢ ~ m u r s . w ~ Tauwharmnllcau R i v e . W h ~ tegaiolg~ Rive~;
462
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17 130
24. - - -
_E × ~ ¢ o
120
B5
110
B2
B~
4_1 I
2
B4
R 25
/t [2627 R
5
FEATHERSTON
•
Fig. 6: Plot o f dextral and vertical displacement o f the Waiohine surfi~ce along a 57 km section o f the Wairarapa Fault. Numbered localities together with displacement values are listed in tab. 2. B1, 2 etc. refer to bulges developed on the upthrown side o f the fault. R = reversal in upthrown side, i.e. to the southeast, otherwise upthrow is to the northwest. P A L M E R & V U C E T I C H 1989). The Porewa agradation is therefore correlated with the sea level low between c80 and c60 ka (fig. 4).
4
Displacement of aggradation surfaces along the Wairarapa Fault
Vertical and dextral displacements of the 11_1 ka Waiohine surface along a 57 km length of the Wairarapa Fault are listed in tab. 2 and the displacements plotted in fig. 6. Displacement data for the c30 ka Rata and c60 ka Porewa surfaces are listed in tab. 3.
CATENA
4.1
Dextral displacement
Dextral displacement of the three aggradation surfaces is recorded from five localities. The Waiohine surface is displaced by 125+5 m as determined from offset channels (Locality 11), an offset riser (Locality 24a) and offset hill slopes at the back of the surface (Localities 6, 17, 19), (tab. 2). GRAPES & WELLMAN (1988) estimated that the total cumulative Holocene dextral displacement of 125-t-5 m is the result of at least ten large (cM8) earthquakes with about 12 m of dextral movement for each event, the last occurring in 1855. At Waingawa River the .riser of the Rata surface has been dextrally offset
An Interdisciplinary Journal of SOIL SCIENCE
HYDROLOGY
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R 31
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
Locality
Displaced
No.
feature
Upthrown side
Vertical
Displacement Dextral
1
Surface
NW
11.5
--
463
Dextral/ Vertical
Comments Minimum
value.
Downthrown
side has
H o l o c e n e silt c o v e r 2
Surface
NW
t4.0
--
Stepped scarp. Minimum
value.
c. 1 m silt c o v e r on d o w n t h r o w n 3
Surface
NW
11.0
--
-
4
Surface
NW
17.0
-
5
Surface
NW
12.5
.
6
Hill edge
7
Surface
NW
12.0
--
8
Surface
NW
11.0
--
9
Surface
NW
13.0
--
10
Surface
NW
7.0
--
11
4 channels
NW
18.5
c. 125_+5
12
Surface
NW
19.0
--
.
.
.
c. 120
side
-.
.
--
Protruding
halfscarp
---7.0
--
13
Surface
NW
20.5
--
14
Surface
NW
7.5
--
--
--
15
Surface
NW
10.5
--
16
Surface
NW
11.5
--
--
17
Hill edge
126_+6
--
Protruding halfscarp
18
Surface
NW
3.0
--
--
--
--
19
Hill edge
. . . . . .
20
Surface
SE
2.0
21
Surface
NW
6.0
--
--
--
22
Surface
NW
5.0
--
--
--
23
Surface
SE
0.1
--
--
--
24a
Riser
SE
1.0
25a
Surface
SE
6.5
--
26
Surface
SE
3.0
.
27
Surface
SE
3.0
--
--
28
Surface
NW
1.3
--
--
--
29
Surface
0
--
--
--
30
Surface
SE
1.0
--
--
--
31
Surface
NW
2.6
--
--
-
Tab.
Tab.
(AIENA
2:
110±5
--
.
.
124_+5
Minimum
124.0
--
-.
.
value
.
.
-. --
Displacement data for the Waiohine Surface along the Wairarapa Fault.
Locality
Displaced
No.
feature
24b
Surface riser
24a
Surface
25b
Hill slope
?SE
--
3"
-
.
Upthrown side
Displacement
Dextral/
Vertical
Dextral
Vertical
SE
7.0
385_+5
55.0
NW
18.0
-c. 8 0 0
Comments Rata surface Porewa surface ? Porewa surface
Displacement data for Rata and Porewa surfaces along the Wairarapa Fault.
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464 soo-~
/
600 t
q.~x / 4D~\X)
400
/Y
-
E 200 L2
÷ Wa
0
~1855 EQ 1=0
210
3r0 410 Age (x103yrs)
510
6~0
7~0
Fig. 7: Dextral slip rate on the Wairarapa Fault Ji'om displacements of Waiohine,
Rata and Porewa aggradation surfaces. Surface ages are from fig. 4, by 385+5 m (Locality 24b, tab. 3) but there are no features that record dextral displacement of the Porewa surface. By extrapolating rates derived from younger surfaces, the c60 ka Porewa surface should be displaced about 770 m. About 7 km N E of Waingawa River a loess mantled hill that may be of Porewa age has been displaced about 800 m (Locality 25b, tab. 3). Dextral displacements for the three surfaces are plotted against their inferred ages in fig. 7 and indicate an average uniform dextral slip rate for the Wairarapa Fault of 11.5 mm/yr over the last 60 ka.
4.2
Vertical displacement
South-east of the position where there is a 10 ° change in the strike of the Wairarapa Fault (fig. 2), variation in the amount of vertical displacement of the
Waiohine surface (fig. 6) appears to be related to the distribution of growing bulges along the upthrown side of the fault. From fig. 6 it can be seen that the amount of throw increases towards the SW end of each bulge. Three of the bulges (Nos. 1, 4 and 5) provide information for determining rates of deformation of the Waiohine and Porewa surfaces. Bulge 4 immediately south of the Waiohine River (fig. 3B) is composed of the Waiohine aggradation gravels and is the result of a 110 m left-sidestep on the Wairarapa Fault. The top of the bulge is now 20 m above the surrounding undeformed surface indicating an average uplift rate of 1.8 mm/yr. This value is similar to the maximum uplift rate of the undeformed Waiohine surface on the NE side of the Waiohine River (Localities 11 13; tab. 2) with an uplift rate of 1.7-1.9 mm/yr. Minimum vertical dis-
CAPENA An Interdisciplinary Journal of SOII SCIENCE HYDROLOG~t (]EOIMORPHIJIO(iY
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand placements of the Waiohine Surface are 7.0 m and 7.5 m. Both localities are at the NE ends of bulges 4 and 5 respectively (Localities 10 and 14, tab. 2) and indicate average uplift rates of about 0.6 mm/yr. Bulge 1, south of Featherston, has developed from a 180 m left-sidestep on the fault (fig. 3B). The surface of the bulge is composed of fluvial gravel of inferred Waiohine surface age and slope washed solifluxion debris that mantles a core of sheared greywacke exposed along two stream gorges that cut through the bulge. The maximum height of the bulge is about 25 m above the same surface to the west of the bulge and about 45 m above the silt-covered downthrown side of the Waiohine surface to the east. The 25 m maximum height difference implies an uplift rate of 2.3 mm/yr, twice the uplift rate of 1.(~1.3 mm/yr for the undeformed surface further to the SE (Localities 1, 2, tab. 2). The two streams that cut the bulge are older and antecedent. They are dextrally displaced by c260 m. Assuming a uniform dextral slip rate of 11.5 mm/yr the 260 m indicates 22 ka of cumulative offset. If the bulge began to grow at this time the uplift rate would reduce to 1.1 mm/yr, in agreement with the lower rate for the undeformed Waiohine surface. Vertical deformation of the Porewa surface occurs in the vicinity of Bulge 5 fig. 3B, NE of Waiohine River, that is the result of a 190 m left-sidestep on the fault. The bulge is composed of a core of highly sheared greywacke covered by loess and solifluxion material that can be traced SW to merge with an undeformed but dissected Porewa surface preserved of the upthrown side of the Wairarapa Fault NE of Waiohine River. The top of the bulge is 50 60 m above this surface indicating that it could have formed with
CA'lENA
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465
an average uplift rate of about 1 mm/yr over the last 60 ka. Where the trace of the Wairarapa Fault cuts late Cenozoic rocks that underlie aggradation gravels in river valleys, the direction of throw changes from NW to SE at several places (tab. 2, 3). At Waingawa River the Waiohine and Rata surfaces are upthrown to the SE and the Porewa surface to the NW (photo 2). This indicates at least one reversal in the sense of vertical movement over the last 60 ka along the Wairarapa Fault. The very small throw of only 1 m for the Waiohine surface compared with the 7 m throw for the Rata surface suggests the possibility that another reversal has occurred sometime in the last 11 ka and that successive Holocene earthquakes have diminished a previously greater vertical displacement of both surfaces. Where the upthrow of the Waiohine surface is to the NW (Localieties 21, 22, 28, 31 ; tab. 2), uplift rates are low and range from 0.1-0.5 mm/yr.
4.3
Dextral/vertical ratio
At Waiohine River the average dextral/vertical ratio for the Waiohine surface is about 6:1. At Waingawa River the Waiohine surface ratio is 1124:1; for the Rata surface the ratio is 55:1 and for the Porewa surface (assuming 770 m dextral displacement) the ratio is 39:1. The apparent increase in the dextral/vertical ratio since c60 ka BP at Waingawa River has been caused by the reversal of the upthrown side of the fault.
5
Discussion
Displacement of aggradation surfaces extending back to 60 ka along a 57 km length of the Wairarapa Fault indicate; GI~OMORPHOLOGY
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Photo 2: Oblique aerial photo looking northwest across the Wairarapa Fault showing dextral and vertical offsets of the Waiohine ( Wa), Rata ( Ra) and Porewa ( Po ) surfaces at Waingawa River. The Waiohine and Rata surfaces are upthrown to the southeast and the Porewa surface is upthrown to the northwest. Arrows indicate the trace of the Wairarapa Fault. Foothills of the Tararua Range in background. Photo: Lloyd Homer, New Zealand Geological Survey.
1. a uniform dextral 11.5 mm/yr, and
slip
rate
of
2. highly variable vertical uplift rates ranging from 0.6-1.9 mm/yr due to the development of surface and possibly subsurface bulges along the fault.
CAI'ENA
The uniform dextral rate is consistent with a uniform rate of plate convergence at least over the last 100 ka or so. The Wairarapa Fault dextral rate is high and accounts for about one third of the 33 mm/yr oblique convergence component of the Australian and Pacific Plates.
An Interdisciplinary Journal o[ SOil SCIENCE
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(;J:OMORI'HOLOG~
Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
The uplift pattern in the time <100 ka has been complicated by surface bulges along the fault. Stepovers of the type that produce bulges appear to be fundamental features of strike-slip fault systems (e.g. A Y D I N & N U R 1985) but their genesis remains an unsolved problem. Data from Bulge 4 at Waiohine River indicate that the bulges may be of shallow origin. If the amount of gravel and underlying greywacke being upbulged is calculated from the offset and the dextral rate and then compared with the volume of the 20 m high bulge at Waiohine River a thickness of about 60 m is all that is required to form the bulge. If the effect of the growing bulges on vertical displacement is ignored then there appears to be a general decrease in the amount of uplift, or conversely an increase in the dextral/vertical ratio, northeastward along the Wairarapa Fault, at least for the Waiohine surface which provides sufficient data. This is consistent with a N E decrease in the amount of uplift along the fault that occurred during the 1855 earthquake (and probably previous earthquakes), i.e. from 2.9 m at Turakirae Head, Palliser Bay, to c0.3 m near Mauriceville a distance of about 100 kin. On a larger and longer time scale the NE decrease in vertical displcement might be related to the N E divergence of the Wairarapa Fault from the axes of greatest uplift for the Rimutaka and Tararua Ranges (fig. 2). Where the axis of the Rimutaka Anticline intersects the coast near Turakirae Head the uplift rate along the submarine extension of the fault determined from the 6.5 ka Holocene bench is 4 mm/yr (WELLM A N 1967). This is twice the rate at Waiohine River (cl.8 mm/yr), 60 km inland, and about eight times the rate for the Waiohine Surface 17 km further to
CA[ ENA
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the N E on the SW side of Waingawa River (0.5 mm/yr, Locality 22; tab. 21. An increase in the amount of vertical displacement of the Waiohine Surface towards the SW end of the bulges suggests propagation of successive earthquake uplifts along the Wairarapa Fault proceeded in the N E direction, as was apparently the case for the last earthquake in 1855. Reversals in throw of aggradation surfaces along the Wairarapa Fault may be related to bulges developing in basement rocks and are restricted to those areas where the fault cuts gravels that overlie relatively soft Tertiary rocks. The trace of the Wairarapa Fault is generally less clear where it cuts Tertiary rocks than where it cuts gravel surfaces on greywacke bedrock and it often appears as multiple, discontinuous subparallel traces that have variable senses of throw. A seismic reflection profile across the Wairarapa Fault (near Locality 20) by CAPE et al. (1990) shows that the fault is one of several subsurface faults arranged in a high angle "flower" structure (CAPE & WELLS 1988) associated with a Tertiary anticline above updomed greywacke basement. Reversals in throw might therefore be caused by differential movement on one or more faults in such a cluster during growth of anticlinal structures adjacent to the Wairarapa Fault.
Acknowledgements Many thanks are due to Professor Paul Veila and Dr. Mike Crozier of the Research School of Earth Sciences, Victoria University of Wellington, for helpful comments that clarified several parts of the manuscript. During the field work Professor Harold Wellman pro-
GEOMORPHOLO(kY
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v i d e d s t i m u l a t i n g c o n v e r s a t i o n w i t h res p e c t to t h e f o r m a t i o n a n d a g e o f a g g r a dation surfaces and aspects of faulting along the W a i r a r a p a grams were draughted
Fault.
The dia-
by R o b i n
Mita
a n d h e r e x p e r t i s e is g r a t e f u l l y a c k n o w l edged. References
GRAPES, R.H. & WELLMAN, H.W. (1988): The Wairarapa Fault. Victoria University of Wellington, Geology Board of Studies Publication 4, 55 pp. HUBBARD, C.B. & NEALL, V.E. (1980): A reconstruction of Late Quaternary erosional events in the West Tamaki River catchment, southern Ruahine Range, North Island, New Zealand. New Zealand Journal of Geology and Geophysics 23, 587 -593.
ADYIN, A. & NUR, A. (1985): The types and rates of stepovers in strike-slip tectonics. Society of Economic Paleontologists and Mineralogists, Special Publication 37, 35~44.
KAEWYANA, W. (1980): Late Quaternary alluvial terraces and their cover bed stratigraphy. Eketahuna and Pahiatua Districts, New Zealand. Unpublished MSc thesis, Victoria University of Wellington.
CAPE, C., LAMB, S.H., VELLA, P., WELLS, P.E. & WOODWARD, D.J. (1990): Geological structure at Wairarapa Valley, New Zealand, from seismic reflection profiling. Journal of the Royal Society of New Zealand 20, 85 105.
KAYAL, J.R. (1984): Microseismicity and tectonics at the Indian/Pacific plate boundary: southeast Wellington province, New Zealand. Geophysical Journal of the Royal Astronomical Society 77, 567-592.
CAPE, C., WELLS, P.E. (1988): Seismic expression of the Wairarapa Fault system.Abs. Workshop on the active deformation of the Wellington region, 19 20 September 1988, Wellington, Geophysics Division, Department of Scientific and Industrial Research.
LAMB, S.II. & VELLA, P. (1987): The last million years of deformation in part of the New Zealand plate-boundary zone. Journal of Structural Geology 9. 877 891.
CARTER, L., LEWIS, K.B. & DAVEY, F. (1988): Faults in Cook Strait and their bearing on the structure of central New Zealand. New Zealand Journal of Geology and Geophysics 31, 431-446. CHAPPEL, J. & SHACKLETON, N.J. (1986): Oxygen isotopes and sea level. Nature 324, 137 140. COW1E, J.D. & MILNE, J.D.G. (1973): Maps and sections showing the distribution and stratigraphy of North Island loess and associated cover deposits, New Zealand. 1:1,000,000. New Zealand Soil Survey Report 6. COWIE, J.D. & WELLMAN, H.W. (1962): Age of the Ohakea terrace, Rangitikei River. New Zealand Journal of Geology and Geophysics 5, 617 619 DAVEY, F.J., MAPTON, M., CHILDS, J., FISHER, M.A., LEWIS, K.B. & PETTINGA, J.R. (1986): Structure of a growing accretionary prism, Hikurangi Margin, New Zealand. Geology 14, 663 666. GHAN1, M.A. (1978): Late Cenozoic vertical crustal movement in the southern North Island, New Zealand. New Zealand Journal of Geology and Geophysics 22, 117 126.
CAIENA
LENSEN, G.J. (1968): Sheet N 158 Masterton. Late Quaternary map of New Zealand, 1:63,360. New Zealand Department of Scientific and Industrial Research, Wellington, New Zealand. LENSEN, G.J. (1970): Sheet N 153 Eketahuna. Late Quaternary map of New Zealand, t:63,360. New Zealand Department of Scientific and Industrial Research, Wellington. New Zealand. LENSEN, G.J., STEVENS, G.R. & WELLMAN, H.W. (1956): The earthquake risk in the Wellington district. New Zealand Science Review 14, 131 135. LENSEN, G.J. & VELLA, P. (1971): The Waiohine River faulted terrace sequence. Royal Society of New Zealand Bulletin 9, 117 119. MARDEN, M., PAINTIN, I.K., LEES, C.M. & NEALL, V.E. (1986): Woodville neotectonics and Quaternary stratigraphy field trip. Geological Society of New Zealand 16th Annual Conference Field Trip Quides. Massey University, Palmerston North, B3-1, B3-30. MILNE, J.D.G. (1973): River terraces in the Rangitikei Basin. New Zealand Soil Bureau maps 142/1, 142/2, 142/3, 142/4. Wellington. New Zealand Department of Scientific and Industrial Research.
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Aggradation Surfaces, Displacement Rates, Wairarapa Fault, New Zealand
M1LNE, J.D.G. & SMALLEY, l.J. (1979): Loess deposits in the southern part of the North Island of New Zealand : an outline stratigraphy. Acta Geologica Aeademica Scientarium Hungaricae 22, 192-2-4. ONGLEY, M. (1943): The trace of the 1855 earthquake. Transactions of the Royal Society of New Zealand 85, 205 212.
WELLMAN, H.W. (1972): Rate of horizontal fault displacement in New Zealand. Nature 23% 275 277. WILSON, C.J.N., SWITSUR, V.R. & WARD, A.P. (1988): A new 14C age for the Oruanui (Wairakei) eruption, New Zealand. Geological Magazine 125, 297 300.
PALMER, A.S. (1984): Quaternary geology of Wairarapa. Geological Society of New Zealand Miscellaneous Publication 31 B, 20..59. PALMER, A.S. & VUCETICH, C.G. (1989): Late Glacial loess and early Latc Glacial vegetation history of Wairarapa Valley, New Zealand. New Zealand Journal of Geology and Geophysics 32, 499 514. ROBINSON, R. (1986): Seismicity, structure and tectonics of the Wellington region, New Zealand. Geophysical Journal of the Royal Astronomical Society 87, 379-409. SUGGATE, R.P. (1990): Kawakawa tephra and Ohakea loess. Geological Society of New Zealand Newsletter 88, 36-37. SUGGATE, r.P. & LENSEN, G.J. (1973): Rate of horizontal fault displacement in New Zealand. Nature 242, 518 519. TE PUNGA, M.T. (1952): Radiocarbon dating of a Rangitikei river terrace. New Zealand Journal of Science and Technology B35, 45~48. TOMPK1NS, J. (1987): Late Quaternary pollen stratigraphy, geology and soils of an area south of Greytown, Wairarapa. Geological Society of New Zealand Newsletter 77, 5 6. VAN DER LINGEN, G.J. (1982): Development of the North Island subduction system, New Zealand. In: Trench-Forearc Geology. Special Publication of the Geological Society of London 10, 259 273. VELLA, P. (1963): Upper Pleistocene succession in the inland part of the Wairarapa Valley, New Zealand. Transactions of the Royal Society of New Zealand 4, 63-78. WALCOTT, R.I. (1984): The kinematics of the plate boundary zone through New Zealand: a comparison of short- and long-term deformations. Geophysical Journal of the Royal Astronomical Society 79, 613 633. WELLMAN, H.W. (1955): New Zealand Quaternary tectonics. Geologische Rundschau 43, 248 257. WELLMAN, H.W. (1967): Tilted marine beach ridges at Cape Turakirae, New Zealand. Journal of Geosciences, Osaka City University 10, 1-6.
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Address of author: R.H. Grapes Research School of Earth Sciences Victoria University of Wellington RO. Box 600 Wellington New Zealand
(3EOMORPtIOLOGY