Mapping in the Oman ophiolite using enhanced Landsat Thematic Mapper images

7-ecton~@~rts.

3x7

151 (1988) 387-401

Elsevier Science Publishers

B.V.. Amsterdam

Mapping

- Printed

in The Netherlands

in the Oman ophiolite using enhanced Thematic Mapper images M.J. ABRAMS ‘, D.A. ROTHERY

Landsat

’ and A. PONTUAL ’

’ Jet Propulsion Lahorufor3;. California Institute of Technology, 4NO0 Ouk Grow Driw. Pasudenu, CA 91 100 (ti. S.A.) ’ Department

MK7 frAA (Great RriruinJ

of Earth Sciences, The Open C’nicersr(v. Milton Kqvnes, Butkinghum.~hire (Received

July 3. 1987: revised version

accepted

December

8, 1987)

Abstract Ahrams,

M.J., Rothery.

Mapper

images.

D.A. and Pontual,

In: F. Boudier

The large area1 extent programmes changes

and minor

images

ophiolite

outcrops

mapping.

the capacity products,

and complex

in the Oman

exhaustive

ultramafic

it is possible

intrusions.

to delineate

gossans

the area of a lo x lo

mapping

areas

across

dispersed

than

previously

appreciated.

provided

by the imagery.

Such data

semi-arid

regions,

even where already

infrared

in gabbro mantle

The significance

alteration. of certain

The Oman ophiolite is the largest and complete example of well-exposed ocean

mapping

faults

field-based. Recognition

Thematic

rock

types

stretching

to identify mantle.

Off-axis

Thematic

15 1: 387-401.

mapping of subtle

in the field only by unrealistically

Decorrelation

composition,

largely quality.

Landsat

of different

from the deeper

be a valuable

mapped

spectrum.

reflectances

to chloritic-epidotic could

inconsistent

Landsat

Tectonophyxicr.

that previous,

of internally

the whole ophiolite.

variations

the uppermost

subject

has meant

using enhanced

of Oman.

map sheet is possible

in the spectral

to recognize

to discriminate and

maps

ophiolite

The Ophiolites

of the terrain

of the short-wavelength

on distinctions

and to allow consistent in which

nature

in the Oman

(Editors),

have produced

across

By using parts

to capitalize

A., 1988. Mapping

and A. Nicolas

Mapper

and

their

data

of these data produces

small acidic.

gabbroic

to locate the Mohn precisely.

plutonism

is reassessed

tool in similarly

have

weathering

is more

widespread

and and and

in the light of new insights exposed

terrains

in arid

and

at the 1 : 100,000 scale.

contains a complete ophiolite sequence from tectonized and variably serpentinized harzburgite (the mantle sequence), passing upwards into

most floor

rocks now outcropping on land. Knowledge of its structure and petrology can cast much light on our understanding of processes occurring at and near modern-day oceanic spreading axes. It was formed by sea-floor spreading at about 95 Ma and had

layered cumulate gabbro and peridotites and isotropic or ‘high-level’ gabbro (and occasional less mafic differentiates) which formed in axial magma chambers, and sheeted dykes and pillow lavas fed from these magma chambers. Off-axis magmatism is manifested by a later series of lavas and ultra-

been emplaced over the Arabian continental margin by about 70 Ma. The ophiolite forms a sheet-like nappe covering an area of about 20,000 km2 (Fig. 1). Much of the recent work on the Oman ophiolite was synthesized by Lippard et al. (1986). It

mafic to acidic late-intrusive complexes. The ophioiite has virtually no soil or vegetation cover and forms rugged mountainous terrain at elevations mostly between 500 and 1500 m. Exposure of the rocks is excellent, though there is often a large amount of virtually in situ debris

O~O-1951/88/%03.50

@ 1988 Elsevier Science

Publishers

B.V

5WE

58%

ophiolite nappe)

24*1

L 58*E

Fig. 1. Generalized map of the northern part of tbc Oman ophiolite indicating the areas shown in more detail in Plates l-3 and 4-6. The area illustrated by Rothery (1987a) is on the south side of the Plate l-3 box.

which hinders precise mapping of specific contacts. The only map showing the whole of the Oman ophiolite in any detail is that produced by Glennie et al. (1974) at 1: 500,000. Thiswasproduced by interpretation of black and white air photographs backed up by a limited rmmber of field traverses. It was an excellent piece of work but contained many errors, omissions and oversimplifications, and was on too small a scale to i&&ate many important relationships. Subsequently, the northcentral part of the ophioiite (from the U.A.E. border to the Sumail Gap) was mapped at 1: lOtI, by the Open University Oman Ophiolite Project using extensive fiddwork tog&&r with p~t~t~~tat~~n (Smewing, 1979; Lippard, 1980; Lippard and Rothxy,1981; Browning and Lippard, 1982). Because of the nature of this project, during which eleven Ph.D. theses were

produced, parts of the sequence in certain areas were mapped in very great detail at much larger scale while several areas were covered on a reconnaissance-only basis. Other 1: 100,000 maps include a 35 km strip running south from Muscat based on the work of a U.S. team (Bailey, 1981), and thirteen sheets which cover the southeastern part of the ophiolite published in 19861987 by the Sultanate of Oman Ministry of Petroleum and Minerals Directorate of Minerals, based on field mapping by geologists from the French Bureau de Recherches C%ologiques et Mini&es. In this paper we show how enhanced imagee from the Landsat Thematic Mapper (TM) can provide coverage of the whole ophiolite to a consistent standard by taking advantage of the shortwavelength infrared reflectance properties of the ophiolite Cthologies. By way of example, we illustrate two areas (in different degrees of detail) which were mapped some years earlier by the Open University Oman Ophiolite Project. These are representative of the several areas of the opbiolite which we revisited in 1987 in order to investigate incompatibilities between the existing maps and our interpretation of the satellite images. In almost all cases, our new fieldwork showed the remote sensing imagery to be showing real lithological information which the earlier field and air photograph mapping had either misinterpreted, oversimplified or simply been unable to detect. We have previously outlined the level of apparent lithological discrimination possible with TM images in the Oman (Abrams, 1986; Rothery, 1987a), but now we are able to substantiate and refine this by incorporation of our new field observations. We are confident that the lithological discrimination revealed by the imagery is sufficiently uniform throughout the ophiolite to be used as a reliable mapping criterion. The TM date are adequate for producing lithological maps at 1: 100,000 scale and can show many details more appropriate to mapping at 1: SO,000 or better. Because of the large area and rugged nature of the terrain, it would be extremely difficult to map the ophiolite consistently and in such detail by conventional techniques alone. Map revision of a further area of the Oman ophiolite is presented in Rothery et al. (1988).

389

Spectral response of igneous rocks

Spectra of ophiolite lithologies

Spectral absorption features

Laboratory

reflectance

range of spectral In addition by remote

to the synoptic

sensing,

this method

another

range

surface,

The

provided

advantage

beyond spectrum

on the mineralogical which is usually

ophiolite

amples

of the idealized

spectral

reality

is partly

of its

of the whole-

rock mineralogy and weathering minerals. Absorption bands in the visible- and short-wavelength infrared can arise from either electronic or

covered

the following

and Salisbury

(1970)

review derived

Hunt

(1977). Electronic processes duction bands, charge

spectra

lithologies response,

or completely minerals.

surfaces

includes

masked

as exwhich in

by that of

The wavelength

by the TM are indicated

regions

above the reflec-

tance curves. The reflectance curve for limonite shows several broad and medium-width absorption features. The sharp fall-off in reflectance

vibrational processes in these minerals. Because most geologists are unfamiliar with these effects, we present

by some of the

in Fig. 2. The figure

the weathering

of the

weathered

from two unweathered

between

indicative

produced

is

of a rock

composition

a mixture

features

which may dominate

are shown

the visible

to discriminate

reflectance

minerals

of

field techniques

properties

of the spectrum types.

depends

major

over conventional

the use of reflectance rock

coverage

spectra

4 0.8 ir--T---TI

1.2

1.6

2.0

2.4

from Hunt

et al. (1974) and Hunt

include transfer

three types: and crystal

confield

effects. The first two phenomena are manifested by a pronounced decrease in reflectivity at wavelengths shorter than about 0.6 pm. The most likely source for such effects in igneous rocks is charge transfer in the Fe-O bond, as minerals containing this bond are ubiquitous on natural rock surfaces. Crystal field effects in the ions of transition metals give rise to several broad absorption bands, whose positions are a function of the metal involved. In this study,

iron ions are probably

all the detectable

bands.

responsible

The presence

for

of ferrous

iron (Fe”) can produce absorptions centred at about 0.65 pm, 1.0-1.1 pm, 1.8-1.9 pm, and 2.2-2.3 pm, depending on its lattice environment. Ferric iron (Fe3’) can produce absorptions at

HARZBURGITE

about 0.65 pm and 0.87 pm. The vibrational processes which cause visibleand short-wavelength infrared absorptions are bending and stretching vibrations of bonds within radicals or molecules. On igneous rock surfaces, the most important are due to Al-OH and Mg-OH in clays, micas, amphiboles and serpentine. Al-OH produces absorptions centred at about 2.2 pm, whereas Mg-OH produces features at about 2.3 pm. There are also features in the infrared due to vibrations within molecular water.

ABSORPTION Fe3+ -----I

Fe2+ H OH

wFe3+

I

I

CO3

% +P

Hz0

Hz0 1

I

I

0.8

4

FEATURES

Fez+

Fe3+ CI

I.2

I

I

I

I.6

2.0

1

2.4

WAVELENGTH, urn Fig. 2. Laboratory minerals lites,

reflectance

and rocks of importance

showing

obtained

spectral

the

Landsat

from coarse

and serpentinite

TM

particulate

are our own data;

(1971); and harzburgite

measurements

in remote band

passes.

samples: limonite

is from Hunt

sensing

of some of ophio-

Spectra

haematite,

were chlorite

is from Hunt et al.

et al. (7974).

390

short of 0.7 pm is due to a charge-transfer band in ferric iron, and the deep, broad band centred at about 0.9 ym is a ferric ion crystal field effect. The two small absorption bands at 1.4 pm and 1.9 pm are due to adsorbed water. The distinctive natural red-brown colour of limonite is caused by the relatively high reflectance in the red part of the spectrum near 0.7 pm. The chlorite spectrum shows characteristics of many hydrous minerals. The absorptions at 1.4 and 1.9 pm and the series of features between 2.0 and 2.4 pm are hydroxyl bands. The absorptions at 0.7 and 0.9 pm are due to ferric iron. The reflectance spectrum of haematite is dominated by ferric iron features; a strong fall-off in reflectance short of 0.55 pm due to charge transfer and a crystal field absorption near 0.9 I-Lm. Serpentinite shows a relatively flat spectral response. The shallow feature at 0.45 pm is due to ferric iron and the broader absorption centred near 1.0 pm is due to ferrous iron. The rather sharp band at 2.3 pm is due to vibrational processes of Mg-OH, and hydration effects are shown by the absorption near 1.4 pm. The hanburgite spectrum has a broad feature attributable to ferrous iron in pyroxene and olivine, centred near 1.0 pm. Remote sensing TM data

Our remote sensing data are from the Thematic Mapper scanner carried by Landsat(Barker, 1985). This instrument records data in seven spectral bands; six channels in the visible and reflected infrared part of the spectrum and one channel in the thermal infrared (Table 1). The reflected data are recorded with ground resolution cells (pixels) 30 x 30 m in size. Each scene covers an area 180 km wide and 180 km along track. Because the data are recorded in digital form, they can be handled on a computer using image processing techniques to extract, enhance and display that subset of information most appropriate to the problem being investigated.

TABLE Thematic

1 Mapper

bands

Channel

Wavelength (I.rm)

1

0.45-

0.52

2

0.52-

0.60

3

0.63-

0.69

4

0.76-

0.90

5

1.55-

1.75

6 7

10.4 -12.5 2.08-

2.35

The TM data used for this study are from scene number 50340-06140 acquired on February 4, 1985. Two sub-scenes are discussed here: a 25 x 42 km area in the east of the Salahi Block (Plate 1); and a 13 X 19 km area around Jebel Shaykh in the Fizh Block (Plate 4). Image processing

The image processing technique employed is to manipulate TM band triplets using “decorrelation stretching” (Soha and Schwartz, 1978). Decorrelation stretching is becoming a widely used technique, and has been discussed by Gillespie et al. (1986) and Rothery (1987a,b). This technique is based on a principal component transformation of the acquired data. Such transformed channels can themselves be contrast stretched and arbitrarily assigned primary colours for display as a colour composite image. Decorrelation stretch&g differs in that after contrast stretching, the statistically independent (decorrelated) principal component channels are subjected to the inverse transformation into the original coordinate system for display, with one decorrelation stretched band in red, one in green and one in blue. Generally, this enhancement causes little distortion in the perceived hues, but colour saturation is exaggerated which makes it possible to distinguish very subtle spectral reflectance differences. The effect of this is that interpretation is relatively straightforward, since colours can be interpreted in terms of the three TM bands used and related to the differences between the spectral reflectances of the materials in the scene (Plates 2 and 5). A compari-

son

between

decorrelation-stretched

simply processed ophiolite

respectively, nels,

was

reflectance minerals parison

of the three

7, 5 and 4) displayed partly

curves

for

infrared

on

4 contains

bands

(TM

seven TM chan-

analysis

of spectral

representative

ophiolitic

on interactive

other band combinations.

information

relating

as a similar tle, and dotite

in red, green and blue,

and rocks and partly of various

more

(1987a).

out of the possible based

and

of part of the Oman

was made by Rothery

The choice bands

TM imagery

purple

intrusions

text. Where the

colour

cumulate

mantle

the

man-

late-stage

peri-

of geological

peridotite

sequence,

con-

rests directly

two

are

best

on dis-

tinguished by their different characteristic drainage patterns. Trondhjemites, due to their acidic composition,

most

often

appear

show a variety

of colours

Band

of iron

oxide

and

ferences

in primary

of

from

on the basis

com-

to the presence

to the uppermost

is distinguishable

typically magenta.

orange Areas

as pink

areas

due to variable

hydroxide

coating

mineralogy.

Sheeted

but

degrees and

dif-

dykes are

to red, and lavas appear mostly which are especially affected by

iron; band 5 serves to characterize the general albedo of the materials and to highlight certain altered (ferric iron bearing) rocks which have par-

chloritic-epidotic

ticularly high reflectances near 1.6 pm, whereas ferrous iron can cause depression of the reflec-

sheeted dykes and upper parts of the gabbros) tend to appear orange, due to strong ferric iron

tance curve in this region. Band 7 responds to the presence of hydroxyl-bearing minerals by a reduc-

absorption

tion in reflectance due to various hydroxyland H,O-related features. Images using other band combinations proved less satisfactory overall for discriminating rock types in the study areas. Image interpretation The decorrelation-stretched images to make preliminary revisions to

enabled us lithological

alteration

in band

(notably

4, resulting

within

in a degree

of

ambiguity in lithological interpretation. Field traverses were undertaken to check some of the numerous discrepancies between the published maps and our image interpretation. Field observations revealed that in almost every case the geological maps were in error, and that the images properly depicted the geological relations. In addition to correctly allowing identification of erroneously mapped rock types and identifying subdivisions, the images allowed interpretation

new and

boundaries and structures shown on the existing geological maps prior to going into the field (Abrams, 1986; Rothery, 1987a). Colours of rock

mapping at more detailed scales than the published 1 : 100,000 mapping. Our final interpretation maps for the two areas are shown in Plates 3

types on the images and our interpretation of them in terms of the minerals and species whose

maps

absorption

images (Plates

features

we think

the decorrelation-stretched shown in Table 2. Almost

likely

to dominate

spectral response, are all the rock types are

distinct in colour and are separable on the images. The mantle sequence (harzburgite) appears in two distinct colour units: the upper part, for about 1 km below the Moho, is usually a purple colour; most of the rest of the mantle sequence appears as a dark green-yellow colour. Cumulate gabbro ranges

from cyan

through

to greenish-yellow,

re-

flecting differences in mafic content, whereas the high-level gabbros are most often yellow. The petrologic Moho, is usually a sharp boundary between purple mantle (tectonized harzburgite) and cyan cumulate gabbro. Cumulate peridotite, which is much less common than gabbro, appears

and

6 for comparison (Plates

1 and

with the Open 4) and

the

University

enhanced

TM

2 and 5).

Discussion

Following our fieldwork, we are now confident that the colour distinctions revealed by decorrelation stretching of the TM data. correlate with bedrock lithology and are only occasionally controlled by exposure effects such as screen slopes. Almost always they have direct meaning in terms of the petrology of the rocks and the surfaces, enabling us to revise boundaries and structures on previous maps and to subdivide certain previously mapped units, for example by discriminating between more mafic and less mafic gabbros.

392

Within the cumulate gabbros of the Sahthi Block we are readily able to distinguish many previously unmapped ultramafic pods only a few tens of metres thick (Plate 3). There is no clear reflectance difference between concordant peridotites (on-axis lenses within the axial cumulates) and discordant peridotites which we interpret as off-axis, intrusions (Browning and Smewing, 1981). The latter are distinguished on the imagery by their shape and occasional association with late-stage gabbro and/or trondhjemite bodies. We note that recent detailed studies (Juteau et al. 1988a,b) suggest that many concordant peridotite bodies of the type which we have mapped as cumulate peridotite, may in fact be off-axis intrusions. We have also detected many previously unmapped trondhjemites, several of which we confirmed in the field. Structural features within the crustal’ section of the Salahi Block are clarified by the recognition that the northwestwards-trending fault zone on the eastern side of the Salahi Block, which was mapped as a discontinuous feature by Lippard (1980) (Plate l), is in fact continuous for more than 30 km as shown by its prominence as a topographic lineament, juxtaposition of different ophiolite lithologies on either side, and the occurrence of late-stage peridotite, gabbro and trondhjemite intrusions often associated with epidotic alteration along its length. The strike of this feature is at an angle of 40” to the roughly N-S spreading axis demonstrated by sheeted dyke orientation. Unless the fault and associated zone of intrusion were diachronous and propagated as spreading progressed, then intrusions at the further end from the axis must have been at a minimum of about 15 km off-axis. The localization of

intrusive activity along the fault makes it clear that this is a seafloor- rather than emplacement-related fault. In the field there is no obvious and consistent difference between the two types of mantle sequence revealed as green and purple on the enhanced imagery. The Open University work revealed no major variations in primary mantle composition throughout the thickness of the nappe (Lippard et al., 1986), and our two types of mantle do not match the harzburgites (opx > 15%) and depleted harzburgites (opx 5-15s) or fit, except in a gross way, the variation in the degree of hydrothermal alteration of orthopyroxene mapped by Ceuleneer (1986). The general correlation between the “purple” mantle and depth below the Moho suggests that the cause may be related to subseafloor hydrothermal alteration or serpentinization. Nehlig and Juteau (this issue) provide evidence that in the Oman ophiolite, hydrothermal alteration extends at least as deep as the base of the crust though this is fraeture-controlled rather than pervasive. Whatever the cause of the mantle subdivision on our imagery, it is consistent throughout the length of the ophiolite. We await laboratory studies of material collected in the field to identify the compositional and spectral causes of this distinction. The purple colour shown by the mantle near the Moho is similar to the much narrower zones of purple colour running along late-stage fractures in the mantle which are interpreted as a post-emplacement serpentinization effect (Rothery, 1984a,b). The location of the petrological Moho is very clear where, as usual in the Salahi Biock, the mantle sequence passes upwards sharply into an

Plate 1. Geological map of the Salahi Block, based on previous Open University work (Lippard, 1980; Lippard and Rothery, 1981). See Fig. 1 for location. Units are as follows: L= basal serpentinite; MS = mantle sequence (tectonized harzburgite); Cp = cumulate peridotite; Cpg = cumulate peridotite and gabbro; Cg = cumulate gabbro; HLG = high-level (isotropic) gabbro; Df = 30-70s sheeted dykes in gabbro; 02 = > 70% sheeted dykes; 03 = 30-70s sheeted dykes in lava; Eu = upper lavas; El = lower lavas; P’ = late-intrusive peridotite; G’ = late-intrusive gabbro; Tr’ = late-intrusive trondhjemite (plagiogranite); HS = Hawasina sediments. Plate 2. Decorrelation stretched Landsat TM bands 7, 5, 4 (red, green and blue, respectively) colour composite of the area shown in Plate 1. Plate 3. Revised geological map of the area in Plate 1 based on interpretation of enhanced Landsat TM imagery supported by limited new field data. Units are as for Plate 1 except the mantle sequence has been divided into MS1 which appears green in Plate 2 and MS2 which appears purple in Plate 2 and occurs only near the Moho. Colour changes in the mantle sequence due to serpentinization along late-stage fractures have been ignored.

pp.393-395

CJ CJ

•

•

p'

EI

Hs Tr'

G' P'

EI

Eu

Eu

03

03

02

02

01

01

HLG

HLG

Cg

Cg

Cpg

Cp

Cp

MS2

MS

MS1

E

E Fault

- - Fault

Boundary Imprecise boundary ••••••• Trace of layering

Plate 1

Plate 2

Plate 3

pp. 396-398

Hs

D

Hs

D

Tr'

•

D

••

G'

Tr'

G' P'

EI

03

EI

02

03

01

02

HLG

01

Cg2

HLG

Cg1

Cg

- - Fault - - Fault

- - Boundary ____ Imprecise boundary ••••••• Trace of layering

Plate 4

Plate 5

Plate 6

399

TABLE

2

Colours

of rocks on decorrelation

stretched

TM images

(modified

Colour

Rock type

Main ophiolite

from Rothery.

1987a)

’

TM bands

Mineralogical

4B

5G

7R

explanation

sequence

Upper lava (Eu)

Magenta

M

L

M

Magnetite

Lower lava (El)

Dark magenta

M

L

M

Magnetite,

Dykes and lava (D3)

Dark red

L

L

M

Mixture

Dark orange

L

M

M

Fe.’ + (chlorite,

(Dl)

Orange

M

H

M

Mixture

Sheeted dykes (D2) Dykes and gabbro

epidote of El and D2 epidote)

of D2 and HLG

High-level

gabbro

(HLG)

Yellow-green

L

H

M

OH, limonite,

Cumulate

gabbro

(Cg)

Cyan-green-(yellow)

M

M-H

L-M

OH, Fe’

epidote

’

Dark purple

L-M

L

L

Serpentine,

mafic minerals

Mantle:

top (MS2)

Purple

M

L

M

Serpentine,

magnetite?

Mantle:

lower (MSl)

Green-yellow

L

M

L-M

Serpentine,

hematite?

Purple

L

M

L

Serpentine.

magnetite?

Cumulate

peridotite

Serpentinite

(Cp)

(e)

Late-rntrusives Pink, lilac

H-M

M-L

H

Low in OH and Fe

Gabbro

(G’) felsic

Yellow

L

H

H

Limonite.

hematite

Gabbro

(G’) mafic

Cyan-green

L

M-H

L

Limonite,

mafic minerals

Dark purple

M-L

L

L

Serpentine,

TrondhJemite

Peridotite

(T’)

(P’)

’ L = low; M = moderate;

essentially thus more it is now mensional

H = high; B = blue; G = green;

R = red.

scale layering seen on the imagery. The colour change from yellow through green to cyan on the enhanced imagery represents increasing mafic content. An area previously mapped as late-stage gabbro (the large area of G’ in the northeast of

gabbroic crustal sequence. The Moho is tightly constrained than previously, and feasible to attempt to map the three-direlief of the crust-mantle boundary.

The Jebel Shaykh area demonstrates of enhanced TM imagery for mapping

mafic minerals

the utility on a more

Plate ) on the basis of a dark tonal anomaly on 1 : 80,000 black-and-white air photographs did not

local scale. Jebel Shaykh is a region of high ground consisting of gabbros and dykes, separated from the gabbros and mantle sequence of the main part of the Fizh Block to the west by the low lying lava-filled “Alley” region (Smewing et al., 1977). The imagery (Plate 5) revealed colour variations within the gabbros which do not correlate with

consist of ratio-layered (cumulate) and unlayered (high-level) gabbro and a hitherto unrecognized belt of sheeted dykes. Many acidic, gabbroic and ultramafic late-intrusive bodies ranging in size

boundaries shown on the Open University maps (Smewing, 1979; Lippard, 1980) (Plate 4) and suggested the presence of layering in many places.

from about 1 km long to less than 100 m across were identified within the Jebel Shaykh massif, none of which had previously been recognized

Fieldwork showed that 2-20 cm-scale ratio and phase layering occurs parallel to the 30-100 m-

(Plate 6). The ability of yellow-red-magenta

Plate 4. Geological

Open

map of the Jebel Shaykh

correlate with colour boundaries on the enhanced TM imagery. In the field, this area was found to

area, based on previous

University

to map offsets between belts colours representing differ-

work (Smewing,

1979; Lippard.

1980). Units

are as for Plate 1. Plate 5. Decorrelation

stretched

Landsat

TM bands

7, 5, 4 (red, green and blue. respectively)

colour

composite

of the area shown in

Plate 4. Plate 6. Revised geological

map of the area in Plate 4 based on interpretation

new field data. Units are as for Plate 1 except that the cumulate blue on Plate 5. and Cg2. a less mafic member

appearing

gabbro

green-yellow

of enhanced

has been divided on Plate 5.

Landsat

TM imagery

supported

into Cgl, a more mafic member

by limited appearmg

400

ent proportions of gabbros, dykes and lavas, together with fault-controlled late-intrusive bodies, convinced us that there is much more faulting within the Jebel Shaykh region than has previously been recognised. We now know that Jebel Shaykh consists essentially of axial ophiolite lithologies with no large masses of late-intrusive or ambiguous high-level gabbro. This shows that the fault forming the western edge of Jebel Shaykh is probably an emplacement-related structure rather than a major seafloor feature. The Jebel Shaykh massif is therefore a normal ophiolite sequence repeated by a major normal fault near the trailing edge of the ophiolite nappe. Wider applications

The level of map revision described here can be performed throughout the ophiolite on the basis of similarly processed TM images. Our preliminary interpretation was greatly aided by the quality of the geological maps of the area and previous experience of the terrain. In the absence of such preexisting known information, it is unlikely that data with such coarse spectral resolution as Landsat TM will ever be sufficient for unequivocal identification of lithologies, although the data are sufficient for lithological discrimination under such circumstances. However, recent work (Pontual, 1988) shows that decorrelation stretched TM band 7, 5, 4 images similar to those used in this study can distinguish previously unmapped layering and reveal the hitherto unrecognized composite intrusive nature of the Cerro Colorado anorthositic gabbro massif in northern Chile. This shows that the experience gained from our Oman study may be applicable to well-exposed arid and semi-arid igneous terrains worldwide. We are less confident about extrapolation to terrestrial wellexposed temperate and polar regions or to the surfaces of other planets, where the effects of weathering may be very different. In vegetated terrains, the spectral response of the rocks may be completely masked by that of vegetation and soil. Conclusions

ophiolite lithologies. Using this imagery and with only a relatively small amount of field control, it is possible to map with a high degree of confidence, especially within the plutonic lithologies of the ophiolite. From the top of the gabbros upwards, alteration effects become more important and there are some ambiguities. It is especially easy to pick out ultramafic and trondhjemitic intrusions within the cumulate sequence and intrusions of all compositions within the dykes and lavas. Their prevalence shows that off-axis plutonism was more dispersed throughout the ophiolite than was previously appreciated. Such bodies cannot be detected in the field without detailed traverses, nor are they always identifiable on black-and-white air photographs. We recommend the use of enhanced shortwavelength infrared imagery for revision and refinement of lithological maps in Oman and similarly well-exposed semi-arid terrains elsewhere. Acknowledgements

We are grateful to the Sultanate of Oman Ministry of Petroleum and Minerals, particularly Mr Mohammad Kassim, for permission to work in Oman, and to Rachel Haymon and Mike Hughes-Clarke for logistical help. Work by Abrams was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Rothery was funded by NERC Grant GR3/6388. Pontual is supported by a NERC Research Studentship. We thank the referees whose comments enabled us to improve the clarity of the text. References Abrams, M.J., 1986. Mapping the Oman ophiolite using TM data. In: Proc. Thematic Conf., Sth, Remote Sensing for Exploration Geology (Reno, Nevada, 29 September-2 85-95. Bailey, E.H., 1981. Geologic Map of the Muscat-Ibra

area,

Sultanate of Oman. J. Geophys. Res., 86: (pocket map). Barker, J.L. (Editor),

Decorrelation stretching of Landsat TM bands 7, 5 and 4 is an effective way to discriminate

Oc-

tober, 1986). Environ. Res. Inst. and Ann Arbor, Mich., pp.

1985. Landsat-

Science Characteristics,

Early Results. NASA, Washington, D.C., NASA CP-2355, 4 Volumes.

401

Browning.

P.

and

Lippard,

S.J.

Oman

geological

Hawasina-Rustaq. 4/5.

Dir. Overseas

Browning,

P. and

chambers

1982.

ophiolite

J.D.,

spreading

in the Oman

map

axes: evidence

in magma

for magmatic

J. Geol.

Sot. London.

1986. Structure

riage g la dorsale. Gillespie.

des Ophiolites

sous un centre

A.R.,

Thise.

Kahle,

enhancement

A.B. and

Walker,

correlated

stretches.

Nantes, R.E.,

images.

Remote

in the crustal

1986.

Color

Environ.,

K.W..

Boeuf.

M.G.A.,

Moody-Stuart,

M.. Pilaar.

1974. Geology

of the Oman

Geol. Mijnbouwkd.

W.H.F.

Genoot..

Hunt. G.R., 1977. Spectral

the visible and near infrared. of minerals

G.R.,

Verh.

B.M.. K. Ned.

Geol. Ser., 31: 423 pp. of particulate

Geophysics,

minerals

in

rocks:

I Silicate

minerals.

Mod.

G.R.,

J.W. and Lenhoff, spectra

J.W. and Lenhoff. spectra

igneous

T., Beurrier,

Magmatic, area

(Haylayn 4-10

Dahl,

Block,

Oman:

Oceamc

Nehlig,

P., 1988a. at a fossil

of the Haymiliyah

Nappe

and Oceanic

of

Lithosphere

Proc.

(Nicosia,

The

M. Whitechurch,

plutonic

sequence

A review. Proc. Troodos (Nicosia.

Cyprus

H. and

of the

Samail

87. Ophiolites

4-10

October,

and 1987).

S.J.

ophiolite Milton Lippard.

D.A.,

(Editor),

project

1980.

Wadi

Jizi.

map 2. Dir. Overseas

Oman

geological

Surv. Open

Univ..

Keynes. S.J.

and

Rothery.

D.A.

(Editors).

1981.

1984a.

Wadi

The issue):

Conf.,

imagery

In: I.G.

Gass.

S.J. Lippard

of

semi-

6th. Remote

Sen\-

Texas).

Landsat

in mapping

Oceanic

Mapper

accessible

(Houston.

role

(MSS) and

Thematic

of poorly

Environ.

Mich.. in press.

The

Scanner

and

Multispectral

the Oman

A.W.

Lithosphere.

ophiolite.

Shelton Geol.

(Editors),

Sot.

London

Spec. Publ.. 13: 405-413. Rothery,

D.A.,

remote Rothery.

1984b.

Reflectance

of ophiolite

relevance

to lithological

MSS bands: sensing. D.A..

ophiolite. Rothery.

J. Geol. Sot. London.

1987a.

Landsat

Improved

Thematic 1987b.

Rothery,

Landsat Ophiolites October, geological

.A., 19X8. Sub-divl-

ophiolite

images.

Lithosphere

using enhanced

Proc.

Troodos

(Nicosia.

Cyprus,

1979. The Sumeini-Shinas

ophiolite.

Surv.. Open I.G..

8: 1253-1254.

X7. 4- 10

1987). (In press).

J.D. (Editor),

Smewing,

aa an aid to

Sensing.

M.J. and Pontual.

and Oceanic

of rock unlta of the Oman

stretching

units in the Oman Mapper

hy

141: 933-939.

144: 587-597.

Int. J. Remote

Thematic

in the

mapping

imagery

Decorrelation

D.A., Abrams,

sion of mapped

rocks

discrimination

Mapper

J. Geol. Sot. London,

D.A..

J.D.. 1977.

ophiolite

(In press). Lippard.

ophiolite

(Editors). 151 (this

of Landsat

Res. Inst. and Ann Arbor, Rotherg.

Smewing,

1987). (In press).

I., Emewein,

Lithosphere

Oman).

of the Sumail

mapping Geology

image interpretation.

processes

sequence

Samail

October,

R.. 1988b.

Nappe.

R. and

the crustal

T.. Reuber.

IX Basic

rocks. Mod. Geol., 5: 15-22.

87, Ophiolites

Cyprus,

ing for Exploration

using

and rocks:

and hydrothermal

center:

Troodos

III Oxides

C.J., 1974. Visible

of minerals

M., Dahl.

tectonic

spreading

and rocks:

Mod. Geol., 2: 195-205.

Salisbury.

and ultrabasic

of minerals

C.J., 1971. Visible

11: 17X

permeabilitirs

A. Nicolas

In: Proc. Thematic

Ophiolites

42: 501-513.

Mem.,

and fossil thermal

Tectonophysics.

for lithological

Landsat

and near-infrared

Juteau,

Reinhardt,

J.W.. 1970. Visible and near-infrared and

Salisbury,

and hydroxides.

Juteau.

M.W.,

I: 283-300.

and near-infrared Hunt,

and

Mountains.

signatures

Hunt. G.R. and Salisbury, spectra

Hughes-Clarke,

Oman.

A., 1988. Application

imagery

arid regions.

Glennie,

Hunt.

of

and

3.

199-221. Pontual,

20:

sequence

F. Boudier

map

I.G., 1986. The Ophio-

Sot. London

T.. 1988. Flow porosities,

In:

Ophiolites

project Keynes.

data on fluid inclusions

gradients

1. Decorrelation

Sensing

Geol.

(Oman).

338 pp.

ophiolite

Univ.. Milton

A.W. and Gass. Oman.

flux

209-735.

Geol.,

S.J. Shelton,

lite of Northern

et char-

d’Oman:

oceanique

Univ. Nantes,

of highly

and HSI contrast

d’expansion

geological

Surv., Open

and preliminary

G.,

mantellaire

Oman

Dir. Overseas

PP. Nehlig, P. and Juteau,

138: 279-280. Ceuleneer,

Ahin-Yanqul. Lippard,

Keynes.

1981. Processes

ophiolite.

Wadi

project

Surv., Open Univ., Milton

Smewing,

beneath

ashoclations

(Editors),

ophiolite

Univ., Milton

K.O.,

Mineralised

fault

normalization

and

Symp.

Sensing.

Remote

El Boushi, zone

axis. Geology,

Soha, J.M. and Schwartz.

area. Oman

Map 1. Dir. Overseas

Keynes.

Simonian.

spreading

project

1-M. and

parallel 5: 534-538.

A.A., 197X. MultIspectral

contrast

Gabs,

to the Oman

enhancement.

5th (Victoria.

histogram

In: Proc.

B.C.). pp. X6-93.

Can.