The Motunau fault revisited

164

dips of < lo-2”

are typical

above sea level onshore

offshore.

compared

Mesozoic

with offshore

basement

is uplifted

up to 500 m

where the uplift rate is insufficient

to outpace wave erosion. On Fig. 1 we suggest an alternative

structural

which satisfies all the data. It involves

a zone of widespread

interpretation

to Carter

and Carter

dextral shear manifested

in many ways and on many scales and not as a single transcurrent

fault. Major shear

may be taken up on an inferred fault lying just offshore near the coast. Since the Motunau Fault does not exist, the southern limit of the Conway Microplate should be more appropriately called the Porters Pass fault system or Porters Pass deformation zone. REFERENCES

Australian

Gulf Oil Co., 1973. M.V. “Gulfrex”

cruises 94498. New Zealand

region, October

1972-January

1973. N.Z. Geol. Surv., Open File Pet. Rep., 614. B.P.. Shell and Todd Petroleum

Development

Ltd., 1963. Seismic survey of the Canterbury

Plains. N.Z.

Geol. Surv.. Open File Pet. Rep., 328. B.P.. Shell Aquitaine offshore

and Todd Petroleum

seismic

reconnaissance

Development

survey:

operation,

Ltd., 1966. New Zealand data

processing

South Island east coast

and interpretation

report.

N.Z.

Geol. Surv. Open File Pet. Rep., 286. B.P.. Shell Aquitaine

and Todd

Petroleum

Development

Ltd., 1969. Marine

seismic survey.

Canterbury

at the southern

edge of the

Bight. N.Z. Geol. Surv., Open File Pet. Rep., 292. Carter,

R.M. and Carter,

Australian-Pacific

L.. 1982. The Motunau

plate

boundary,

Fault and other structures

offshore

Marlborough,

Oil Co.. 1972. M.V. “Fred

H. Moore”,

New Zealand.

Tectonophysics.

88(1/2):

1333159. Mobil

International

New Zealand

offshore

regional

reconnais-

sance seismic survey. N.Z. Geol. Surv., Open File Pet. Rep.. 587. Mobil

International

Oil Co., 1979. Processed

lines from New Zealand

offshore

regional

reconnaissance

seismic survey. N.Z. Geol. Surv.. Open File Pet. Rep., 738.

THE MOTUNAU

R.M. CARTER

FAULT REVISITED

and L. CARTER

New Zealand Oceanographic Institute, Wellington (New Zealand) (Received

November

12, 1982)

Our paper included an interpretation of the tectonics of the continental shelf in Pegasus Bay. We examined the offshore seismic lines listed by Herzer and Bradshaw and generally agree with them that the poor quality of the lines makes interpretation difficult. In addition, none of the lines was initially planned with detection of the

165

Motunau details

Fault in mind, and their location

is, therefore.

far from ideal for providing

of that structure.

Herzer

and Bradshaw claim the Motunau Fault should cross eight industry to accurately map the seismic lines and therefore allow us “ample opportunity location and character of the fault”. While agreeing with this idealised sentiment it relates

little to the real situation.

As Herzer

and Bradshaw

note. three lines are of

very poor quality and limited use: lines 2 and A end in the vicinity of the fault (the precise location of which is, of course, unknown). and lines 1A and 5 were unavailable to us. Only line 72-5 definitely crosses the projected offshore line of the Motunau structure. The fact that line 72-5 reveals precisely

no major

fault along

why we showed a dotted and question-marked

the Motunau

path between

structure

is

the inner shelf

and slope parts of the structure on our fig. 2. We also pointed out in the text that “an alternative interpretation would be that the inner shelf and slope features represent discrete but broadly colinear faults” (p. 139). Further on in the paper this theme was developed in our discussion of the en echelon nature of other faults such as the Pegasus Bay structure, and the comment made that “the Motunau fault system is not as well developed as a single major dislocation as the more northerly Marlborough faults, suggesting its more recent origin” (p. 156). In sum, we agree completely with Herzer and Bradshaw’s comments regarding the presence of a zone of deep-seated

shear along

the Motunau

structure,

and regret

failed to make this clear in our original

discussion.

Motunau

fault dislocations

system

correspond

to actual

that we apparently

How far individual must,

parts of the

in our view. await

detailed mapping onshore and more and better deep seismic coverage offshore. Herzer and Bradshaw also make detailed comment on our interpretation of the slope sector of the Motunau Fault, and, in particular, the interpretation that we gave to profile NZ-6 (our fig. 5). Again, we agree with many of their comments regarding the difficulty

of interpreting

(a) We identified interpretation

this profile due to multiples

the scarp as the Motunau

now. The scarp is c. 200 m high and separates

slope from the irregular

and probably

much-slumped

(b) We remain doubtful that individual supposed fault with velocity offset only”, possible

and ringing,

interpretation.

The abrupt

“are

under

continuous

we acknowledge

of ringing

to the north of the fault is suggestive of major differences lower parts of the sequence across the fault. (c) We did not infer an anticline

the gently sloping upper

lower slope.

reflectors though

termination

but note:

Fault (fig. 5) and prefer to retain that

the bathymetric

across that

below the bottom

multiple

in seismic response knob

the

this is a of the

at the base of the

slope on profile Gulfrex NZ-6, but under the similarly located knob on line 72-3 (fig. 4). We agree with Herzer and Bradshaw that velocity pull-up is operating on line NZ-6. The second part of Herzer and Bradshaw’s discussion contains much interesting data on the nature of Motunau fault system between the Porters Pass Fault and the

166

coast, and an interesting welcome

we cannot sceptical shown claimed

interpretative

this interpretation, resist

commenting

of the existence by us, Herzer Cretaceous

diagram

of the regional

which we view as complementary on

a detail

of a slope portion

and Bradshaw

age) subparallel

of their

interpretation.

of the Motunau

show not one but to and between

tectonic

situation.

We

to our own. However, Having

been

fault in the location

two subparallel

faults

(of

seismic lines 72-5 and NZ-76, in

an area with no seismic cover whatsoever! Their interpretation that both these faults curve eastwards to cross line NZ-76 gives the lie to their earlier comment regarding the insoluble

geometric

re-emphasizes the need satisfactory interpretation ity.

conundrum

of interpreting

faults

for more and better seismic to be reached of this important

along

line NZ-6;

it also

cover to enable a more zone of tectonic complex-