Preliminary report on the results of geoscience research in the New Hebrides arc-trench tectonic systemPRELIMINARY REPORT ON THE
RESULTS OF GEOSCIENCE RESEARCH IN THE NEW HEBRIDES ARC-TRENCH TECTONIC SYSTEM
(RV Akademik Alexander Nesmeyanov, Cruise 17)
Dr I.K. Pushchin Chief of Cruise
Pacific Oceanological Institute of the Far Eastern Branch of
the USSR Academy of Sciences
NOTE: This cruise report has been published by SOPAC without alteration to the original written by Dr Pushchin except for the inclusion of additions to text and tables supplied by him. All tables and figures are reproduced from the original manuscript. With the exception of Appendix 1, the appendices are not included, but are held by the SOPAC Technical Secretariat with the original manuscript.
[3]
CONTENTS
Page
Bathymetric surveys ........................................................................................ 11 Seismic reflection survey (single-channel) ....................................................... 13 Magnetic survey ................................................................................................ 16 Geothermal studies ........................................................................................... 17 Physical properties of rocks ............................................................................... 30 Petrological studies ............................................................................................. 35 Lithological studies ........................................................................................... 39 Biostratigraphic studies ................................................................................... 43 Hydrogeochemical studies .............................................................................. 44
CONCLUSIONS ................................................................................................................ 48
REFERENCES ............................................................................................................. 57
APPENDICES 1 Preliminary programme of geological/geophysical researches
during Cruise 17 of the RV Akademik Aleksander Nesmeyanov in the New Hebrides Arc-Trench tectonic system ................................. 59
2 Seismic profiles
5 XRF data on magmatic rocks
6 Catalogue of stations Unumbered: lists of 563 dredge samples and 6 cores from Hunter fracture zone, Leg 17
Catalogue of magnetic anomalies and depths
[CR134 - Pushchin]
2 Heat flow probe ............................................................................. 8
3 Map of data ......................................................................................... 12
4 Distribution of sedimentary cover ...................................................... 15
5-15 Profiles ................................................................................................ 18-28
16 Map of magnetic anomalies ............................................................... 29
17 Sound velocity versus density in igneous and metamorphic rocks .... 33
LIST OF TABLES
1 Heat flow data ................................................................................. 30
2 Physical properties of volcanic and sedimentary rocks, Hunter Fracture Zone and Ridge ........................................................... 31
Physical properties of intrusive and metamorphic rocks of Hunter Fracture Zone and Ridge ............................................................... 32
Lithified sedimentary and volcanic-sedimentary rocks dredged in Study area N17-6 ........................................................................... 40
Hydrogeochemistry parameters of sea water along cross-section near
3
4
5 172 degrees E, Southwest Pacific, Study Area N17-6 .......................... 50
172 degrees E, Southwest Pacific (Study Area N17-6) ....................
degrees E, Southwest Pacific (Study Area N17-6)............................... 54
6 Composition of gas dissolved in sea water along cross section near 52
7 Trace elements disolved in seawater along cross section near 172
[CR134 - Pushchin]
1.1. General information on the cruise
Marine geoscience investigations in the New Hebrides Tectonic system were organised in Cruise 17 RV Akademik Alexander Nesmeyanov by the Pacific Oceanological Institute of the Far Eastern Branch of the USSR Academy of Sciences. The main goal of the cruise was to study the structure and composition of the oceanic crust in the Hunter fracture zone and adjacent morpho-
structures. The location of study areas and methods of investigation were co-ordinated with the representatives of the Republic of Vanuatu.
Geoscience works were carried out from January 5 to 29, 1990, predominantly within study
area N17-6 (Figure 1). Bathymetric, seismic reflection (single channel), and magnetic surveys
preceded rock sampling. Samples were taken by dredges and gravity corers. Data obtained were
partly processed onboard ship (petrographic, chemical, and paleontological studies). Several sampling sites were occupied beyond the study area, in the South-Fiji Basin and the North-Fiji Plateau.
Copies of all preliminary data (sample collection, seismic, magnetic and bathymetric records,
and the results of onboard analyses) were transferred to the Department of Geology, Mines, and Rural Water Supply of the Republic of Vanuatu.
All conclusions concerning the geology of the studied area based on on-board analyses are preliminary; they could be substantially revised after further studying of material in onshore
laboratories.
1.2.1. Geomorphological and geophysical surveys
Geomorphological surveys were carried out using 12 kHz onboard echo sounders of CEL-3
and LAZ-72E types ("Honeywell ELAC"). "Tsikada" and "Transit" (JLE-320) satellite navigation
systems were used to fix the ship's position. Echo sounding profiles were referred to these data.
Seismic reflection surveys (single channel) were carried out together with magnetic and bathymetric measurements along planned profiles at a ship's speed of 5.5 to 7 knots. A standard
method was used: the source (air gun) was towed 30 m behind the ship, and a hydrostreamer
[CR134 - Pushchin]
240
Figure 1. Scheme of study areas location. -outline of Study Area 17-6;
,
[7]
200 m behind the ship, providing an essential relationship between signal and noise. Signals were generated by "Impulse-I" firing chambers 31 volume; an EK-7.5 - types air compressor was used.
Operating conditions: water depth above the source 5 m, operational pressure of the air gun
14 MPa, continuous operational time of the source 25 hours, three streamer sections (two of them
are operational), water depth above the streamer 3 m, towing speed 8 knots, filter range 35 to 120
Hz, shot interval IO sec and record range 5 sec. Preliminary routine processing of records,
preliminary seismographic analyses, and time cross-sections were made onboard ship. For our
interpretations, seismic velocity in water was taken as 1500 m/sec. The thickness of acoustic units was estimated using two-way acoustic travel time. No corrections were made for seismic drift.
Seismic stratigraphy was described using terms defined in works of Kunin et al. (1983), and Seismic Stratigraphy ...( 1987). The resolution of the method was estimated as 0.02 sec.
Geomagnetic studies included measurements of the total magnetic field of the Earth (T),
distinguishing its anomaly constituent (ATa), and determining the features of the anomaly magnetic
field.
Total magnetic field data were collected by a MEM proton magnetometer, the operational conditions of which were described by Rotstein & Tsirel (1963), Livotov et al. (1979), and Hood et
al. (1979). It was towed 250 m astern of the ship on a nonmagnetic rope. T was registered by a KCP-4 recorder.
Geothermal studies. A 5-channel heat flow measuring system was used, consisting of heat flow
probe (Figure 2) and on-board computer. The probe is connected with the computer by a 3-
cord wire to control measurements. The probe includes 3 thermal and 2 thermal conductivity
sensors for in situ measurements. The accuracy of temperature gradient measurements is
+0.005K, the accuracy of in situ thermal conductivity measurements is +5%, the accuracy of absolute temperature measurements is +0.02K. Detailed description of technique is given in
work of Balabashin et al. 1985.
1.2.2. Rock sampling and analytical work
Dredging. Rocks were sampled by cylinder dredges 50 to 55 cm in diameter, carried on a
steel rope of variable diameter from 10 to 18 mm.
[CR134 - Pushchin]
[CR134 -Pushchin)
[9]
For sediment sampling we used a gravity corer (operational length of 5 m) and a gravity corer
of increased diameter with a load of up to 100 kg, equipped with a differential pressure valve,
which allowed us to take cores up to 5 m long at depths of 6 to 8 km
All cores were analysed for calcium carbonate content, and some sections chosen at random
for several chemical elements (K, Na, Ca, Fe, Cu, Zn, Ni and Co) using a "Saturn-I atomic absorption analyser.
We employed the classification of compositional and genetic types of sediments developed in
the Shirshov Institute for Oceanology of the USSR Academy of Sciences (Bezrukov, Lisitsyn, 1968;
Murdmaa, 1979). Sediment names are based on a three-constituent grain-size classification.
"MIN-8", "Polam", and "Amplival" microscopes, "MBC-2 binocular microscopes, fractional
disperser, carbonatometer (our design), powder disperser, and photoelectrocalorimeter were used for analyses.
Paleontological studies . Conventional methods were employed for microfossil studies
(planktonic and benthic foraminifera, calcareous nannoplankton, diatoms, spores, and pollen)
(Barash, 1970; Saidova, 1973; Diatoms of the USSR, 1974; Erdtman, 1933; Grickhuk,
Zaklinskaya, 1948; et al.). "BIOLAM-2II" microscope (x 1300) and "MBC-1" binocular
microscopes were used.
Geochemical studies of microelements in rocks, seawater, and the atmosphere included comparison of different techniques, measurements of suspended and dissoloved Cd, Mn, Fe, Zn, Cu, Co, Cr, Ni, Mo, Pb, Ag, and Hg, and the determination of major parametres of equilibrated
carbonate system and gases in seawater.
Rock specimens were taken for analysis with respect to their textural and structural features.
An average specimen weight was 50 g. Surficial seawater was taken from the moving ship at depths
of 0.1 to 0.2 m three times a day (9 am, 2 pm, and 7 pm).
Deep-water specimens were taken using big (51) bathometers made from inert materials. Five levels were sampled at each station (10, 50, 100, 200, and 400m from the sea floor).
About 0.81 of water taken by a bathometer was used to measure hydrochemical parameters; a suspension and a concentrate of dissolved metals was extracted from the rest of the taken water.
The technique was described by Kovarsky (1981, 1987).
[CR134 - Pushchin]
[10]
Bottom water from a sealed bathometer at true pressure was put into an evacuated polyethylene container; the sample was allowed to come to room temperature to extract and analyze
gaseous components.
Dissolved gas was extracted from seawater in a degassing unit using a vacuum pump. CO 2,
O 2, N 2, He, and hydrocarbon gases were measured by LXM-80 and Casochrom-3101 chromatographs.
Mercury content was determined in rocks, atmosphere, and seawater employing a "Mercury- 3M" atomic fluorescent photometer; the technique was described by Stepanov and Trukhin (1986).
TO determine microelements in sea-water, we used a "Saturn-I" atomic absorption
spectrophotometer. Zn, Fe, Co, Cr, Ni, Mn, Cu, Dc, Pb, and Ag concentrations were determined. Detection limits were 0.075, 0.375, 0.375, 1.875, 0.375, 0.0375, 0.375, 0.0375, 0.5, and 0.075, respectively.
Major ore elements were determined in altered rocks using a "Spark-I" X-ray fluorescence
spectrometer and "Staturn-I" spectrophotometer. Iron-group elements, alkalies, copper, lead, zinc, and silver were measured.
Modification of X-ray spectrophotometers and choice of essential conditions of excitation and
recording of X-ray K-spectra allowed us to obtain the following detection limits at 100 sec exposure: Ti:4.10%, V:2.10%, Cr: 8.10%, Mn:6.10%, Co:5.10%, Cu:8.10%m Zn: 1.10%, and Ni:
5.10%.
Approved techniques were used to determine salinity and characteristics of equilibrated
carbonate system (pH, dissolved oxygen, and alkalinity) (Methods ..., 1978). We employed 1-115 pH meter, GM-65 salinity meter, and titration sets manufacture in the Institutes for Chemistry and
Pacific Oceanology of the Far Eastern Branch of the USSR Academy of Sciences.
Conventional techniques were used to study physical properties: density, moisture content,
porosity, velocity for P- and S-waves, absorption of sound, 8-activity, and acoustic non-line-arity
(Kalinina, 1962; Krobanova et al., 1977; Myuir, 1977; Boyce, 1973). Some differences of technique are due to local conditions.
[CR134 - Pushchin]
2. RESULTS
The most important result of our studies at the southern termination of the New Hebrides
arc-trench tectonic system is the revealing of one of the most complete cross-sections of layers and 3 of the oceanic crust. Thus, the main goal of the cruise provided by the Program (see
Appendix 1) was achieved. In addition, data on the seafloor relief, the composition and structure of
the sedimentary cover, and also substantial new paleontological data were obtained.
As it has been mentioned before, further petrographic, paleontological, and chemical analytic
studies will be necessary for final evaluation of works performed.
2.1. Bathymetric surveys
The main goal of the bathymetric survey in the study area, (and also the seismic reflection survey) was to choose sites for rock and sediment sampling. Surveys were carried out along a series of track lines oriented normal to the strike of the island arc system. In addition, several crosscutting
track lines parallel to the arc were surveyed (see Figure 3). Initial construction of a map of seafloor morphology within the study area, was based on a bathymetric map compiled by the French cruise of the RV Coriolis (Monzier et al., 1984).
The study area occurs within a structural transition zone from an island-arc tectonic system to
a transform fault of an arc-arc type. This situation determines the principal features of the seafloor relief in the study area N17-6.
The Hunter Ridge, lying on the projection of the New Hebrides Ridge, was surveyed in
several intersections to the east of Hunter Island. Near the crest of the ridge there are numerous
volcanic edifices which have relative elevations of several tens to a few hundred m, and rarely up to
800-1100 m. Minimum recorded depth was 1150 m (see profile 8).
The inner slope is approximately 45 miles (83 km) wide, and is complicated by numerous
depressions and uplifts with elevation reaching 3500 m. It is notable that the uplifts are arranged
along the inner slope to form a linear ridge, parallel to the Hunter Ridge, but separated from it by a narrow trough (Figure 3). This trough presumably traces a fault zone, that has split a single ridge
into two parts, which we propose to call the North Hunter Ridge and the South
[CR134 - Pushchin]
.II !! N" .E § -'-.~'" " ..j, 2_6=; 0 ., ." .8"11",, .~B-S~ ~., .,--., ~:S'£g""1j3.~~
0( "~.~-"~- E g~!d' ~ ~:~ E".Eo.. .=-0'0( ~~~i'3g2c;;.!-N . 0 -gZ~ .-o",g' \.C') u.. .""~..;"'!!~E 0 .;-g".;,~":,'g ~~-o
Q) Q. Em~ .g-""'.3~"'- =- ."o~~~.-g.".~:J ~ c=E..c,§"~-"C- w .~>~Ej,=' ".8.2.OJ :I: "'=.' .~".cc..~!J'u.-~ .g~7-"c~"-,.e~~!!£ 0 o. =~."~. 0". ..c..- .~ .~ "NM5~38~~~~~~~ I
~ ~~~~~g>8~&.'=E~ w .c~.".2.,.,.2=£"'!!o" ,;,;,~. '."".~.~
~ -1- ~ "~..§"..,,.~" +
). I / :
.~ '---0-;;--;-;:;-- L ~ oj ~~ ~~~o 1 ~ ~ (.J Q.. ~ ~ :J)c ~ ~
,,-0 ~ ~ ~ :J E l Q:: +' q
C/) g
2 g
0 ~~:t: ~ t ~ ~ lJ W
~ '::] ~ 7 QJ C S -
-~ ~ -~ a- ,~ D g-
::t c= -"""'~~~.~R=~
~- --:- .::~- ~~ [CRl34 -Pushchin]
Hunter Ridge
The New Hebrides Trench within the study area occurs as two east-west aligned basins with maximum depths of 7575 and 7660 m, separated by an uplift with a minimum water depth of
6700 m. Along the isobath of 6000 m the trench is less than 20 km wide and to the east of 172'35'E (172'30'E according to the French bathymetric map) it is pinched out.
An east-north-east trending basin up to 6200 m deep, lying further to the east and separated by an uplift with a water depth of 5300 m over it, is not a projection of the trench; but rather is
related to the system of transform faults along the Hunter Fracture Zone (Monzier et al., 1984).
The New Hebrides Trench has asymmetric transverse profile typical of all trenches: i.e., the inner slope is steeper than the oceanic one. At the depths between 4700 and 5000 m the oceanic
slope gradually levels out onto the seafloor of the South Fiji Basin.
2.2. Seismic reflection survey (single channel)
The total length of track lines along which a continuous seismic reflection survey was carried
out in study are N17-6 is approximately 800 kilometers (-430 n.mi). The records are of satisfactory quality (Appendix 2).
Profile 1 has sublongitudinal orientation and occurs at the western boundary of the study area. The seafloor in the northern part of the profile is tilted to the south and covered with a
sedimentary layer, 0.1 to 0.2 sec thick. These sediments cover a very rugged acoustic basement.
The southern part of the profile is confined to the trench. Sediments occur sporadically, their
thickness is small (up to 0.2 sec). The acoustic basement and part of the sedimentary cover are
fractured; separate blocks are shifted relative to each other along these fractures, with displacements
reaching up to 2 km.
The distribution of sediments along profile 2 is similar to that along profile I. In the northern
part of the profile the thickness of the sedimentary layer varies from 0.1 to 0.65 sec; the southern
part (the trench) is practically devoid of sediments. Displacement along faults in the acoustic basement is estimated to be several tens to a few hundred m.
[CR134 - Pushchin]
[14]
Profile 3 is characterised by widespread sediments, the thickness of which reaches 1.2 sec in the northern part.
The thickness and the distribution of the sedimentary layer along profiles 4 to 7 are similar to those of profiles 1 to 3.
Profile 10 lies to the south of the main study area on the projection of profile 7 into the South Fiji Basin. The surface of the acoustic basement is undulating, and virtually everywhere covered with sediments 0.4 to 0.5 sec thick.
Profiles 8-9 are orientated east-west, and cross the whole study area. The rugged surface of the acoustic basement is covered with sediments, the thickness of which varies from 0 to 1.5 sec, being thickest at the foot of seamounts. The average value falls between 0.1 to 0.15 sec.
In the south of the study area, in the axial part of the deep trench, and below the surface of
the acoustic basement, a distinct seismic boundary was recorded where intense diffraction of seismic waves occurred.
The analysis of the thickness of the sedimentary layer within the study area shows that sediment distribution is controlled by morphostructural features of the relief (Figure 4). In the
deepwater of the New Hebrides trench, sediments occur only on the southern slope, whereas the
northern slope is devoid of them. Southwards of the trench, the thickness of sediments is within 0.5 - 0.6 sec except for an uplifted region marginal to the trench, where rocks of the acoustic basement outcrop onto the seafloor. In the northern part of the study area, sediments are concentrated principally in seafloor depressions and at the foot of seamounts, the tops of which are
practically devoid of sediments. The thickness of sediments reaches 1.5 sec.
Profiles 11 - 12 cross a seamount in the South Fiji Basin. The surface of the acoustic
basement near the seamount is an uneven subhorizontally boundary, corresponding to the magmatic
basement (see Appendix 2). Sediments occur subhorizontally, their thickness varies from 0.6 to 1.0
sec.
The analysis of wave pattern in the sedimentary cover and the relationship between reflectors resulted in two seismic units ("A and B") being distinguished in the sedimentary cover.
Seismic unit "A" occurs unconformably over wither the rocks of the acoustic basement, or
seismic unit "B". Its upper surface corresponds to that of the seafloor. According to its
[CR134 - Pushchin]
THE DISTRIBUTION OF THE SEDIMENTARY COVER
f~ -..£ -_foe. -jOKM
171.4fJ 172"00 172'20 17e.4O
;
[16]
seismoacoustic features, seismic unit " A is a stratified layer with clear internal reflectors. Bedding is
subparallel and conformable to the underlying layers.
Long reflectors are consistent along the strike. Rocks of seismic unit "A occur only in the southern part of the study area, along the periphery of the South Fiji Basin. The thickness of the
seismic unit does not exceed 0.15 sec along the southern slope of the New Hebrides Trench, and it gradually increases up to 0.4 sec towards the basin. The sediment distribution in seismic unit "A" suggests accumulation in a tectonically quiet setting. Sediments of this seismic unit were formed
under deepwater conditions. In the upper part of the unit foraminiferal ooze predominates (St. N17-
69) with some volcaniclastic material. The age of these sediments is Upper Pleistocene. The oldest
sediments of seismic unit "A" are proposed to be Pliocene.
Here and further, all data on the composition, age, and genetic conditions of sediments are by lithological and paleontological team of the cruise.
Seismic unit "B" consists of a thin (up to 0.15 sec) well stratified layer resting unconformably
on the acoustic basement. Reflectors within this seismic unit are discontinuous, subparallel, and sometimes clearly dislocated. To the north of the trench these sediments accumulated under the
tectonically active conditions of the region, whereas to the south (in the South Fiji Basin) they have
accumulated under a rather quiet tectonic setting. Deepwater compact clay with admixtures of
volcaniclastic material predominates in the composition of this unit. The age of sedimentary rocks of
seismic unit "B" probably does not exceed Middle Miocene.
2.3. Magnetic survey
The total magnetic field (T) was measured along a series of north-south (main) and east-west (crosscutting) track lines (see Figure 3). Root-mean-square error of results, calculated from T
difference in sites of track line intersections is +16nT (intersections of track lines 6 and 9 in high-
gradient field is eliminated from calculations). Discrepancies between T values on transferred
profiles are mostly due to magnetic variations and coordination error. The effect of the magnetic
mass of the vessel calculated by T changes at the turns does not exceed +5 nT for every track lines.
Magnetic anomalies ( Ta) were calculated using the IGRF-85 international analytical model
and are given as a catalogue (Appendix 3), plots for each profile (Figures 5-15) and an isodynamic
line chart (Figure 16). the catalogue also lists the total magnetic field.
[CR134 - Pushchin]
[17]
Since an average Ta is approximately 100 nT within the study area, we reduced numerical values of isodynamic lines by 100 nT to facilitate reading of the map of magnetic anomalies. In
accord with the precision of surveys the spacing between the isodynamic lines was chosen 50 nT.
Figures 5-15 and 16 show that anomalies of both signs are not very intense, feature typical of most trenches. A zone of lower magnetic field occurs east-west along the lower inner slope of the
trench. Less clear negative regions (anomalies) are also oriented north-south between profiles 3 and 4 (where a deep trough branches off to the north from the trench), and also profiles 5 and 6. To
the south these zones are bounded by the east-west zone along the trend; and we have insufficient data to trace them further to the north. These zones occur between normally magnetised blocks,
and are probably negative regions corresponding to these anomalies. The features of these blocks
and their location could be determined using data on magnetization of recovered samples and the distribution of anomalies.
2.4. Geothermal studies
Heat flow was measured along Geotraverse N17-3; the results are presented in Table 1. At
Station 96, a temperature profile of the water mass from the surface to the seafloor was obtained.
Heat flow values close to normal were obtained at Stations N17-125, 134, 135, 136. At Stations N17-
132, 133, 138, and 139, anomalously high values of thermal gradients and heat flow were recorded.
Increased thermal conductivity values measured in situ are due to a relatively high density of sediments.
[CR134 - Pushchin]
d 0 I-Q Q 0 0 ..0 Q 0 ~ N
~N -"~" fl' , It' \0 ,...
/ '-' , d co ~
~ I< )'-' '-' ~ /~ ~~ ...,/ I ~
_ 0 \ ~ ; 0 ~ g ~ ~ : :.~ : 0
~ = .i" "~~~~o.~ \D ...'2]. 0 ~.. 0 .;: ~ ~ I
\ 0 "...~ .0 ..~ .0 .~ ~ ..~. 0... 0 ~ ..P ~ ...,- " ".." .~ " .o. 0 .n,... ~ ".. po" ..0""::< -..o. ~ 00 ~ 0 0 ~. .. ~ -
\ 0 0 00 o. 0 0 0 -o. O~ ~O ~ ". "0-~ .." o. ~~ .~ .~ ~" "
" ...~. gS ~ ...~ 0 e " -< ~ .t: og ..~ Q i ~ ~ .~ . "
\ .". .0 .0 " " ~. . I< ..~ ~. .0 " ." .~ ~ ~ .
, \ )
I- ~
~g ~ -! ~ ~ - '" "
. [20]
do 0 0 0 1-0 0 0 ~ ~ ., '" -",- I-" ' , I ' ., c ~ I
., '" '" 0 r-~ rt.°i70_2t.r x 0 - S 6o°2G~£G /
..,.-" 8 ..,.-" 8 0
( 0 "t/) " ..." ~ ~ 0 OJ
~ ~ g ~ ~ ~ ..~ ~ r , r'.~.~.~ H ~ ,,~~.,~..
14 \ H ~. ..~ .. 14 Z : g ..~ .~ . rn HI'!. ~ ...0: =H H i ~ ~ ~ ~. . :;) \ ~ .~ Cl:b: ~ 000 () () ~ .to ~ ~;: :.
-< ) § .~~o... ~ ~ I'! ~ = ~ ~ ~: : ~ :I: "'~2~~.
/ ' '" , , ./ :I: mrnBt11~/" ~ ,--
~ / g t u ,,~ B ~ ~ '? /~ r-- ~ ~ ( ~z ...-< : ~ "': ~ \ ia : ..!.: '< .0 . -< ~ .z "' : >- i ~.. ~ > § g 0 ~ "" ~ ~ .!:,~"11: rn ;; ..."..-g: .0 ".. "0 ..~..~. o. ..
~ "t.o; ~fo. ~ ..B" o. .~ ~ "~ ., ". ~~ .~ .o. ~.~:. ~. ". ~.;. ~g :~ ...~ ~. I' , .I
~ 0~oGO02t.r I)l III ~ ~ 8 . S 0009£022 E I.lJ l1J llJ l1I L:J","-
xC --T '" -.Q r-
~6 ,..!. t .!. 0'C~ ~ 0 ~ ~
. [21]
Co 0 0 g t-o 0 0 9 N~N -., ~' ...1 ..I r- co
a 09 ;71oZl.1 ~ 0 -'" '" ...'" oQ
S gl;~~0S:2
x >
~ ; 0 ~ g,"!.:!.~: -0 to.. o;~.~. >
/ ~ .~.. ..~ : ~ :g .0 g: 0 -=..: 0:: ~ z -" ; 7 g: ~~ ~!. ~ .;,:. I c( \D -" 1: e ; ~ 0 0
~ I / ~ ~ '0 ~ -;~ go 0 ~ 8 g ~~ ~:s C'- ~ o. u~ ,"0 ~ """~ 0
:/) ~ 00 ~~ .~ 0 ~ .~ ~ ...~ ~ z 0 ~c ~ .~ ~ 0 C .. z 0 ~:. ~5 .: ~ ~ ~ ~ ~ .~ 0 ' -< : o.?- :g :g .: : ~ ~ ~ ;:. :
; ~ \ ' I , I , I ~(, ~ I
"" ~
:J .,f;~1oZl.1 '", S ""'rfoZ;Z Ed I ~o --r '" '0 r-
I- ~ ~ I ~ -!. I ~ .. ":-0 0 Q ':0 ' goo 0'" -..~
[CRl34 -Pushchin]
~(-.I -I I
~ 29"~~o2(,I ~o -~ ~ ~ ~ ~
.S (,~"917o22 / (. 1717"9I ~
\ j
/ ~ ./ .~ ! : ~/ fl :.. .~ ;;
C'-C'- Ij ~"' ~p c.. H ~ :. ~.. .~ :.~:.~.~ I 0 ~... ~ c .~ .~ .',-.. re;. ~ g..:.~:.~~~ 0 c ~ .~ .(/) H ~ ...~ H ~ ~ .~ ~~ ~ :: ~ ~ ~ ~~ ~
::) I 0 :. ~ ~:..~ : g ..:. .': ; "" " ~ .~ ..p~ . ..."'1: p. >e .~ ..0 ..0 {) {) 9 ~P8. :.g~~...< \ 0 0'. 0.. 0 0
o. o~... ~ ..,"~.
~ ~ " o. .,~ .~ .~ .~ ~ ..."
e.. ". ~c ~ .~ ~ 0 e .. .~:. ~g .: ..Q ~ ~ ~ .~ .\ : .~ :~ :g .: ; ~ ~ ~ ~l : I I " I " ,
~ \ ~g]~rn0~rn8~~B ~ \D "" ~ I " 0 ~ ~ " 0 ~ z ) 0 z ~ g" < J (\J 0 < ~ AII-:"IN"" ~ ,. 0 r.:I ~ ~ (' 6oI-(,IR~ H If\ ~
< HI.. Q < .-IH H= ~ ) <all ~ ?- § {) d '"tit " e.. 0 U ~ ~ (I) (l)N r.:I
./ ~ e.. ~ ~ z
S 8i"GIOZ2 E U- u ~o -tV ~ 0.7 il" 10
~ I - f-~- c!, ~ ~ .!,
I I .. ;
[CRl34 -Pushchin]
[25]
do g o 8 8 -0 0 ~ N ':"~~ "' .,. '. , " 'C -_I .0
~SO.f~u:!.I!o -'" '" ...It' ~~ \ 96-lII/ "" ~6-LII/ 06.10080 0~.91
ir6
/ to." / : 5 ( ~...0 . H - \ ..cc ..0 ~..." " ...~
~\ a .:.. .~ ;;..~ ~",~ "\ i ..~ ~.. .~ :.~:.0 ~... ~ 0 " ~ o~ ..\ re ;. ~ ~'":.:~~:. > i .r,.. "~~.~. 0 ) N ~ ~;~.. ,,~:. :. ~ .~ N.. " ~'"~. ~ .-;.~..
1 .0 "";'.. 0 :.~g:..~ :~..::..~: I
/ H " ~ .~ ~ .to. ~ 't... >c ~ ~ " ~ " ~ ~ ~ ~ ': ~ -;~ g" .: ~ ~ t. ~ ~
O §~ 0" ".. "~'"" ~ """~. ... ...~ r4..". ~~ .~ .~ .~ ~ ...~-~ ". 00 ~ .~ ~ 0 " ~" ", ..-'" > ~'"... "- o. ."~,,...,,~..,".,.'" ...". ." ." " .c. ..e '" ..~ ~. .0 ...'" ~ .c ~ ~ . ~ ~ "- I I I I I I I I ,"-
2.!:.!..~~'" 06.10060 Olo~O rn III ~ I1l G f11 ~ B~ ~ B"s 91 L'oGG ~o '" ~ "".0 ~ t1J l1J ill i.j ~ LtI b;I ~ ':0.. .I , .. ~8 0 0 ~ 8 8 ~,
"'- ~ : N ~., I ',
[CRl34 -Pushchin]
'c-~ Ir"9~o:lr~! :: '" ~ ~ -D r- S 69"90o17Z
06'IO'OI 90.00 /
H C\I 0
/ U! d IG
\ .~.. Ua~.~.~ ~.~.. ..~ : .., !. g . ..~ ".. .., ..o~ "
I ..~ o .~ 0 ..0 ~ . 1: po >f ; ~ " ...U
e g: ~ ~;!: ~ ~ ~ ~ :~ g \ , U 0 ~~ .~ .~ .~ ~ 0.. ..,
" ..~. ~ 0 .~ .~ ..
0 .~:. ~6 .: ..~ ~ ~ ; .': 0r-. ...: o?' :~ :g ~ : ~ ~ ~ ~:. :... I I I I ..I I I .
~~ I)lf)ll\lm~m~ B I=j~[] ~~ \ l..)JDJL\J~t.:JiliL.t/ ~~D
.,1 Eo4 \
.11:~ -0: \= >- > § \" Eo4 :c 11: In \ u
2: ~:;J .- " E- a>" ~
OJ~ OI.2~o2l1 \ i:i: ...S 00.6~o22 E...~o -'" '"' ~t- -1- ~
~ 1.. ,I , -' ., I ~o 0 0 c' . Co 0 Q C
i N Q -'" -"
l- t: In \D I'-
~'? -:- ":" ~ ~ , ' I
I /
---0 »... ~.. ~ .~ ~0" .o. 0" .0" ~ ~ ~~ ~" 0 "" o;~.~. "-. ~ ~ i'J.. ..~ ~ ..~ g .
" ..~ ...0 ...'.~.. " 0 :.~ §: ,,» : g " ~ .'; :orr ' IN ..». 0". 0 ~ ..,,~ .-" t ";' ~f ; ~ ~ ~ f. 0
'" ) GOT-loT N- ~ g : ~ ~2 ;: 2 ~ : ~ :. E g~ '" ~ ..". ~~ .~ .~ .~ ~ '" ..
.-" ..." ~ .~ ~ H \ ...~. 00 "~~ ~ 00 ...~ ..~ ...~ 2: : ..? :~ :g .: : ~ ~ 2 ;:. : :.') H "" " '" I (. I I I H ~ ""
~~; ~/l]~/]J0ill[]B~~B ""'-
= ') ~ 2: - <~ '" 0..~ I --0 :;e r'-- --0:J) H 0 0 ;4:Z; '- u"\o z --~NO
.< 0
~ It)
{} ..- I- ;, , ..I #,
0 0 Q <30 0 0 0 = "N 9; I- ~ "
r:
~: or-! or-! oM ~ i a>S ~ <!>E, >m m >('6
s I } /;! ~ I I o!'"il:l 1:1 oM~~' 1.' I ~ I" ~ ~ ~ ~ ~ W~\, ~\,\ (I( \U!! (0 Q ..-IrtjCJ OtCa> tn,r.,Q).'~ ,~..J~ /'-, / ~ 2 WO~ HOQ bOo~ ; ~,~ I ' , / " ~ ~ 0 f1) Ot-i Q) 0 OM 4) t/).rf ;; ~\'.I! ( f I IC: ::J .C:J ~.,..j,.; N"'" r-I P oM ~,.. , .OJ ~ J I Il ~ " \ '"II! iI: H [SJ LJ U~( " .) 0 ' \ g .~ .:., -\ , ' '.- --~
,; ~ \ '\ .~~~., \ \ \ I': ,'~/ ( \ '/ 1 J ..(' / I ~ ,- V, Q~' ,...'-- /
J '& ~ .~.\'\ .( /'!/~~~~ ~T ) I " ,(r,...,--) '_/ Ii ' / ;' , \, -"
1 -=---1" {./ ( \.8 I f ,"... JI! / / II II (~,' \ (~ \' ~ ( ' " / /' ::/ ",I / I ,,\ = ":u""\ \. ",-,'::, ' '. (' ( , I \', \, \ ,I \
1&)' ,/ ',\\, \, ",I IcUJ I I ', ,\ 1 I,r"" , I ,.--:;/ '\ ' I ~ " \ \ /.~ ! 'II / /, \ III ,\, I I / ~ J !~,'\ [ ~ 1111111 1 \11 \ \ \ \ \ I I "" Q ~'i-"; \lr\\ \1 ~6 ~-/(I'.-l-..' ,.,1 _'L_\~\-j; I-c--I'--/~.- ~~~:::~:::::;::;~ ~ I / I \ I , I \ I \ \ \ \. I / .I
~ I (I \ , / III II '\ ,/ I : \ \ \ \ -I)' II ) -"" I i
S '" ,,' , / I ~I I / , \'- , I I / , .~ " , 1/ I / , \
:::"\ .~~'""\, ~'~":', '\ \ 1/ I / ( \ \" I~, I.D A (£(~"'\ \ ! \. \ \ I /1 I ~ I/'"- -, \ \ ' I :
f ,\111{"§.\~\\ll/f'l/ "1\ : t'- 'J) i (1\./1" ! fOi j II \11 I' I ( \ \ \ '!ol ; ~ ~ r .' ! ~ l~ .111 I' '~I I 1. ( \ \\ '
H J 'I I ,I I \ I' I I I ; / \\\ =-< I. --. l , \ \ , I I ;/\--\ 1, .. -w / /11 ~r ",,--I-, ~ -,-" ,',\ -,~
, > Q) =a}: II I 0 \'rr-", \ , , I I Ii./' \ \ \\, t 0 ~ -0 ::c \ R ~ \, f r ' \ \\ \ .d < ~ <::0 /1; I j'.;:::--'T /' / ' ""'-""' I; , \ \ ;
m C'I ( \ J I I: . \ \ h.- ---"" I .j \ \ ~r~ ~ \' ",,' ---/, ' \\...G> ~ 0 \.. \.,_.-' '" I .\. \ rn ~ H / ! '. \,\
I m~ ~ ' / \\ ° \ ' ~ t/) ~ /. . (Q ' 2 ! I '" '" / ( 0 \ \' ,
,-zo ~ ,/ .,...'-" .'1t.9..: '. C.'.) / .",' ~", I I C) \,\ - < 0 / ,/ ' \ ( . ~ '
-~ / /' \. (\ \\ .-:" I /' \ \ \'\ .\ ~
~ ~ ' n .0 ~ \ ~- ~ ~ o~ (i!2!1 /' \; !I!
l'- t:1.. \ ! , 1 i". ! : I ~ H ~ \. !I ( (O, i \ If ~O> \ II \ \...1 ..1 I,
.CI) ~ . ",' .,1 II oM =! ,'\ / ., ~ Eot \\ \ ,I r, './. ! I Co) 'j ,~'\ I~I\ / I,
\ 1 \ \-1 I ,/ I { I , / / I .
I ~~- ~--' 1.,1_- I ! ( 1 - ! !
Table 1. Heat flow data
I Station I Latitude I Longitude I Depth I Depth I Gradient I Conductivity, I Head I I number I South I East I of sea I of I mK/m I W /m*K I ~ I I I I I lpenetr I I I mW/m I I I I I I I I I I
I N17-96 I 22°48'0 I lno54'4 I 3UO I temperature profile of water mass I I I N17-110 I 22°19'8 I 173°11'1 I 1500 I -I no penetration I -I I N17-111 I 22~'2 I 173°10'4 I 1560 I -I no penetration I -I I N17-Ul I 22°04'8 I ln049'8 I 2600 I >1.5 I 31 I 1.09 I 34 I I N17-125 I 20°54'4 I 1no20'O I 3050 I >1.5 I 54 I 1.17 I 63 I I N17-126 I 20°55'0 I 1no19'l I 3200 I >1.5 I 41 I 1.19 I 49 I I N17-132 I 21°35'5 I lno34'8 I 3280 I > 1.5 I 154 I 1.16 I 179 I I N17-133 I 21°35'4 I lno35'O I 3280 I > 1.5 I 156 I 1.21 I 181 I I N17-134 I 2O0U'l I lnoO4'O I 2950 I >1.5 I 65 I 1.24 I 81 I I N17-135 I 20°11'7 I 1noO4'2 I 2930 I >1.5 I 75 I 1.17 I 88 I I N17-136 I 20°11'5 I 1noO4'3 I 3000 I >1.5 I 54 I 1.29 I 70 I I N17-138 I 19°54'4 I 171°56'0 I 3120 I >1.5 I 146 I 1.25 I 182 I I N17-139 I 19°54'1 I 171°56'1 I 3120 I > 1.5 I 132 I 1.18 I 156 I I I I I I I I I I
2.5. Physical properties of rocks
Velocity of longitudinal waves, density, and anisotropy of sound velocity were measured in rocks
of the sedimentary cover and basement dredged from the New Hebrides Trench (Hunter fractures
zone) (Tables 2, 3).
Sedimentary rocks consist of weakly lithified siltstones (St. N17-93). Density and sound velocity in
these rocks increased with the grade of lithification; from 1.38 g/cm3 and 1.58 km/sec in
bioturbated, poorly cemented rocks to 1.74 g/cm3 and 1.84 km/sec in more compact tabular
siltstones. These increases are also accompanied by increasing sound velocity anisotropy, from 0.9%
to 6.5% (see Table 2).
Volcanic rocks: dolerite, boninite, and basalt form slopes of seamounts at varying depths. They
gave variable density (from 2.02 to 2.60 g/cm3) and sound velocity (from 2.65 to 5.83 km/sec)
values. The lowest values (2.02 -2.09 g/cm3 and 2.6-3.7 km/sec) were measured in vesicular basalts
from Stations N17-84 and 88 (Samples 84/1, 2, and 88/2. Maximum density and velocity of
longitudinal waves were observed in massive rock varieties from Stations N17-76 and -79. On the
plot of velocity versus density, basalt points make up an elongated field above the field for
.serpentinites (Figure 17, see Table 2).
,
sedimentary rocks (Hunter Fracture Zone and Ridge)
I IDensi-lVelocity of longitu- 1 .Sample No I Rock name 1 ty/ 3 ~~ ~~-y-~.(~~)---1 A, %
I g cm I1 VII IVIIIl VI I V I
NI7-88/I Basalt 2.28 4.63 4.55 3.67 4.28 24.9
84/2 Basalt 2.08 3.57 3.24 3.08 3.30 I4.9
84/2a Basalt 2.08 3.7I 3.50 3.20 3.47 15.0
84/I Basalt 2.09 3.37 3.28 3.00 3.22 I2.1
84/4 Basalt 2.24 4.68 4.554.54 4.59 3.5
e8/2 Basalt 2.02 2.98 2.89 2.64 2.84 I2.4
88/3 Basalt 2.3I LJ..55 4.504.41 4.50 3.1 87/6 Basalt 2.32 5.32 5.20 5.07 5.20 4.7
76/58 Basalt 2.38 4.93 4.784.62 4.77 6.6
82/1 Doleri.te 2.42 4.87 4.7I 4.26 4.61 I3.7 87/7 Basalt 2.43 5'.63 5.43 5.Ia 5.4I ~-'.3 a2/I3 Basalt 2.45 5.72 5.54 5.35 5.54 6.7
76/35 Clastic lava of boninites 2.~5 5.07 5.204.93 5.07 5.3
81/2 Basalt 2.42 5.51 5.144.87 5.18 12.0
76/59 Basalt 2.49 5.05 4.944.86 4.95 3.8
79/I Dolerite basalt 2.50 5.20 5.I65.II 5.I6 1.7
79/6 Basalt 2.52 5.62 5.56 5.50 5.56 2.1 .
79/7 Basalt 2.51 5.30 5.29 4.93 5.17 b.l 79/4 Basalt 2.54 5.52 5.40 -5.46 -
76/62 Basalt 2.57 5.82 5.~O 5.09 5.41 14.0
79/5 Basalt 2.60 5.81 5.78 5.74 5.78 1.1
93/ Siltstone 1.38 I.59 I.59 1.58 1.58 0.9
93/ Siltstone 1.62 I.9I I.89 I.89 I.90 1.5
'13/ Siltstone I.74 1.90 I.85 1.78 'I.84 6.5
Notes: VII' VIII' VI -sound velocity in three orthogonal
directions, A- velocity anisotropy of longitudinal
waves. ,Analyses ..'{ere perf ormed by A. T. Svininnikov
;
Table 3. metamorphic rocks of Hunter Fracture Zone and Ridge
1 IDensi-IVolocity or longitu- I Sample Nol Rock name It~ 3 l!!J,.n~w~ye~__<,~~S:.)- -I A,~ -I :~_c: ~V+I _~~Il~.. 'l_~- I V I --
I I 2 131415161718 ' NI7-IJO/I Sorpentinite 2.38 3.14 3;08 2;9I 3~04 7~7
80/Ia " 2.39 3.20 3.20.2.9I 3.IO IO.9
76/8 " 2.40 3.62 3.58 3.55 3.58 2;0
76/7 " 2.42 3.52 3..5I3.35 3.46 5.4
8I/I Gabbro-norite 2.46 3.85 3.DI 3.'~ 3.72 I1.0
77/I ~erpentinous harzbur8ite 2.49 4.33 4.02 '.93 4.09 IO.2
76/IO " 2.49 4.01 3.923.43 '.79 16.5
Serpcntinou8 76/' Dunite 2.11-9 './JI '.653.49 3.65 2.7
76/2 Serpentinous harzburgite 2.53 4.0U 11-.02'.79 '.96 7.8
78/'0 Gabbro-norite 2.56 4.8I 4.6' 4.I9 4.5'+ I4.0
76/I Serpentinou9 hnrzburgite 2.57 5..I6 4.9I 4.90 1+.99 6.0
r~o/u ~erpentinou8 lherzolite 2.57 4.94 4.694.I8 4.6I I7.0
76/9 Serpentinous . harzburgite 2.59 '.92 3.79 3.73 3.8I 4.8
78/I" 2.60 4.&4 4.59 4.26 4.50 9.2
80/3" 2.69 5.I& 4.704.09 4.6523.0
bO/3I Amphibolitized gnbbro 2.71 5.50 5.394.94 5.2(1 II.3
50/32" 2.72 5.64 5.3U 5.35 5.46 5.9
80/30 Gebbro-norito 2.74 4.56 4.323.96 4.28 14.0
NI7-76/32 Gnbbro 2.79 4.74 4.63 4.0I 4.46 I7.5
82/3 Cabbro-norite 2.85 7.30 7.23 6.38 6.97 I4.8
78/29 Metagabbro 2.88 6.88 6.6I 6.54 6.68 5.3
76/22 Gabbro 2.94' 6~36 5.8I 5.64 5.94 I2.6.. 78/I9 " 2.96 6.9I 6.76 6.59 6.75 4.7
77/2 Lherzolite -2~9I 5.42 5.I2 4.92 5.I6 9.8
78/25 Serpentinite 2.92 5.03 4.94 4.28 4.75 I7.4
76/21 " 2.95 5.9I 4.56 4.2'~ 11-.90 36.2
77/9 1Jebsterite 3.I4 6.87 6.09 5.86 6.27 I6.2
77/6" 3;17 6.69 6.'~0 5.95 6.35 n.7
77/5" 3.20 8.62 7~43 7.01 7.69 2I.7'
77/6a " 3;20 7;13I 7;50 7.21 7.5I 8.0
77/8" 3.22 6.69 5.90 5.35 5.98 23.0
77/7" 3.26 8.47 6.76 6.59 7.28 28.6
77/4 n 3.28 7.5b 7.04 6.90 7.17 9.7
76/6 Serpentinoll9
Notes: vII' "'III' VI -sollnd velocity in three orthogonal
directions, A -velocity anisotropy or longitudinal
waves. Analyses vlere performed by A.I.Svininnikov
[CRl34 -Pushchin]
8.0
6.0
;.0 111 1
j J:(-clastic lava of .~. ! '1l bonini t e
I. ..Z,~ I cm3
;
[34]
Serpentinite and secondary altered gabbroic rocks have very low velocity of longitudinal waves values at high density. We have obtained similar data for the same type of rocks from Tonga
Trench.
These studies show decreasing velocity and density of rocks during their serpentinization; the
density decreases from 2.95 to 2.36 g/cm, and sound velocity - from 5.9 to 3.0 km/sec (see Table
3).
Gabbroic rocks are altered and metamorphosed to different grades. The density and velocity of longitudinal waves in these rock type are 2.7 - 2.96 g/cm and 4.0 - 7.3 km/sec, respectively (see
Table 3).
Websterites have the maximum density (3.14 - 3.31 g/cm) and velocity of longitudinal waves
(5.40 - 8.64 km/sec) values of all the rock types measured, and also pronounced sound velocity anisotropy, up to 28.6% (see Table 3, Figure 17).
The data obtained show a wide variety of density and sound velocity values, encompassing the
total range of crustal seismic velocities. In several samples of ultramafics, velocities of longitudinal waves, typical of those observed in the upper mantle, were measured.
The distribution of density values of basalts dredged from different depths of the study area
seems to be useful for the reconstructions of paleoconditions of their solidification and subsequent movements.
As is well known, porosity (and, hence, density) is affected by hydrostatic pressure of the
overlying water column during the solidification of basalts (Jones, 1966, Moore, 1972; Svininnikov,
1986, 1989). In the interval between the depths of 2400 and 2600 m the density of basalts (Stations
N17-76, 79, 81, and 88) increases from 2.02 to 2.57 g/cm downward through the cross-section. The
density gradient in the upper part of the interval (2400 - 3400 m) is similar to that obtained for the volcanic rocks of the Reykjanes Ridge, where the porosity decreases from 40% at a depth of 100 m
to 5% at a depth of 1000 m (Jones, 1966). This suggests seafloor subsidence of approximately 2.3
km since the formation of the basalts dredged in this study.
The density of basalt recovered at the other stations (Stations N17-82 and 84) also increases from 2.08 to 2.45 g/cm with depth, and the density gradient is similar to that given above, but in the
depth interval between 4700 and 6900 m. Following the idea outlined above we can conclude that
these rocks have subsided from more shallow levels to a depth of approximately 4.5 km. Rocks
[CR134 - Pushchin]
[35]
dredged at Stations N17-76, 79, 81, 87, and 88, and Stations N17-82, 84 could occupy sites on
opposing sides of a fault with a vertical shift of about 2.5 km.
2.6. Petrological studies
Igneous and metamorphic rocks of the acoustic basement were predominantly sampled along sublongitudinally oriented profiles following seismic and bathymetric surveys. The most successful results were obtained on the inner slope along profile 5. There, at depths between 6280 m and 3040
m, a cross-section of the plutonic rocks of an ephiolite assemblage more than 3000 m thick was sampled (Stations N17-76, 81). Similar rocks were recovered from depths between 6920 and 6840 m
(St. N17-82 to the west of the profiles cross-section. Different extrusive and hypabyssal rocks were
dredged from the eastern part of the study area (Profiles 7, 8, 10, 20, 27, and 33). Some of these
rocks are probably genetically related to the plutonic rocks and form the upper levels of the ophiolite assemblage. The following rocks have been distinguished within this assemblage.
Ultramafic rocks include various predominantly serpentinised rocks of dunite, Iherzolite,
harzburgite, wehrilite, and websterite lithologies. Harzburgite and dunite predominate.
Dunities were recovered at Station N17-76, 78, 87, and 92 as blocks and fragments, 7 to 40 cm in size. These are greenish-grey to brown in colour, depending on their degree of serpentinization, with
massive, sometimes banded or brecciated structure, and often are cut by serpentine (sometimes ore
mineral) veinlets. A cumulate texture is observed in dunite, consisting of olivine, sometimes rare
grains of cumulus clinopyroxene, and chrome spinel, which was observed under the microscope.
Chromite content reaches 50% in some rocks (Sample N17/21).
Lherzolite was sampled at Station N17-76-78, 80, and 82). It is a grey-green with greenish-brown
tint, banded and fractured serpentinised rock, sometimes strongly altered and often containing serpentine or zeolite veinlets (Samples N17-76/4, 6 and others). Lherzolite samples are often
brecciated. These rocks consist of olivine, ortho- and clinopyroxene, and additional chrome spinel. In
some cases the content of chrome spinel was rather high (Sample N17-76/6). Thin sections often
reveals a cumulative texture, with large idiomorphic crystals occurring in a serpentine aggregate.
Harzburgite was recovered at Stations N17-76-78, 80, 87, and 92 and from the central part of
profile 5, where it comprises up to 30-35% of total material recovered (St, N17-76). These are
greenish-brown, sometimes grey-green or yellow-green rocks which have been serpentinised to variable degrees. These rocks are often brecciated and commonly have glides planes. They generally
[CR134 - Pushchin]
include relics of primary minerals: olivine, orthopyroxene, and spinel, though some samples have
fresh grains of these minerals (Samples N17-77/1, 13).
Wehrlite and dredged as rare small fragments (5-9 em) only along Profile 5 (Stations N17-77, 78
and 80) at mid-levels of the inner slope of the trench. It is a grey-green or dark green to black
rock with a coarse-grained texture, consisting of cliopyroxene crystals replaced by bastite up to 1 cm in size (Sample N17-78/18).
Websterite was sampled at two stations (Station N17-77, 78). It is a greenish-grey, sometimes muddy-greenish, commonly coarse-grained, brecciated rock with zeolite and serpentine veinlets, consisting of bastite pseudomorphs over clinopyroxene with subordinate serpentinised olivine, spinel,
and probably orthopyroxene. A white, metasomatically altered variety, consisting of serpentine with
ortho- and clinopyroxene relics was observes (Sample N17-77/17).
Carbbroic rocks, common in the study area, consist of gabbro, gabbro-norite, and gabbro- anorthosite (?) Cranodiorite (?) recovered at Station N17-87 could be also grouped with these rock
types.
Gabbros were dredged at Stations N17-77-80, 82, 97, and 92. These gabbros are typically fine- grained, greenish-grey rocks cut by veinlets of pyroxene, zeolite, or feldspar.
The structure of the gabbroic rocks in massive, rarely spotty (Samples N17-78/28), banded
(Sample N17-78/44) or brecciated (Samples N17-78/34, 36). Their mineralogy consists of plagioclase, amphibolitised pyroxene, amphibole, and, often, serpentinised olivine (Samples N17-
77/26, 78/19, 35, 43, 44 and others). Metamorphosed varieties were also observed (Samples N17-
78/46, 54, 56-59, 80/31, 32, 82/8, 87/38, and 92/18, 27.
Gabbro-norites occur as melanocratic fine-to medium-graine cataclastic (Samples N17-82/2-6) and
greenstone-altered (Sample N17-82/7) variants. They consist of plagioclase, clinopyroxene,
orthopyroxene, and sometimes,m serpentinised olivine. The ration of rock-forming minerals varies substantially, with orthopyroxene-rick variety (norite) being observed (Sample N17-92/19 20).
Dike suite includes dolerite, dolerite-basalt, and microdiorite, recovered predominantly along
profile 5 (Stations N17-76, 78-80, 82, and 87).
Dolerite is a greenish-grey, aphyric and porphyritic (sparsely phyric), fine-grained, generally
slightly porous, often fractured rock with veinlets of zeolite (Sample 82/1) or feldspar (Sample
[CR134 - Pushchin]
[37]
76/2). Both fresh (Sample 76/23) and slightly altered (Sample 80/36, and even metamorphosed
(Sample 87/40, 42, 55) dolerites were observed. The most extensively crystallised metamorphosed
varieties are gabbro-dolerite (Sample N17-78/49). Phenocrysts in dolerite generally consist of chloritize pyroxene and plagioclase, and groundmasses comprise plagioclase and pyroxene with
additional olivine (Sample N17-80/47) and amphibole (Sample N17-76/22).
Doleritic basalts were recovered at Stations N17-79, 80. Their mineralogy is similar to that of the dolerites, but differing in their lesser degree of crystallization. These rocks are generally fresh,
porous, and aphyric or phyric with phenocrysts of plagioclase and clinopyroxene, and may contain
altered olivine in the groundmass (Sample N17-79/1, 3, 89/37).
Microdiorite (Sample N17-82/12) is a single small fragments, consisting of a tine-grained, weakly metamorphosed rock of an intermediate composition.
Volcanic rocks include basalt, andesite-basal andesite, andesite-dacite, and boninite. Volcanic rocks were recovered at each successful station.
Basalts are the most abundant rock type. Several varieties have been distinguished on the basis
of phenocryst mineralogy.
Clivine-clinoyroxene-plagioclase and olivine-plagioclasephyric basalts were dredged at most
stations (N17-76-82, 87, 89, 92 and others). They have pillow cleavage and black glassy chilled
margins 1-2 mm thick or more. These are generally GREY sparsely-phyric vesicular rocks with
glomerocrysts of olivine and clinopyroxene (Sample N17-76/62). Groundmasses are usually glassy, and rarely crystalline with dolerite texture (Sample N17-79/2). Phenocryst content varies from
isolate crystals to 15-20%. Vesicles reach 3.5 cm (Sample N17-81/14).
Olivine-clinopyroxene-phyric basalts were recovered at Station N17-87 only. They are grey,
porphyritic, vesicular rocks with well-crystallised, though sometimes glassy groundmasses. Some
samples contain magnetite phenocrysts (Samples N17-87/21).
Clinopyroxene-plagiophyric basalts were taken at Station N17-79-82,88. These are grey, brown-
grey, sometimes black vesicular rocks with porphyritic texture (up to 10% phenocrysts). Groundmasses are glassy, sometimes micric or variolitic (Samples N17-80/38, 81/2, 82/13, 88/2, 9, 11). Class is completely replaced by smectite.
[CR134 - Pushchin]
[38]
Pyroxene-phyric altered basalts were dredged as individual fragments at Stations N17-77, 80, 87. These are sparsely phyric vesicular rocks which have experienced greenstone alteration. Phenocrysts
are strongly amphibolitised pyroxene.
Plagiobasalt was dredged at Station N17-80 as a single fragment with a vesicular structure. Vesicles are filled with zeolite, and the groundmass is glassy (Sample N17-80/46).
Two-pyroxene-plagiophyric basalts WERE dredged at Stations N17-81, 84, 86, and 88. These are grey, porphyritic, commonly massive or slightly porous rocks with a glassy, (rarely well crystallised)
(Sample N17-81/16) often fluidal groundmass. The amount of vesicles varies markedly, sometimes
being substantial (Sample N17-84/1, 6), and the amount of phynocrysts reaches 20% (Sample N17-
84/3,4). The vesicles in basalts are commonly hollow, but sometimes are filled with zeolite or iron
hydroxides (Sample N17-86/8, 9). Some samples are coated with a ferromanganese oxide crust up to
5 mm thick (Sample N17-88/1). Most samples studied form fragments of pillow lavas (Sample N17- 87/2, 10 and others).
Aphyric basalts were recovered at many stations (Stations N17-78-80, 84, 86, and 88). These rocks are vesicular, with glassy or crystalline groundmasses of clinopyroxene-plagioclase, and some samples are slightly brecciated (Sample N17-79/19) and contain iron hydroxides (Sample N17-80/40, 43). Sometimes these rocks contain clinopyroxene microphenocrysts, zeolite veinlets (Sample N17-
80/41) and amigdules (Sample N17-84/9, 86/6). Blackslaggy variants (Sample N17-88/7) also occur.
Basaltic andesite was sampled at Station N17-84. Macroscopically, this lithology is similar to the two-pyroxene basalts, differing only in their lighter colouring.
Andesite was dredged at two stations N17-82, and 86) as single fragments. These are light-grey
with a green tint, slightly porous rocks with a glassy, microlitic, or variolitic groundmass texture
(Sample N17-82/11, 23, 88/13). They are metamorphosed to greenstone facies (Sample N17-82/19)
(Sample N17-82/11). Phenocrysts commonly consist of plagioclase and hornlende, and the
groundmass is glassy, though sometimes crystalline.
Dacitic andesite was recovered as small fragments at Station N17-8). These are light-grey,
vesicular, sparsely - and finely-phyric rocks with orthopyroxene, clinopyroxene and plagioclase
phenocrysts.
Boninite is one of the most common rock types within the study area. It was dredged from
different parts of the inner slope at seven stations (Stations N17-76, 77, 78, 80, 81, 92, and 115).
[CR134 - Pushchin]
[39]
Boninite was dredged predominantly as fragments of clastic lava up to 3 cm in size. Clasts are composed of grey and light-grey, aphyric and porphyritic boninites, with phenocrysts of olivine and pyroxene. The matrix of the clastic lavas is yellowish-green, and partially or completely smectitised
(Samples N17-76/35-40). Vesicular (Samples N17-76/41, 51, 53), melanocratic (Sample N17-76/43) varieties with fresh black clasts (N17-78/61), often brecciated (Sample N17-79/64) were also
observed. At Stations N17-92 and 115, olivine boninites were recovered, exceptional in their fresh
appearance and highly melanocratic composition. Porphyritic segregations in these rocks consist exclusively of olivine, partially serpentinised and inserted in a black glassy ground mass with
numerous quenched pyroxene crystals (Sample N17-92/37).
2.7. Lithological studies
Sedimentary rocks were sampled using gravity corers (nonlithified sediments) and dredges (lithified sedimentary and volcanic-sedimentary rocks).
2.7.1. Lithified sedimentary and volcanic - sedimentary rocks
Several groups have been distinguished among the dredged sedimentary and volcanic-sedimentary rocks with their different degree of lithification (Table 4). In the majority variants distinguished,
microfossils were identified which indicate their Miocene-Pleistocene age. In many cases, mixed
fossil assemblages of different ages were found (foraminifers and nannoplankton); and forms characteristic of Paleogene (Oligocene) were also identified.
Reddish-brown and chocolate-brown mudstones are very common within the study area (Table 4,
group V). They are devoid of fossils. These rocks were found on both the Outer Slope of the New Hebrides Trench, a position where such sediments are typical, and the Inner Slope, where grey
sedimentary deposits normally occur. These mudstones are probably similar to mudstones occurring
at the base of Malekula Island (Marfarlane et al., 1988). On this basis, the sampled mudstone are
grouped with Red Mudstone Formation rock type of presumably Oligocene age.
Additional petrographic and geochemical data obtained in onland laboratories will help in the
final identification and classification of the rocks described here.
[CR134 - Pushchin]
Lithified oedimentary rocks dredged in Study Area N 17-6
Site Gro- Representative Colour Lithifica- . -.u~ rock samDle tion ~rade Structure Texture GenesJ.s AgeR~~
1 2 '4 '5 6 7 8 9;0:-=-
South-1iji Ba- I Vitroclastic Brown Weakly Bioturba- Clastic, Pleis- sin, seamount tuff N11-67-1 lithified ted, ca- silty- tocene to the south of vernous pelitic Study Area
II Vitroclastic White Semilithi- Massive Clastic, Late Fe -Mn tuff fled silty- Plio- microno- N 11-68 pelitic Pleis- dules
tocene The Outher Slo- I Sandstone Brow- Weakly li- Massive Clastic, pe of the New N 11-13-11 nish- thified silty psam- Hebrides Trenuh grey mitic
II Mudstone Brow- Weakly li- Massive, Clastic, N 17-73-16 nish- thified in some silty-
grey places pelitic bioturba- ted
III. Limestone Light We~ly li- Organic Hemipe- Early- N 11-1)-13 grey thified Btoturba- lagite., middle
ted Miocene The Inner Slo- I Breccia Grey Semil~thi- Clastic, Debrite pe of the New N 11-1.16-41 fled mictitic Hebrides Trench
II Sandstone Brow- Weakly li- Massive, Clastic, Turbi- Late Pli- N 11-84-31 nish- thified graded psammi- dite ocene -
grey bedding tic Early Plei- stocene
III Mudstone Brownish W~ak~y ~assive, Clastic, Turbi- Late Plio- Nq-18-94 grey lJ.thJ.- J.n some silty- dite cene-
tied places pelitic Early biotur- Pleistocene bated, thin bed- ded gra- dational
IV Limestone Light Weakly Biotur- Clastic, Ooze Late Plio- N11-81-1-1 grey lithJ.- bated organic cene -Early
fled Pleistocene
Transitio- V Mudstone Reddish- Weakly Massive, Clastic, nal g1'OUp N11-13-15 brown lithi- in some silty-
fied Places pelitic biotur- bated
[CRl34 -Pushchin]
2.7.2. Nonlithified deposits
In the study area N17-6 non-lithified (loose and silty) sediments were taken at 8 stations (N17-69,
70, 72, 97, 98, 101, and 109) along, or near, profiles N17-7 and N17-10. To the south, profile N17-10
ends in the South-Fiji Basin, and to the north, in the New Hebrides Trench. The profile N17-7
crosscuts the Hunter Ridge (see Figure 3).
In the planar area of the northern part of the South Fiji Basin, at a depth of 4400 (St. N17-69), ooze occurs to a depth of up to 5 m below the sediment surface. This ooze is a biogenic-
terrigenous siliceous clay, brown in colour and massive. It consists of up to 30% siliceous biogenic
material (mostly radiolarians, rarely diatoms), terrigenous clay and clastic materials, and iron hydroxides. The amount of clastic materials in the sediment increases downward through the
sediment cross-section, and at a depth of 210 cm from the seafloor, redeposited clastic material appears, first as rare pockets but increasing up to 40% of the core, where the sediment becomes
redeposited-terrigenous and silty in size. The redeposited material consists of fragments of opaque
rocks (probably extrusives), plagioclase and pyroxene grains, and fragments of volcanic glass. In the lower cross-section (between the interval 480 to 500 cm) the amount of redeposited material
increases up to 60%, and big (up to 1 cm) pumice fragments appear.
The age of sediments was determined using diatoms. In total 43 species and their variants were
observed. The distribution of diatoms is uniform throughout the core and includes recent tropical-
subtropical species. The most common species (more than 5% of total abundance) are: Coscinodiscus tabularis, C. nodulifer, C. crenulatus, C. asteromphalus, C. radiatus, C. perforatus,
Hemidiscus cuneiformis, Nitzschia marina, Thalassiosira oestrupii, Pseudoeunotia doliolus,
Rhizosolenia bergonii Rh. firma, Thalassionema nitzschioides et var. parva were also identified.
Within the interval from 40 to 100 cm the sediment contains sublittoral allochthonous diatoms,
namely Cocconies scutellum and Cyclotella striate . This suggests the occurrence of erosional processes in the coastal zone of nearby islands.
The absence of the zonal species Nitzschia reinholdii, general appearance of the population, and
the absence of such tracers as Thalassiosira leptopus var. elliptica, suggest a late Pleistocene- Holocene age for the origin of these sediments (the upper zone, Pseudoeunotia doliolus). The
boundary between the Holocene and Pleistocene occurs at a depth of approximately 40 cm below the seafloor. A typical subtropical-tropical flora was observed above 40 cm, and below 40 cm species of Arctic and Antarctic areas wee identified among the diatom population (Coscinodiscus marginatus, Thalassiothrix longissima, Nitzschia kerguelensis), marking the cooling in the late
[CR134 - Pushchin]
[42]
Pleistocene. In cross-sections, the core shows that at the beginning of late Pleistocene, the region
experienced more intensive tectonic activity compared with the Recent Time. This was accompanied
by intense redeposition of material from surrounding seamounts, and coincided with period of colder climate.
In the area of the South-Fiji Basin adjacent to the New Hebrides Trench (St. n17-70), and in the
trench itself (St. N17-72), silty sediments are absent, and compact clay (St. N17-72) and ashy tuff
(St. N17-70) outcrop on the seafloor. They are completely devoid of fossils, suggesting they have originated from the destruction (halmyrolysis) of bedrock, and its conversion into compact clay. This may have been accompanied by their limited transportation.
On gently slopes and individual uplifts of the New Hebrides Ridge (Hunter Ridge) loose and
silty Quaternary sediments are virtually absent, and basalts (St. N17-101) and their breccias (St.
N17-94) outcrop on the seafloor. Only depressions between highs (St. n17-94) and in seafloor valleys
(St. N17-109 have thin biogenic redeposited sandy sediments accumulated. A core recovered at St.
N17-109 is an example of such sediments. From the surface to a depth of 5 cm this core consists of yellowish-grey biogenic, redeposited, calcareous-clay, and soft, low to medium-density pelitic silt
(probably mictite). The silty and psammitic portion of the sediment consists mostly of foraminifers
and their fragments (3% CaCO 3 ), and lesser amounts of redeposited material (plagioclase and pyroxene grains and fragments of basic volcanic glass). Clay minerals predominate in the pelitic portion where carbonate material is subordinate. These sediments are separated from a lower layer
by a boundary of marked compositional and density difference.
Sediment of the lower layer, between an interval from 5 to 74 cm, consists of fine, grey with
yellowish tint, biogenic, redeposited, foraminiferal, homogeneous, massive, and dense sand. This
sediment also comprises foraminifers and their fragments, calcareous nannoplankton (17 to 34%
CaCO 3 ), fragments of plagioclase, pyroxene, rocks, and basic volcanic glass (predominate).
The age of sediments was determined from foraminifers. Throughout the entire sediment section,
occurs the Pleistocene zonal species Globorotalia truncatulinoides . The presence of species
Sphaeroidinella dehiscens, Globigerina calida calida, Globorotalia hessi restricts the age of the fossil-containing sediments to the Brunhes epoch (0.69 M.a.).
In the upper part of the interval, Globorotalia fimbriata was identified. This species appeared at the end of late Pleistocene and developed during the Holocene. The most morphologically
developed forms of this species were identified within the interval 0-5 cm. Further down the core,
beginning from 30 cm to the base of the core, Globorotalia tumida flexuosa was found. The
[CR134 - Pushchin]
[43]
extinction of this species occurred, according to different sources, between 80 and 20-30 thousand years ago. Thus, we have split the core in the following way 0-5 cm - Holocene (subzone GI.
fimbriata); 5.74 cm - late Pleistocene (subzone Globigerina calida calida).
Loose and silty sediments are completely absent in the isolated basins occurring between the northern and southern ridges of the Hunter Ridge system (St. N17-97). Only fragments of ferro- manganese crusts, comprising only a sooty film of manganese, were recovered from this area. This
evidence suggests a rather young age for the morphostructures present and the occurrence of
intense bottom hydrothermal activity.
It is concluded that the arc-trench tectonic system within the study area is a young structure. Upper - Quaternary sediments are virtually absent, and their presence only in depressed areas
suggests a more intense tectonic activity at the beginning of late Pleistocene and cessation prior to Recent Time.
2.8. Biostratigraphic studies
Neogene-Quaternary deposits occur in the study area. Miocene, Pliocene - Lower Quaternary,
and Quaternary deposits were distinguished using microfossil studies (for details of systematic compositions see tables provided). Pliocene-Quaternary deposits are also common (Appendix 4).
Assemblages of the oldest microfossils were identified in silty-pelitic sediments from Station N17-
73 at the base of the Outer slope of the New Hebrides Trench. Foraminiferal and calcareous
nannoplankton fossils (Sample N17-73/19) Globigerina venezeulana, G. obesa, pseudodruryi,
Globorotalia continuosa, Coccolithus eopelaquicus, Discoaster woodringi, D. deflandrei and other
forms, indicative of the Early-Middle Miocene boundary, were identified.
An assemblage of nannoplankton (Discoaster deflandrei, D. variabilis, D. pansus) typical of
Middle Miocene, was also identified in compositionally similar rocks from the same station. A common occurrence of Early Miocene spore and pollen was found.
Pliocene-Quaternary deposits were sampled from the Inner Slope of the Ridge.
An assemblage of nannoplanktonic foraminifera, corresponding to the lower Pliocene zone Globorotalia miocenica (Blowscale), was observed in Sample N17-84/32, which is comprised of
calcareous siltstones. These nannoplankton suggest a young age for the deposits.
[CR134 - Pushchin]
[44]
The stratigraphic unit of the Upper Pliocene - Lower Pleistocene is represented by terrigenous rocks from Stations N17-84, 86, 87, and 88. A Pleistocene species index Globorotalis
truncatulinoides ) of nannoplanktonic foraminifers has been identified. In addition to Pleistocene
species, typical Pliocene species (Gl. pertenus, Discoaster brouweri, D. quingueramus) were observed, providing evidence for the reworking of sediments in this area. Older fossils were rarely
observed in the young structures of the inner slope, examples of which are Late Oligocene Catapsydrax dissimilis and Globigerina euaperta identified in Sample N17-87/1.
A common assemblage of Pleistocene species of foraminifers and calcareous nannoplankton was identified in weakly lithified and unlithified sediments from a series of Stations (N17-67, 78, 81, and
others). A more detailed division of these sediments was not possible.
2.9. Hydrogeochemical studies
Preliminary analysis of data obtained concerning the geochemistry of seawater within the study
area may be summarised as follows.
The water mass is characterised by an increased geothermal field in the bottom layer, a peculiar
distribution of oxygen and pH, rather high concentrations of hydrocarbon gases, and a complex and
very irregular microelement distribution (Table 5, 6, 7, see pages 49-55).
The observed distribution of temperature indicates a free exchange of water between the South-
Fiji Basin and the Fiji Plateau. A zone of mixing occurs along the New Hebrides Trench (the
Hunter Fracture Zone). To the north of the trench, the water stratification is typical of oceans, with
a steep gradient in the depth interval of 300 to 1000 m, and minimum temperatures occurring at the bottom (1.81 - 2.39'C). In contrast, water temperature in the axial part of the trench changes monotonously from 20'C at the surface (water interval of 15 m) to 2.2'C at the bottom (5783 m
deep). Cold water (~5'C) occurs at a depth of 2500 m, whereas elsewhere in the region it occurs
at depths below 100 m. Warmer bottom water (3.28 - 3.73'C) was detected to the south of the
trench, where lower temperatures (2.32'C) occur higher in the water column at depths near 3200 m.
The cause of this distribution is as yet poorly understood. Because of the large temperature
difference (more than 1'C), it may be presumed that the geothermal field is due to a shallow magmatic chamber present beneath the zone of strong fracturing in this area.
[CR134 - Pushchin]
[45]
Different vertical and lateral oxygen distribution was detected in waters on opposite sides of the Hunter fracture zone. A regional minimum of oxygen was determined in bottom water of the Fiji
Plateau, in intermediate water layers of the South Fiji Basin, and in the trench axis. A normal subsurficial maximum (3.3 ml/l), occurring in the South Fiji Basin within the water interval 250-600
m, passes into the subsurficial maximum (4.19 - 4.36 ml/l in the Fiji Plateau within the water
interval 400-500 m to 1000 m). In bottom water, the oxygen content increases further, up to a maximum in the Hunter Fracture Zone, at a depth of 4500 m (4.19 ml/l). The similarity in the
pattern of isotherms and isocontents of dissolved oxygen suggests a peculiar hydrological regime in
this region of the Pacific. pH is the most interesting parameter of the carbonate system. system.
Within the Hunter Ridge, intermontane basin, and Fiji Plateau areas, pH monotonously decreases with depth, from 8.26-8.28 at the surface to 7.73-7.71 at the bottom. Quite different pH behavior is
observed in the area of the Hunter Fracture Zone and the South Fiji Basin. Similar to the other
regions pH decreases to 7.75-7.71 in surficial and intermediate layers, but in contrast, a bottom
maximum occurs, as is also observed for temperature and oxygen content. The cross-section shows a rapid pinching out of these waters towards the Hunter Ridge. To the south, the zone of water with
an increased pH (7.83, Station N17-71 gradually becomes narrower, as it passes outside the study
area. Such pH behavior, caused by the breakdown of the carbonate quilibrium, remains to be fully
explained.
Additional processing of the data obtained is needed before a complete description of
microelement distributions in the seawater of the study region can be provided (Table 7 ). The
water of the Fiji Sea is enriched in iron, nickel, lead, and, partially, also cadmium.
Manganese and cadmium form anomalous areas in the deepest submarine sections of the Hunter Ridge. Along with vast areas of chemical subtraction of dissolved silver and zinc, high
concentrations of zinc and ferrum in suspension are located here; besides, very high concentrations
of murcury and hydrocarbonaceous gases were stated. These variations are thought to be important
to use them for controlling geochemical nature of the said minerals in hydrothermal and exogenic
processes.
Endogenic manifestations are marked by chromic ores in ultrabasic rocks. The specific
characteristic of the ores is their being of transitional type from segregational to hysteromagmatic
one. Two substages are distinguished in the later stage of secondary alteration (probably, of regional
nature); they are as follows: the earliest one is characterised by serpentization connected in space with ore-mineralization; the latest one - by veins of serpentinite crosscutting rock-forming and ore- bearing minerals.
[CR134 - Pushchin]
The main ore-minerals are chrome-spinellids. They occur in pockets, in phenocrysts, in
glomerophitic excretions and breccia-like accumulations of fine grain aggregate and dust-like mass.
Dominating ores are those of pocket- and phenocryst-type. Among phenocrysts, the mottled and
mottle-shadowed veinlets are observed.
Studying of morphostructural peculiarities of ore-minerals allows us to distinguish three
differentiates: 1-idiomorphic grains of nearly cubic configuration with right-angled faces in cross- section (comparatively rare, occur detached, mainly in poor-serpentised major mass; 2- highly xenomorphic grains and accumulations of accreted fine grains (major part of ore-aggregates); 3-
intermediate phases, occur rarely.
The stated structure-textural peculiarities of ores are in good agreement with the conceptions on a proper magmatic stage of formation of chrome-spinellids, suggesting that the main phase of ore-
formation process should take place at the last substage of peridotite crystallization.
A usual periodotite with an admixture of accessory chromespinellid (Sample H 17 - 76/6 - 1)
contains Fe/3.88, Ni/0.145, Cr/0.12, Mn/0.0875, Co/0.00938, Zn/0.003, Cu/0.00075 and Hg/1.5.10-
6x).
Areas mostly inriched in ore-minerals containing chrome-spinellid no less than 30-40% from the total mass (Sample H17 76/62) are characterised by the following consentration series: Fe/3.21,
Cr/1.75 (Cr 2 O 3 / 2.56), Mn/0.0744, Ni/0.058, Co/0.0109, Zn/0.0048, Cu/0.0029, Hg/1.5.10%.
There are Cr/5.52 (Cr2O3/6.06), Fe/3.79, Mn/0.0793, Ni/0.052, Co/0.0086, Zn/0.0083, Cu/0.0028,
Hg/4.1.10% present in massive ores (fraction containing about 20% of ore-minerals (Sample H 17
- 76/6-3).
According to the obtained data, the studies ore samples are represented by ores of inferior quality with poor chrome mineralization. The distinctive feature of the mineralization is enrichment
of ore minerals in mercury and depletion in cobalt and nickel. Such elements as ferrum and
manganese are present in equal proportions in both major rock mass and in ore-minerals. Plumbum
and cadmium were found in no one ore-differentiate. As impurity elements, there were stated ruthenium (0.05ppm) and rhodium (0.01ppm).
[CR134 - Pushchin]
Two different types can be distinguished between ferrum-manganese concretions: concretionary crusts on basalts, and hydrothermal crusts of zone-layered texture with coatings, ochre, gouges and
pockets of brown and greenish-grey clayey minerals. Thin (up to 1 cm) concretionary crusts coat
basalt fragments (Site H 17 - 88). Admixture of argentum (31-54 ppm) considerably exceeding an average content of Ag in concretions and crusts from other oceanic areas, is a property of these concretions.
The data on ferrum-manganese crusts of the second type from the stations H 17 - 122 and H 17
- 117 must cause the acutest interest. The direct volcanic exhalations as well as hydrothermal fluids
and thermal alterations of basalts in active zones of the Fault Zone Hunter cab be a source of
these ore-manifestations. The crusts are represented by essentially monomineral aggregate with
manganese content from 35 to 45.5%, owing to which they have bright metallic lustre and a good
maleability. Their geochemical particularities are marked by manganous modulus (more than 20),
their being rich in argentum (14-35 ppm), platinum (0.01 ppm), and lean content of cobalt and
nickel.
3. CONCLUSION
Major new data concerning the geology of the New Hebrides tectonic system were obtained within the study area N17-6, the region where the New Hebrides island-arc system undergoes structural transformation into the Hunter left-lateral strike-slip fault. Additional important data were
also obtained from the Fiji Plateau and the South-Fiji Basin, neighboring the study area.
Bathymetric and geophysical surveys were used to select sites suitable for rock sampling, and additionally have supplied data on the seafloor morphology and the distribution and thickness of the
sedimentary cover. Within the study area the Inner Slope is complex, with a narrow ridge (South
Hunter Ridge) striking parallel to the main Hunter Ridge, from which it is separated by an almost
linear trough. The morphology of this ridge is complicated by more or less isometric, sometimes
enclosed depressions. It is still difficult to interpret the origin of this ridge. It may be separated
from the Hunter Ridge along a series of listric fractures, which dip towards the trench. The trough
separating the ridges could trace a fault belonging to the Hunter Fracture Zone. In both cases the
South Hunter Ridge is complicated by a series of transverse faults, some of them steep upthrusts along which rocks of Layer 2 and even 3 outcrop onto the seafloor.
The single-channel seismic reflection survey revealed the sedimentary cover to be thin within the
study area, and that rocks of the acoustic basement outcropped in many areas. The results of sediment sampling confirmed these interpretations of the geophysical survey data. Sediment sampling
has also shown that peculiar reddish-brown and chocolate-brown silt-stones and mudstones devoid of
fossils were common on the Inner Slope. These rock types occur at the base of the cross-section of Malekula Island, and on this basis, could be grouped with the Red Mudstone Formation of presumed Oligocene age. Our study demonstrates that these mudstones are much more wide-spread than was previously regarded (Marfarlane et al., 1988). Thus, they could represent relic sedimentary
cover common to the oceanic crust of the region, prior to the initiation of the New Hebrides tectonic system.
Microfossil studies have revealed common Miocene - Pleistocene deposits within the study area.
Paleogene forms were identified in several Neogene foraminiferal assemblages, and sediments
containing mixed microfossil assemblages are also very common. This is a feature also characteristic of other regions in the South-West Pacific.
The most important result of our studies is the establishment of a complete cross-section of an ophiolitic suite on the Inner Slope of the New Hebrides Trench (the northern slope of the South
Hunter Ridge). The cross-section begins with serpentinite (apoharzburgite?) similar to rocks
[CR134 - Pushchin]
[49]
occurring at the base of the ophiolitic complex cross-section in the northern part of the Tonga
Trench (Sharaskin et al., 1983). These rocks are the uppermost depleted portion of the mantle
(Coleman, 1977). Overlying gabbroic rocks and peridotite are compatible with the cumulate rocks of ophiolitic complex, and dolerite and boninite of the upper part of the cross-section are probably
sheeted dike complex equivalents and the extrusive "cap" of these assemblages. In addition to
orthopyroxene boninite, olivine boninite was identified within the study area; - until now being found
only in the Tonga Trench. On the whole, data obtained on the composition and structure of the
acoustic basement at the southern termination of the New Hebrides Trench provide firm evidence for the hypothesis of prior spatial continuity of the Tonga and New Hebrides arc-trench tectonic systems.
In conclusion, it is also noteworthy to mention the discovery of chromite ore in the ophiolitic
complex, which provides tangible evidence for the potential applied value of our research.
[CR134 - Pushchin]
section near 172~R, South-West Pacific (Study Area NI7-6)
' '- Station De)th ToO 02 pH Salinity ilk
.!!.wn~~r-l- ~2 --1- ~-- ~!-(~/~) -.-! r (~LO~)~~'1 ~,:,"_~uLe- ,- -'- --~._' -_0 .2, -.'.- -_7-
NI?-71 IO 26.18 4.71 8.30 35.0;I 2.35
200 LB.-59 3.52 8.I4 35.650 2.42 500 9.90 3.30, 7.94 .34.768 2.34
IOOO 4.I9 3.90 7.78 34.489 2.;7
S -~ 2420 3.I4 2.92 7.75 34'.576 2.44 ..,... 'j' ~.ov 3290 2.32 2.86 7.75 :34.654 2.45
3775 x 3.41 7.83 34.5~ 2.38
4158 3.28 3.IO 7.76 34.576 2.42
l-t;~8. -_.2.7,__- --3.~~3_,- -__7.~2 _3~.!.428 -?.!...4.Q._. NI7-90 500 12.IO 3.52 7.91 ;5.0I2 2.40
1850 7.~ 3.94 7.85 34.564 2.39
-r(""~JJ'-((" 2630 4.I5 3.49 7.79 34.450 2.;9 3I38 3.24 3.I5 7.7I 34.540 2.43 ?;Q5 x ___~37~.'L-2_4~..QL-b-4-L-
ll.I7-91 I5 19.07 4.66 8.28 35.0I2 2.;6 200 x 3.83 8.11 35.673 2.42
T.r(.,I1'- tt: 3850 x 2.4I 8.13 35.4-05 2.41 4830 x 2.40 8.12 35.031 2.40
5360 2.20 2.44 7.73 34.603' 2.44 --,--- 578"5 x :5:.5,9- -~-- :?~.72I...;- g.5Q NI7-95 IO 26.87 3.74 8.28 34.988 2.38
~ I95 I2.40 3.63 8.I5 35.063 2.40 ~..,.~-.. " 'j -,. 4-90 7.76 3.64 7.79 35.685 2.;8
, I685 3.I2 2.90 7.74- 34.595 2.45
2635 2.13 2.82 7.73 34.697 2.48
-" "- -..'-.. .
Table 5 ...continued
I I 2 I 3 f 4 1 5 1 6 1 7 , --f. '--' -'- ..-
NI7-IOO IO X 4.46 8.26 34.894 2.38 I50 X 3.84 8.I5 35.685 2.38 3IO X 4.30 8.08 35.437 2.40
N'f='13 475 X 4.36 7.93 :;4.8I5 2.38
955 6.00 4.I3 7.77 34.446 2.38 I/+O5 3.65 3.05 7.73 34.544 2.42
-,. ,---~- ~.3L-.-?:-~(~ 7_.7~ .,~.65;4 -_.?~ '0 NI7-IO6 I5 26.94 3.82 8.27 34.894 2.3I
200 20.14 3.32 8.I4 35.717 2.33 545 IO.25 4.19 7.92 34.8I5 2.34 900 4.74 3.49 7.80 34.427 2.38
<;~fj'" I525 2.92 2.9I 7.72 34.623 2.~ ~ 2435 2.I7 3.32 7.7I 34.697 2.48
~~~J 33I5 1.87 3.62 7.72 34.729 2.48 4225 X 3.69 7.7I 34.733 2.48 4525 I.86 3.47 7.7I 34.729 2.48 4700 X 3.53 7.12 34.737 2.50
472-9- !!.~.~-- ---=. --=-- NI7-120 IO 27.47 4.4I 8.26 34.858 2.37
205 20.I4 4.31 8.16 35.685 2.42
580 9.83 3.69 7.85 34.743 2.37 JF2 I2IO 3.67 3.08 7.72 34.529 2.42
I700 2.63 2.84 7.7I 34.642 2.45 2000 2.26 2.69 7.72 34.678 2.49 2IS'5 X 3.34 7.7I 34.682 2.50
22.!0 _.-"" ~~..- -=- '--" -:" ;--
[CR134 -Pushchin]
Table 7 ...continued
, L."\ t!'\ Lf'\ 1/\ 1/\ 1/\ l1'\ ~ 1/\ 11'1 1/\ I/'\i L."\ 0 1.1'\ L."\ 1/\ 1/\ ll\ 11'1 1/\1.0 : ...
HIOOOOO'::>;OooOOOOI.OOOOO'oo'o: V"""" v v " " " v "!" y v v y v V V ,- I: tC\ r'- :j" '-0 0 ~ C\J :j" I-! :j" 0 r'- I-! '" ci' C' :~ C\J ! ~:j":j"tC\ ~tC\tC\tC\~~:j"C\Jt!'\~ ~11'I11'ItC\1.1'\ xx x ~' 0 0 0 0 0 0 0 :) 0 OJ 0 0 0:) 0 0 0 ::>
; i
: -1/\ 0 1.0 !1' C\J ~ C\J ~ C'- tC\ 1.0 ~ ~ C\! ~ ~ 0 or (1\ C\J C\J
:j" C\J t."\ t1"\ 0 ~ C\J '-' :j" C\J N 1."\ ~ :j" ~ :j" 1.0 ~ ~ ~ ::- '.. I:j" tC\ 1.0 ~ 1/\ ::- \;0 i tC\ 1/\ 1/\ :j" tC\ 1.0' N C\J r'- 11'1 0 ~ ~ 1.0 1/\
,~ I i- I I : , 0'\ 0 C\J ::> 0 C7'\'::> '" C\J Lf'\ 1/\ 0'\ 1('\ ~ ~ '" ~ 1.0 ll\ W ~ j ,1/\1.0 1.0 tC\1.O tOll.O::>I/\",~~ (:() 0'\1.0 1.0 1.0 ~ tC\1/\~
! " ...' '1('\ ~::- C\J 1/\ 1/\ 1/\ I 1('\ ::- t<\ ~ N l."\ ::- ~ :j" Lf'\ ~ tC\ ~ ::- ~
~ I I I J I,- I : '
I '" 0'\ ~ :j" ~ u'\ I 1.0 N C\J ::- N 0 i C\J 0 1.0 1/\ ~ '" N N : ID ~ ~ ~ ~ 1.0 ~ ~ N ::- ~ ~ N C\J (:() "" N! ro 0 1/\
I x.". N: I-!~I-!~ HI-! ~H I-!H~~ H~~ H ~OI-!H ~' 'I
-I I ~ 0 1/\ N ~ tC\ ~ N N 0 0 O! N \D ~ C'- ~ 0 t<\ 0 1/\
i ~ ~ 1/\ "" 1/\ C'\ t !1' ~ 0 0 ~ I('\! ::- N 0'\ ~ N 1('1:. t<\ "" ~ ~ " ...
.~ H ~ H ~ ~ I-! 10 ~ H H!-I H H ~ 000 HH H!-I i !
-C'- 1/\ (:() H C\J 1('\ I ~ 1/\ ro r'- ro ~ 1.0 0 (0 :0 N 1/\ ~ 0 ::- , ro 1/\ 0 tC\ tC\ t<\ I 1<'\ N H 1/\ C\J 0 t'- (:() 1/\ ~ 1.0, tC\ 1.0 ~ :0 ,HI 00 H H ~ H:~ H NH N H H 0 0 000 H ~ 0
I : I I
I-! (:() '" 1/\ ~ 0 Ilr\ "" ~ ~ N 01 C\J ~ ~ t<\ ::-11/\ "" ~ ! I H 0 ::-.;:j- tC\ I' ::- H ~ 0 ~ 1.01 ~ 0'\ C\J:O 0' ~ "" 0
X. ~ 0'\' d" lr\ ::- tC\ ::- 1('\ lr\ d" If\ d" 1/\ lr\ 1/\ 1.0 l."\ C\J::- t<\ \-0
I I
-" I I ' ! ~ 0' lr\ H 0 0 f 0 N 1/\ lr\ C\J H ~ 0 1.0 0 ~ 1.01 0' C\J ~ : (:() .;:j- ,X) 0 C\J N H ' lr\ N co H N Ji : C'- 0'\ ro H ~ .;:j-, 0'\ .::- 1('\ , .,..,' ' ', ... i : N C\J I('\N Lf'\C'-IH I('\C'-::- C'-~C\J ~1.I'\Lf'\~ I('\N tC\C\J
,-, ' , t 1'::- 0::0 ro OJ ""Ilr\ tC\0'0'\ 1\-00 C'- lr\
"" 1('\ C\J N lr\ ll'\ .H C\J t<\ N tC\.;:j- ~ HI I .~ ...~ I ...M X X .M M t'o 000000,0 000 000 0
I I ' -I I
i ! ~ 0'\ ~ tC\ N ~' lr\ N N "" tC\ 1-1' 1.1'\ 0 0'\ 0 tC\ 1.0 ~ N t'o I i ~ 1/\ C'- 0'\ "" lr\ i ~ N \;0 C'- 0'\ '" ~ 1.1'\ ::- H H ::- " C) ::- ~
! ' ,
j -lr\ ~ N ~ N 1.0 ! ~ 0 ~ 0 N 1('\1 0 t'o CD lr\ lr\ ~; t<\ H N I I N CO::-NOJC\J'I.OH ""~H~(:()I.OHU'\Nll'\O:O""lr\ I 1 ...
.1('\ ~ ro Q) "" IX>I' ::- .:f" lI\ ~ l!\ \-0 I 0'\ \-0 0'\ IX> N K'!::- tC\ lr\ I I " I ' .- , N C\J 0 (\J 1.0 i 1('\ lr\ H 1.1'\ U'\ lI\ 0 I co , N ~\-OHIX> (:()I('\OOO'\~ N .:f"I j .x I ~ M M H ..j ~ .~ .H
.:f" H N.:f" H 0 0 H 1('\ H H H H: H
I " -I 1('\ C'- 0 ~ 0 "" ! t'- 0 C'- 0 '" (:() I 0'\ C\J ~ tC\ 1/\ ~ tC\ tC\ N
I I"::> IX> C'- 1.0 ~ tC\ 1.0 H "" .:f" C\J H; t'o 1.0 C\J co C\J tC\ 0 tC\ "": ... I ~ C\J.;:j- l!\ tC\ 1/\ ~ I .:f" ~ U'\ tC\ .:f" I.t\! 1.0 Lf\ ~ "" N C\I' d" N 1/\
, i I I = I
1 -I lI\ 0 0 0 0 tC\! 0 0 1/\ 1/\ 0 01 0 0 Lf\ 1/\ lr\ 0, lI\ 0 1/\ I I H 0 1.1'\ tC\ "" (:() I H '" (:.) t<\ ::- C\I H 1.1'\ C'- 1/\ 0 ~. H 0 .::l-
N : C\I ~ IX> tC\ C'- .;:j- "" "" 0"' H !-I.;:j--!1'.;:j- ,J.) : N U'\ I; 1('\ ~ 1/\ lr\ !-I C\I C\J tC\ ~ ~ , ,i i--i ,
i iO"" I H I U'\O 10
.0'\ 0'\ H HI Hi' , I , 'I ' ~C'-~ ~ ~ H H ,H; I z I~ 1= 1= '..
[CRl34 -Pushchin)
[57]
REFERENCES
Balabashin V.I., Matveev V.G., Novikov A.A., Rot A.A Methods and technique of thermal conductivity coefficient measurements in bottom sediments. In: Geophysics and tectonics of a transtional zone of West Pacific type. Vladivostok, 1985, p. 100-106. (Russian).
Barsh M.S. Panktonic foraminifers in the sediment of the Noethern Atlantic. Moscow, Nauka, 1970, 103 p. (Russian).
Bezrukov P.L., Lisi