Mantle flow patterns at an oceanic spreading centre: The Oman peridotites record

Tectonophysics. 151 (1988) 1-26 Elsevier

Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Mantle flow patterns at an oceanic spreading The Oman peridotites record G. CEULENEER,

centre:

A. NICOLAS and F. BOUDIER

Lahoratoire de Tectonophysique, Unioersitt! des Sciences et Techniques du Lmguedoc, place Eugtke Bataillon. 34060 Mantpellier Cedex (France)

(Received July 30,1987: revised version accepted

December

1, 1987)

Abstract Ceuleneer,

G., Nicolas.

peridotites

record.

The mantle

section

Earth’s surface. to unravel have

mantle

processes

“asthenospheric”

thrusting).

The diapir

pattern,

of the oceanic

the first

particularly

feeding zones of the overlying The second

flow pattern

flow patterns

relevant

to the mantle

a diapiric

of magma magma features

pattern

very intense

plastic

structural the drift estimated

direction.

is frequently

observed,

pointing

the existence

upstream

of mantle

homogeneity

of the oceanic

of the overlying

at the in order

peridotites

of Oman (the

of the peridotites criteria.

ridges, features

vertical

flow lines

transition

recognized

these mantle

zone a few hundred

in a few places diapirs,

which

metres thick

along

the Oman

are probably

the main

spreading

to a forced

lithosphere,

One

section.

features

very regular

structures

over

the flow plane weakly dips away from the ridge axis, and

This Bow pattern

diapirs revealed

by the partially

plate. This occurs

along the ridge axis, away from a diapir. than the diapir

molten

at a distance

is frozen

regime.

asthenospheric

making

space

during

the gradual

A shear-sense flow. Forced for themselves

inversion below

studies,

by the more regular

the ridge which.

depth

the rtdge

diapirs from

of the

flow on the ridge flank

by seismic anisotropy is superseded

accretion at shallow

is acquired

in the Oman

is

axis. The

farther

from the

flow Induced

by

case, can be roughly

to be a few tens of kilometres.

The last flow pattern

has been observed

zone that

strikes

described

along present-day

at a right

angle

in only one area and it corresponds

to the ridge

fast spreading

axis, This zone could

The formation the asthenosphere

represent

to a 20 km thick asthenospheric a broad

diffuse

transform

shear zone as

ridges.

Introduction

~~-1951/88/~03.50

spreading

(- 70% of the Oman peridotites)

away from the ridge in a steady-state

ridge when this forced flow induced

been

flow channelled

to the spreading

with

beneath

is several times longer

the flow line is parallel

consistent

range

of the lithosphere

the first step of the emplacement

through

along the strike of the palaeo-ridge;

below the Moho

exposed

chamber.

which is by far the most common

mantle

The mantle

to the accretion

in a narrow

has

several tens of kilometres lithospheric

mantle the Oman

of which along the ridge axis is in the order of 10 km. These

has circulated

fed in such a way by one diapir

The pattern

oceanic

on the basis of microstructural

process

in a pipe, the extension Such

The Oman

have been documented.

and diverge in every direction

discontinuity.

A large amount

ridge segment

during

centre:

Te~ta~a~hysi~s. 151: l-26.

throughout

lithosphere.

one related

spreading

of Oman.

piece of the uppermost

These two events have been distinguished

rotate to the horizontal

the Moho

palaeo-ridge.

deformations:

asthenospheric

at an oceanic

The Ophiolites

of these rocks has been conducted

with the generation

plastic

and elliptic flow plane trajectories below

is the largest

mapping

associated

flow patterns

(Editors),

shear flow), and the second one imprinted

Four well-contrasted

structures

ophiolite

structural

two successive

F., 1988. Mantle

and A. Nicolas

of the Oman

Extensive

recorded

(intraoceanic

A. and Boudier, In: F. Boudier

neath mid-oceanic ridges (e.g., Le Pichon et al., 1982; Nataf et al., 1986) and relies on two distinct processes: crust formation from mantle partial melts and accretion of the residual lithospheric

of oceanic lithosphere involves rising to a shallow depth be8 1988 Elsevier Science Publishers

B.V.

mantle. nomena

The greater the depth, the less these pheare understood. Geophysical studies and

observations clarify

from submersibles

the structure

anisms

of crust formation

Study

Group,

Francheteau However,

1981;

(e.g., East Pacific

Macdonald.

and Ballard, the actual

ing mantle

are beginning

1982.

Rise 1983:

1983; Orcutt et al., 1984).

flow structure

and the mechanisms

from peridotites

to

of active ridges and the mech-

are poorly

in the underlydue to the lack

of direct evidence.

crust and the linearity

of the magnetic

anomalies pattern of the oceans led naturally to the view that the processes of oceanic lithosphere generation

were

completely

homogeneous

along

the strike of the spreading centres. A very simple mantle flow pattern, classically depicted as the ascending

limbs of large convective

rolls and con-

tinuous magma discharge to the surface was invoked to explain such first-order homogeneity. The along-strike variability of ridge processes has emerged

from

recent

geophysical,

petrological surveys, especially Pacific Rise. It has been shown segmented

1982). Furthermore.

composition

of the peridotites

distribution

and the composition

trapped ascent for

within towards

on a much smaller

structural

and

along the East that the ridge is

scale than the spac-

1987; Lorand,

or whether

mantle

processes.

it does actually

As the detailed

reflect

flow structure

in

the mantle can hardly be unravelled through geophysical methods, this question is still open. A complementary approach is to analyse the structure of ophiolites, fragments of fossil oceanic lithosphere which outcrop at the Earth’s surface. Peridotites from the mantle section of ophiolites, when unaffected by the processes associated with their abduction, have kept a record of solid-state mantle flow in their structures induced by high-temperature, low deviatoric stress plastic deformation (Nicolas and Poirier, 1976; Nicolas et al., 1980). Mapping these structures and turning them into kinematic maps gives a precise image of the asthenospheric flow geometry in the uppermost parts of the mantle beneath the oceanic

of

the

liquids

during

their

a lot of data

magma

(e.g., Nicolas,

such studies,

migration

1986a;

it has been

flow can adopt

demonstrated

by physical

Sleep,

shown

a diapiric

modelling

et al., 1984). Unfortunately,

that

pattern

ophiolitic

(Rabinowicz massifs

are

usually small relative to the representative dimensions of the oceanic structures under consideration. Furthermore, these massifs may have been seriously dislocated by deformation events during or after their emplacement. Reconstructing large asthenospheric structures from such a patchy and dismembered puzzle can be quite tricky. In this respect, the Oman ophiolite (Fig. 1) is a particularly

favourable

field of investigation.

to an area of the Alpine Arabia

and

Coleman, lithosphere

feature

of basaltic

section provide

models

with

this issue).

asthenospheric

can be asked whether crustal

the petrological together

(Nicolas and Violette, 1982) the importance of which in oceanic accretion mechanisms has been

continental Obducted

is a purely

mantle

the surface

the mantle

ing between two major transform offsets (see Macdonald et al. (1984, 1986) and Langmuir et al. (1986) for reviews of the main evidence). Now, it this segmentation

the

constraining

Through

In the early days of plate tectonics, the great homogeneity of the structure and composition of the oceanic

and Violette,

through

of melt extraction

known

ridges provided that the spreading centre has been restored in its structural framework (e.g.. Nicolaa

Eurasia

range

have

not

It belongs

where converging yet

reached

the

collision stage (e.g., Ross et al., 1986). onto the Arabian margin in Campano-

Maastrichtian

times

(Glennie

et al., 1973, 1974;

1981), this fragment of the Tethyian has retained the structures acquired

during its formation and emplacement. With a total area of 30,000 km2, it is the largest ophiolite in the world. Outcrop conditions are excellent. The trend of the range is, on average, parallel to the axis of the presumed palaeo-ridge (Pallister, 1981) and this allows an approximately 400 km long segment of an oceanic spreading centre to be studied along strike. Hence, the scale of the structures found in Oman may be comparable to those studied along present-day ridges. Continuous outcrops often provide a complete section of the lithosphere, from a maximum depth of 9 km beneath the palaeo-Moho to the volcanosedimentary formations of the upper levels of the crust (Hopson et al., 1981). This allows mantle structures to be analyzed within a plate-tectonic reference

25

50 km 24

2:

Fig. 1. Simplified

framework

(see next section)

between

mantle

structure

and composition

onstrated.

map of the Oman ophiolite

processes

and the relationships and

variations

in the

of the crust to be dem-

with location

of the massifs and of the main wadis.

Our work in Oman the systematic section.

was essentially

structural

mapping

The data are presented

at the end of this issue.

devoted

to

of the mantle

in the pocket

maps

The Oman mation

peridotites

episodes:

tion of the lithosphere intraoceanic two

are

and

clearly

help of microstructural following

section.

structures

are

and

In

arately

(Boudier

during

1981).

distinguishable criteria, this

only

Intraoceanic

models

are

the

These

with

as discussed

paper

accre-

lithospheric

Coleman.

presented.

emplacement

during

which preceded

(Boudier

events

two plastic defor-

and the second

thrusting

abduction

record

The first occurred

the

in the

range

(Boudier

and

Smewing,

Ceuleneer paper). ditions. peridotite

sep-

onstrated in Oman, focusing on the areas where such structures are the most clearly exposed. Their contribution to our processes at oceanic

understanding of spreading centres

cussed.

description

mantle is dis-

of the mantle

1981;

Bartholomew,

with

crystals

is oriented

axis

in a direction

while the [OlO] and

[OOl] axes spread out in a girdle perpendicular to the [loo] axis and the [OlO] axes are usually preferentially grouped around a direction which is subperpendicular

to the S. The

[loo]

axis and

the

(010) and {Okl} planes correspond respectively to the preferential slip direction and planes of olivine

is generally

et al., this

of mantle

their origin (see review

close to that of the lineation

Oman

(Nicolas

majority

con-

1986b). The [loo] crystallographic

crystals in high-temperature

is proposed

of

in asthenospheric

the

structures throughout the Oman range is given in a companion paper where a reconstruction of the palaeo-ridge

1983; et al., this

in Fig. 2 is representative

fabrics, whatever

in Nicolas,

1981; Christensen

1985: Ceuleneer

deformed

It agrees

of the olivine

et al., 1985, this issue).

Coleman,

and Nicolas, The fabric

thrusting

considered

and

Oman peridotites

accretion

The purpose of the present paper is to describe the various asthenospheric flow patterns dem-

An exhaustive

lattice fabric of the olivine in the Oman peridotites has been measured at numerous points in the

(see review in Mercier, strong

conditions

( > 800 o C)

1985). As the lattice

and

slightly

oblique

fabric to the

issue). Along-strike variability of ridge processes and ridge segmentation are discussed in the light

shape fabric, it may be deduced that the rock has recorded strong plastic deformation under a sim-

of these new data.

ple shear regime. The average (010) plane is equated to the shear flow plane and the average

Method

Plastic deformation of mantle peridotites occurs through slip and climb of dislocations accompanied by dynamic recrystallization (Nicolas and

[loo] direction to the flow direction. The shear sense is that of the rotation the foliation would require in order to coincide with the shear plane. It is currently determined under the optical microscope in thin sections cut in a plane perpendicular to S and parallel to L. The trace of the shear plane is deduced from the orientation of maxi-

Poirier,

to the

mum

Shape

fabric

between

(L),

corre-

Kinematic

rock

analysis of mantle peridot&es

1976).

a shape

features,

These and

foliation

mechanisms

a lattice (S)

and

fabric. lineation

confer

spond respectively to the plane of flattening and to the stretching direction of the minerals that make up the rock. The common extinction angle of minerals under the polarizing microscope reveals the lattice fabric, which may be measured precisely by means of a universal stage. Kinematic analysis, permitting a precise definition of the geometry of flow lines and the shear sense in non-coaxial strain, relies on relating the shape to the lattice fabric (Nicolas and Poirier, 1976). Oman mantle peridotites are very depleted (harzburgites to dunites). Their deformation is thus controlled by the behaviour of olivine. The

extinction

in olivine

crystals.

shape and lattice fabric than 15”, so a map of foliations gives a good approximate picture

The obliquity is usually less and lineations of the plastic

flow geometry recorded by a peridotite massif. These orientation data can then be turned into maps of flow planes and lines by taking into account the obliquity and the shear sense. Field studies In the mantle section, the plastic deformation structures of the peridotites were systematically measured. In the case of depleted peridotites such as those in Oman, they are revealed by the flattening and lengthening of chromian spinels, dispersed

Fig.

2. Typical

Arrow-shear projection.

within

preferred sense:

orientation

dashed

lower hemisphere.

contours

the rock at about

black metallic the superficial oriented

of olivine

line-shear

l-5%

plane:

crystallographic horizontal

per 0.45% total area:

of the volume

was taken

(the

and lineation were measured in on bleached oriented specimens.

Layering, the

shown

modal

up by a regular

composition

(olivine/pyroxene

ratio)

of

the

variation

the

in

peridotites

is most often

time-com-

posed of deformed crystals and is parallel probably derives from dykes (pyroxenitic

to S. It layers)

or dyke walls (dunitic layers) emplaced below the ridge before the cessation of plastic flow although a more pristine origin is not excluded (Allegre and Turcotte, 1986). The same origin is ascribed to the concordant chromite pods (Cassard et al., 1981; Ceuleneer and Nicolas, 1985). In contrast, undeformed pyroxenite and gabbro dykes, together with discordant dunite veins and chromite ore bodies, are emplaced after the cessation of the plastic flow. Their

orientation

can

be related

to that

trace;

of the

dotted

equigranular

harzhurgite

line-lmeation.

Equal-area

of Oman. stereographlc

I. 2, 3. 4 and 5%.

Contours:

to the palaeo-Moho and can also be used reference plane for the horizontal (Reuber. issue). The trend assumed

on a 1 km sampling

mesh in order to determine the shear sense and the microstructure. When not seen in the field, the foliation laboratory

in a coarse-gained

100 measurements.

colour of the spinels contrasts with ochre colour of the peridotite). An

sample

axes

line-foliation

to be parallel

ridge (Cann,

of the emerging

Ridge (Helgason

other

is formed

within

is

to the axis of the palaeo-

by the seismic anisotropy crust (Shearer and Orcutt, complex

dyke complex

1974). This hypothesis

by field studies Atlantic

of the sheeted

as a this

is confirmed

parts of the Mid-

and Zentilli.

1985) and

of present-day oceanic 1985). The sheeted dyke

by dykes

a particularly

intruding

narrow

revealed by the narrowness axial zone along present-day

into each

axial zone,

as

of the neovolcanic ridges, which is usu-

ally less than 2 km wide (Macdonald. 1983), and is only 500 m wide along some parts of the East Pacific Rise (Choukroune et at., 1985). ~orrnatioI1 models for the dyke complex suggest that this process of self intrusion often causes dykes to break in their middle (Cann. 1974: Kidd and Cann, 1974). Theoretically, a statistical study of the chilled-margin facing direction should allow the side of the ridge to which the ophiolite belonged

to be determined.

be of limited

use in Oman.

This criterion

seems

where the majority

to of

and

dykes have two margins, and ,wher,e. of those which are split in their*middle, the designation of

The crustal structures necessary to establish the internal plate-tectonic reference framework for in-

chilled margins to one or the other facing direction is quite weak (Pallister. 1981; Dahl. 1984; Ceuleneer et al; this paper (personal observutions)). We have instead tried to determine the original position of the ridge axis from the layering and lamination attitudes within the upper layered gabbros (Nicolas et al., this issue), and more exceptionally from the attitudes of slump direction and the polarity of synmagmatic normal faulting.

stress in the lithospheric Jackson, 1982).

mantle

(Nicolas

terpreting mantle structures were also systematically measured. The contact between mantle peridotites and the basal cumulates of the crustal section (palaeo-Moho) is taken as the palaeohorizontal (Nicolas and Violette, 1982). Our experience in Oman is that the layering of the basal cumulates of the crustal section is always parallel

The flank of origin of the ophiolite can also be from the shear sense in the mantle peridotites

However, it can be estimated indirectly (Nicolas. 1986b). There are several ways of estimating the deviatoric stress that peridotites have recorded. Of

(Nicolas

these palaeo-piezometers,

deduced from the palaeo-dip of the foliation and and Violette,

1982;

Rabinowicz

et al.,

1984).

the size of recrystallized

grains is the most reliable (Nicolas, gives information

1978). This

on stress during the major de-

formation episode responsible for the macroscopic

Microstructural studies

structures measured in the field. In a recent comApart from determining

the shear sense, the

main purpose of studying microstructures estimate the physical conditions tion. These data are fundamental

is to

during deformafor distinguish-

pilation,

Karat0

(1984)

experimental calibrations

showed that the various of this paleo-piezometer

were coherent to a factor of three as long as dry experimental

conditions

were considered.

A one

ing between the structures acquired during accre-

order of magnitude increase in the size of recrys-

tion (HT o or “asthenospheric” deformation) from those recorded after the lithosphere formation

tallized grains corresponds to about a one order of magnitude decrease in the deviatoric stress. According to this outline, there are two main

(LT o or “lithospheric” deformation). Ophiolitic peridotites are derived from a very superficial level of the oceanic mantle where temperature varies far more than any other physical parameter. Hence, the deformation history of ophiolitic peridotites is controlled by their thermal evolution. The deviatoric stress prevailing during deformation is itself linked to the temperature through the flow law (e.g., Gueguen and Nicolas, 1980;

Kirby,

1983).

Beneath

the ridge axis, the

mantle temperature is estimated to be 1275O C assuming dry melting and is controlled by the melting of the peridotite in the 01-Ens-Diop-Plag

textural categories of Oman peridotites (Fig. 3): The first includes the coarse-grained equigranular texture (Fig. 3a) featuring a unimodal grain size distribution around an average of 3-4 mm (corresponding to a stress of 0.2-0.8 MPa (Karato, 1984)). The grain boundaries are sharp. often slightly curved, and make up 120” triple junctions. The grains are equant to slightly flattened and elongated, with a thickness to length ratio of no less than l/2. Most of them lack optically distinguishable substructures. Only the characteristic lattice fabric (Fig. 2) clearly shows that these

four-phase field (e.g., Maaloe, 1985). At such a temperature, the flow rate of the peridotites under stress of the order of mega-Pascals is compatible with the rate of shearing induced by plate move-

rocks have undergone plastic deformation, thus distinguishing them from cumulates. Some grains

ments (about 1O-‘4 s-t). On spreading away from the axial zone, the mantle cools through heat conduction to the surface, thus transforming into lithosphere (Parker and Oldenburg, 1973; Forsyth, 1977). Stress conditions prevailing during deformation of the mantle after its transformation into

tation. Due to the strong shear strain undergone

lithosphere are characteristically

higher than dur-

ing asthenospheric flow. A drop of temperature from 1200 o to 1000 o C, for example, corresponds to an increase in stress of about 10 MPa, all other parameters remaining constant (e.g., Goetze and Evans, 1979; Darot and Gueguen, 1981). It is impossible to determine the temperature which prevailed during plastic deformation of ophiolitic peridotites from a simple petrographic investigation within the P-T field considered.

are polygonized into subgrains; two adjacent subgrains usually present a strong degree of disorienby these rocks, most of the subboundaries have evolved into grain boundaries through progressive rotation. This mechanism is accountable for the nearly equant texture of the rocks. It requires a temperature close to the solidus as far as low stress conditions are concerned (Nicolas and Poirier, 1976). This is the most common texture of Oman peridotites. The orientation of the corresponding structures is not random relative to the crustal reference frame, which indicates, a posteriori, that these structures are indeed related to the accretion event. Solidus to hypersolidus temperatures are indirectly recorded by the presence of variously deformed impregnation minerals (plagioclase and clinopyroxene) in a matrix of

7

size distribution of millimetre-scalp porphyro~lasts being worn down into a fine-grained matrix. The grain size distribution scattered

between

micronmetres tens

in the matrix

(corresponding

of mega-Pascals

to stresses

to several

of a few

hundred

mega-

Pascals (Karato,

1984)). In this latter case, matrix

recrystallization

occurred

tion and growth

at the edge of the porphyroclasts.

mainly

through

Most of these are very elongated, is quite dense, only ence

skew to one another.

of these

of numerous

boundaries This

are roughly

to their long axes. The subgrains

slightly

extinction

nuclea-

the substructure

and the subboundaries

perpendicular a

is often widely

100 and 200 pm to a few tens of

subgrains

that are invisible

moderate

to poor

and the wavv

indicates

free dislocations

and

with optical

recovery

are

means

the presof suhmethods. that

the

porphyroclastic texture had developed at temperatures sufficiently low with respect to the solidus of the rock to prevent dislocations migrating. A temperature of

from significantly 900°-1000°C is

classically ascribed to this type of n~i~rostru~tur~ (Mercier and Nicolas, 1975). The porphyroclastic texture becomes increasingly developed close to the basal thrust plane of the ophiolite. The agreement between the deformation recorded in the

b

peridotites

here and in the garnet

the subophiolitic

metamorphic

the porphyroclasti~ thrusting confirms

amphibolites

sole indicates

in that

texture relates to the ophioliti~

event (Boudier and Coleman, 198 1) and the temperature range ascribed to this

deformation.

Where

lithospheric

deformation

very intense. the po~h~r(~~lasti~ texture into a mylonitic texture (Fig. 3~).

was

evolved

A coarse-grained equigranular texture is required if plastic deformation is to be ascribed to the accretion episode. However, this condition can Fig. 3. Microstructures of Oman Nicolas, 1986b). a. Coarse-grained Porphyroclastic areas---olivine Dashed

texture. (dotted

c.

mantle harzburgites (after equigranular texture. h. Mylonitic

texture.

lines are the traces of dislocation

areas-orthopyroxene:

Black areas-chromian

Open walls); spine].

plastically deformed olivine and orthopyroxene (Nicolas, 1986b). The second textural category is the porphyroelastic texture (Fig. 3b) featuring a bimodal grain

be misleading. When affected by the lithospheric deformation olivine only recrystallizes into fine grains. and hence the porphyroclastic texture may only begin when the plastic strain reaches 4O- 60% (y of 1-2) (Karat0 et al., 1980). The lithospheric deformation may have affected the mantie section to a lesser degree, thus disturbing the asthenospheric structure without the development of a fine-grained texture. We therefore developed more accurate criteria capable of revealing the onset of low-temperature deformation. Near the mylnnitic

x

shear zones affecting et al., this issue), evolution

the mantle

of the olivine

gressive

structures.

zone (depending structures

of

the

with the pro-

high-temperature

km from

the shear

of the size of the zone), no sub-

by optical

methods

olivine

of coarse-grained

Closer

to the shear

are found

equigranular

zone,

weak and only affects stage, the HT”

the

One-two

(Boudier

to correlate

substructure

disorientation

(H7”)

section

we are able

in the

the wavy extinction is virtually

is

at this

boundaries appear at this stage. Closer to the shear zone (a few hundred to a few tens of metres

the HT” being

structure

strongly

is severe, all grains and the orientation

is severely

rotated

disturbed.

where grains

are of

it starts

begin

to dif-

ferentiate into porphyroclasts and fine recrystallized grains. When fine-grained areas coalesce to form a matrix surrounding the porphyroclasts, low-temperature

(LT o ) orientation evolution

they

result

from

peridotites the

of

asthenosphere

provides

plastic

flow

variation

a qualitative

gradient

in

at the time of its accretion

the

to the

lithosphere. Mantle flow patterns in Oman: Description of typical situations Considering vealed by our

the kilometre-sized structural mapping

features reof the Oman

peridotites (pocket maps; Nicolas et al., this issue), four well-constrained asthenospheric flow patterns can be distinguished ship

between

palaeo-tectonic

on the basis of the relation-

plastic

flow

reference

section,

we examine

patterns

are particularly

directions

system

and

the

(Fig. 4). In this

a few areas where these flow well illustrated.

was inmithe

The first flow pattern (Fig. 4a) is by far the most common. It has been observed along about 70% of the Oman palaeo-ridge segment (figs. 3 and 4 in Nicolas et al., this issue; pocket maps)

mantle flow patterns in processes in cases where

but it is described here by considering the relationship developed within the Fizh and Sal&i

the interference

of LT”

and

aim of the microstructural

to quantify asthenospheric between the foliation and

massifs. along

HT o deformations. Another

in the mantle estimation

Strain

of the peridotite

when a lithospheric shear zone is approached observed in several places. Where the direct fluence of shear zones is not apparent, these crostructural criteria permit the avoidance of interpretation of certain terms of asthenospheric

of the strain.

Homogeneous mantle flow away from the ridge axis: The Fizh and Salahi massifs

has definitely

been acquired. This microstructural

estimate

undisturbed.

It begins to be more severely affected when the wavy extinction is strong in all grains; sub-

away), the wavy extinction polygonized into subgrains

qualitative

fabric give a

peridotites.

some of the grains;

structure

criteria such as the strength of the lattice and the degree of enstatite recrystallization

analysis

is

strain. The angle (Y the shear plane is re-

lated to the shear strain y according to y = 2 cotg 2 (Y,assuming grain boundary recrystallization. Usually, the angle (Y varies erratically from one station to the next, even in zones where the flow appears very homogeneous, so that its use to precisely define a local value of y is dubious. We have, however, found that the average value of (Y, at the scale of zones where the flow is homogeneous, varies in a consistent fashion from one zone to the other. Such average values have been used to evaluate the magnitude of the shear strain recorded by the Oman mantle peridotites. Other

It features

very homogeneous

strike on a scale of about

structures

100 km (Figs.

5

and 6, and pocket maps). The flow plane dips slightly, the palaeo-dip varying between 0“ and 25O away from the spreading axis (Fig. 7). The flow line is at right angles to the ridge axis and follows the steepest dip line of the flow plane (Fig. 5 and 6). The shear strain y increases rapidly towards the top of the mantle section (Fig. 7): From an average shear strain of - 3 in the main part of the mantle section, it reaches a value of - 10 at about 500 m beneath the palaeo-Moho. The most intense strain is measured just below the cumulates of the magma chamber; however, the latter show virtually no sign of plastic strain and a very rapid gradual decrease of y within the top few metres of the mantle section has been locally

spreading

b

a

C Fig. 4. Sketch

of the four asthenospheric

(sheeted

and Moho

planes

dykes

at right angles

parallel strike-slip.

to ridge

discontinuity).

Lower

to ridge axis. b. Vertical

axis. d. Flow planes

For further

flow patterns

discussion

strongly

of flow patterns,

boxes:

d

recognized

in Oman

Asthenospheric

mantle

flow planes

flow lines. down-dip

flow planes

dipping

normal

and

striking

peridotites. and lines.

and curved

to the ridge

Upper

boxes:

M-Moho. flow plane

Crustal

reference

a. Homogeneous trajectories.

axis. Subhorizontal

flov.

c. Flow line

flow lines indicate

see text.

observed. In the zone of very strong shear strain, at the top of the mantle section, the palaeo-dip of the flow plane is close to 0 O. In the deeper parts of the mantle section, it increases gradually, downsection up to a value of about 25” (Fig. 7). The shear sense is very consistent throughout the mantle section: the uppermost parts of the

mantle

flowed

away

from

the

ridge

axis

at a

higher rate than the deeper parts (Fig. 7). In Wadi Hilti, a detailed study revealed a reversal in shear direction at the top of the mantle section (Fig. 7). The reversal zone closely corresponds to the - 500 m thick zone of very strong shear strain mentioned above.

1(J

a

Fig. 5. Preferred orientation best-computed

of the structural elements in Salahi and Fizh massifs from Wadi Ahin to Wadi Zabin. Black triangle -

axis. a. Dykes

Magmatic laminations with an asthenospheric

from the sheeted

dyke complex;

in the basal cumulates; 46 measurements. texture; 119 measurements.

Mantle flow in asthenospheric Batin and Shamah areas

Contours:

Biapirs: The Maqwd

231 measurements. Contours:

Contours:

1.3, 2.6, 4.3, 10.4 and 18.2%. b.

2, 4, 6, 8 and 10%. c. Foliations

1, 2, 4 and 7%. d. Associated

in the mantle peridotites

lineations.

is hidden in places below the crustal formations but is no longer than 20 km. It may, therefore, be described in terms of a vertical pipe, slightly el-

This second configuration (Fig. 4b) is more exceptional. It features vertical flow lines, down-

liptical in cross section, elongated along the ridge axis. The asthenospheric flow does not remain

dip flow planes and curved flow plane trajectories. Such a pattern has only been found without doubt in three areas along the Oman palaeo-ridge segment (figs. 3 and 4 in Nicolas et al., this issue) namely in the Maqsad, Batin and Shamah areas. In the Maqsad area (Fig. 8) the width of the vertical flow zone is 8 km at a right angle to the local orientation of the diabase dyke swarm; it may be followed for about 10 km along the axis. It

vertical up to the Moho; it breaks up a few hundred metres below the cumulates (Fig. 10). Within a radius of at least 30 km around the centre of the pipe, the flow planes are, on average+ subhorizontal and the flow lines are radial with respect to the pipe axis. The directions parallel and perpendicular to the ridge axis are clearly preferred (Figs. 8 and 9). This confi$uration is very clear to the west and north of Mqsad.

11

FIZH and HILTI masslfs

LINEAR STRUCTURES ____~~

5&/n

i__.J.:.*)

‘. .;;,.:;. ;

;’

,. ,.:.,/.

; ,’

.,

;.

56’3O’E

“E

Fig. 6. The linear structures sheeted

“.:-:;,

.

4’OO’N

5

dvke complex

~

in the Fizh and Saiabi massifs.

to the south of Wadi Zabin.

Note the orientation

Departure

from

of the asthenospheric

this orientation

is clearly

Iineations

at right angles to the

due to late lithospheric

shear zones.

x

E

-1 ----

13

a

Fig. 9. Preferred contours Andam

of the structural

sheeted dyke complex;

cumulates; texture;

orientation

per 0.45% total area. Black triangle 80 measurements. 217 measurements.

elements -

177 measurements. Contours:

Contours:

in the Sumail

best computed Contours:

massif.

Equal-area

axis; open triangle

stereographic best computed

projection,

0.9, 1.8, 2.7, 3.7 and 4.2%. d. Association

the lattice fabrics are often very strong. The average obliquity of the foliation to the shear plane is recrystallized. fit in well with

in the mantle

lower hemisphere,

girdle. a. Dykes from the Wadi

1.13, 2.26, 4.52, 8.47 and 14.69%. b. Magmatic

1.25, 2.50, 3.75 and 5.00%. c. Foliations

11 o ( y - 5); enstatite is severely The shear sense determinations

-

peridotites

laminations

in the basal

with an asthenospheric

lineations.

has an elliptical shape with a 12 km long axis, oriented NW-SE, parallel to the local trend of the sheeted dyke complex. The vertical flow breaks up a few hundred

metres

below

the Moho,

at a level

this pattern: they indicate that the upward flow was faster in the centre of the pipe than around its edges (Fig. 10). In the radially diverging zone, they

where large tabular dunite bodies invade the harzburgitic framework. Interestingly, in these dunites the foliation is mainly vertical whereas the

show that the upper levels of the mantle section were flowing away from the vertical pipe faster than the lower levels. Despite several extremely closely spaced cross sections at the top of the mantle section of the Sumail massif, we were unable to detect any sign of reversal in the shear sense on nearing the palaeo-Moho (Fig. 10). In the Batin area (Fig. 11) the diapiric structure

lower dunite-harzburgite horizontal.

contacts

are

mainly

Within a radial distance of 5 km, flow lines diverge rapidly, become horizontal and attain a perpendicular trend to the ridge orientation. Shear sense determinations indicate that the flow was faster in the centre of the diapir than on its margins; in the radially diverging zone they show

14

MAQSAO

See tlon

SSE

NNW

2 km

_

asthenospheric llow foliations in mantle peridotites

HT”

J>‘,.,.“I _ cwstal section

Fig. 10. NNW-SSE cross section through the Maqsad diapir. This cross section is parallel to the palaeo-ridge axis. M-Moho.

that flow away from the diapir was ftister in the upper level than in the lower one level. In the Shamah area (Fig. 12), the vertical flow zone has an elliptical shape with the long axis (12 km) subparallel to the regional ridge trend. This diapir is truncated by the topographic surface,

I

BATIN btAPlR

BATIN DIAPtFt ‘.,

,

*

b

1 FOLtAT@t$

i.

-?S .. :

-

approximately 2 km below the Moho, providing the opportunity to observe such a diapiric structure at a lower level than in the two previous cases. Inside the pipe, the harzburgite is rich in pyroxenitic layering, often folded and oblique to the foliation. Dunites are rare, as are pyroxenite

/

L

TRAJWTORY ... ....__ ,SD,P

-. . . .. .. .

TRAJECTORY ,WD,f#

Fig. 11. Flow line (a) and flow plane (b) trajectories in the Batin area, Wadi Tayin massif. Smallest dots indicate dunk

limits.

‘I,,’

I

1

,I=,’

SHAMAH DIAPIR FOLIATIONS

/”

-

trajectory

...

lsodip

,_-I

bllatlon I”

‘,

I_-

(

t

Fig. 12. Flow line (a) and flow plane (b) trajectories

in the Shamah

and gabbro dykes. Within a radial distance of 6 km, the flow lines diverge into a flat attitude, trending parallel to the assumed ridge orientation. Channeling

of the mantle flow along the ridge axis:

The Wudi Fayd- Wadi Ragmi and Sayma The

third

configuration

is exemplified

areas by the

Wadi Fayd-Wadi Ragmi area and features very intense and linear plastic deformation (Fig. 4~). The flow line is parallel to the ridge axis (Figs. 6 and 13). The flow plane is in a zone around the flow line. Although on average subhorizontal, the dip of the plane is irregular, in agreement with the linear character of the deformation, on average, it is parallel to the Moho, especially in the upper-

area, Khawr

Fakkan

massif.

most level of the mantle. Lattice fabrics are very strong; the obliquity between the shape and lattice fabrics has an average value of 5 o which corresponds to shear strain in the order of 10. Enstatite is usually entirely recrystallized, indicating exceptionally intense deformation at high temperature. The shear sense is quite constant throughout the thickness of the mantle section, and in particular, no shear sense reversal was observed on nearing the palaeo-Moho, despite close sampling in this area. Such a flow geometry has been recognized along about 15% of the Oman palaeo-ridge segment (figs. 3 and 4 in Nicolas et al., this issue). In the Maqsad area, mantle flow parallel to the ridge axis has been shown to be genetically linked to the diapir flow pattern, as discussed in the previous

Fig. 13. preferred orientation of foliations and hneations in mantle perjdotites with an asthenospheric texture. Biwk triangle best-computed axis; open triangle - best-computed @die. a and b. Sayma area of the Sumail massif (crustal structure orientations are sbown in Fig. 9); 47 measurements. contours: 2,4 and 6%. c and d. Ragmi area of the Fizh massif (crud structure 0rierrtlltion.s are given in Fig. 5). Foliations: 30 measurements. Contours 3 and 6%. Lineations: 26 measurements. Contours 4 and 8%. e and f. Fayd area of the Fizb massif (area where the palaeo-Moho is tilted into a vertical orientation (see Nicolas et al., this issue)). Foliations: 39 measurements. Contours: 2.6, 5.1, 7.7, 10.3 and 15.4%. Line&ions: 27 measurements. Contours: 4, 8, 12 and 16%.

17

section. nized

In the Ragmi-Fayd without

stream

a diapir

area,

being

it was recog-

found

farther

(Fig. 6). The Sayma area is separated

the Maqsad

from

area by the Sayma shear zone (Fig. 8).

As this shear zone is dextral tion is mylonitic metres,

up-

and as the deforma-

over a thickness

the southwestern

of a few hundred

part of the Sumail massif

may be considered

to have originally

been at a few

tens of kilometres

to the southeast

of its present

position,

i.e., in the SE-diverging

The flow pattern

parallel

area of the pipe.

to the palaeo-ridge

axis

and divergence to the southeast recorded by this zone is very consistent with this hypothesis. Asthenospheric

flow in a broad mantle shear zone.

The Wadi Tuyin massif

et al., 1984). somewhat

The

characteristically two diapirs

found

fourth

(50-100

in the central

(Fig.

part of the Wadi Tayin

in a 20 km thick zone oriented the

presumed

planes

ridge

are steeply

4d) was only

axis

dipping

massif

at a right angle to

(pocket

maps).

to the southeast

Flow and

strike normal to the ridge axis; flow lines are subhorizontal indicating a strike-slip movement. The shear direction is consistent along the full length of the zone and indicates a sin&al shearing, also confirmed by the rotation of the foliation on each side of the zone. The peridotite found here has the classical coarse-grained structure typical of asthenospheric deformation. In the crustal section overlying the shear zone, cumulate gabbros are undeformed but the sheeted dyke complex is locally at 45” with respect eral trend in Oman (Pallister, 1981).

to its gen-

Discussion

Small mantle diapirs are involved in the spreading process at ocean ridges. The zone of vertical mantle flow in such diapirs can be viewed as a pipe slightly elliptical in cross section (Fig. 14). Normal to the ridge axis, its width does not exceed 10 km, a value very close to the width of the bottom of the magma chamber deduced from thermal models (Morton and Sleep, 1985) and from seismic experiments at present-day fast spreading

centres

(e.g., Herron

et al., 1980; Orcutt

can

km; Nicolas

horizontal

attitude

and

be

axis but

than the spacing

is

between

et al., this issue).

is channelled

into a

along

the

ridge axis (Fig. 14). One ridge segment mantle

diapir

long as the section of diverging diapirs

fed in such a way by one

can be at least three-four of the vertical

horizontal

and within

times as

pipe. In the zone

flow at the

top

of the

the zone of longitudinal

Mow

away from the vertical pipe, the absence of a shear sense reversal near the Moho is explained by the fact that the crystalline chamber

matter

mixture

at the bottom

was not yet solidified structures

were recorded.

of fact. it has been shown

line mixture

suffered

coupled

with

mantle

(Nicolas

the plastic

of

when As a

that this crystal-

an important

viscous

flow

Mow in the underlying

et al., this issue).

The most distal part of an asthenospheric current flowing along the ridge axis away from a diapir has not yet been clearly observed in Oman. The Wadi Ragmi-Wadi Fayd area, which recorded longitudinal flow and which lies far from any recognized diapir, might be such a zone (see discussion, petrological arguments and fig. 19 in Nicolas et al., this issue). Several

observations

show

that

the

astheno-

spheric diapirs have drained a considerable amount of magma and have also served as the main feeding pipe for the magma

chamber

above

as more

amply discussed in a companion paper (Nicolas et al., this issue). In the mantle section, the exceptional size and abundance

Mantle processes at an ocean ridge

section

the ridge

At the edges of the pipe, the flow rotates

the magma

configuration

pipe

along

smaller

the asthenospheric This

vertical

elongated

of chromite

pods and of

pyroxenite and gabbro dykes is noteworthy (Ceuleneer and Nicolas, 1985). Some of the chromite pods were chilled in the vertical flow zone and were not foliated by plastic deformation; the ore has remarkably well-preserved magmatic textures which bear witness to the intensity of magmatic circulation in these dykes. In the Maqsad area, the dunitic transition zone between mantle harzburgites and basal chamber cumulates is on average much thicker than in areas where the flow lineations have regular trajectories at a high angle to the ridge.

---+I

c -----

_-_

19

In the diapiric

structure

area of the Wadi Tayin transition

zone outcrop

with a thickness It

is the

Oman.

suggesting

body

of magma

The dunites

observed

and

the

mafic

has drained in Oman

dykes

a consider-

diapirs

could be

between

peridotites

of Oman

diapirs

the

in the

flow is rotated

for such melt percolation

ous in the dunites

in

horizonis numer-

(Nicolas

et al.,

this issue). area, it is chromite

deposit within the gabbro cumulates, a relationship which is unique in Oman, indicating an ex-

and Nicolas,

strong discharge of primitive melt in chamber above the diapir (Ceuleneer 1985; Nicolas

The most unexpected

the 0.05 MPa km-’

is indeed

positive

et al., this issue).

and consequential

struct-

ascend

toward

(Nicolas,

the

1986a;

dynamic

increase

pressure,

the

until a few hundred

that

be broken the

melt

in the

migration

to overcome material

generated

ceases

this overpresis very soft and

of peridotite of enstatite,

This implies

at depth

is filtered

and

again

moving

through

the

just below the Moho. The and thus the generation of

dunite is thought to occur in this highly impregnated zone as discussed above. Compaction operates in this magma mixture: subhorizontal gabbroic lenses observed in the Maqsad area where the compressive stress is clearly vertical may be frozen

solutions,

a typical

In fact,

there

of the plastic

10). In a

et al.. 1987). In the

by hydrofracturing.

the surface

framework dissolution

hydrofracturing

of a few bars

magma

sure. This impregnated

rotation

(Fig.

as it is

due to the

metres of fully interconnected

melt has accumulated cannot

by

Rabinowicz

rotates

attitude

gradient

surface

in deforming

a horizontal

pressure

but,

difference in density between the matrix and the basaltic magma (5 MPa km-‘), the melt can

ural feature of the Oman mantle diapirs is the thinness of the zone in which the vertical flow into

dynamic

upward:

much weaker than the pressure

towards

In the crustal section of the Maqsad worth noting the presence of a stratiform

ceptionally the magma

tive plume, gradient

case of a sudden

interaction

surrounding

area where asthenospheric tally. Evidence

metres.

mapped

at its apex.

the scar left by prolonged magma

ever

by numerous

that this diapir

able amount

of the

at a few hundred

dunite

It is cross-cut

in the Batin

the dunites

over an area 13 by 2.5 km,

estimated

largest

located

massif,

media

(Scott

feature

of porous

flow

and Stevenson.

is a feed-back

effect:

flow below

1986).

The

sharp

the Moho

im-

constant viscosity mantle, it might have been expected to be of the size of the radius of the vertical

plies a viscosity drop at the top of the diapir that we explain by a sudden increase in the

channel. The fact that it is more than ten times thinner implies that there is a major rheological

magma/rock ratio in this zone. In turn, such a magmatic impregnation is possible thanks to the

discontinuity recent study

overpressure

at the top of the mantle diapirs. In a (Rabinowicz et al., 1987), we have

mantle

due

to the

sharp

rotation

of

flow itself. Once it has been initiated,

the this

shown that a drop in viscosity of several orders of magnitude can allow a considerable proportion of

configuration is stable because it is self perpetuating (Rabinowicz et al., 1987). This condition is

the mantle flow to be channelled into such a narrow slot. This drop in viscosity is attributed to

probably not transient because this flow pattern has been observed at various places along a 400

a catastrophic

km long palaeo-ridge

increase

in the magma/rock

ratio

segment.

in the transition zone between the peridotites and the magma chamber, in agreement with the petrographical observations mentioned above. The dynamic pressure in the rising diapir is found to increase suddenly by a few bars in the zone of rotation of the vertical flow into the horizontal. The dynamic pressure, which in a porous medium can be viewed as the force exerted by the deforming matrix on the interstitial melt (McKenzie, 1984; Richter and McKenzie, 1984), acts against the rise of the buoyant magma. In a rising convec-

In the present-day oceanic mantle. the most direct evidence of plastic flow orientation is provided by the anisotropy of seismic wave propagation. In the uppermost parts of the oceanic mantle, the direction of maximum seismic wave velocity is usually parallel to the spreading direction, especially in fast-spreading oceans (Hess,

20

1964; Raitt et al., 1971; Shearer Nataf

and Orcutt,

et al.. 1986). This fast direction

to match hence

the [loo]

axis of olivine

to be the plastic

crystals

flow direction

with the spreading

process

nick

1978). The present

and

Nicolas,

anisotropy

(Francis,

in the oceanic

mantle

with a subhorizontal

orientation

tallographic

of

(Francis, Nataf of

plane 1969;

Nicolas

1969: Peseldegree

of the (010) crysin

olivine

Christensen,

1986;

et al., 1986).

tectonic

structure

4a) leads under tation attitude

of

is compatible

of most Oman

us to attribute

to the accretion

structure and the

peridotites

this monotonous of the lithospheric

at some distance

centre

down

4 km heiow the

Moho

has been computed

to a depth

and

Sleep

(1985). This model shows that the uppermost

1 km

of the mantle

section

Ma X half in

needed

the

spreading

temperature

(Nicolas

Boudier

and Nicolas,

and

Violette,

The

maximum

sampled

rate,

litho1982;

1985).

the deepest parts of the mantle ophiolite are also those which

pattern

of the

ophiolite

a

C is flowing

( <: 1 MPa).

mantle

section

is 9 km (Hopson

when the lithosphere and Sclater, 1977).

is about

observations

et

by the 1000 * C

relative

2 Ma old to the flow

of Fig. 4a must now be interpreted

scheme. The flow structure peridotites is and direction

in this

more valuable information on the in the accretion zone of the Oman more concerned with the intensity of the plastic flow gradient (magni-

tude of the shear strain and shear sense) and the precise slope of the accretion surface. It has been shown in the previous section that the shear strain increases dramatically in the uppermost 500 m of the mantle section and that this zone

closely

shear

sense.

more

moderate

accreted farthest from the ridge axis, a point we must keep in mind when reconstructing astheno-

increases

spheric flow patterns using the structures recorded by the peridotites. The distance of the accretion zone from the ridge as a function of depth cannot be determined accurately: Firstly, the actual ther-

parts

mal structure of the mantle near the ridge is poorly known due to the intense hydrothermal circulation taking place there (e.g., Davies and Lister, 1977). Secondly, in a steady-state expansion regime, the asthenospheric structures are not suddenly chilled when crossing a given isotherm but are progressively frozen as the mantle cools by a few hundreds of degrees, then remaining virtually undeformed under the same stresses which caused plastic flow at higher temperature. Finally, the actual spreading rate of the palaeo-ridge where the ophiolite formed is largely unknown. How-

from

stress conditions

thickness

that

200”-300’

the peridotites

in the Oman

Our structural

0.25 and 0.50

assuming

of about

to preclude

at a distance

to between

isotherm (Parsons

from the ridge axis. This

by the slope of the thermal

boundary

is accreted

from the ridge equivalent drop

by Morton

flow mantle

spreads away from the ridge (Parker and Oldenburg, 1973; Forsyth, 1977), the flow plane attitude sphere

of about

al., 1981). Such a depth is reached

interpretation calls for gradual cooling of the mantle by heat conduction towards the surface while moving away from the spreading centre: The lithospheric mantle gradually accretes as it

Consequently, section of an

boundaries: The of a spreading

(Fig.

steady-state spreading conditions, after roof the ascending flow into a horizontal

being controlled

ever, we can try to fix plausible most realistic thermal structure

under weak deviator&

The consistency between the seismic present-day oceanic upper mantle

pattern

and

associated

dislocations and

1985;

is presumed

corresponds

to an inversion

Below this zone, and

somewhat

very homogeneous of the mantle

the shear

the dip

of the

with depth:

of the strain

is

flow plane

The shear sense is

and shows that the uppermost section

flow away

from

the

ridge axis at a higher rate than the lowermost parts (Fig. 14). Such a vertical evolution of the mantle flow structure has already been observed in a few other ophiolites (Girardeau and Nicolas, 1981; Nicolas and Violette, 1982). This can be explained in the following way: Plates slide away from the ridge due to the traction applied in subduction zones, and the plastic flow in the asthenosphere is driven mainly by the drift of the overlying plate, leading to the shear sense recorded by the ophiolitic peridotites below the inversion zone. At shallow depths beneath the ridge axis, partial melting occurs and induces a drastic drop in density and viscosity in the asthenosphere. It leads to the formation of a

21

small

convective

portion

cell which channels

of the mantle

spheric

matter

is forced

ridge

through

this

(Rabinowicz

a large pro-

flow. When

the astheno-

to spread

away from the

so-called

“rolling

mill”

et al., 1984), its flow rate is greater

than

that

induced

by the overlying

plate

and the shear

lithospheric

of the massifs

discussed

graphs

considerably

varies

variations

in the preceding

were probably

in the partial

melting

tions which occurred

along

inherited and

than

the

(Nicolas

et al., this

far from the axial zone, where

the

probably

the mark of diapirs

asthenosphere

is driven

by the drift of the overly-

interac-

has not been chilled

away from

in a companion

one expected

sense is the reverse

These

from variations

magma/rock

prior to spreading

the axial zone. As discussed

para-

strike.

issue),

these

in Oman

paper

variations

are

whose vertical

pipe

peridotites.

ing plate. The

uppermost

parts

of the

mantle

section,

which accreted close to the ridge axis, have recorded the forced flow pattern whereas the deepest parts,

which accreted

have recorded The

rolling

numerically

farthest

the flow induced mill

effect

(Rabinowicz

has

flow, very high strain

computed

(about

ssl).

The flow pattern of the Wadi Tayin

recorded

massif and depicted

is more enigmatic.

With the exception

been

of diapiric

the flow plane

reproduced

rates

have been

explaining

the very

trolled

ascent.

by the thermal

boundary

steeper

part

in Fig. 4d of the zones

attitude

is con-

lithosphereeasthenosphere

orientation.

surprisingly

wull.~

by the central

by the plate drift.

et al., 1984). In the zone

of forced

lo-”

from the ridge,

Mantle flow channelled along steep lithospheric

In than

Wadi

Tayin.

elsewhere

it

was

in Oman

and

strong finite deformation found in the uppermost part of the mantle section of ophiolites. Farther

at a right angle to the ridge axis. suggesting that the asthenosphere there flowed along a pre-ex-

from the spreading axis, deformation rates drop rapidly to within the range of lo- I4 s- ’ imposed

istent lithospheric wall. Shearing at a right angle to the ridge trend suggests a transform origin.

by plate velocity. The depth of the reversal zone in Oman is found to be about 500 m (Fig. 14) i.e..

Such fossil transform faults have already been recognized in other massifs in the Bogota peninsula (Prinzhofer and Nicolas, 1980) and in the Antalya

four-five times shallower than that observed in two complexes in Bay of Island (Girardeau and Nicolas. 1981). This shallower depth might result from a higher spreading rate at the Oman palaeoridge. Indeed, the higher the spreading rate, the weaker

the dip of the isotherms;

the mantle of influence

accreted

of the rolling

reduced. The palaeo-dip be tangential

the thickness

of

close to the ridge in the zone mill

is consequently

of the flow planes,

to the accretion

supposed

isotherm,

ophiolite (Reuber, 1985). This interpretation in the Tayin

shear

to

found to increase somewhat with depth (Fig. 14). Granted that the deepest parts of the mantle section are also those which accrete farthest from the ridge, the increase in flow plane dip with increasing depth suggests that isotherms close to the ridge are convex upwards and not concave as predicted by extrapolating the conductive cooling law up to the ridge axis. This isotherm shape is compatible with the small-scale convection under the ridge axis inferred by the rolling mill model. In contrast with the structural homogeneity of the peridotites, the petrology of the mantle section

presents

case

of the

Wadi

a few difficulties.

In

Bogota and Antalya. the rotation of the mantle flow structures into a transform orientation coincides with the development temperature

has been

zone

of higher stress-lower

microstructures

in

the

peridotites.

Such a microstructural evolution does not occur in the Wadi Tayin shear zone where the deformation structures

of the mantle

peridotites

asthenospheric type described above. Wadi Tayin, solid-state deformation recognized in the overlying crustal magmatic structures are preserved

remain

of the

Moreover. in has not been section where (Pallister and

Hopson, 1981; Miss&i. 1982). This situation also contrasts with the Antalya ophiolite where mylonitic shear zones affect both the mantle and the crustal formations (Reuber. 1985). If the Wadi Tayin asthenospheric shear zone actually represents a piece of upper mantle deformed at a transform fault, one must admit that the classical “cold edge effect” (e.g.. Sleep and

Biehler,

1978) could

This

thermal

with

a fast-spreading

Gallo,

1984; Forsyth

that crust generation the Wadi Tayin found

along

transform

here.

(Fox

and

1984). The fact above

deviatoric tions

prevailing

zones

eanic

ridges.

with the broad

chambers

transform

ridges which may ex-

component

(e.g., Madsen

with the narrow ridges

transform

where

is frequently

zones

the continuity impeded

et of

and crust

generation considerably reduced (Fox and Gallo, 1984; Whitemarsh and Calvert, 1986; Potts et al., 1986). In this respect it should be noted that the Wadi Tayin shear zone coincides with a change in orientation

of the sheeted

proximately

45 o with respect This sheeted

plies a dextral

Nicolas

dyke

complex

dyke complex

sinistral

mantle

deformation

mapping

throughout

et al., this issue),

Nicolas the

generation

mantle

processes

of oceanic

(fig. 18 in

Gallo, in the

sampled from the

condiunder

oc-

plastic

the Oman range (pocket allow us to un-

associated

with

the

lithosphere.

Only a few well-defined mantle flow patterns have been recognized in Oman. Among them, the most common ( - 70% of the outcrop of the mantle section) features very homogeneous structures along the strike of the ridge axis on a scale of 100 km. The flow plane

im-

a re-

the weak

of such

flow structures

the ridge

Extension oblique ( - 45 * ) to the general trend of the ridge axis is well documented along fast-

shear zone suggests that the lithosphere in the Wadi Tayin massif originates

Extensive

from

with the

ridges (Fox and dip of the foliation

in the asthenosphere

maps; ravel

provide

under

stress and the high-temperature

about

et al., this issue).

spreading present-day 1984). The southeastern

of Oman

flow acquired

trend

pattern

this offset is consistent

peridotites

of ap-

to the general

ridge offset. In the case of a trans-

form movement, observed

The mantle

cord of solid-state

which

of slow-spreading

in Oman.

Conclusion

be consistent zone

and Wilson,

fast-spreading

al., 1986), than

occurred

has not been affected

an extension

magma

have might

shear zone is also a feature

is more consistent perience

hardly

configuration

( < 25 ’ ) and

dips slightly

away

the flow line

is at

right angles to the ridge trend. This flow pattern also

the

most’

ophiolites

frequently

belonging

observed

in

to the “ harzburgitic

all

is the

sub-type”

defined by Boudier and Nicolas (1985). Moreover, the seismic anisotropy of the present-day oceanic lithosphere

allows us to extrapolate

this result on

a world-wide scale (e.g., Nataf et al., 1986). This flow pattern is attributed to the gradual accretion of the lithospheric mantle at some distance from the ridge axis after rotation of the ascending flow into a horizontal attitude. The uppermost levels

intersection of the transform zone with the western ridge segment (fig. 18 in Nicolas et al., this issue). This is in agreement with the fact that the

(- 500 m) of the mantle section close to the ridge have recorded

western

mantle is driven by internal forces, probably the buoyancy force due to partial melting. In turn, the

part of the Wadi Tayin

massif is the distal

edge of the Maqsad diapir which has been shown to be a sample of an active ridge. Finally, direction above

it is worth noting on

the

the mantle

floor

that the magma

of the

magma

flow

chamber

shear zone of Wadi Tayin

is at

right angles to its general orientation in Oman, i.e., parallel to the strike of the presumed transform faults. Magmatic lineations in the cumulate section of ophiolitic massifs have been interpreted as the imprint of viscous flow coupled with the plastic flow in the underlying peridotites (Nicolas et al., this issue). The orientation of magmatic lineation and of the sheeted dyke complex is thus the only evidence for a transform origin of the central Wadi Tayin crustal section.

pattern

showing

that

which accreted a forced flow

close to the ridge

axis the

flow pattern recorded by the lower levels of the mantle section is consistent with the view that, far from the ridge, the asthenospheric by the overlying

plate. Realistic

flow is driven

thermal

models

of

spreading centres (Morton and Sleep, 1985) allow us to deduce that the active mantle flow is superseded by the passive flow induced by the drift of the plate at a distance from the ridge axis equivalent to a value of less than 0.25 Ma X half the spreading rate, i.e., a few tens of kilometres in the Oman case. (A half-spreading rate of about 5 cm.-’ yr -I is a reasonable estimate for the Oman palaeo-ridge (Pallister and Hopson, 1981; Pallister, 1984; Nicolas et al., this issue).)

In a few massifs, vant to mantle have

been

structures

processes

recorded

the three-dimensional

pattern

has been cooling

tiation

of the intraoceanic

sifs provide structure

releridges

In these

asthenospheric

fossilized,

of the mantle

itself (Boudier

spreading

by the peridotites.

massifs, rapid

more directly

beneath

flow

of the

phy, gravity

or heatflow.

ferred

asthenospheric

rising

authors

diapirs

by the ini-

up to 100 km along the ridge (Crane.

to study

asthenospheric

and spreads

et al., 1985; Schouten agreement and

flow

Oman

peridotites

the

conclusion is that the ascent of material beneath the ridge axis is along strike: it involves small di-

the structure

flow

of about

10

spaced

by

1985; Crane

of present-dav

structure

recorded

is noteworthy.

This

our work will be more thoroughly companion

in-

et al.. 1985). Here also, the

between

ridges

in the horizon-

have

more or less regularly

the

tal plane. The major asthenospheric discontinuous

some

induced

at the ridge

in

characteristics

ridges, such as topogra-

km in diameter

thrusting

ascending

of geophysical

fast-spreading

due to the

opportunity

and the way it rotates

variations

present-day

probably

et al., 1985, this issue). These masa unique

strike

paper (Nicolas

by

the

aspect

of

developed

in ;I

et al.. this issue)

Acknowledgements

apirs, the sections of which do not exceed 10 km at the Moho. Such diapirs are driven by the body forces induced by partial melting and are the main

This work was made possible thanks to facilities in Oman provided by the Ministry of Petroleum and Minerals. We are very grateful to M.

feeding zones of the overlying magma chamber. The detailed flow pattern in the diapirs is, to a

constant

large extent,

conditioned

tion in the partially

by the magma

molten

in a recent study (Rabinowicz a strong

interaction

and solid-state

mantle.

distribu-

As discussed

et al., 1987) there is

between

magma

migration

Mohammed

de la Recherche

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