Origin, structural and tectonic history of the Macquarie Island region

Tags: Macquarie Island, Macquarie Ridge, oceanic crust, Macquarie Ridge Complex, magnetic anomalies, New Zealand, Australia, J. Geophys, Australian plate, Hayes, Macquarie Trench, palaeontological, tectonic history, Antarctic Research Series, sea-floor spreading, American Geophysical Union, Macquarie Island Symposium, Island Region, structural model, Bowers Ridge, South island of New Zealand, Jo Geophys, Macquarie triple junction, The Ridge, Washington, Puysegur Trenches, island arc, characteristic, landward side, Hjort Trench, intrusive rocks, Eagle point, geoid anomalies, Scientific Committee on Antarctic Research, Plate tectonics, Government Printing Office, Shirshov Ridge, J. Geoph, massive rocks, J. Geophyso Reso, Universitets Forlaget
Content: Papers and Proceeding, of the Rlwa!
of Tasmania, Volume 122(1), 1988
27
ORIGIN, STRUCTURAL AND TECTONIC HISTORY OF THE MACQUARIE ISLAND REGION
by P.E. Williamson
(with eleven lcxt·figurcs)
WILLIAMSON, P.E., 1988 (viii): Origin, structural and tectonic history olthe Macquarie Island region Pap. Proc. R. Soc. Tasm., 122( I): 27 43. Papers presented at the Macquarie Island Symposium, Hobart, May 1987. ISSN 0080-4703. Bureau of Mineral Resources, Canberra, A.C.T., Australia. Macquarie Island, in the Southern Ocean, was formed by oceanic crust uplift due to transpressivc forces between the Indian! Australian and Pacific Oceanic plates, in a transpression'll regime which has persisted over the last 10 Ma. The amount of uplift is affected by regional isostatic compensation for crustal thickening; accompanying effects are tilting of rucks and rotation of the southern segment oflhe island. Gabbro and serpentinite, in the north, and basalts, in the sOllth, all of which were formed in the primary oceanic crust, are now exposed, Consequently, magnetic properties of igneous rocks on the island correlate with similar features on the Indian plate which is on both sides of it. In conflict with evidence from younger palaeontological and potassium-argon (K-Ar) dating, which may reflect later episodes, this suggests that the original oceanic crust composing the island was formed at the Indian-Antarctic accreting mid-oceanic ridge around the time of anomaly 7 (27 Ma BY.). Key Wurds: oceanic crust uplift, magnetic anomalies, sea-floor history, Macquarie Island.
INTRODUCTION The tectonic and physiographic expression of Macquarie Island and the surrounding region (fig, I) is the result of oblique compression (transpression) between the Indian/ Australian and Pacific oceanic plates near to their common boundary. This boundary, south of New Zealand, lies within the Macquarie Ridge Complex, an arcuate system of ridges and troughs, extending from the Macquarie triple junction between the Indian, Pacific and Antarctic plates near 62° S, 160° E (Sykes 1970) to the Alpine Fault of the South island of New Zealand. The major physiographic element of the complex is an arcuate ridge (the Macquarie Ridge) with a trench (the Macquarie Trench) to the east in the central area and a trench to the west in the northern area (Puysegur) and the southern area (Hjort). Geological and geophysical studies of the Macquarie Ridge began with the New Zealand programme to investigate the marine geology of the New Zealand subantarctic sea floor (Summerhayes 1969), Reconnaissance geophysical investigations in the region were later carried out by Lamont-Doherty Geological Observatory vesstls Vema and Conrad, and were expanded between 1968 and 1971, using the USNS Eitanin. Data were also collected during a cruise by the Japanese vessel Umitaka-Maru and during aeromagnetic studies
by the US Navy Oceanographic Office. In the Macyuarie Island region, marine magnetic and bathyMetric Data were collected, primarily by the author, from the Australian National Antarctic Research Expedition (AN ARE) relief vessel Nella Dan between 1970 and 1973. Magnetic lineations have been used to resolve the sea-floor spreading history between Australia and Antarctica (Weissel 1972, Weissel & Hayes 1971, 1972, Weissel el ai, 1977) and in the Pacific Plate south of New Zealand (Christoffel & Falconer 1972). The broad crustal structure and tectonic history of the Macquarie Ridge Complex and the Macyuaric Island region has been discussed by Hayes & Talwani (1972), Woodward (1973), Williamson & Johnson (1974), Williamson (1974, 1(75), and Weisse! ('I al. (1977), utilising a predominantly marine geophysical datil base, and by Ruff & Cazenave (1985), using SEASAT data. Analyses of sea·floor spreading magnetic lineations and gravity from Macquarie Island imply that the origins of the rocks of Macquarie Island were mainly at the Indian/ AustralianAntarctic spreading centre, probably around 27 Ma B.P. anomaly 7 time (Williamson 1974, 1978, Williamson el al. 198!). Subsequent rotation of island segments, probably during uplift of M acq uarie Island, has been demonstrated by Williamson (1978) and Yarne & Rubenach (1972).
28 p, E Williamson 160'E
FlU 1 - IJ)((/lily map. lsohalhs ill 2, 4 and 6 km.
This paper represents a review, and, for the palaeomagnetic component, a reassessment of the geophysical and geological studies relating to the broad structure and origins of the Macquarie Island region, as a basis for future study,
MORPHOLOGY
Macquarie Ridge Complex
The morphology of the major segments of
the Macquarie Ridge Complex (figs 2 and 3) has
been described earlier (Brodie & Dawson 1965,
Summerhayes 1969, Hayes & Talwani 1972). Except
for a number of breaks (sometimes pronounced),
the ridge lies at a depth less than 2000 m between
latitudes of 47°5 and 57°S. South of 57°S it
becomes less regular and branches into two
segments. The
Trench lies east of the
MacqU(lrie Ridge between about 51°S and 56°S.lt
is a continuous deep over a distance of 550 km. It
reaches a maximum depth of 6000 m and is typically
than 5200 m.
The Emerald Basin, east of the Macquarie
Trench, is characterised numerous large sea"
mounts and pronounced depressions, especially in
the vicinity of 55°S 59° S (
& Ta!wani 1972).
The basin lying to the west of the Macquarie Ridge
is characterised by relatively low relief, generally
less than 400 m, and variable sediment thickness
(Bout? et aL 1971, Hayes & Conolly !972).
Macquaric Island Region
The bathymetry of the Macquarie Island
region (Summerhayes I shows gentle gradients
to the west of the island which continue down to the
level of the ocean basin (fig. 3), Much steeper
gradients occur to the east of the island and extend
into the Macquarie Trench. The major bathymetric
trends in the area parallel the Macquarie Ridge,
wilh only minor irregularities along these trends.
The
Ridge is consistently shallow and
rises above sea level in Ihe cases of Macquarie
Island, Bishop and Clerk Islands to the south, and
and Clerk Islands to the north.
Macquarie Island itself trends northnorth-
easterly, paralleling the Macquarie Ridge in the
region. The island is characterised by a narrow
coastal strip with steep slopes up to an undulating
plateau al a maximum elevation of about 400 m.
The island plateau is cut by a major valley between
Bauer and Sandy Bay.
GEOLOGY Macquarie Ridge Complex Dredge hauls along the Macquarie Ridge (Summerhayes 1969, Watkins & Gunn 1(70) ohtained in silU tholeiitic basalt, harlburgite, peridotite, sodic olivine gabbro and troctolite, similar to such rocks observed on Macquarie Island. The Macquarie Ridge Complex south of 51°S from these data to be composed of rocks to normal oceanic crust. The Macquarie Ridge is virtually devoid of sediment cover, and sedimentation in adjacent areas is extremely variable (Houtt. & Markl 1972, Hou!1 e! al. 1971, Hayes & Talwani 1972). The sediments in the Macquarie Trench are generally variable in thickness (from 7ero to I km) but arc locally undisturbed and horizontally layered (Hayes & Talwani 1972), with thick accumulations of sediments occurring in small enclosed basins (H outz el al. 1971).
200km -----' fATHOMS o 1000 2000- 2500 2500 - 3000 > 3000
Tectonic history 29 46"
FlU 2 Generalised bathymetry
McCue. this volume) and vec/or.s
mOlion
Minsler & Jordan 1978); A-A' is the location
showing seismicity (after Jones & Indian/ Australian and Pacific plales in gravity profile shown in figure 6
30 P E. Wiiliamson FIG. 3 -"- Bathmletry of the Macquarif' Island reKion contoured in melres (afler Summerhayes 1967).
Tectonic history 31
Macquarie Island
work carried out on Macquarie Island
by Mawson (1943) has largely been superseded by more detailed work (Varne et at 1969, Varne &
Rubenach 1972, Quilty el al. 1973, Griffin & Varne
19HO, (Jriffin 1982, Christodoulou et at, 1984).
Varne & Ruhenach (1972) concluded that the
island is formed of fault-bounded blocks. The
northern part ofthe island is described as composed
mainly of serpentinised peridotites, gabbros, and
dolerite
swarms (fig. 5), and the southern part
of
rocks (fig.4) which occur both as
basaltic lava and as basaltic volcanic breccias.
Parallel dolerite dykes cut the lavas, volcanic
breccias and associated sediments in all parts oflhe
island, but there is no continuous gradation from
areas of extrusive rocks cut by dykes into the areas
of dyke swarms. Globigerina ooze occurs as
interstitial materials in the pillow lavas, and poorly
preserved coccoliths have been found at North
Head.
The detailed structure and stratigraphy of the
extrusive rocks of the island are complex and
poorly understood. The pillow forms of some lavas
imply that these have been tilted or near-vertical
attitudes, but the steepest dips which could be
reliably measured on lava flows with interbedded
mudstone are about 45°. These Hows were pre-
sumably horizontal or near-horizontal at the time
of deposition. Many high and low angle faults cut
the extrusive rocks.
Dykes cut the interbedded lava flows, breccias
and sediments as a series of parallel sheets and do
not intersect one another. Acute angles between the
planes of the dykes and the bedding planes of the
extrusive rocks are almost always large, no matter
how steep the inclinations of the bedding planes,
implying that the dykes were originally steeply
inclined intrusions into relatively flat-lying extru-
sive rocks, and that the tilting occurred sub-
sequently. In addition, the strikes of the tilted lava
flows, breccias, and sediments are parallel or near-
parallel to the strikes of the dykes that cut them.
Seemingly, therefore, the axes about which the
tilting took place must have been approximately
horizontal and parallel to the dyke-bedding plane
intersections (Varne & Rubenach 1972).
In the northern part of the island, coarse-
grained intrusive rocks representing deeper levels
in normal oceanic crust are exposed. The serpen-
tinised peridotites are usually massive rocks. The
only layering interpreted as compositional occurs
at Eagle point, where adjacent layers differ in their
proportions of serpentinised olivine and ortho-
58' '--_ _ _ _ _---'15 km
POint
D dyke swarms '-fault strikes on lavas strikes on dykes ~ ·· <.o. gradational or uncertain contact
FIG. 4 - Generalisedge%gica! map of Macquarie Island with superficial deposits omitted (after Varne & Ruhenach 1972).
32 PI.,'. vVil/iwmun
pyroxe nc. The
and
are sim ilar to that found in cumulate
A layer-ed gabbro and dyke swarm west section of the showinil spectacular Varne & Rubenach lack of schistosity or
volurne) shcn,\
of in!
at the
,ollthern end
and ;.H about 56° S, First rnotion studies
panied by deformation, and
to burial metamorphism.
Petrographic
of rocks from Mac-
quarie Island by Varne Rubenach (I
show
the majority oftlle dolerites and basalts are similar
to normal ocean crust. On the basis of trace
element studies, the rocks from
Island
also clearly correspond to the
basalts,
although some variation in the trace element
content in the rocks occurs (Varne Ruhenach
1972, Griffin & Varne 1980).
Although
forms part oflhe crest of the
Ridge, the
rocks of the island were not, in general, formed
during tectonic evolution and emergence of the
ridge within the last !O Ma. This is
the
observation that some of the freshest and pre-
sumably youngest volcanic rocks on
Island are associated with sediments that
be of M ioeene age (11-25 Ma) (Quilty et
(1972) concluded that the
of rotation of the Australian Pacific plate is
located cast of and close to the ridge, at about
50 57° S. Ruff et at. (I
however, have generated
focal mechanism solutions, in basic agreement with
the
derived RM2 rotation
of Minster
& Jordan (1978), that
the pole at 60.5°S and
for
boundary a recent
is
tectonic reconstruction of
and the studies of Walcott
(! 978) and Scholz et af. (1973). These studies
indicate thaI, prior to 10 Ma RP., the Macquarie
was characterised by strike-slip and spreading
CRUSTAL STRUCTURE AND TECTONIC HISTORY
During the last 10 Ma and lip to the present
time, the dominant motion between the
Australian and Pacific plates south of New Zealand
has becn interpreted as mainly transcurrent, with
compression particularly at the
and
Trenches, wherc varying
duction of the Indian
are
earthquake studies by Sanghar &
from studies of relative plate
Weisse!
(1972), Hayes & Talwani (1972),
and Wcissel el al. (1977).
Seismicity Studies of the seismicity of the Macquarie Ridge Complex have been made Cook (1966) and Barazangi & Dorman ( 19(9). (Barazangi & Dorman 1969, Jones & McCue, this
Gravity
The gross crustal structure associated with
the central Macquarie Ridge Complex has been
investigated using
data. Marine gravity
aata over this area have been inverted to show a
thickening of the !ndian/ Australian
oceanic crustal
from approximately 7 to
14 km, over a 200 km distance a!> the ridge is
from the west
& Johnson
A root W!l(;, some 30 km wide and approxi-
mately 4 km
is indicated nol symmetrically
beneath the
Ridge itself but displaced
towards the
Another root lone, approxi-
50 km wide and 3 km thick, within the
Emerald Basin to the east of an unthickened
segment of oceanic crust immediately west of the
Macquarie Trench, appears to be associated with a
local bathymetric high, possibly caused by volcanic
(Hayes & Talwani 1972) or the presence of
an ancient spreading ridge associated with the
opening of the Emerald Basin.
S8:pentlnlseo periaoilte layered gabbro D QClDbro ether symbols as in flgum '" 54" 30' S Eagle Point Langdon Point
33 krn
() 54" 33' S
158' 58'
23/09/110
FIG. 5 -- Geological map o/Ihe northern part 0/ Macquarie Island with superjicial deposits omitted (ajier VarnI' & Ruhenach 1972).
34 P.E. Williamson
DISTANCE I km)
200
400
600
IY'
(Macquarie Ridge, using denslt!e~ shown below) r----r--~:_':"--------~-'1 _/
\,
Ie l\q.$~··"1
\ Bowers /'1 lij \ Ridge I f/
Macqu8ne
\
1\
\
I \ r'ueito Rico
\-,-/
B
r"~::.'."·;~] Vo!~metric shortening/thickening of
25
'." '.. :.". IndIan Plate near plate boundary
FIG, 6 - (A) Crusta! structure (lower surfaces) over the central Macquarie Ridge Comp/e.x, arrived at hy inversion of Bouguer gravity anomaly data, and hathymetry (upper surfaces); demonstrating incipient suhduction compared to progressively more developed suhduction for the Bowers Ridge and Puerto Rico Trench region M R is Macquarie Ridge; MT is Macquarie Trench; EBH is Emerald Basin High. Densities used for the inversion are a/so shown Location ofprofile is shown infigure 2, (Alier Williamson 1975.) (B) Stippled area shows the crustal thickening relative to the original Indian/ A uslralian oceanic pialI' during theformation of the Macquarie Ridge, which could accommodate crustal shortening of65 km, Asymmetry similar to tj1at observed for the central Macquarie Ridge (excluding the Emerald Basin feature) is typical of island arc systems (fig, 6), The Tonga-Kermadec area has been investigated by Talwani et at. (1961), the mature Puerto Rico Trench region, by Talwani, Sutton & Worzel (1959), the nascent Bowers Ridge Complex by Kienle (197!), and the Japan region by Tomoda (1973). All the above regions exhibit asymmetry in the free air and Bouguer gravity anomalies, and in their inferred crustal structure. Also, in island arc regions, negative isostatic anomalies are associated with the trench and positive isostatic anomalies with the arc or ridge (Heiskanen & Vening Meinesz
19511, Tomoda 1971), Similar relationships are
observed for the Maequarie Ridge Complex (Hayes
& Talwani 1972), For well-developed island arc
systems, such as that associated with the Puerto
Rico Trench (Talwani, Sutton & Worzd 1959),
where extensive loading by island arc volcanics has
occurred, this asymmetry is not as marked as i, is in
(he case of partially developed systems, such as in
the western part of the Bowers R
In the latter
area, crustal thickening increases towards the
trench, with the root zone occurring beneath the
trench. This is most similar to that observed for the
central region of the Macquarie
A
similar, though less pronounced, crustal structure
occurs in the Mendocino Escarpment, where a root
zone is present and is displaced from the bathy-
metric high towards a buried trough feature masked
by thick sediments (Talwani, Worzel & Landisman
1959). This structure would appear to be the result
of secondary intra-crustal compressive forces and
to represcnt the oppposite case to that of the "leaky
transform fault" which rellects secondary intra-
crustal extensional forces.
The various cases cited can be used to show
progressive stages in the development from an
incipient to a mature subduction lOne (fig, 6). The
asymmetry of gravity signature and inferred crustal
structure of the central Macquarie Ridge Complex
(along with the shallow seismicity) correspond to
those of an incipient subduction system and support
the predicted compressional component of relative
plate motion between the Indian and Pacific plates
in the region, as shown in figure 2. It is further
impled that the Indian/Australian-Pacific plate
boundary occurs at the Macquarie Trench where
incipient westward dipping subduction of the
Pacific plate is now occurring. For the northern
and southern ends of the complex, gravity
anomalies and inferred crustal structures (Wood-
ward 1973) both show thickening towards the
ridge. In the case of the Puysegur Trench, however,
a more pronounced asymmetric root lone is
defined. This, together with the occurrence of a
considerably more pronounced component of
underthrusting of the ocean plate in the area
(Johnson & Molnar 1972), is consistent with a
belter developed subduction zone in that region.
Loss of around 65 km of oceanic crust of the
Indian plate, due to crustal shortening, and
thickening to around 150% (fig.6) is implied by
gravity results in the centra! Macquarie Ridge
Complex north of Macquarie Island. The area is
north of the zone where Weissel et al. (1977)
interpreted crustal loss from sea-floor spreading
magnetic anomaly patterns, which they considered
Tectonic history 35
to be due to possible subduction or entrapment by the Pacific plate. Investigation of crustal loss in the area of the gravity profile was not possible from magnetic studies due to lack of data (J. Weissel, pefS. comm.). The oceanic crust in the Macquarie Island region has probably been further thickened and shortened, compared with the location of the gravity profile, to accommodate the additional uplift of the ridge to above sea level. Crustal thickening at Macquarie Island and close to the boundary between the Indian/ Australian and Pacific plates in general, is the result of transpression"al forces, and is thus likely to be associated with structural features, including "flower structures ", that usually occur within transpressional regimes (M" Etheridge, pefs. comm.). Uplift of some rocks of Macquarie Island by approximately 10 km during crustal thickening is indicated, since harzburgites representative of the lowerocean crust formed at depths of 5 km (Griffin & Yarne 1980) have been uplifted to outcrop on Macquarie Island from water depths of around 5 km. The high dips of lavas on the island and the rotation of the south of the island relative to the cent.ral island block, by 55° clockwise around a vertical axis, which has been demonstrated from palaeomagnetic results (Williamson i 978), are also apparently associated with the transpression along the ridge crest (fig. 7). Rotation of rocks on the island is also supported from geological argument (Yarne & Rubenach 1972). The Bouguer gravity anomaly associated with Macquarie Island shows only a Bouguer gravity gradient across the island (Williamson & Rubenach 1972). This is inconsistent with local isostatic compensation of the island but consistent with deepening of the Moho towards the plate boundary at the Macquarie Trench east of Macquarie Island, as indicated by inversion of the gravity profile (fig. 6). The characteristics of the Bouguer gravity anomalies and the inferred crustal structure over the central Macquarie Ridge Complex (fig. 6) are dissimilar to those over accreting mid-ocean ridges (e"g., for the Mid-Atlantic Ridge and the East Pacific Rise (Talwani ef aL 1965), for the PacificAntarctic Ridge (Tomoda 1971), for the North Atlantic and Rykjanes Ridges (Talwani et al. 1971». These typically show gravity lows which are symmetrical about the ridge axis. SEASAT Data SEASAT data, along with earthquake seismicity data collected over the Macquarie Ridge Complex
(Ruff & Cazenave 1985), show geoid anomalies over the Hjort and Puysegur Trenches which arc interpreted as being characteristic of su bduction zones (fig. 8). They display the characteristic geoid signature of mature subducting trenches. This includes a narrow geoid minimum of 5-20 m amplitude and 100-400 km width directly at the trench axis. From the trench to the landward side there is a geoid high, coinciding generally with the island arc. Superimposed on this short variation is a broad positive anomaly in the with a maximum in the vicinity of the trench. The Hjort Trench region is interpreted & Cazenave 1985) as an oblique subduction zone
· n::>rma.! palaeomagnetic directions o reversed directions
-+--- 54°30'
Block 1
22 8 Assumed rotation of Block 3
Block2
j
810<:1<3
FIG. 7 -- Rotation of the southern hlock (3) of Macquarie Island relative to the central hlock (2) during crustal thickening and uplift of Ihe island, inferred from requirements for statistical (ompatahility of apparent palaeomagnetic poles from the central and southern areas of Ihe island. (After Williamson 1978.)
36 P.E. Williamson
/ A
I
"U'","U' m'~\
{J f
!
Profile / 128_
l
5m
':;{ 223_
~.~
c
50'S 239
I '-----'~~_+os FIG. 8 --- SEA SA Tgeoid anomalies over the Macquarie Ridr;e Complex (A and B) and locations o(proflles (C)(afier RuJI& Cazenave /985). Anomalies over the Puysegur (A) and Hjort Tre,!ches (B; profiles 43-353) are interpreted as indicating nascent suhduction. Anomalies over the Macquarie Trench (fig. B; profiles 239/36) are non-diagnostic consistent with incipient suhduction.
37
similar to the A!eutianKomand0rski
consistent
a nascent subduction lone ellviron o
menL Thc geoid anomaiics ovcr the central Mac-
quarie
are
but
clearly not consistent
subduction in that
characteristic narrow
Regional Pattern of Sea-Floor Magnetic Anomalies The formation of the eastern Indian/ Australian oceanic plate south of Australia, to which the Macquarie Island rocks belong, has been investigated from sea-floor spreading magnetic anomalies (Le Pichon & Heitzler 1968, Weissel & Hayes 1971, Weisse! ef at 1977) tied to the geomagnetic time-scale of Heitzler el al, (1968)0 The magnetic anomaly lineations on the Indian/ Australian plate south of the Resolution Ridge at 48° S strike eastnort heast and are interpreted as resulting from formation of Cainozoic oceanic crust at the Indian-Antarctic Ridge (figo 9). Anomalies to anomaly 21 are identified. Anomaly lineations are offset in detail (figo 10) by fracture zones striking northnorthwest. To the west of the Macquarie Ridge Complex north of 48°5 (fig. 9), the magnetic anomalies trend in a northnorthwesterly direction and have been correlated with specific sea-floor spreading anomalies associated with the opening of the Tasman Sea (Ringis 1972, Hayes & Ringis 1973, Weissel el at 1977). An interpretation of the magnetic anomalies on the Pacific plate to the southeast of the Macquaric Ridge Complex shows the maximum age of oceanic crust in the region is about 80 Ma, anomaly 32 (78 Ma) occurring close to the Campbell Plateau (Christoffel & Falconer 1972)0 A sequence down to anomaly 25 occurs south of thiso The next reliable identification, however, is of anomaly 5, which occurs close to the Pacific Antarctic spreading ridge. The results imply that distinct periods of sea-floor spreading have occurred within this region, as many of the sea-floor spreading anomalies between 5 and 25 do not appear to be represented in the area (Christoffel & Falconer 1972, Adams & Christoffel 1965). To the east of the Macquarie Ridge, at about 51.5° Sand J 6205° E, anomaly lineations are poorly
55 c SOUTHWEST P4C!FIC OCEAN L_o_ _o._ _1______o-l-_____.L_ _ _---I60" FIG. 9 - Generalised !fends marine magnetic lineal ions surrounding the Macquarie Ridge Compiexo For the Irenches 2500 fathom con/ours are shown. defined but appear to trend approximately parallel to the ridge (Hayes & Talwani 1972). However, the tentative correlations may reflect structure in the sea floor rather than the normal polarity reversals. The direction of magnetic lineations has also not been established in the Emerald Basin which lies to the south of the above area. The orientation of tracks and the apparent wavelengths displayed in that area, suggest that, if lineations are present, they are probably oriented in an approximately northnorlheasterly direction. Spreading Magnetic Anomalies in the Macquarie Island Re!,rion The history of oceanic crust around Macquarie Island was investigated, using magnetic data collected along profiles parallel to the Macquarie Ridge from the Nella Dan (Williamson ar el 1981)0 Anomaly 7 (27 Ma) is identified, on Nella Dan data, to the west of Macquarie Island (figo 10), and anomalies 8 and 9 north of the island.
38 P. E. Williamson
These allomaiies have been identified with greater confide!1ce due to their existence as part of a longer sequence of anomalies, which extend from anomaly 7 to ana maly 21, and are observed in the Nella Dan data between Melbourne and Macquarie Island. The magnetic lineations parallel sea-floor spreading anomalies on the Indian/ Australia plate west of
\
I
\
I
\
\
\
\ -\
\
\
-\ \ \B \
\
Interpreted zone \1 \ 54' 30' S \
the island and terminate east of M acg uarie Island at the Macquarie Trench, which is interpreted to be the Indian! Australian-Pacific plate boundary; this implies that Macquarie Island occurs on the Indian/ Australian plate. Offsets in the magnetic lineations suggest the presence of two minor transform fall Its 10 the west of the island. These are as a del1ection to the north in the magnetic lineations interpreted in the broader ar study of Weissel et (1977). To the west of the island, anomalies g and 9, observed in the Ellanin 44 data, occur about 80 km south of the same anomalies on the Nella Dan tracks east of a separating transform fault. A second transform fault, with a small offset, appears to trend in a northnorthwesterly direction close to the West Coast of Macquarie Island. Here, anomaly 9 is interpreted to occur about 20 km to the south of the same lineations further west. Anomaly 9 is tentatively correlated over the ridge north of Macquarie Island. The shape of the anomaly is distorted, however, probably as a result of the magnetic anomaly associated with the ridge. Sea-Floor Spreading Magnetic Anomaly Data on Macquarie Island A magnetic anomaly lineation is interpreted as crossing Macqllarie Island (fig. 10). The preferred identification is that it is anomaly 7 (27 Ma), since the broad features of the upwardly continued magnetic anomaly profile (10 I km) along the island can be correlated with anomaly 7, interpreted on the adjacent marine magnetic profiles. In the onland data, the character and anomaly widths of the magnetic high and the associated low to the south are also similar to those of anomaly 7 and the associated magnetic low in the adjacent marine
FIG. 10 - Marine magnetic anomaly profiles in the vicinity of Macquarie Is/and to the west ofthe plale houndary at the Macquarie Trench. Identified anomalies are from the Heirtz/a ct al. (1968) sequence. Inferred fracture zones are interpreted from offsets of the anomaly sequences. The lower part ofIhefigure shows marine magnetic anomaly profiles close to Macquarie Island. Also shown is the main north-south profile along the island, upwardly continued to a level of I km, which is similar 10 the depths to sea floor on the marine profiles. The len/ative correlation of the anomaly on either side and across the island corresponds to anomaly 7 (27 Ma B.P.).
Tectonic hisiOY)' 39
data. This implies the presence of remanent seal100r spreading anomalies over, and generated by, the island lavas. Magnetic propertie~ of the Macquarie Island rocks, confirming the ability of the basalt lavas on the island to contribute to sea-noor spreading magnetic anomalies, have been reported by Williamson (1974, 1978), Butler & Bancljee (1973) and Butler et al. (1975, I Modelling of the main north-south onland magnetic profile from Maequarie Island (William- son ei aI. 1981) demonstrates that using palaeo- magnetic, magnetic property and structural data from the island, the upwardly continued magnetic anomaly profile can be modelled as a remanent profile caused by a sea-floor spreading sequence of magnetic reversals. The model is complex, but mainly reflects the effect of primary remanent magnetic properties of the rocks, the directions of which have been adjusted by subsequent block faulting and tilting, probably related to the formation of the ridge and island. Minor magnetic features of the profile appear to be related to demagnctisalion, possibly due to local weathering associated with major zones of faulting (fig. II). The magnetic anomaly profile cannot be modelled as induced, using the magnetic susceptibilities measured for the island rocks (Williamson 1978). These could produce magnetic anomaly amplitudes which are only around 50% of those observed. However, the observed anomalies can be produced by remanent magnetic polarity reversals, since the Koenigsberger ratios (normal remanent magnetisationjmagnetic susceptibility ~ NRMjSUS) of the island lavas are close to unity (Williamson 1978). The final model for the distribution of norma! and reversed directions of remanent magnetisation within the pillow lavas of the island has a broad section of reversed polarity followed, to the south, by a broad normal polarity section, interrupted in the centre of the island by a narrow lone of reversal (fig. II). This is qualitatively similarto the sequence of reversals surrounding anomaly 7 of the Heirtzler et al. ( 1968) series. If the pattern of reversals along the island does correspond to the anomaly 7 sequence, however, an apparent compression of the sequence relative to that further to the west is observed. This could be associated with transpressional structures at the ridge crest which accommodated greater crustal thickening in the formation of the island.
I
A
1=-3,°(+35°)
1=-15C(+2~)cl
o · normal mognetisotion Do reversed magnetisation ~ assumed to be demagnetised c
o 5 km '---_--'I E FIG. I 1--· Modelfor remanent magnetic anomalies a/on;; the main north south track of l\1ucquarie Island, incorporating ma;;nelic, palaeomagnetic and structural data; with (A) as the anomaly profile, (B) the inferred model, (C) the synthetic vertical mmponent at a leVl'1 of 2 m, (D) at I km and (Ј) the upwardly continued profile at a level of I km,
40 PE. Williamson
Dala
P 21aeomagnetic apparent
data from
Macquarie Island (Williamson 1(78) have been
reviewed and recalculated
In
relation to the revised polar wander
Austra lian plate (ldnurm I
apparent palaeomagnetic pole for
is now ccmsidered possible from presem data, due
to the inability t.o resolve rotations around
vertical axis in relation to any
structural
model generated from other data, The
range 0 f poles in not inconsistent with """cr""',"
from marine magnetic data but cannot be said to
uniquely support them.
DISCUSSION
Geophysical results can be summarised as
follows:
( I) The delineation of a remanent sea-floor
spreading magnetic anomaly lineation across
Macquarie Island, with a trend similar [0 those on the Indian plate to the west, suggests that the rocks
of the island were formed at the !ndian/
Australian-Antarctic
(2) The coincidence of the trends of Indian/ Australian plate sea-Hoor spreading lineations with the strike of dykes in the central region of the island is consistent with the model of dyke injection at a mid-ocean ridge, and further argues for the forma-
tion of the island rocks at the indian/ AustralianAntarctic spreading ridge. (3) The interpretation of crustal structure from gravity data implies that the eastern extent of the Indian/ Australian plate is at the Macquarie Trench, east of Macquarie Island, which again implies that
the rocks of Macquarie Island were formed as part
of the Indian/ Australian oceanic plate.
(4) The preferred interpretation of the remanent
magnetic lineation crossing Macquarie Island is
anomaly 7 (27 Ma), This
is an exten-
sion of interpretations of remanent magnetic anomalies over the oceanic crust for the
Australian plate west of Macquarie Island, The interpreted age is considered to be correct within
approximately +3 Ma, which corresponds to the anomaly being misinterpreted anomaly 6 or 8" Due
to the structural complexity of the region such a misinterpretation may be possible. Detachment of
oceanic crustal blocks at the ridge could contribute to this so that the age is more likely to be of
anomaly 8 than 6, given that the motion of the
Indian/ Australian plate relative to the Pacific plate is dextral. That is, the age is more likely to be
underestimated than overestimated. An age for
appear to be the casco
of
the
data, are broad scale and do
rock
and certainly
those data argue that the
of Macquarie
Island in generai were formed at the Indian/
Antarctic spreading ridge at around 27 Ma B.P.
A discrepancy exists, however, between geo-
physical and some palaeontological dating and all
K-Ar age estimates. Palaeontological data asso-
ciated with rocks from North Head ("some of the
freshest volcanic rocks") on the island have given
ages of middle to
Miocene(QuiltyelaI.1973),
Recent K-Ar data
by Varne (pers. comm.),
however, give young ages, normally within the last
10 Ma, for rocks oflhe island. Those ages essentially
correspond to the normally accepted time of uplift
of the ridge and island estimated from geophysical
data. K-Ar ages could not be obtained for the rocks
used in the palaeomagnetic study because their
preservation was not considered sufficiently good
for that purpose.
The apparent discrepancy between the two
sets of ages couid reflect two factors:
(I) essentially different data sets relating, on the
one hand, to original oceanic crust and later
intrusive and extrusive activity (such as seamount
activity) as,ociated with the already formed oceanic
crust Of, on the other, to crustal compression
during formation of the Macquarie Ridge; this
could result in the fresher
being more
recent, which may be
relevant to
from North Head which are unusually
"fresh"
or
(2) inaccuracies in the relatively small number of
palaeontological age determinations, due to poor
preservation of specimens and misidentification;
loss of resid ual argon, due to tectonic activity
during the last 10 Ma, giving young and incorrect
K-Ar dates, although some of these are 30 ArtO Ar
dates, which are not as susceptible to Ar loss or
alteration; dating simply reflecting time of uplift;
or fortuitous correlation of unrelated geophysical
anomalies.
"Tectonic
41
Some igneous episodes after the formation of the original oceanic crust are to be expected, however, particularly given the level of tectonic activity of the region and the nearness to the plate margin. This could suggest I hat factor (1) above is the dominant cause of the discrepancy. Nonetheless, it is important that investigations be carried out to resolve the discrepancy, and to this end more extensive palaeontological studies on sediments between lava layers from all areas of the island could be employed.
CONCLUSION
Macquarie Island is a rare example of uplifted
oceanic crust. As such it is a geological enigma and
a critical area for the study of ocean tectonics. The
structure and physiographic expression of the
island and the surrounding region are the result of a
tectonic history of oblique compression (trans-
pression) between the Indian/ Australian and
Pacific oceanic plates at their common boundary
within the Macquarie Ridge Complex south of
New Zealand. Intra-plate transpressive forces in
this
resulted in the uplifted of oceanic crust
of the Indian/ Australia~ plate to form the
Macquarie Ridge and Macquarie Island, and in
incipient subduction of the Pacific oceanic plate at
the Macquarie Trench. This uplift has been effected
by thickening and shortening of the oceanic crust
of the Indian/ Australian plate, the amount of
uplift being reduced by regional isostatic compen-
sation of the increased crustal thickness. The
transpressional regime has persisted during the last
10 Ma; prior to that time, strike-slip and exten-
sional regimes occurred along the Macquarie Ridge
Complex.
The uplift of Macquarie Island has been
accompanied by tilting of rocks and rotation ofthe
southern segment of the island, relative to the
central segment, by about 55° in a clockwise
direction around a vertical axis. In the north,
gabbro and serpentinite fonned at depths of up to
5 km in the primary oceanic crust, are currently
exposed abovc sea leveL Structural tilting of the
island lavas has not been sufficient. to reverse the
primary in situ directions of remanent magneti-
sation. Consequently, long wavelength remanent
magnetic sea-floor spreading anomalies associated
with areas of outcropping lavas in the centre and
south of Macquarie Island correlate, with the same
with similar features in marine magnetic data on the indian plate on both sides of the island. Measured magnetic properties of the isJand's igneous rocks confirm that their remanent magnetisations could contribute to sea-floor spreading magnetic anomalies, with the dominant contribution coming from the lava sequences and iesser contributions from gabbros and dnierites. The sampies measured do not exhibit stable
Sea-l1oor spreading magnetic
7 is
correlated across Macquarie island,
indicating that oceanic crust, of which the island is
composed, was formed al the Indian-Antarctic
accreting mid-oceanic ridge around
7
time (27 Ma B.P.).
This is in conflict with evidence from younger
palaeontological and K-Ar dates from the island
(from Miocene to Recent) which may reflect later
metamorphic or igneous episodes after the estab-
lishment oflhe original oceanic crust. The youngest
K-Ar date, for instance, is on a metamorphic dyke
from the sheeted dyke complex and reflects the
timing of the closure of the metamorphic minerals
as they cooled (R. Varne, pel's. comm.).
To date, broad tectonic studies have presented
insights into the mode of formation of Macquarie
Island, the origins of its dominant rock types and
the nature and result of the large..scale structural
movements which affected the region. The detailed
structural configurations involved are, however,
poorly understood. Future work could usefully
investigate the inferred presence of transpressional
.structures on the island, associated with the crustal
thickening and uplift of deep crustal rocks,
assuming sufficient data reflecting stress and strain
directions can be obtained. In addition, more
extensive palaeontological studies could help to
resolve discrepancies in the ages of the island rocks
as interpret.ed from existing palaeontological,
K Ar and geophysical data.
ACKNOWLEDGEMENTS The work described here was carried out under the auspices of ANA R E with assistance from many !\N ARE personnel. Constructive reviews by c.J. Pigram, B.D. Johnson and R. Varne have helped bring the text to its final form.
42 FE. Williamson
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