Lateral and vertical variability in crustal velocity structure in a small area of the North Atlantic

Tectonophysics,

153

90 (1982) 153-166

Elsevier Scientific

Publishing

Company,

LATERAL AND VERTICAL STRUCTURE IN A SMALL

ROBERT

S. WHITE

Department

Amsterdam

- Printed

in The Netherlands

VARIABILITY IN CRUSTAL VELOCITY AREA OF THE NORTH ATLANTIC

and G.M. PURDY

*

of Earth Sciences, Bullard Laboratories,

Unioersity of Cambridge,

Cambridge EC3 OEZ (Great

Britain) * Woo& Hole Oceanographic (Final

version

received

Institution,

Woods Hole, Mass. 02543 (U.S.A.)

January

23, 1982)

G.M.,

1982. Lateral

ABSTRACT

White,

R.S. and Purdy,

area of the North

Atlantic.

sphere in Europe The detailed Atlantic

and the North

velocity

and vertical

In: E.S. Husebye

structure

Atlantic.

Tectonophysics,

using 358 explosive

fired into an array of eight ocean bottom,hydrophones. the upper crust increasing probably

anisotropic

a normal

oceanic

mantle.

Delay

associated

with a minor

similar

to that

median

valley extends

fracture

of the top, indicating

with a much flatter

down

charges Inversion

mapping

variability

with a small region of high delay times in the northwest probably

of the first arrival overlying

16.4-l airgun refracted

a typical oceanic

using two datasets is generally

corner caused

the normal

faulting

which

at least into layer 3. Our observations

Moho which constrains

in a small

Ma old crust on the flank of the mid and several hundred

shots

travel times

possible

models of crustal

containing

in

layer 3 and a arrivals

from

low over most of the survey

area,

by the presence

crust

zone. We find that the topography

that

structure

90: 153- 166.

at the seafloor

time function

show that lateral

velocity

of the Lithosphere-Astheno-

section about 5 km thick with a steep velocity gradient

from about 3.5 km/set

layer 3 and from the mantle

in crustal

The Structure

of a 70 by 35 km area of 6-10

Ridge at 24ON was studied

shows that the crust comprises

variability

(Editor),

of abnormal

of the base of layer 2 is

occurs

along

from mantle formation

the margins

arrivals

of the

are consistent

at the spreading

centre.

INTRODUCTION

Marine the general

seismic refraction features

profiles

of the crustal

throughout

the world’s oceans have shown that

seismic velocity

structure

are everywhere

remark-

ably uniform. Uniform layer solutions suggest that normal oceanic crust can be characterised by three layers; at the top are sediments of variable thickness and velocity overlying layer 2 (the “volcanic” layer) with a mean velocity of 5.07 * 0.63 km/set, beneath which is layer 3 (the “oceanic” layer) of velocity 6.73 * 0.19 km/set. Beneath the base of the crust, marked by the Mohorovocic discontinuity, is the mantle with a mean velocity of 8.15 -C 0.13 km/set (Raitt, 1963; Christensen and 0040-1951/82/ooOO-0000/%02.75

0 1982 Elsevier Scientific

Publishing

Company

(a)

3500-3800

between

location

of OBH’s.

numbers

b. Detailed

spreading

immediately

m.

Areas

of seafloor

deeper

Bottom lineations

array

within

are indicated area. Heavy

shaded

“H”

EXP; shading,

this area is shown

and areas shallower

than 3500 m lightly

shaded,

f-8

of refraction “H” EXP on Fig. la. Numbers

line shows position

in Fig.

and

over depths

stars are identification

by Detrick

with no shading

against

line discussed

is

lb. circles.

rift valley in numbered

Ridge

enlarged

N

offset in the mid-Atlantic

identifications

of the Mid-Atlantic with anomaly

The location

“H” EXP survey area and the right lateral

by fainter broken

boxed area marked

survey

than 3800 m are heavily

shot positions

of the detailed

in box labelled than 4000 m shaded.

Zone to the south of the detailed

anomaly

greater

Hydrophone

of 500 m and depths

of Ocean

magnetic

to the north

map of OBH and explosive

Ridge which commences

Purdy ( 1980).

position

offset at the Kane Fracture

lines. Seafloor

Note the 160 km left-lateral

by diagonal

indicated

map showing

et al. (1978). with an interval

location

from Purdy

Contours

Fig. 1. a. General

45” 10’ (b)

24”OO’

24’ 10’

24” 20’

156

Salisbury,

1975; Kozminskaya

the velocity predicted

structure

by synthetic

by a discontinuous

and Kapustyan,

by matching seismogram

(White,

studies

velocity gradient

or more in layer 2, and rather 1979; Spudich

1975). Refinement

observed

waveforms

and

small gradients

typically

better

of 0.7 see-’

’ in layer 3

of the order of 0.1 set-

1981). Superimposed

of

to those

suggest that the crust is described

with the highest gradients,

and Orcutt,

of the details

amplitudes

on ‘this general

velocity

structure are variations which we can attribute to the formation of anomalous crust in fracture zones (Detrick and Purdy, 1980; White and Matthews, 1980), to systematic changes with age (Christensen and Salisbury, 1975), to azimuthal variations in propagation velocity caused by crack or by crystal alignment (Raitt et al., 1969; Keen and Tramontini, 1970; Christensen and Salisbury, 1975; Bibee and Shor, 1976), or to lateral the bathymetric

inhomogeneity

expression

70 by 35 km area of 6-10 old that

most

portion

of the experiment

of normal

either correlated

Ma old crust in the North

of the systematic

occurred. The objective

in structure

of the crust. The survey reported

crust unaffected

velocity

changes

Atlantic,

caused

with or unrelated which is sufficiently

by ageing

have already

was to map the degree of lateral variability by fracture

zones,

to

in this paper is over a

though

large localised

in a delay

time residuals found in the northwest corner of the survey may be related to the presence of anomalous crust associated with an offset in the spreading centre. A full report containing an analysis of the stability of the delay time inversion is in preparation (White and Purdy, in press); for the present we will confine ourselves to reporting

the major results and their imp~cations

EXPERIMENTAL

east-west

of oceanic

crust.

(OBH; Koelsch and Purdy, Zone in the North Atlantic

1979), in an

METHOD

An array of eight ocean bottom hydrophones was deployed to the north of the Kane Fracture extended structure

for the formation

“H” configuration (Fig. la). In order to control the upper crustal velocity a 16.4-l (lOOO-inch3), airgun was fired once every 100m along two long lines crossing longer

all the OBH but the central

ranges

were generated

using

one. Deeper penetrating

arrivals

from

varying

in size from 3.6 to 116 kg (8 to 256 lbs.), fired along lines joining

358 explosive

charges

crustal of TNT the edges

and diagonals of the OBH array (Fig. lb). Ranges from each shot to the receivers were calculated from the direct water wave travel time using a velocity versus depth profile

within

the water

column

derived

from

nearby

temperature

and

salinity

profiles. The analogue tapes from the continuously recording OBH’s were digitised at a 4 msec sampling intervai, record sections plotted and first arrival travel times picked to give a total of 1613 different range-travel time pairs. Prior to using the set of travel time data to determine the crustal structure, corrections were made for the varying water depths at the seafloor ray entry points near each shot. We use a modified water delay correction (White and Purdy, in

157

press), which corrects shots had been

the travel times to those that would have been measured

detonated

mean

water

about

the configuration

a delay entry

depth

different

arising

azimuth

height,

interfaces,

Uncertainty

partly

6 above

the seafloor,

area. This correction

of sub-seafloor

time inversion.

point,

a uniform

over the survey

requires

many

to the track followed

of 24 * 20 msec in the topographic

so leaves us free to use the data in

of the shot to receiver

by the shooting correction.

where & is the no assumptions

of 30 ‘-t 25 m in the seafloor

because

if the

depth

at the ray

ray paths

lay at a

ship, lead to possible

When

these

errors

errors

are added

to

uncertainties in the time of the first arrival of typically 10 msec, together with the possibility of small errors in the ranges and seafloor dips, the overall average uncertainty of corrected travel times is about 45 msec. VELOCITY

STRUCTURE

Normal

oceanic

crust

about

5 km thick was found

from conventional

reversed

refraction interpretations of arrivals from the explosive sources. The higher resolution of the velocity structure afforded by the denser coverage of airgun shots suggests

that

the compressional

velocity

in the

lf

to 2 km

“volcanic” layer), increases from approximately 3.5 km/set km/set at the base. This steep velocity gradient is typical presence

of open voids and cracks

increase in velocity deposited minerals, density

of cracks and fractures

deeper

material

associated

with a small fracture

variability

in the upper crustal

velocity

2 (the

with a downwards by secondarily and to a lower

than an abnormal

corner

profiles

layer

lf km

of the survey probably

record

only minor

lateral

structure,

lies beneath

km/set. Horizontal anisotropy is frequently observed beneath the spreading direction (Raitt

in the crust. Other

in the northwest

zone, the airgun

layer 3 (the ‘“oceanic”

of 6.7 km/set

at the top of the basement

due to a combination of partial crack infilling to crack closure under the increasing pressure

thick layer of 3.6 km/set

A normal

thick

near the seafloor to 6 of that caused by the

layer),

almost

layer 2. Mantle

3 km thick with a mean refractions

seismic

yield a velocity

of 8.0

in the velocity of the upper mantle of typically I-390 oceanic crust, with the maximum velocity parallel to et al., 1969; Keen and Tramontini, 1970; Keen and

Barrett, 1971; Christensen and Salisbury, 1975; Bibee and Shor, 1976). The azimuths of the mantle arrivals are similar to the spreading direction and simulations with artificial data having the same ~imuthal distribution show that the velocity we determine is close to the maximum velocity if the mantle is anisotropic (White and Purdy, in press). On a nearby north-south refraction line, which is perpendicular to the spreading direction, Detrick and Purdy (1980) found well-controlled mantle velocities of 7.62 kmj’sec over 17 Ma crust to the north, and 7.70 km/set over 7 Ma crust to the south of the Kane Fracture Zone (see Fig. la for Iocation). Since the north-south elsewhere, anisotropy

lines will sample the minimum the mantle is anisotropic, with of the order of 0.3 to 0.4 km/set.

velocity this suggests that a peak to peak horizontal

here, as velocity

158

DELAY

TIME FUNCTION

METHOD

The set of 1613 range-travel time pairs inverted using an iterative delay time function lateral

variability

basement

depth.

and the possibility

attempt inclusion

assumes

area on which

to explain

and Hanson,

of correlation

The delay time function

Raitt et al., 1969) the survey

that there is a uniform

the delay time and the 1969; Morris et al., 1969;

velocity-depth

perturbations

listed below,

free parameters velocity.

between (Morris,

travel times by making

1974) to the variables

(a) Mean refraction

method

are superimposed

the observed

of additional

from the array of eight OBH’s was technique to investigate the degree of

significantly

A fundamental

structure

in the delay least squares

testing

within

times.

at each stage whether

improves

assumption

We

fits (Lawson the

the fit.

of the delay time method

is that refractions travel in uniform velocity layers, so we do not use arrivals from layer 2, which is a region of high velocity gradients. We only illustrate results from subsets comprising layer 3 or mantle arrivals which have been returned from layers with small velocity gradients. (b) Mean delay time to refractor. (c) Two dimensional polynomial fit to order n of deviations in delay time to refractor as a function of geographic position. The delay time has been considered as represented at a position offset from the ray entry point through the seafloor near the source and the receiver by a constant distance along the azimuth of the raypath. The displacement is chosen such that the geographic location of the associated delay time lies above the region in the crust where we consider that most of the lateral inhomogeneity is present. Provided that lateral changes in the delay times occur over distances

that are large compared

offset is not critical (d) Systematic

and different

variations

to the offset, solutions

as they do, the magnitude

of the

give stable results.

in delay times as a function

of seafloor depth (and hence

basement depth since the sediment cover is negligible). This simple linear relationship is included to test whether there are systematic seismic velocity or thickness variations whether

beneath

bathymetric

highs

and

lows.

In particular,

the base of layer 2 and the base of the crust exhibit

the top of layer 2 (the seafloor)

or whether

deep crustal

we wish

to find

the same topography

interfaces

are flatter,

as since

this helps constrain possible models of processes at the spreading centre. There is a danger with delay time function mapping that artefacts will be introduced into the solution by, for example, the presence of refractor anisotropy or refractor topography (Whitcombe and Rogers, 1981) or by the poor distribution of sources and receivers. We have tested extensively the stability of the delay time inversions using sets of artificially generated arrival times from known velocity sructures, but with a random component added to each travel time to represent the errors to which each is subject. Results of these tests are discussed in detail by White and Purdy (in press) but, briefly, it suffices to say that we are satisfied that our inversions for the mean velocity, and the geographic delay time distribution, ((a)

159

through

(c) above),

is our survey

correctly

represent

area. The apparent

needs more care in interpretation, POLYNOMIAL

DELAY

TIME

We find that a fourth represent

the delay

of velocity

time-seafloor

depth

structure (A~/A~)

as it exists relationship

as we discuss later.

SURFACE

order double

times

the variation

delay

polynomial

satisfactorily,

delay time surface is sufficient

and that the significance

of adding

to

extra

ORDER 4

24” IO’ N

24”OO’

45” 10’ w

44” 30’

(a)

I

LAYER 3

%

At /Ah

I=

= -0.09

V = 6.7 km /set

G

d 5

5 & 2 n

-0.30

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -1000 -500 0 500 1000

DEVIATION

FROM MEAN DEPTH (Ml

lb1 Fig. 2. a. Fourth

order double

areas show those regions constrained b. Deviation

polynomial

of the seafloor

delay time surface illuminated

by the data in these areas. Contour in delay time versus difference

the mean depth

for fourth

order solution

interval

between

from inversion

by the explosive

of layer 3 arrivals.

Unshaded

shots: the delay time surface

is only

is 0.05 sec.

seafloor

using layer 3 arrivals.

depth

at the ray entry and exit points

and

160

ORDER 4 24” 10’ N

24” 00’ 44” 30’

4i”lO’ w (a)

G i g F

0.30

--F-T

I ’ 7I ’ ’ 1’ I 1 , ’ ’ I 7 7 I

’I

7’ / ’ ’ ’ 7I ’

’

MOHO

‘.

0.15 -

yx _,_ .:

At/Ah

'.... ,,

=-0.15

G > iti

-0.30 ,,',.,",,""",'..~,'~~,,',,~~"~,,"~"L~,,"~~:',i 1000 -1000 -500 0 500

DEVIATION

FROM MEAN DEPTH (Ml

(b) Fig.3. a. Fourthorderdouble polynomial

delay time surface

interval 0.05 sec. b. Deviation in delay time versus difference the mean depth

for fourth

order solution

between

using mantle

seafloor

from inversion depth

of mantle

arrivals.

Contour

at the ray entry and exit points

and

arrivals.

terms to the polynomial series diminishes as higher orders are considered. In Figs. 2 and 3 we illustrate the fourth order polynomial surface, together with the corresponding correlation between delay time and seafloor depth, for inversions using the layer 3 and the mantle arrivals, respectively. The geographic delay time surface is normalised to the well-controlled delay time at OBH 7 in the centre .of the survey area, and the unshaded areas depict the Fresnel area beneath the shotpoints to

161

emphasise

how little of the upper

dense coverage

Delay

even with such a relatively

of shots and receivers.

The polynomial generally

crust is illuminated,

delay time surfaces

small variations

from both layer 3 and mantle

in delay times over the majority

times from layer 3 (Fig. 2) are the best constrained,

which they occur as first arrivals (8-25 receivers, ern corner

because

show

survey area.

the range over

km) is similar to the average spacing between

thus giving good ties between

layer 3 delay time deviations

arrivals

of the central

source and receiver

of a little over +O.l

of the survey, whilst elsewhere

delay times. Maximum

set are observed

the variations

in the northwest-

are typically

of the order of

0.05 sec. Since our average error in travel time is about 0.045 set, this represents only a small resolvable degree of lateral inhomogeneity over the majority of the survey area. Delay time variations from mantle arrivals (Fig. 3) show a similar pattern to those from layer 3 with a broad area of small deviations in the centre of the survey and a region of large delay times reaching over +0.25 set in the northwestern corner. The only significant difference in the pattern is that there is an area of high delay times along the southeastern edge. reaching over +0.20 sec. However, this is poorly controlled

because

predominance

it is on the edge of the survey

of arrivals

the northwestern

corner.

originating

and

in the anomalously

The somewhat

greater variations

may be corrupted

by a

high delay time region

in

in delay times sampled

by

mantle arrivals compared to those from layer 3 may be evidence for some lateral inhomogeneity in the lower crust. Variation may be introduced into the lower crust subsequent

to its formation

by hydrothermal

and

metamorphic

activity

or by

off-axis intrusion in layer 3 over the first 30 Ma (Christensen and Salisbury, 1975). The small lateral variations in delay times of around 0.05 set over the majority of the survey area must originate in layer 2, since they are recorded by the layer 3 refractor. Lateral thickness changes in layer 2 of up to a few hundred metres will explain

the observed

the superimposed result, may km/set

for example, cause

delay time variations

effects

of lateral

velocity

et al., 1973). Interbedding

we cannot

changes.

from low grade metamorphism

the compressional

(Fox

although

velocity

to drop

distinguish

Lateral of basalt

from

of sediment

velocity

may

to zeolite facies, which

5.0-6.6 with

this from

changes

km/set

to 4.4-5.0

the extrusive

igneous

material, together with changes in the thicknesses of shallow basement layers and with variations in the amount of rubble and pillow lavas will also cause minor lateral heterogeneity. The high delay times in the northwest corner of the delay time surface are probably related to spreading centre jumps at this latitude. The survey area is positioned over a swathe of crust to the north of the Kane Fracture Zone exhibiting normal seafloor spreading magnetic anomaly lineations. However, near the north of our survey the lineations terminate and are offset some 60 km to the east (Fig. la). Unlike the offset across most fracture zones, which occurs over a narrow zone, this offset is spread

over a broad area (Rona

and Gray,

1980) which suggests that it may

162

be accommodated Fracture

zones

by a series commonly

1980; Sinha and Louden, in the shallow

steps

atypical

or jumps

crustal

in the spreading

structure

1981) often with thicker than normal

crust (Detrick

times in the northwest

of small

exhibit

and Purdy,

of our survey

(Schouten

centre.

and White,

low velocity

material

1980) and we suggest that the large delay

may lie over crust modified

by such a ridge

jump. DELAY

TIMES CORRELATED

Inversions

from

both

TO SEAFLOOR

layer

3 and

DEPTH

mantle

arrivals

show

a strong

correlation

between the delay time and the seafloor depth (which is equivalent to the depth to the top of the basement) at the ray entry and exit points. The magnitude of the ratio, At/Ah, increases as the order of the double polynomial delay time surface is increased up to the fourth order, thereafter tending to stabilise at about -0.10 set/km for layer 3 and -0.18 set/km for mantle arrivals (Fig. 4). A fourth order polynomial surface is required to describe adequately the geographical variations in delay times; At/Ah

values from lower order solutions

are corrupted

by the residual

delay times unaccounted for by the polynomial surface and so will be ignored. One of the major shortcomings with the delay time function method is that we represent

the delay time down to the specified

to one specific

-0.15

location,

whereas

in reality

refractor

the raypaths

as a vertical delay pertaining follow curved

paths

which

-

0

2

ORDER

4

6

8

OF POLYNOMIAL

Fig. 4. Plot of delay time-seafloor depth (Al/Ah)

ratio versus order of double polynomial delay time

surface from layer 3 and mantle arrivals for those cases up to order 8 where the inclusion of the depth-delay

time relationship significantly improves the fit to the observed dataset. We show results

using only those raypaths within 30° of the ship’s track (i.e. where the depth at the ray.entry point is reasonably we.11determined).

163

sample

an oblique

improve

section

of the crust.

the representation

of the geographical

delay time to apply at a location the depth

we cannot

we take

the seafloor

variable,

whereas

the point

some distance

of the lateral

a similar

depth

at which

and therefore

section,

delay time surface by considering

the majority

follow

because

in the previous

lying above the region where the raypath

at which we consider

Unfortunately,

As discussed

we the

intersects

inhomogeneity

to reside.

procedure

with

At/Ah

at the ray entry

point

as the independent

relationship,

the ray will meet the refractor

lie beneath

basement

at a different

will be offset

depth.

Raytracing

through models of the crust with the same velocity-depth structure throughout, and basement topography taken from our survey tracks shows that continuous refraction of the raypaths arrivals that

through

to exhibit

the At/Ah

artificially

a At/Ah ratios

generated

topography

the velocity

as the

gradient

ratio of typically

we measure

values, top.

indicate

The

degree

-0.10

in layer 2 causes

the layer 3

set/km.

We therefore

3, which

are the same

as those

that the base of layer 2 has roughly

the same

from

layer

of scatter

topography on the base may be a somewhat surface (the seafloor), and still produce definitely

present

say that our data do not permit

in the

results

conclude

is such

that

the

subdued version of that of the basement acceptable At/Ah ratios, but we can an interpretation

with a flat base to layer

2. Measured At/Ah ratios from mantle arrivals are rather larger in magnitude than those from layer 3 (Fig.4). This is consistent with a model of the crust with a flat Moho, although the higher velocities of the lower crust mean that the resolution of the Moho topography is much poorer than that of the layer 2-layer 3 interface. Our conclusions are that the topography of the upper crustal layers is similar

to

that of the top of the volcanic basement, and that the observed At/Ah ratios are consistent with greatly subdued or flat relief on the Moho. The rugged seafloor relief in the Atlantic Ocean is created primarily by normal faulting at the margins of the median

valley

parallel

to the spreading

(Harrison

1977; Macdonald the Atlantic (Laughton constructional

and Stieltjes,

1977) the faults

centre (Laughton

and Rusby,

and Luyendyk,

Ocean,

and

and Searle, volcanic

form

1977):The within

1979) beyond relief

(Atwater, 1979), it is frequently around the volcanoes suggests

of up

2-4

forming

long linear

scarps

1975; Ballard and Van Andel,

faults are typically

2-2.5

km apart

km of the axis of the median

which little further

faulting

to 200 m is present

occurs.

in the

in

valley

Although

median

valley

blanketed by subsequent lava flows and faulting that the volcanoes sink into the crust after and

probably during their growth (Ballard and Van Andel, 1977). We are therefore left with the primary control on basement relief as the faulting in the crestal mountains. The presence of 7-8 km deep earthquakes on the Mid-Atlantic Ridge (Lilwall, 1980), is evidence for brittle faulting down to the Moho, and Lister (1974) and Williams et al. (1974) have postulated that fracturing with associated hydrothermal alteration may also penetrate as deep as the Moho. This then provides an explanation for the topography we infer on the layer 2-layer 3 interface; the upper crust,

164

already

formed by the time it is faulted into the crestai mountains,

into blocks and uplifted Although

by normal

the topography

faults extending

on the Moho is not so well constrained

that on the base of layer 2, we suggest

two main

much flatter

there is the geometric

than the basement.

at the margins crust.

This

tectonic

of the median

would

allow

tilts ranging

is simply chopped

down at least into layer 3.

Firstly,

of the crust

IO0 to 30” or 40

and

explain

in the fault

donald and Luyendyk, 1977; Atwater, 1979; Hall, 1979; Laughton Our second explanation is that the thermal structure of ridges temperature

isotherms

extend

at depth considerably

beyond

that the faults

out at the base of the

would

observed

by our results as for why it may be

reason

valley may be listric, flattening

extension

from

explanations

the outward blocks

(Mac-

and Searle, 1979). is such that high

the median

valley, and

that a normal Moho is not generally found at the spreading centre, but has developed by 10 or 20 km off axis (Fowler, 1976, 1978; Keen and Tramontini, 1970). There is also evidence of a growth in layer 3 over the first 30 Ma after formation (Christensen and Salisbury, 1975). Since the faulting occurs within 2-4 km of the axis, only f Ma after formation, the Moho structure will only be fixed subsequent to the faulting basement

after the deep crust has cooled and so will not reflect the shape of the in the way that the layer 2-layer

3 interface

does.

ACKNOWLEDGEMENTS

We thank the officers, crew and scientists aboard R/V “Atlantis” II during the cruise 96, leg 3, whose hard work and co-operation made this experiment possible. The ocean bottom hydrophones were built and maintained by D. Koelsch and C. Grant. C. Dean, R.S. Detrick, Dickson Ellison, L. Gove and M. Rosser assisted in various aspects of the data reduction and computer programming, for which we are most grateful. This research was sponsored by the Office of Naval Research under contract NO~l4-74 00262 NR083-004 to the Woods Hole Oceanographic Institution (WHOI). R.S. White acknowledges support from WHOI and Natural Environment Research Council

Postdoctoral

Cambridge bution

Fellowships.

contribution

number

number

Department

of Earth

Sciences,

194. Woods Hole Oceanographic

University

Institution

of

contri-

5025.

REFERENCES

Atwater,

T., 1979. Constraints

M. Talwani, Ocean Ballard,

CC.

Crust. R.D.

36”50’N

from the Famous

Harrison

Am. Geophys.

and

Van Andel,

on the Mid Atlantic

Bibee, L.D. and Shor, Jr., G.G., Geophys.

and D.E. Hayes Union. T.H.,

area concerning (Editors),

the structure

of the oceanic

Deep Drilling

Results

and

of the inner

section.

in the Atlantic

In:

Ocean:

pp. 33-42. 1977. Mo~hology

tectonics

rift valley

at lat.

ridge. Bull. Geol. Sot. Am.. 88: 507-530. 1976. Compressional

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in the crust

and upper

mantle.

165

Christensen,

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