Download Extinct spreading center in the Labrador Sea

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B2, PAGES 2261-2278,FEBRUARY 10, 1995
Extinct spreading center in the Labrador Sea: Crustal structure
from a two-dimensional seismic refraction velocity model
John C. Osier• and Keith E. Loudcn
Departmentof Oceanography,DalhousieUniversity,Halifax, Nova Scotia,Canada
Abstract.TheLabradorSeacontains
a rareexampleof anabandoned
mid-ocean
ridgewhere
activeaccretion
of oceanic
crustceased
dueto a change
in thespreading
geometry
of lithospheric
plates.Seismic
refraction
datawerecollected
alonga linewhichtransversely
crosses
theextinct
spreading
center.Two-dimensional
analyses
of therefraction
data,usingmy-tracing
andsynthetic
seismogram
techniques
revealmajorvariations
in crustalthickness
andvelocityin relationto the
axisof theextinctspreading
center.In theextinctspreading
center,a crustalthickness
of
approximately
4 km is determined,
compared
with5.5km for theflanks.Substantial
lateral
variations
in P wavev½locities
of theupperandlowercrustareobserved
witha markeddecrease
withintheextinctspreading
center.Low v½1ociti½s
arealsoobserved
in theuppermost
manfie
underlying
theextinctspreading
centerandareinterpreted
asbeingtheresultof hydrothermal
alteration.
The anomalously
low crustalv½1ociti½s
andcrustalthinningareattributed
to a
decreasing
supplyof partialmeltandincreasing
degreeof tectonism
at theslowspreading
rates
preceding
extinction.
Introduction
syntheticseismogram
techniques,showsthe crustwithin the
extinctspreading
centerto be thin andof anomalously
low P
Mid-ocean
ridgesmarktheboundaries
between
diverging wave velocity relative to its flanks. Likewise, low velocities
lithospheric
plateswherenewoceanic
crustandlithosphereare observedin the uppermostmantle,underlyingthe crust
areformed.Extinctspreading
centers
aremid-ocean
ridges within the extinct spreading center. Two potential
wherethe active accretionof oceaniccrusthasceaseddue to a
mechanismsfor forming these crust and upper mantle
structuresare explored.The preferredinterpretation
is that
decreasesin spreadingrate led to an enhancedconductive
cooling of the crust renderingit brittle and subjectto
tectonism.
The tectonism
produces
low crustalvelocitiesby
fracturingthe crust and may causesomecrustalthinning
throughthe rotationof crustalblockson major faults.The
tectonismpromotesdeeppenetrationof hydrothermal
fluids
reducingpartial melt supplywhichyieldsthinnercrust.The
circulation
penetrates
throughthecrustallowing
theCentralBasinFault[LewisandHayes,1980];theShikoku hydrothermal
Ridge [Tomodaet al., 1975];andridgesin the Tasman serpentinizationof the upper mantle therebyreducingits
change
in thespreading
geometry
of thelithospheric
plates.
Theyarelarge-scale
features
formed
byridgejumpsin excess
of 400 Ion betweentheextinctspreading
centerandthenew
locus
of rifting[Batiza,1989].Theycanbeidentified
bytheir
morphological
expression
[Mammericloc
andSandwell,1986]
and magneticanomalypattern.Extinctspreading
centers
identified
onthisbasisaretheMathematician
Ridge[Klitgord
andMammerickx,
1982];theGalapages
Ridge[Anderson
and
Sclater,1972];theAegirRidge[TalwaniandEMholm,
1977];
[Weissel
andHays,1977],Coral[Weissel
andWatts,1979] velocity.
The anomalous
crustat the extinctspreading
centerformed
andLabradorSeas[Srivastava
et al., 1981].Few refraction
studies
havebeenconducted
at these
extinct
spreading
centers.
in the time interval from Chron 21 until its extinction.
PhilippineSea, and seismicrefractionlines have been shot
protractedover the periodfrom Chron21 to 13. The variation
Nagumo
et al. [1980]published
refraction
profiles
samplingModeling of magnetic anomaly patterns [Osler, 1993]
but was
theuppercrestin thevicinityof theShikoku
Ridgein the suggeststhat spreadingdid not ceaseinstantaneously
in the observedcrustalstructurein relationto the palcorift
thismaterial
remains
unpublished
(R.B. Whitmarsh,
personal axis supportsthe idea of a systematicrelationshipbetween
acrossthe Aegir Ridgein theNorwegian-Greenland
Sea,but
slowspreading
ratesand(1) increasedtectonism;(2) decreased
partial
melt
supply;
(3) hydrothermal alteration of the
Thispaperpresents
a two-dimensional
analysis
of seismic
uppermost
mantle;
and
(4)
crustal thickness.
refractiondata collectedon a line which traversesthe extinct
communication,1992).
spreading
center
in theLabrador
Sea.Thetwo-dimensional
(2-
D) velocity-depth
structure,
developed
by ray-tracing
and GeologicalSetting
•Now
atDefence
Research
Establishment
Atlantic,
Darmouth,
Nova
Scotia, Canada.
Copyright1995by theAmericanGeophysical
Union.
Paper number 94JB02890.
0148-0227/95/94JB-02890505.00
The Labrador Sea is a small ocean basin conf'med between
the coasts of Labrador and Baffin Island to the west and
Greenlandto the east (Figure 1). The presenceof a buried
extinctridgein the LabradorSeawasfirst suggested
by Drake
et al. [1963] basedon seismicreflectionprofiles,althoughat
that time the oceanicaff'mity of the crust in the basin was
unclear[Van der Linden, 1975]. From detailedanalysisof the
2261
2262
OSLER AND LOUDEN:
EXTINCT
SPREADING
CENTER IN LABRADOR
SEA
60
55
65
60
55
50
45
Figure 1. Locationmap for the seismicrefractionexperimentin the LabradorSea.The thick solid lines are
refractionprofiles; open crossesare locationsof oceanbottom seismometer(OBS) receivers.Thinner solid
lines are fracturezones[Roestand Srivastava,1989b].Selectedbathymetrycontourlevels(as indicatedin
meters) are shadedin grey. Selected magnetic anomaly linearionsand correspondingChronological
identifiersare from Srivastavaet al. [1988]. OceanDrilling Program(ODP) Sites646 and 647 are indicated
by stars.
magnetic anomalies in the basin, its seafloor spreading
history has been most recently documentedby Roest and
$rivastava [1989a], as follows.After a periodof continental
stretching,spreadingwas initiated southof the Cartwright
FractureZone at Chron34 (84 Ma, followingthe timescaleof
Kent and Gradstein[ 1986]) andwas established
in the northern
LabradorSea, southof Davis Strait, by Chron 31 (69 Ma).
Spreadingcontinuedin an east-northeast
directionuntil Chron
[Kristoffersenand Talwani, 1977]. Spreadingrate historiesare
providedby Roestand Srivastava[1989a] for a sectionof the
extinct spreadingcenternorthwestof the Snorri fracturezone
(Figure 1) and by Osler [1993] for the segmentof the extinct
spreading
centerunderconsideration
in thisstudy.Roestand
Srivastava [1989a] proposehalf spreadingratesof 18.2, 9.4,
and 2.7 mm/yr for Chrons25 to 24, 24 to 21 and 21 to 13,
respectively. Osler [1993] proposeshalf spreadingrates of
25 (59 Ma) whena majorreorientation
of spreading
occurred. 6.9 and 7.0 mm/yr for Chrons 25 to 24 and 24 to 21,
The reorientation
to a moreobliquespreading
direction(north- respectively,followedby a lineardecreasein spreadingrate as
northeast)was coincidentwith the onsetof hot spot activity a function of time from 5.01 to 0 mm/yr between Chrons21
in Davis Strait [Hyndman, 1973;$rivastavaet al., 1989] and and 13. The modelingof spreadingrate historiesis hampered
the separationof Greenlandfrom Eurasia.A triple junction by the low amplitudeof the magneticanomaliesfrom Chron
south of Greenland existed during simultaneousseafloor 24 to the extinct spreadingcenter.They may be muted due to
spreadingbetweenNorth AmericaandGreenlandand Greenland the probability of rocks with mixed geomagneticpolarities
andEurasia,betweenChron24 (56 Ma) andChron20 (45 Ma)
occurringin a verticalcolumnat slowspreading
rates[Denham
Spreading
continued
in theLabradorSeauntilChron20 (45 and Schouten,1979] or greatersegmentation
of the ridge axis
Ma, Figure 2) and was abandonedsometimebeforeChron 13 during obliquespreadingjuxtaposingcrustalblocksof normal
(36 Ma) which is the first anomalyin the North Atlantic andreversedpolarityacrossmany smalloffsettransformfaults
parallelto andcontinuous
withthepresent
Mid-AtlanticRidge [Roots and Srivastava, 1984].
OSLER AND LOUDEN: EXTINCT
SPREADING CENTER IN LABRADOR
SEA
2263
1978]. A reexaminafionof the magneticanomalies[Roestand
Srivastava, 1989a], following the collectionof the refraction
Two longrefractionlineswereshotat theextinctspreading data, alteredthe strike of this flow line by approximately20ø
centerin the LabradorSea(Figure2) in 1987.Line R1, 150km to the north. Hence line R2 does not exactly follow a flow
in length,was situatedto samplecrustalongits strike,and line.
The refraction lines were positioned to avoid the known
line R2, 220 lcmin length,crossedit transversely
to sample
crustawayfrom the areaof the extinctspreading
centerout to fractureszonesin the area(Figure 1): the SnorriFractureZone
Experimental Procedure
Chron 24 in order to measure variations in crustal thickness as
to the northwest and the Minna and Julianhaab fracture zones
the extinct spreadingcenteris approached.R2 was oriented to the southeast[Srivastavaet al., 1988]. The determination
parallel to the generaldirectionof spreadingfollowing the of fracturezone locationsin the LabradorSea is guidedchiefly
major reorientation at magnetic anomaly 25 [Srivastava, by their signaturein the free-air gravity field, where they
56 W
52W
54
6O N
.......
......
.........
_ - .... ::::::::::::::::::::::::::::::::
:::- .. _:
................
"':.::'(i
.. 7 :i.
.
-..-...
:•.."•
...
.....
::..
'-".'2::::
........
.......
•,
.'2':':'.':::
.
:-:-_.:.'
..............................
ß ;:-::..{:'!.'" %
.....
.-
-.:•..;..:...........
.-i'i!ii.'
..............
..........
:-.
..............
:;iii:ii!ii•iE';::
....
•.<;:i•:.•::::"-w:ii•:::i.::':...
-.--•-•-•.•:..
,!•!•i•...'..i•i.,'.i•!•.•::.:...•..::-.,::.:..{-::•.:.-•R::!'i:
I
'?::
:'"i.'•:.'::•;{:.::::
...:: :.-:::::::::::
i.::':!-!.:::•
.......
! :•.:-•.-':•ii•:!
...
59
..
:..:...
..........
.
::.:..:.:
':'!lil...:.:.:.:.:.:.:.:.:.:.:.:iR.:.:...-_-.:
.::
if::.:
.:.
'.:::::::2::::'
.........
...
ß
\,.
..
-.:..-.:..-
\
\.\
58
::.:.:.:::.:.:::.
===========================================================
.........................
'2
..........................................
57N
-40
-30
-20
-10
0
10
20
30
mGals
Figure 2. Shot point and receiverlocations(OBSs, solid letteredcircles,and sonobuoys,
opennumbered
squares) for the seismic refraction experiment. The axis of the extinct spreadingcenter is along the
prominent low in the free-air gravity anomaly [from Woodside, 1989]. Magnetic lineafions and
chronologicalidentifiersfollow the interpretationof Roest and $rivastava [1989a].
2264
OSLER AND LOUDEN: EXTINCT SPREADING CENTER IN LABRADOR SEA
componentmay be up to 1/4 of a seismic period between
successiveshots,but its effect is removed by trace mixing.
Shot point locations were obtained using a combinationof
Loran C and transit satellite fixes, smoothedby a running
seismically
determined
crustalstrdctures
alongtherefraction mean applied along a 6-min window. Instrument locations
lines is thusconsideredminimal but carmotbe excluded.
were determinedby creatinga referencegrid andthensearching
Free fall/pop-up ocean bottom seismometers (OBSs) this grid to find the location where there is a least squares
minimumbetweenrangesfrom water wave arrival times,ray
[Loncarevic, 1983] from Dalhousie University and the
traced through a water velocity structureto their respective
Atlantic
Geoscience Center (Bedford Institute of
Oceanography)were deployedat locations(Figure 2) along shotpoints, and rangesfrom a positionon the referencegrid
lines R1 (five OBSs) and R2 (eight OBSs). The spacing to the shot points. A detailed discussion and listing of
between OBSs was approximately 20 km at the extinct uncertaintiesin OBS and shot point positionsis given by
spreading center and up to 50 km on the flanks. In the Osler [ 1993].
following sections,each seismic record is referred to by the
Two-DimensionalVelocity Structure of Line R2
OBS letter and the directionof shooting(e.g., OBS Msw ). An
From
Travel Time Modeling
array of 6 x 16.4 L (6 x 1000 in3) air guns was used as the
seismicsourcewith a traceseparationof approximately250 m Model
Parameterization
(shot point locationsin Figure 2). Single-channelreflection
TheRAYINVR packageof ZeltandSmith[ 1992]wasusedin
data were collected simultaneously. Disposable sonobuoys
developing
a 2-D seismiccrustalstructure.
Computed
arrival
were deployedbetweenOBS positions(Figure 2) andrecorded
times
for
user
selected
refracted,
head
wave
or
reflected
phases
on analogFM tape. Two of these,S/B 11 and 12, were usedin
axedetermined
by raytracingthroughan irregularnetworkof
the subsequent
analysisbecauseof a failure of one OBS.
blockswhichareusedto parameterize
the
For each OBS, timing correctionsare appliedto accountfor layeredtrapezoidal
The ray tracingthroughthevelocitymodel
(1) offsets and drifts in the OBS clock and (2) offsetsbetween velocitystructure.
usingzero-order
asymptotic
ray theory[Cerveny
analog data channelsdue to tape head misalignment.The first is performed
appear as prominent, low-amplitude lineations. In
positioningthe refraction lines, theseprominentfeaturesin
the free-air gravity anomaly field were avoided. The
possibility that fracture zone features are influencing the
et al., 1977]. Criteria which guidethe specification
of nodes
for eachlayer are (1) ray coverage;(2) auxiliarydatasources
(e.g., 12 kHz bathymetry and single channel seismic
component,
whichisdetermined
every24hours
byapplying
a reflectiondata); and (3) trade-offsbetweenRMS travel time
TRMs, andresolution.
The numberanddistribution
calibration tone to all channels, and a dynamic component residuals,
that arisesfrom irregular tape transport.The offset due to this of nodesalongline R2 areshownin Figure3.
correction
could leave a travel
time error of no more than 12
ms, which is equivalent to one sample point and is not
significant.The secondcorrectionis treatedas having a static
Line R2: Parameterization
of Velocity-DepthStructure
Southwest
0
40
Distance
(km)
80
120
Northeast
160
200
0 .
10-
15•,
20o
o _Dept.h
o Dept•andVelocity
Above
1- Water
.-.
o _Dept.h
andVelocityBelow
4,5,6,7- Crust
* Depth andVelocityAbove andBelow 8- Mantle
Figure
3. The2-Dmodel
of thevelocity-depth
structure
along
refraction
lineR2isparameterized
bya series
of layered
trapezoidal
blocks.
Eachlayerisdefined
bya series
of boundary
nodes
alongitsupper
surface
and
velocitynodesalongitsupperandlowersurfaces
(values
for eachnodeaxelistedin Table2, electronic
supplement).
OBSandsonobuoy
positions
areindicated
bystarsandcircles,
respectively.
OSLER AND LOUDEN:
EXTINCT
SPREADING
Employingthesecriteriaresultsin a basicalongtracknodal
spacingof 10 km, althoughthereare severalareasalongthe
refractionline with a differingnodaldensity.A first example
is thesediment-crust
boundary(betweenlayers3 and4 or 3 and
5 in Figure 3) wherenodesare specifiedevery 2.5 km as
requiredby observedtravel time variations(e.g., oscillation
of the observed travel time of Pn arrivalsfor OBS Msw
between60 and 80 km in Figure9a). Anotherexampleis the
absenceof boundaryand velocity nodesin the crustbetween
30 and60 km (layers5 and7 in Figure3), wherethepaucityof
raypaths traveling through this region precludes the
specificationof nodes.A third example is the lower crust
withinthe extinctspreading
centerfrom 70 to 90 km (Figure
3), where lateral velocity variationsare moderateand do not
CENTER
IN LABRADOR
SEA
2265
Vuppe
r andVlowe
r arethevelocities
at thetopandbottomof
the layer respectively,the normal incidencetravel time, AT,
within a layer is
Vupper
(1)
The layerdepthandlowerboundary
velocityare
AT(Vlower-Vupper
)
Vupper
Zlower
=In(1+Vlower_Vu
) (2)
requirea smallernodal spacing.
Vlowe
r = Vuppe
r + kZlower.
Initial
Velocity-Depth
(3)
Model
The water colmnnis treatedas a singlelayer with a uniform
velocity of 1.49 km/s. Bottom boundary nodes were
determinedfrom the 12-kHz bathymetry and specified every
2.5 km. The sedimentcolumn is divided into two layers. The
farst layer is between the seafloor and an internal sediment
reflector at an averagetwo way travel time of 0.68 s below the
seafloor, maxking the transition from the reflective upper
section to the transparent lower section (Figure 4). This
For the upper sedimentlayer, the velocity log at ODP Site 646
reflector
Vupper--2.2
km/sandk=0.2s-1.Thenodes
computed
withthese
correlates
with
reflector
R1
identified
on other
reflectionprofriesin the LabradorSea [Srivastavaet al., 1989]
and correspondsto the bottommostoccurrenceof ice-rafted
debrisat OceanDrilling Program(ODP) Site 646, datedas mid
to late Pliocene. The second layer is defined between the
internal sediment reflector and the sediment-crustboundary.
For a linear increasein velocity as a functionof depth,wherek
is the velocity gradient, Z•oweris the layer thickness, and
[Jarrardet al., 1989,Figure16]is usedto assign
Vupper=l.5
km/s and k=0.96 s-1. Using thesevalues, the computeddepth
to the internal sediment reflector averages 0.63 km for
refractionline R2, which coincideswith the depthat which the
velocity gradientdecreasesmarkedlyin the Site 646 velocity
log. For the lower sedimentlayer, the observedtravel times
from rays refractedwithin the layer are usedto selectvaluesof
values require little modification in the modeling, except in
the areaof theextinctspreading
center,whereVuppe
r andk
both
increase.
The specificationof boundaryand velocity nodesfor the
upper crust, lower crust, and mantle layer is based on 1-D
travel time solutions.For each OBS, the x-p velocity-depth
structures[Osler and Louden,1992] are dividedinto upperand
Line R2 reflection profile
25km
SW
SP lOO
200
300
400
500
600
700
NE
800
4.0
5.0
6.0
7.0
Extinct
Rift
Figure 4. Single-channelreflectionprofile alongline R2. Individual tracesare band-passfiltered from 6 to
36 Hz, amplitudesquaredto increasesignal to noise, and multiplied by a time-varyinggain function to
compensatefor energy loss due to spreading.OBS locationsare indicatedby solid circles; pointerslocate
sediment-basement
(B) and upperand lower sediment(S) interfaces.Vertical exaggerationis 30:1.
2266
OSLER AND LOUDEN:
EXTINCT
SPREADING
lower crustallayers at a depthwhere the velocity gradientmay
be split into two linear segments offset by a velocity
discontinuity,if required.The layer thicknesses
and velocities
are used to define the structureof the crust at a point offset
laterally from the receiver by 10 km in the direction of
shooting. In caseswhere the lateral offset causesstructures
from adjacentreceiversto overlap (e.g., OBS ONE and OBS
Esw ), the structuresare averagedand placed at a distance
midway betweenreceivers.
Mantle velocities are taken from the slope-intercept
velocity-depth structures [Osler and Louden, 1992]. The
mantle velocity nodesare offset laterally from the receiverby
10 km in the direction of shooting.For defining the lower
boundary of the manfie layer, a velocity of 8.24 km/s is
specifiedat a depthof 20 km. This introducesverticalvelocity
gradientswhich range from 0.02 s-1 to 0.04 s-I dependingon
the velocity at the upper boundaryof the mantle layer. For
most OBSs, this gradient provides a suitable match for the
critical range beyond which mantle refractedphasesbegin to
be observed. The irregular distribution of boundary and
velocity nodesfor the crustaland mantlelayersis interpolated
to form a regulargrid with a nodal spacingof 10 km.
Model
CENTER
IN LABRADOR
SEA
providing an acceptable model revision. This minimum
standardis only relaxedwhenmultipleray pathsfrom different
receivers overlap, but no structurecan be devised which is
capable
of allowing
Z2 < 1 for allreceivers.
Final
Model
In Figure5, the final velocity-depth
structureis represented
by smoothedisovelocitycontours,calculatedevery 1 km by a
linearinterpolation
betweenthe velocityandboundarynodes
for the trapezoidin which they are bounded.Contourscan
becomecoincident along model boundarieswhere there is a
velocity contrast(e.g., the sediment-crust
boundary).Figures
6 and 7 presentray paths throughthe layers of the final
velocity-depth structure and their associatedobserved and
computed
traveltimes.Thevaluesof n, TRMs, andZ2 in the
final velocity-depth
structureare presentedin Table 1•
(electronicsupplement).
Sediment Layers. The sedimentlayers are numbered2
and 3 in the modelparameterization
(Figure3). Ray pathsand
associated travel times for sediment refractions
are found in
Figure 6a. Comparisonof computedand observedarrivalsfor
OBSsL, M, and QNE exemplifythe excellentfit providedby
themodel(Z2 valuesfrom0.057to 0.140).Maximumshot-
Revisions
receiveroffsetsfor computedsedimentrefractionscorrespond
An optimal fit betweenobservedand computedtravel times closelyto observations,
indicatingthat the vertical velocity
is soughtby adjustingthe initial velocity and boundarynodes. gradients have been appropriatelyspecified. For sediments
By assigninguncertaintiesto observedtravel time picks, the within the extinctrift valley (nodesfrom approximately80 to
quality of a computed travel time fit may be assessed 120 km), the velocity of nodesalong the top and bottom of
numerically (equation(5) below). For all travel time picks, a
the lower sediment layer (layer 3) must be increased.The
baseuncertaintyof :k 0.02 s has been chosento accountfor the requirementfor highervelocitiesis indicatedin Figure 6a by
timing uncertaintiesinherent in the refraction data. A further
uncertaintyhas been added to reflect the quality of the pick
itself. The largestuncertaintiesof + 0.12 s have been assigned
to Prop and sedimentrefractionarrivals,which are difficult to
pick becauseof interference from first arrivals, and to Pn
arrivals,which are difficult to pick becauseof their low signal
to noise.
To assessthe quality of successivemodel iterations,three
parameterswere compared.First, one seeksa velocity-depth
structure
which
maximizes
the number
of observed
locations
to which rays may be traced,n. The parametersTRMs and Z2
quantify the fit between the observedand computedarrival
timesfor a givenray groupand aredeemedasfollows
n(Tøbsi
1/2
TRMS
i=•l
-Tcalc
i)2,
=
Z2
= /=1('•r•b•/
(4)
OBS E andOBS ONE' whichhavevelocitiesexceeding
2.5
km/s, while receivers outside this region have velocities
slower than 2.5 km/s. The topographyof the sediment-crust
boundary(interfaces3 and 4 or 3 and 5 in Figure3) has been
modified to improve the travel time fit of the crustal and
mantle arrivals in regions where the basement reflector is
difficult to pick. Two notable examples are (1) the shortwavelengthoscillationsintroducedon the basementhigh from
60 to 80 km to reproducethe travel time oscillationsobserved
in Pn for OBS Nsw, OBS Esw, and OBS Msw and (2) the
deepeningof the sediment-crustboundaryaround 150 km to
delay crustalarrivals.
Crustal Layers.
The 2-D velocity-depth structure
quantifiesthe lateral variationsin the crust acrossthe extinct
spreadingcenter.The model layerswhich define the crustare
numbered4, 5, 6, and7 in the modelparameterization
(Figure
3). Ray pathsand travel times are presentedin Figure6b for
the upper crust and in Figure 6c for the lower crust. Two
principal variations in crustal structure emerge: (1) the
variationin the velocity and thicknessof the crustacrossthe
(5) •Anelectronicsupplement
of TablesI and2 maybeobtained
ona
disketteor AnonymousFTP from KOSMOS.AGU.ORG. (LOGIN to
AGU's FTP account using ANONYMOUS as the usemarne and
GUEST as the password.Go to the right directoryby typing CD
APEND. Type LS to seewhat files are available.Type GET andthe
where Tobsis observedtravel time, Tcalcis calculatedtravel nameof the file to get it. Finally, type EXIT to leavethe system.)
(Paper 94JB02890, Extinct spreadingcenter in the Labrador Sea:
time, Uobsis uncertaintyof the observedtravel time, and n is
Crustalstructurefrom a two-dimensionalseismicrefractionvelocity
number of calculatedtravel times reachingobservedarrivals. model,by J. C. Osier and Keith E. Louden).Diskettemay be ordered
Forvaluesof Z2 < 1, thecomputed
arrivalsfit theobserved from American GeophysicalUnion, 2000 Florida Avenue, N.W.,
arrivals within the uncertainties of the observed travel times,
Washington,DC 20009; $15.00. Paymentmust accompanyorder.
OSLER AND LOUDEN:
EXTINCT
Southwest
SPREADING
CENTER
IN LABRADOR
SEA
Distance(km)
0
40
80
2267
Northeast
120
160
200
I
01.,
11
12
Line R1
O
E
N
M
, ,
L
10
Mantle
15
Figure 5. The 2-D velocity-depthstructureis represented
by isovelocitycontours,drawnevery 0.25 km/s,
which can becomecoincidentalong model boundarieswhere a velocity discontinuityis present.Layers in
the model parameterization(Figure 4) are distinguishedby different grey tones. Receivers are denotedby
solid circles. Arrows and correspondingchronological identifiers mark the intersection of magnetic
anomalylineationswith line R2. Selectedvelocity contoursare labeled in kilometersper second.
(b) Upper Crust
SedimentLayers
12--
-•- E ---' •-M --•
Q•.--•
•
6
•
•:i:•
12
%..
11--
•
11•
•o•N
•
• 4
!
•
........
o
3
•0
•
m
6
0
40
sw
80
120
160
Distance(km)
200
NE
'•
6
•
12
0
SW
..........
HII
IllHi
[
.........
40
80
120
160
Distance (km)
200
NE
Figure6. Computed
raypathsandtraveltimesusingthe2-D velocity-depth
structure
alonglineR2 for (a)
refractions
in thesediment
layer(b) refractions
in theuppercrust,P2 (c) refractions
in thelowercrust,P3,
and(d) mantlewide-angle
reflections,
PmP.Observed
traveltimesandtheiruncertainties
arerepresented
by
the stippledareasthroughwhichthe computed
arrivaltimesare traced.Ray pathsconvergeto their
corresponding
receivers.
For clarity,thediagrams
showeveryotherOBSfor eachline so thatarrivaltimes
can be discriminated.
2268
OSLER AND LOUDEN: EXTINCT SPREADING CENTER IN LABRADOR SEA
(c) Lower Crust
(d) PmP
m
6
•, 6 12-"
•,
•••f
• 5Q-•.•--E?...•-M-,'u•!
5
m12• ....
6
11•
• 5
•O•••
•
5
4
o
go
• 6
............• 6
m 12
Z
Z
'"
'
Z
"''•
m 12
0
40
SW
80
120
160
Distance(km)
200
NE
0
40
SW
80
120
160
Distance(km)
200
NE
Figure 6. (continued)
(b) Pnfor OBSsN andE
(a) Pnfor OBSsO andQ
6
6
•--N
.•- O --•.
--,-
;%...".--,•
' .-.*,•
o
• 10..........:..
:,•• .............
lO
•
20
6
20
,
6
,,•- E .--,,.
0l•'ill/lllI 1/1111111111'11111
IIIII
•
10
20
0
40
80 120 160
Distnnce (km)
200
0
40
80 120 160
Distance (km)
200
Figure
7. Computed
raypaths
andtraveltimesforrefractions
in themantle,
Pn,using
the2-Dvelocitydepth
structure
along
lineR2for(a)OBSs
OandQ,(b)OBSs
N andE,and(c)OBSs
L andM.Observed
travel
timesandtheiruncertainties
arerepresented
by thestippled
areas
through
whichthecomputed
arrivaltimes
aretraced.
Raypaths
converge
totheircorresponding
receivers.
Forclarity,
thediagrams
showeveryother
OBS for each line so that arrival times can be discriminated.
OSLER AND LOUDEN:
EXTINCT
SPREADING
CENTER
IN LABRADOR
SEA
2269
have many overlappingray paths in the lower crust and some
in the upper crust.The four refractionprofilessamplingcrust
(c) Pnfor OBSsL andM
in thisregionhaveTRMs < 0.41s andZ2 < 0.49 (Table1,
electronic supplement).
On the flanks of the extinct spreadingcenter,0 to 60 km to
the southwest and 130 to 220 km to the northeast, the seismic
behavior
o
lO
20
6
I I I I I IIIIIIIIIIIIIIIIIIIIIII
M
Illill/Ill
I1l
lO
0
40
80
120
160
200
Distance (km)
Figure7. (continued)
extinct spreading center; and (2) the asymmetry between
flanks of the extinct spreadingcenter.
The transitionfrom flank to extinctspreadingcentercrustal
typesis markedby a combinationof (1) thinningof the crust;
(2) decrease
in P wavevelocity;and(3) a morelinearvelocity
gradient throughoutthe crust. Within the extinct spreading
center,arrivalsat OBS ONE,OBS E, andOBS Nsw definethe
upper and lower crustalstructure.Low upper crustalvelocities
are readily apparentin Figure 6b by noting that the refractor
velocities do not exceed 5 km/s. Further evidence is the
markeddelayin lower crustal(P3) arrivalsat OBSsE, Nsw, and
ONE (Figure6c) which travel throughthe regionof low upper
crustal velocities within the extinct spreading center. For
arrivals from the lower crust(Figure 6c), the observationsof
low velocitiesat theseOBSs are evenmore pronounced.They
are expressedin the final model by a downwarpingof the 6
km/s isovelocity contour as the extinct spreadingcenter is
enteredand the proximity of the 7 km/s contourto the crustmantle boundary.
The influenceof the extinct spreadingcenteris broad, with
crustalthinning observedfrom 50 to 120 km (Figure 5). The
transition to the extinct spreadingcenter crustal type, thin
low-velocity crust with a uniform velocity gradient, occurs
between
60 and 80 km on the southwest rift flank and 110 to
130 km on the northeastflank, approximatelycoincidentwith
Chron 21. The crustal thicknesswithin the paleorift valley
from
80 to 110
km
varies
between
3.8
and 4.2
km.
The
thinning is defined by (1) the short shot-receiveroffsetsover
which computedand observedrefractedphasesare present;and
(2) the Pn arrivals, particularly the critical range at which
mantle arrivals begin to be observed.The 2-D velocity-depth
structure within the extinct spreading center is well
constrainedbecausereceiversin this region were situatedto
of the crust differs
from the crust within
the extinct
spreadingcenter, particularly to the northeast,with higher
crustalvelocities;greaterrangesat which refractedarrivalsare
observed;2 to 3 times more observed arrivals; and clearly
discernibleProp arrivals.The southwestern
flank is definedby
arrivalsfrom OBS Q, sonobuoys
11 and 12, andOBS Osw. The
range from 0 to 30 km is well definedby the ray pathsfrom
OBS Q and sonobuoy 11. A typical oceanic structure is
observedin this region, with a high-velocitygradientin the
upper crust (isovelocitycontours4 to 6 km/s in Figure 5), a
weaker velocity gradient in the lower crust (isovelocity
contours 6 to 8 km/s in Figure 5), and a nominal crustal
thicknessof 5.5 km as definedby Pn arrivalsfrom OBS Q,
OBS Osw, OBS Nsw, andOBS Esw.
For the northeastflank, the P3 (Figure6c) andProp (Figure
6d) arrivalsfrom OBS NNE, OBS M, and OBS Lsw define the
featuresof this region. The arrivalsare of considerablyhigher
velocity and extend to greater shot-receiveroffsets than the
arrivals at OBSs samplingthe southwestflank or the extinct
spreadingcenter.To model the lower crustrefractorvelocities,
upper boundary velocities from 6.9 to 7.2 km/s and lower
boundary'velocities from 7.5 to 7.7 krcdsare required in the
fastest region from 130 to 170 km (Table 2, electronic
supplement). In addition, a thickening of the crust (by
approximately1 km) and a weak velocity gradientare required
to match the shot-receiveroffsets to which P3 and PmP
arrivals are observed. From 190 to 220 km, the crustal
thickness (Figure 5) is comparableto that observedon the
southwest flank from 0 to 30 km, but the velocities in the
lower crustare higher.
The Prop arrivalsobservedon thenortheastflank areusedto
definethe velocitygradientstructureof the lower crustand,in
conjunctionwith Pn arrivals,the depthof theMoho. They are
second arrivals which are difficult to pick because of
interference
with
the
wavetrain
from
earlier
arrivals.
Accordingly, they are assigned the largest travel time
uncertaintiesof all arrivals and are intentionallythe most
poorlyfit (0.6 < Z2 < 3.8). The final velocity-depthstructure
which accommodates
the four setsof observedProp arrivals
fromOBSM, OBSLsw, andOBSNNErepresents
a compromise
betweenthe modelrevisionswhich individualOBSs require.
For example,in the regionof rapid crustalthinningfrom 140
to 120 km (Figure 6d), OBS Msw requiresa combinationof
slope on the crust-mantleboundaryand high lower crustal
velocitiesto reproduce
the high (greaterthan8 km/s) apparent
move-outvelocitiesof ProP. The fit of computed
Prop arrivals
for thisOBS (Z2=0.58)andfor OBS NNE(Z2=1.78)couldbe
improvedby more rapid crustal.thinning.
However,the Pn
arrivalsfrom OBS NNE (Figure7b) arevery sensitiveto the
slopeof this boundary,as it controlsthe extentof the shadow
zone encounteredby rays enteringthe mantle. As the shadow
zonebroadens,the numberof observedarrivalsto whichrays
can be traced decreases.
The asymmetrybetweenlower crustalstructuresobservedon
southwest
andnortheastflanksof the extinctspreadingcenter
is consistent
with previousseismicand gravityobservations.
High-velocity lower crustal material has been observed at
2270
OSLER AND LOUDEN: EXTINCT SPREADING CENTER IN LABRADOR SEA
widely spacedlocationsnortheastof the extinctspreading Mantle Layer. Ray paths and travel times for Pn (Figs.
center(LinesA andF [Stergiopoulus,1984];ODP site 646 7a, 7b and 7c) are used to detimethe thicknessof the crust and
[Srivastavaet al., 1989];andline 15/77[Hinzet al., 1979]), the velocity of the uppermostmantle. These two quantities
thoughall but oneof theserefractionprofilesareunreversed exhibit significant variations along the length of line R2,
and may be influencedby sedimentthicknessvariationsor primarily in relation to distance from the extinct spreading
dippinginterfaces.
The presenceof this materialmay be the center.In fitting the observedarrivalsfor Pn, TRMS was kept
cause of the regional asymmetryin the free-air gravity within the uncertaintyof the travel time picks (i.e., Z2 < 1) for
anomalynotedby Srivastavaet al. [1981]northof theMirma seven of the 10 sets of observedarrivals (layer 8 in Table 1,
andCartwrightFractureZones.It is largelypositivenortheast electronicsupplement).Model revisionswere terminatedwhen
of the extinctspreading
centerandnegativeto the southwest reductionsin TRMSfor onereceivercausedTRMSto increasefor
with the degreeof asymmetry
increasing
northwards.Vogtet another.For example,thelargestPn traveltimeresidual(TRMs
al. [1982], in studying this regional asymmetryin the = 0.12 s), at OBS MNE (Figure7c), may be reducedby thinning
Labrador and Norwegian-GreenlandSeas, find that the the crust or increasingthe mantle velocity between 170 and
asymmetryis most pronouncedfor crust formed between 220 km. However, the arrivals for OBS L shootingsouthwest
magneticanomalies
25 to 29 andweakerfor theyoungercrust. (Figure 7c) also travel throughthis region of the manfie but
The formationof high-velocitylower crustmay be relatedto alreadyarrive too early (TaMs= 0.86 s). The remainingtravel
hot spot activity from the time of anomaly24 until the time residuals may be attributed to (1) limitations of 2-D
cessation of spreading [Srivastava et al., 1989], but methodology,with rays traveling outside the distance-depth
knowledgeof its regionalextentwill be requiredto address plane assumedin the 2-D velocity-depthstructure;(2)
variationsin the sumctureof an overlyinglayer which are not
this speculation.
(c) Method2: TRMS(ms)
(a) Method 1' ReduceCrustalThinning
.......................................
r.•
I
,•,
, ,/ _,•,r"•
"•;// •,//l•
o.o
//,'7//./,/./' ,/1/if///
,•/ /•
•
•.
/I.
/-
,
/
,.
>-0.21
.......................................
0.2
12
60
40
80
100
140
120
d.)
Method
2:Z2
.....................................
!
I
Distancealongline R2 (km)
•0.0
(b)Method
1'Z2
-o.21.......................................
I
0.2 e!
.....................................
In
Method
2:Observed
arrivals
reached,
0%
20%
40%
60%
80%
Percentageof crustalthinningremow•d
>-0.21........................
-2.0
-1.0
0.0
,,. ............ I
1.0
Depth Perturbation(km)
Figure8. Erroranalysis
alonglineR2. Formethod1, (a) percentage
is shownof crustal
thinning
removed
withintheextinctspreading
centerfrom40 to 140km and(b) thecorresponding
influence
is shownonthe
qualityof fit between
observed
andcalculated
arrivaltimesfor all mantlearrivals.
Formethod
2, velocity
anddepthperturbations
(stars)areintroduced
to nodeswhichformthecrust-mantle
boundary
in thefinal
velocity-depth
structure
for line R2. The effectof theseperturbations
on thequalityof themodelfit is
assessed
by plottingcontoursof (c) TRMs, (d) Z2, and(e) n.
2.0
OSLER AND LOUDEN: EXTINCT SPREADING CENTER IN LABRADOR SEA
adequately resolved by the existing data; or (3) instrument
timing errorswhich remain at variousreceivers.
The isovelocitycontoursin Figure 5 show low velocities(<
8 kin/s) in the uppermost mantle at three locations. The
thickest of the low-velocity regions underlies the extinct
spreadingcenter and extendsto a maximum depth of 4.3 km
below the crust-mantleboundary.The upper mantle velocities
within the extinct spreading center are the most well
constrainedin the model becauseof the numberof overlapping
ray paths and the confidencein the crustal structureoverlying
the mantle. The quality of the travel time fits is particularly
2271
perturbations (crustal thinning). For positive velocity
perturbationsexceeding0.05 km/s, the number of observed
arrivals which are reached by the rays begins decreasing
markedly (Figure 8e). The increasesin the upper mantle
velocities decreasethe velocity gradient of the mantle layer to
the point where it is too weak to refract rays to the shorter
shot-receiveroffsets.Using the dimensionsof the 80 ms TRMS
contour and rejecting positive velocity perturbations,average
uncertaintiesof q- 0.25 km and q- 0.08 km/s can be assignedto
the depth and velocity nodes respectively which comprise the
crust-mantle boundary.
good(0.24< Z2< 0.58)for thereceivers
whichhavePn ray
paths crossing or entering the mantle within the extinct
spreadingcenter(OBS E, OBS ONE,OBS Nsw, andOBS Msw).
The depth of the low-velocity zone underlying the extinct
spreadingcenter complementsthe results obtained by 1-D
WKBJ syntheticseismogrammodeling for OBS G on line R1
along the axis of the extinct spreading center [Osler and
Louden, 1992], which suggests that a relatively strong
gradientexistsin the uppermostmantle and extendsto a depth
of 3 km below the crust-mantleboundary.
Mantle arrivals help define the transition from typical
oceanic crustal structures on the flanks to the distinct crustal
type within the extinct spreading center, as previously
mentioned.On the southwestflank, the slope on the crustmantle boundaryis revealed by the Pn arrivals at OBS Osw
(Figure7a) from 45 to 60 km, whichareup to 0.4 s earlierthan
Synthetic Seismograms
Synthetic seismogramswere generated for all receivers
along line R2 using TRAMP, a geometrical ray theory
algorithm for inhomogeneousstructures(C. Zelt, personal
communication, 1992). The impulse responseseismograms
were convolved with a source function, the exponentially
dampedcosinefunction of Cervenyet al. [1977]:
+v)
(6)
f(t)=
exp
- T t2cøs(2•fmt
where fm is dominant frequency of source (6.8 Hz), T is
dampingfactor (5.6), v is phaseshift (•c/3), and t is time. This
thosefrom 10 to 45 km. OBS QNE(Figure7a) alsodefinesthe reproducesthe long wave train (approximately450 ms) which
of the air gun sourcearray.
crust-mantle boundary on the southwest flank. For this is characteristic
A comparisonof synthetic seismogramscomputed for a
receiver, the P n arrivals terminate abruptly at 55 km at a
shadow zone generatedby the rapid crustal thinning. On the typical 1-D velocity-depth structure between TRAMP and
northeastflank, the transitionis not as easily discerned.The WKBJ [Osler, 1993] is consistentwith previouscomparisons
high velocity lower crust in this region counteractsthe delay of asymptotic ray theory and its extensions (the WKBJ
which P n would otherwise exhibit as the crust thickens. approximation)[e.g., Chapman, 1985]. These differencesare
However, there is evidence for slope on the crust-mantle most prevalent around the critical point for mantle arrivals,
boundaryfrom the move-out of Prop and the shadowzone where asymptoticray theory breaks down. The result is that
the peak amplitudeof PmP arrivalsis observedat 6 km greater
generatedfor Pn arrivalsfrom OBS N shootingnortheast.
rangein the WKBJ syntheticsandthe amplitudeof PmP builds
up very rapidly in the TRAMP synthetics,while it is more
Error Analysis
gradual in the WKBJ synthetics.The amplitudesof refracted
Error analysisof the modelparameters
definingthe crust- arrivals comparefavorably except for Pn in an interval from
mantle boundarywas performedby perturbingthe model just beyond the critical point to a range of 40 km, where the
parameter
valuesandthenray tracingthroughthe perturbed amplitudeof the TRAMP syntheticsare lower than the WKBJ
model. In the first of two approaches,
the degreeof crustal
counterparts.
thinningwithin the extinctspreading
center,as definedby
Refractionprofiles at two OBS have been selectedfor
boundarynodesfrom 40 to 140 km along line R2, was inclusionin this paper. Observedprofiles for all OBS are
progressively
decreased
(Figure 8a) until the crust-mantle available on microfichet (FiguresA1 to A10 and B1 to B4).
boundary
waslinearbetweenthe40 km and140km endpoints. The synthetic
seismograms
andobserved
profilesfor OBSM
In Figure8b,Z2 valuesusingthemantlearrivals
at all OBSs locatednortheastof the extinctspreadingcenterare presented
are presentedfor eachof theseperturbedmodels.Following in Figures9a and9b. Shootingsouthwest,
the high amplitude
the criteria of fitting computed arrivals to within the Prop arrivalsfrom 18 to 48 km in the observeddata are
uncertainty
assigned
to theobserved
traveltimepicks(Z2<1), matchedin the syntheticsfrom 18 to 39 km, at whichpoint
no more than 25% of the crustal thinning may be removed theProp arrivalsterminateabruptly.The gapwith no second
before the travel time fits are no longer acceptable.
arrivals and weaker first arrivals from 15 to 19 km in the
In the secondapproach,a constantvelocity and/or depth observeddata is also producedin the synthetics,but the
perturbationwas appliedto all nodescomprisingthe crust- intervalis shiftedto 17 to 21 km. The observedPn arrivalsfor
mantleboundary.
Contours
of TRM
s,Z2,andn arepresented
in this receiverare particularlystrongfrom 80 to 90 km as rays
Figures8c, 8d, and 8e, respectively,as a functionof the enter the southwestflank of the extinctspreadingcenter.The
velocityand depthperturbation
magnitude.The TRMs = 80 ms
andZ2 = 1 contoursshowa preferredorientationto plausible
1Appendix
figuresareavailablewith entirearticleon microfiche.
perturbations.
Increasesin Pn velocity are accompanied
by Orderfrom AmericanGeophysicalUnion, 2000 FloridaAvenue,N.W.,
positive depth perturbations (crustal thickening), and Washington,DC 20009. Document B95-001; $2.50. Paymentmust
decreases
in Pn velocityare accompanied
by negativedepth accompany order.
OSLER AND LOUDEN:
EXTINCT
SPREADING
CENTER IN LABRADOR
SEA
2273
syntheticsdo not replicatethis behavior.Shootingnortheast,
the syntheticis a particularlygoodrepresentation
of the data.
The low amplitudessurrounding
the basementhigh (shotpoint
700) are presentin both the syntheticsand observeddata from
19 to 24 km. On either side of this feature, the synthetics
match the high-amplitudearrivals from 10 to 19 km and 25 to
supportedby a number of studies.Huang and Solomon [1988]
present observations that the maximum centtold depth of
active ridge crest earthquakesincreasesfrom 2-3 km at full
spreadingrates of 4045 mm/yr to 5-6 km at full spreading
rates of 5-10 mm/yr. Purdy et al. [1992] show that a
systematicrelationshipexists between spreadingrate and the
45 kin.
depth to which a P wave velocity inversion is observed at
The syntheticseismograms
andobserved
profilesfor OBS E active spreadingcenters. Their preferred explanation is that
locatedwithin the extinct spreadingcenterare presentedin the "major faults that extend to greater depths on slowerFigures10a and 10b. The amplitudeof the upper and lower spreadingridges [Huang and Solomon,1988] and allow a more
crust refracted arrivals is strong and well modeled in the vigorous circulation of cooling sea water to reach greater
synthetics,as the energy is refractedby the nearly linear depths,blocking the upward migration of large melt bodies"
velocity gradient throughout the crust within the extinct [Purdy et al., 1992]. The maximum depth to which faulting
spreadingcenter.The high amplitudesin the observedcrustal extends is an indication of the thicknessof the mechanically
arrivalsare followedby strongPn arrivalsfrom 20 to 30 km. stronglayer which is prone to brittle failure in responseto the
The synthetics
put very little energyinto thesePn arrivalsjust extensionalstressescausedby the divergingplates. Through
of the throw on normal
faults at slow
beyondthe criticalpoint, despitea relativelystrongvelocity an examination
spreadingcenters,Macdonald and Luyendyk [1977] suggest
gradient(0.04 s-l) for theuppermantlein thisregion.
The syntheticand observedrefractionprofiles compare that up to 20% of the plate divergencemay be accommodated
favorablyas the major amplitudevariationsfor P2, P3, and by these faults. Last, there are compilations of seismic
Prop arrivalsare matched,althoughthe patternsof amplitude refraction data which document that crustal thickness becomes
variationsare shiftedby severalkilometersin someinstances. a function of spreadingrate when the spreadingrate is less
The greatest shortcomingsof the syntheticsare the low than 15 mm/yr [Reid and Jackson, 1981; Bown and White,
amplitudeof thePn arrivalsandthevery rapidterminationof 1994].
A systematicrelationshipbetweenspreadingrate, faulting,
the PmP arrivals.However,basedon the comparison
of the
TRAMP and WKBJ algorithms,theseshortcomings
stemin hydrothermalcirculation,and the depthbelow the spreading
part from limitationsof the asymptotic
ray theorywhichthe centerat which an appreciablepartial melt fractionis observed
TRAMP algorithmemploys,andnot solelyfrominadequacies couldbe readilyappliedto the spreadingcenterin the Labrador
Sea precedingits extinction.The zone in which crustal and
in the f'malvelocity-depthstructure.
Discussion
Nature and Origin of the Crust
The seismicrefractionresultsalongthe strikeof the extinct
spreadingcenter [Osler and Louden, 1992] and crossingit
transversely (this paper) find a prevalence of anomalously
thin and low velocity crust in the extinct spreadingcenter,
which is underlain by low velocities in the uppermostmantle.
There are two frameworksby which theseobservations
may be
explained. These consider the crust as (1) having formed
through igneous extrusive and plutonic processes, then
subsequentlyaltered by tectonism;or (2) being composedof
hydrothermally altered upper mantle. Oceanic crest, in the
prevalentview, is igneousrock whichsolidifiesfrom partial
melt after it migratesto the accretionary
axis.The detailsof
this process remain equivocal, but analogies between
ophiolitestudies[e.g., Salisburyand Christensen,1978] and
marine seismic studies [e.g., Spudich and Orcutt, 1980]
suggestthat the structureof matureoceaniccrestis primarily
vertical, with (1) the upper crust (layer 2) composedof
extmsive volcanics (pillow basalts), underlain by a dense
seriesof vertical dikes (sheeteddiabasedikes) stemmingfrom
upper mantle velocities are anomalouslylow and the crustal
thinning occursis boundedwithin anomalies21 west and 21
east(Figures1 and 5) wherethe slowestspreadingratesare
encountered.
It is within this region that the crest shouldbe
most susceptibleto brittle failure, thinn2ngby tectonism,and
a reducedpartialmelt supplydueto conductive
coolingof melt
bodies. This hypothesis is supportedby a multichannel
seismic reflection profile crossing the extinct spreading
center to the south of line R2. Srivastava et al. [1993] find
evidence for crustal extension by rotated crustal blocks
between anomalies 21 west and 21 east.
A second view of the anomalous crustal structure within the
extinct spreadingcenter is one in which it is formed by the
serpentinizationof upper mantle peridotitc [Hess, 1962;
Lewis and Snydsman,1979]. The Moho in this casewould not
representa compositionalchangebetween crest and mantle
but the depth to which upper mantle was hydrothermally
altered.
The
crustal P wave
velocities
observed
would
be a
linear function of the volume percentageof serpentinite,
velocities of approximately 5 km/s for 100% serpentinite
increasingto 8.0 km/s for unalteredperidotitc [Christensen,
1966].Serpentinite
hasa Ve/Vsratiowhichis muchhigher
than other rocks likely to constitutethe oceaniccrest. It is on
the basisof Vt•/Vsstudies[Christensen,1972] that the
episodesof injectionfrom an underlyingmagmachamber;and serpentinitemodel of normal oceanic crest has been largely
(2) the lower crest (layer 3) consistsof plutonicallyeraplaced rejected. However, it remains a viable alternative for the
gabbros.
Assuming that the crust at the extinct spreading center
formedfollowingthis generalmodel,a pervasivefracturingof
the entire crest is a mechanism for thinning the crest and
lowering its velocity. In this scenario,decreasesin spreading
rate precedingextinctionwould be associated
with a decrease
in magmatismand an increasingrequirementfor tectonismto
accommodatethe divergenceof the plates.This associationis
structure of the crust at oceanic fracture zones [Calvert and
Potts, 1985; Louden et al., 1986] and has been applied as an
alternativemodel for the East Pacific Rise basedon gravity
data and thermal models [Lewis, 1983].
In the serpentinitemodel, the variable thicknessof the
crust along the strike of the extinct spreading center, the
linear velocity gradients and the low crustal velocities (line
R1, 1-D travel time solutionsand z-p inversions[Osler and
2274
OSLER AND LOUDEN: EXTINCT SPREADING CENTER IN LABRADOR SEA
(a) Line R2: ObservedandSyntheticRefractionProfilesfor OBS E ShootingSW
Shot Point
100
200
300
I
I-',....
,._-:._,.;,:,,
?,,..-..,..:•
,,,;..,:,:..:..,
;;....,•....
......
,. ,, ,:"
,'.,, ,,. , ",.... .
,.,.,"".
• ,,.,'
'.''.,,.,"c..,.r';.;•.',,.;.,...
',;',',:,;.,'-'.,,
;......
,a.....
,,..,
'f:.,.,
.......
F:..,.,.,,,;:,,•,.,.,..,.,•,.z,:';,,,,Z.
,.,"r,
.,,;.,,,..,;•.,
::.......
,:,,-,,,.-1
......
-,.,',•'.",
.,•,.
,,,, ,,,' ,,, ,-,., ........
,,', ,,, .,,.,.'
'{-•-:,;.,:-d"Z::;"-•:-":'•Z
:";•" '-':":'•';".:'":;
'. :"'T..,-'..,•: ,, ,: ','.,,":.'.,,,-,.,,
' -.,,, ,-: '• • ,'
'i) ) w,)' ',,)•.'
,,, ',. "' . '), k)n)
)• ',"" ,,,......
• ), ,,*'
,","•,, • ß ','..,.,.,,':.,..,,,"
,,. ")'•"•
"..,,--'-,.,'"•.,':.,,..,.,')
{ ;,
'M
;,,d,,)'.."'•?,,•_
,.",'' ,•, , ))'•)
' ,,,,.. M,.' "7•'
,'.•,,
,',,•),',,,,",
,.,,....,
'I,'.'..'
,j ,,, , ,, ,)•k•
,,,,,
,,,,.
,..,') ,'• ,.,,,',
,),•)
") ,.
'"---
-' •
•"ik.'•
-.
i,,.,-'.,,
"" "2
. :' '"
.,_ ,
,-. "'
•-"')"'• •
,,
.,
' ,, ,,.
'
:.,::,,
II '...,,_..
['""-;";•
•/ •,1,,
....•
,'.,._
•'.,_ •,,•,,,
' ,-;..,.',,•,,.
,_.,:,,'
,,..,:,.,;;.
-'..r,
,,
," , ' , , ,, ,
,,,o I me) ,,,),,,
), ),•)l) ,,) ),,
M,)t) i) , ) w
,
,w
, •
{]
•/•j,•
IL )'/%
Ih
I1/
•
If--','"•
%
•
I "."'.."-•-'
;'•.,
- ,-.:...'
,:"'",.*""'.".<
....... '.::."-',,.,' ,:;',
"',,.. ";',,
..............
J ;•,
-'":-'•,
"..' ..',,,•• ".,'
...:;. •..' ,• !;',. ;, e':.',--' ",
,- ", .........
-L....
'...,,',i,•.,,,,"'
•.,
._.,
,:_'./•'..',,•,,,,, ,,,,.' ,,,•,.,'" ,,,,; ,,, '.-'"' ,,';,, ,',
,' ',,
,L"'"
'.,....••",
".,'.
'-"•'
;',.•"'''
.-':,
' -•.,.' '_':.,',':3
,..,.'..... .•.,....',, ....
'• '-i"
',..,•,.,
'.._.t_•_
ß
',.,
',"'• '-."' •.,'.:.,-_•L,'..
',T,-,;";,:'.,;,,',5.,:, ".'..'..',.
•-•
-,so
,
,
I
' .....
:.... ,, :'
,. , , ,,
'";,
,
,
""
Di•..,•(kin)
-so
7
3
I
I
-90
I
450
Distance0an)
-30
0
(b) Line R2: ObservedandSyntheticRefractionProfilesfor OBS E ShootingNE
500
,..::.
.....
. .... .
600
,,,•,-;......•,...,;, .,,,J,........ ß ,...
, ,,,,,.,,...,.,. ,, ', ,,
6
,•...),
....... ',;.........
M))
' ,' ..............
• ), , )'
., ,
. , , , , . ' ..
,
,
•,
0
)') )) ) ) ,
, , ,.
I'l)n•,)I
)))'*'
,.
) )), ) ))"
;...,
'.;•., -. '.. .,.,•.. ,.:',.,,. , ,;
",.'
.:'•"
") ',:"'" M))1'):-i
! i1....)
) .i.14,1, i•
,))) ) •
,...... ' ." '' :.r, . , '. ,',,
.....:';..'.;
:;'....:
........
i."!'.-:...
.......
".....; ....
'"::"•'"::'
:.:.:.,.,,,,
.......
:
,,•.....,
r'•..,
,,
.;:,.,;.
""•
....
. .....:,,,,,.,:,:'.:,, .:,..:"•:•
))) ß
)L ' ) ) ) .),))
, .,,, . ,.,.
20
,,,,•. :.. ,,.......
,..w
',: ,,,', ,. ) .....
ß
..' . )
) ,•).1,) )1. )) ) ) ) )M)w) ) )14.
•
.')) ) ,)•
' , )
)) ,• )) ,.
Shot Point
ß .::;
Distance 0ma)
, ) P ) ) )')))'•) '))))
..... . ::...';
40
60
6
I
0
20
I
Distance (kin)
I
I
40
Figure10. Observed
andsynthetic
refractionprofilesfor (a) OBSEsw and(b) OBSENE. Observed
tracesare
band-pass
filteredfrom 4 to 12 Hz; threetracesaremixed.Gain is appliedas a functionof rangeandtraces
are plotted at a reducingvelocityof 8 km/s. Arrivalsfrom differentphasesare identifiedon the synthetic
profiles.
60
2275
OSLER AND LOUDEN: EXTINCT SPREADINGCENTER IN LABRADOR SEA
Chron
(a)
21
20
13
20
21
;
.
;
.
;
;
:
:
;
;
Thin.
rang
iRegion.
orCru!tal
(b)
•
i
o
o
I
;
;
;
:
:
:;
:
Southwest i.;
:
;
o
:
;
:
:
Northeast
0
i•::•::::i•i:i•i•i•:::::•ii?:•::•:::•
................
•i!iiii::iii!!::•::!::iiiii,i::??:::!:'
'!:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
.......
'i":;:;:;
.......................
ii::•::!::i::•??•
.......
•..................
iil!!iiiiiiii::..
':i':i,.'::::..
"i:i:i:i;i
..:ii!!i
:•(•
:'
10
iii,,i:ii!ii
:.:.:.:.:::::::::::
•::iii'i::i::
•!!!!!!!•
-
i
15
Mantle
I
I
40
!
i '*•-......
• 'F,% PervaSive
Faulting
oftheCrust
I
!
I
!
I
"• Serpebtinized
Upper
mantle
80
I
I
120
I
I
160
I
200
Distance (km)
Figure11. Summary
of results
alonglineR2 transversely
crossing
theextinctspreading
centerin the
Labrador
Sea.(a) Crustal
thickness
at eachmantleboundary
nodein the2-D seismic
modeling
and(b) the
suggested
relationship
between
thecrustal
thinning,
low crustal
P wavevelocities
andhydrothermal
alteration
of theuppermantledueto faultingandreduced
partialmeltsupplyat theslowspreading
rates
precedingthe extinctionof the spreadingcenter.
Louden, 1992]) may be explainedin termsof the regional Nature and Origin of the Upper Mantle
variabilityin the depthto whichhydrothermal
circulation
has
penetrated
andalteredtheuppermanfie.
However,
thepresence Low velocitiesare observedin the uppermantle in four of
of velocitygradients
in the uppermantlebeneaththe Moho the sevenrefractionprofilesshoton line R1 alongthe strike
requiresa furthercomplexity.
Two stagesof hydrothermalof the extinctspreadingcenter[Oder andLouden,1992].The
alterationwouldneedto havebeenoperative:onethatis quite averageuppermantlevelocityis 7.74 km/s, whichis lessthan
vigorousandshortlived to establish
the distinctboundary the velocities of 7.92, 7.82, and 7.94 km/s determined for the
interpreted
astheMohoanda second
thatis moreprotracted
to threeuppermantlenodeswithin the extinctspreadingcenter
alter rocks below this boundary.While this mechanismis
in this study(Table 2, electronicsupplement).
This difference
largelyspeculative,
multiplestages
of hydrothermal
alteration in velocity may be attributedto azimuthalanisotropyin the
arerequired
to explain
thepetrology
of gabbroic
rocksdredged mantle [MacKenzie, 1972; Bibee and Shor, 1976].
The low velocities are interpreted as resulting from the
from the failed MathematicianRidge in the easternPacific
Ocean[Stakesand Vanko,1986]. An initial high-temperature hydrothermal alteration of upper mantle peridotire. The
vaporphaseis purported
to be followed
by a morepervasive pathway for hydrothermalalteration of the upper mantle
(Figure 11) couldbe providedby faultsextendingthroughthe
lower-temperature
brinephase.
A crustallayerwhichis discretefrom the mantleis more
easilyexplained
by theigneous
modelin whichthevelocity
of the crustis loweredby pervasive
fracturingandthinnedby
increasedtectonismand decreasedpartial melt supply.The
faults providea pathwayfor fluids to penetrateinto the
uppermost
mantleand alter the peridotite(Figure11). A
preexisting
Moho is thusoverprinted
by one episodeof
hydrothermalalteration.Discriminationbetweenthese
crust. No more than 10% of the peridotire must be
serpentinized
to producethe observedvelocities[Christensen,
1966]. There is evidence for hydrothermal alteration and
serpentinization
at oceanicfracturezones[e.g., Calvert and
Potts, 1985; Christensen, 1972; Minshull et al., 1991].
Hydrothermalalterationis called upon by Calvert and Potts
[1985] to explain the widespreadoccurrenceof low upper
mantle
velocities
in
old
Atlantic
fracture
zones
and
an
alternatives
wouldrequireVp/Vsratios
for thecrustal
level apparentdecreasein upper mantle velocity in fracturezones
with increasingage. Given the similarity betweenthe crustal
structures
observed within the extinct spreading center to
arenot well developed
andonlyshowarrivalsfor the mantle
those at oceanic fracture zones [Oder and Louden, 1992] and
refraction, Sn.
material. S-waves are observedon some OBSs; however, they
2276
OSLER AND LOUDEN:
EXTINCT
SPREADING
the likelihood of faults penetratingthe entire crust at the slow
spreadingrates preceding extinction, the extinct spreading
center is an environmentin which uppermantle peridotitcare
liable to be serpentinized.
Comparison to Crustal
Spreading Centers
Processes at Active
Slow
At the Mid-Atlantic Ridge, recent studies suggest that
within a ridge segment there is considerable along-axis
variation in the thermal and mechanical properties of the
lithosphere.The studiesinclude along axis seismicrefraction
[Purdy and Derrick, 1986], high-resolution multibeam
bathymetry mapping [Sempere et al., 1990], multichannel
seismic reflection [Derrick et al., 1990], 3-D gravity studies
[Kuo and Forsyth, 1988; Linet al., 1990], 'microearthquake
source locations and delay time tomography[Tooracyet al.,
1988; Kong et al. 1992], and geochemicalstudies[White et
CENTER
IN LABRADOR
SEA
extinctspreadingcenterin the LabradorSea.Informationfrom
12 kHz bathymetrydata, seismicreflection data, and plane
layerand'r-p 1-D traveltime solutions
wereusedto developan
initial velocity-depth structure. The final velocity-depth
structurewas obtainedby revising this initial velocity-depth
structureto improve(1) the fit betweenarrivaltimescalculated
by ray tracingand their observedcounterparts;
and (2) the fit
between amplitude patterns in synthetic and observed
refraction profiles.
Crustwithin the extinctspreadingcenteris foundto be thin
andof anomalously
low P wavevelocitywhencomparedwith
mature oceanic crust. It is underlain by low upper mantle
velocities which may exhibit some anisotropy. The low
crustalvelocities are attributedto an increasein the degreeof
tectonismprecedingextinction, which pervasivelyfractured
the crust(Figure 11). This faultingalsoprovidesa pathwayfor
hydrothermalalterationof the uppermantleperidotitcand is
al., 1992].
the manner by which the low velocities in the uppermost
An important consensusemerging from these studies at mantle are produced.The lateral extent of the thinning and
slow spreadingcentersis that crustalaccretionis an episodic occurrenceof low-velocity crust are coincidentwith the time
and not a steady state processand crustal accretionmust be
interval in which spreadingrate in the LabradorSea decreased
focused at the center of the ridge segment [Ballard and most rapidly (Chron 21 until the cessationof spreading).This
Francheteau, 1983; Solomonand Toorney,1992; White et al.,
observation supports a systematic relationship between
1984]. As there is no seismic evidence supporting the spreadingrate, faulting, hydrothermalcirculation,and partial
presence of steady state axial magma chambers at slow melt supply at active spreadingcenters.A subsequentpaper
spreadingcenters [Derrick et al., 1990], magm•ifismat the will extend the seismic results presentedin this paper by
crustal level is consideredto be an intermittent process. consideringgravity models which are basedon the proposed
Consequently,
plate divergenceat slow spreadingcentersis seismicvelocity-depthstructureand plausiblespreadingrate
partially accommodatedthrough lithospheric stretching historiesfor the Labrador Sea basedon modelingof magnetic
[Macdonaldand Luyendyk,1977]. Along-axis,ridge segment anomalies.
ends appear to experiencingmore tectonismthan the axial
high wherecrustalaccretionis focused[Huangand Solomon, Acknowledgments. We would like to thank the officers and crew of
1988; Lin and Bergman, 1990]. The ends of the ridge
segmentsare cooler than any other portion of the ridge
segmentbecauseof their proximity to the cold lithosphere
againstwhich they are juxtaposed.Consequently,
they are
mechanicallystrongerare more prone to brittle failure than
otherportionsof the ridge crest.The endsof ridge segments
are also areas where the production of partial melt supply
and/or its transportis most severelylimited [Phipps Morgan
and Forsyth, 1988].
the C.S.S.Hudson,BordenChapman,Guy Fehn,andNeil Hamilton,for
their technicalexpertiseduringthe courseof the seismicrefraction
experiment.The analogrefractiondata werereplayedand digitized
usingcomputing
facilities
attheBedford
Institute
of Oceanography
with
the assistance
of StevePearyand Ian Reid. The development
of
processing
algorithms
for therefraction
datawasassisted
in itsinfancy
by GregLegerandthestaffof theDalhousie
University
Computing
and
Information
Servicesandmorerecentlyby DepingChianandSaeid
Yazdanmehr.The researchwas supported
by Energy,Mines and
Resourcesgrant 30/04/87 and Natural Scienceand Engineering
ResearchCouncilof CanadagrantsOGP0008459,EQP0027418,and
INF0001469.J.C.O.acknowledges
financialsupport
providedby a
fellowshipfrom DalhousieUniversity.Comments
on the original
manuscript
by J. Austin,D. Hutchinson,
andananonymous
reviewer
are
The processeswhich occur during the extinction of a
spreadingcenter have an analog in the spatial relationship
betweenthe center of a ridge axis and its fracturezones.The
parallels are (1) the presenceof brittle crust due to thermal
regime, hot and cold lithospherejuxtaposedat a fracturezone greatlyappreciated.
and a dissipatingthermalanomalyat a dying spreadingcenter;
References
(2) the effectsof tectonismwhich thin the britfiecrustdirectly
through crustal extension accommodatedby rotated crustal Anderson,R.N., andJ.G. Sclater,Topography
andevolutionof the East
PacificRisebetween5øSand20øS,EarthPlanet.Sci.Lett.,14, 433blocks or indirectly by reducingpartial melt supply;and (3)
441, 1972.
the provision of a pathway for hydrothermalalterationof the
Geologicprocesses
of the mid-ocean
upper mantle. At the extinct spreadingcenter,the decreasesin Ballard,R. D., andJ. Francheteau,
ridge and their relation to sulfide deposition,in Hydrothermal
spreadingrate which precede the extinction of a spreading
Processes
at SeafloorSpreadingCenters,editedby P. A. Ronaet al.,
center enhancecooling of the crust by conduction.This leads
to an increase
in tectonism
as the crust becomes britfie.
The
increased tectonism promotes deeper penetration of
conductive cooling which restricts partial melt supply.
Serpentinizafionof the upper manfie ensuesonce the crusthas
been faulted and deephydrothermalcirculationoccurs.
Conclusion
A 2-D seismic velocity-depth structure (Figure •) was
developed for refraction line R2 transverselycrossingthe
pp. 141-168,Plenum,New York, 1983.
Batiza,R., Failed rifts, in The Geologyof North America,vol. N, The
EasternPacific Oceanand Hawaii, editedby E. L. Winrefer,D. M.
Hussong,and R. Decker, pp. 177-186, Geological Society of
America,Boulder,Colo., 1989.
Bibee,I• D., andG. G. J. Shot,Compressional
waveanisotropy
in the
crustanduppermanfie,Geophys.
Res.Lett.,3, 639-642,1976.
Bown,J. W., andR. S. White, Variationwith spreading
rateof oceanic
crustalthickness
andgeochemistry,
EarthPlanet.Sci.Lett., 121,435449, 1994.
Calvert, A. J., and C. G. Potts, Seismicevidencefor hydrothermally
OSLER AND LOUDEN: EXTINCT SPREADING CENTER IN LABRADOR SEA
alteredmantle beneaththe old crustof the Tydemanfracturezone,
Earth Planet. Sci.Lett., 75, 439-449, 1985.
2277
seismotectonicsof the Vema fracture zone, Atlantic Ocean, J. Geol.
Soc.London,143,795-805, 1986.
Cerveny,V., I. Molotkov,andI. Psencik,Ray Methodin Seisinology, Macdonald, K. C., and B. P. Luyendyk, Deep-tow studiesof the
Universityof Karlova,Prague,Czechoslovakia,
1977.
structure
of the Mid-AtlanticRidgecrestnearlat 37øN,Geol. Soc.
Am. Bull., 88, 621-636, 1977.
Chapman,
C. H., 1985,Raytheoryanditsextensions:
WKBJandMaslov
seismograms,
J. Geophys.,
58, 27-43,1985.
MacKenzie, D. B., Peridotitefabricsand velocity anisotropyin the
Christensen,
N. I., Elasticityof ultrabasicrocks,J. Geophys.Res., 71,
Earth's mantle, Mem. Geol. Soc. Am., 132, 593-603, 1972.
5921-5931, 1966.
Mammerickx,J., andD. Sandwell,Riftingof old oceaniclithosphere,
J.
Geophys.Res.,91, 1975-1988, 1986.
Christensen,
N. I., The abundance
of serpentinites
in the oceaniccrust,
J. Geol., 80, 709-719, 1972.
Denham,C. R., andH. Schouten,On the likelihoodof mixedpolarityin
oceanicbasementdrill cores,in Implicationsof Deep Drilling Results
in the Atlantic Ocean: Ocean Crust, Maurice Ewing Ser., vol. 2,
editedby M. Talwaniet al., pp. 160-165,AGU, Washington,D.C.,
1979.
Derrick, R. S., J. C. Mutter, P. Buhl, and I. Kim, No evidencefrom
multichannel reflection data for a crustal magma chamberin the
MARK areaon the Mid-Atlantic Ridge,Nature, 347, 61-64, 1990.
Minshull, T. A., R. S. White, J. C. Mutter, P. Buhl, R. S. Detrick, C. A.
Williams,andE. Morris,Crustalstructure
at theBlakeSpurFracture
Zone from expandingspreadprofiles,J. Geophys.Res.,96, 99559984, 1991.
Nagumo,S., T. Ouchi,andS. Koresawa,Seismicvelocitystructurenear
the extinct spreadingcenterin the ShikokuBasin,North Philippine
Sea, Mar. Geol.,35, 135-146, 1980.
Osier, J. C., Crustal structureof the extinct spreadingcenterin the
LabradorSea:Implicationsfor dynamicmodelsof flow beneathmid-
ocean ridges, Ph.D. thesis, Dalhousie Univ., Halifax, Nova Scotia,
Drake, C. L., N.J. Campbell,G. Sander,and J. E. Nafe, A mid1993.
LabradorSea ridge,Nature, 200, 1085-1086,1963.
Hess,H. H., Historyof oceanbasins,in PetrologicStudies:Buddington Osler, J. C., and K. E. Louden, Crustal structureof an extinct rift axis in
the Labrador Sea: Preliminary results from a seismic refraction
Volume,pp.599-620,Geological
Societyof America,Boulder,Colo.,
1962.
Hinz, K., H. U. Schlfiter,A. C. Grant,S. P. Srivastava,D. Umpleby,and
J. Woodside,
Geophysical
transects
of theLabradorSea:Labrador
to
southwest
Greenland,Tectonophysics,
59, 151-183, 1979.
Huang,P. Y., and S.C. Solomon,Centroiddepthsof mid-oceanridge
earthquakes:
Dependence
on spreading
rate,J. Geophys.Res.,93,
13,445-13,477, 1988.
Hyndman,R. D., Evolutionof the LabradorSea,Can.J. Earth Sci.,10,
637-644, 1973.
Jarrard,R. D., K. A. Dadey, andW. H. Busch,Velocity and densityof
sediments
in the Eirik Ridge,LabradorSea:Controlby porosityand
mineralogy,
Proc. OceanDrill. Program,Sci.Results,105, 811-835,
1989.
Kent, D.V., and F. M. Gradstein,A Jurassicto recentchronology,in The
Geology of North America,vol. M, The WesternNorth Atlantic
Region, edited by P.R. Vogt and B. E. Tucholke,pp. 45-50,
GeologicalSocietyof America,Boulder,Colo., 1986.
Klitgord, K. D., and J. Mammerickx, Northern East Pacific Rise:
Magnetic anomalyand bathymetricframework,J. Geophys.Res.,
87, 6725-6750, 1982.
survey,Earth Planet. Sci. Lett., 108, 243-258, 1992.
Phipps Morgan, J., and D. Forsyth, Three-dimensionalflow and
temperatureperturbationsdue to a transform offset: Effects on
oceaniccrustaland upper mantle structure,J. Geophys.Res.,93,
2955-2966, 1988.
Purdy, G. M., and R. S. Detrick, Crustal structureof the Mid-Atlantic
Ridgeat 23øNfromseismic
refraction
studies,
J. Geophys.
Res.,91,
3739-3762, 1986.
Purdy, G. M., L. S. L. Kong, G. L. Christeson,and S.C. Solomon,
Relationshipbetween spreadingrate and the seismicstructureof
mid-oceanridges,Nature,355, 815-817, 1992.
Reid,I., andH. R. Jackson,
Oceanspreading
rateandcrustalthickness,
Mar. Geophys.Res.,5, 165-172, 1981.
Roest,W. R., andS. P. Srivastava,
Sea-floorspreading
in the Labrador
Sea:A new reconstruction,
Geology,17, 1000-1003, 1989a.
Roest,W. R., and S. P. Srivastava,Seafloorspreadinghistory,I,
Magneticanomaliesalongtrack, in East CoastBasinAtlas Series:
LabradorSea,coordinated
by J.S.Bell, p. 98, GeologicalSurveyof
Canada, Atlantic Geoscience Centre, Dartmouth, Nova Scotia,
1989b.
Kong, L. S. L., S.C. Solomon,and G. M. Purdy, Microearthquake Roots,W. D., and S. P. Srivastava,Origin of marinemagneticquiet
characteristics
of a mid-oceanridge along-axishigh, J. Geophys.
zonesin the Labradorand GreenlandSeas,Mar. Geophys.Res.,6,
Res.,97, 1659-1686, 1992.
395-408, 1984.
Kristoffersen,Y., and M. Talwani, Extinct triple junction southof
Greenlandand the Tertiary motion of Greenlandrelative to North
Salisbury,M. H., andN. I. Christensen,
The seismicvelocitystructure
of
a traverse through the Bay of Islands ophiolite complex,
America, Geol. Soc.Am. Bull., 88, 1037-1049, 1977.
Newfoundland,an exposureof oceaniccrustand uppermantle,J.
Geophys.Res.,83, 805-817, 1978.
Kuo, B., and D. Forsyth, Gravity anomaliesof the ridge-transform
systemin the SouthAtlanticbetween31 and34.5øS:Upwelling Sempere,J., G. M. Purdy, and H. Schouten,Segmentationof the MidAtlanticRidgebetween24øN and30ø40'N,Nature,344, 427-431,
centersandvariationsin crustalthickness,
Mar. Geophys.Res.,10,
205-232, 1988.
Lewis, B. T. R., The East Pacific Rise and the thermal model, J.
Geophys.Res.,88, 3348-3354, 1983.
Lewis, S. D., and D. E. Hayes, The structureand evolution of the
1990.
Solomon,S.C., and D. R. Toomey,The structureof mid-oceanridges,
Annu.Rev. Earth Planet. Sci., 20,329-364, 1992.
Spudich,P., andJ. Orcutt,A new look at the seismicvelocitystructure
of the oceaniccrust,Rev. Geophys.,18, 627-645, 1980.
CentralBasinFault, West PhilippineBasin,in The Tectonicand
GeologicEvolutionof Southeast
AsianSeasand Islands,Geophys. Srivastava,S. P., Evolutionof the LabradorSea andits beatingon the
earlyevolutionof the North Atlantic,Geophys.J. R. Astron.Soc.,52,
Monogr.Ser.,vol. 23, pp.77-88,AGU, Washington,
D.C., 1980.
313-357, 1978.
Lewis, B. T. R., andW. E. Snydsman,Fine structureof the lower crust
Srivastava,S. P., R. K. H. Falconer, and B. MacLean, Labrador Sea,
on the CocosPlate,Tectonophysics,
55, 87-105, 1979.
Davis Strait,Baffin Bay: geologyand geophysics-A
review,Mem.
Lin, J., and E. A. Bergman,Rift grabens,seismicity,and volcanic
Can. Soc.Pet. Geol., 7, 333-398, 1981.
segmentationof the Mid-Atlantic Ridge: Kane to Ariantis fracture
zones(abstract),Eos Trans. AGU, 71, 1572, 1990.
Lin, J., G. M. Purdy,H. Schouten,
J. C. Sempere,and C. Zervas,
Evidencefrom gravitydatafor focusedmagmaticaccretionalong
theMid-AtlanticRidge,Nature,344, 627-632, 1990.
Loncarevic,B. D., Oceanbottomseismometry,
in CRC Handbookof
Geophyscial
Exploration
at Sea,editedby R.A. Geyer,pp.219-263,
CRC Press,BocaRaton, Fla., 1983.
Louden,K. E., R. S. White,C. G. Potts,andD. W. Forsyth,Structure
and
Srivastava, S. P., C. G. Powell, A. G. Nunns, L. C. Kovacs, D. G.
Roberts,M. T. Jones,C. Uruski,andD. Voppel,Magneticanomalies
of total intensity,in GeophysicalAtlas of theNorth AtlanticBetween
50øto 70•Bland0 øto 65øW,editedby S. P. Srivastava,
D. Voppel,
and B. Tucholke,pp. 3-4, DeutschesHydrographisches
Institut,
Hamburg, 1988.
Srivastava, S. P., K. E. Louden, S. Chough, D. Mosher, B. D.
Loncarevic, P. Mudie, A. de Vernal, and B. MacLean, Results of
2278
OSLER AND LOUDEN: EXTINCT
SPREADING CENTER IN LABRADOR
detailedgeologicaland geophysical
measurements
at ODP sites645
in Baffin Bay and 646 and 647 in the LabradorSea,Proc. Ocean
Drill. Program,Sci.Results,105, 891-922, 1989.
Srivastava,S. P., C.E. Keen, and P. Potter, Crustal extensionin the
Labrador Sea: Evidence from multichannel seismic reflection data
(abstrac0,Eos Tram. AGU, 74, Fall Me•ting suppl.,605, 1993.
Stakes, D., and D. A. Vanko, Multistage hydrothermalalterationof
gabbroicrocksfrom the failed MathematicianRidge, Earth Planet.
Sci. Lett., 79, 75-92, 1986.
Stergiopoulus,
A. B., Geophysical
crustalstudiesoff the SW Greenland
margin,M. Sc. thesis,DalhousieUniv., Halifax, Nova Scotia,1984.
SEA
Weissel,J. K., and A. B. Watts,Tectonicevolutionof the Coral Sea
Basin,J. Geephys.
Res.,84, 4572-4582,1979.
White,R. S., R. S. Detrick,M. C. Sinha,andM. H. Corntier,Anomalous
seismiccrustalstructureof oceanicfracturezones,Geephys.J. R.
Astron.Sec., 79, 779-798, 1984.
White, R. S., D. P. McKenzie, and R. K. O'Nions,Oceancrustal
thickness from seismic measurements and rare earth element
inversions,
J. Geephys.
Res.,97, 19,683-19,716,1992.
Woodside,
J.,Geophysics
II, Gravityanomaly,
in EastCoastBasinAtlas
Series:Labrador Sea, coordinated
by J.S. Bell, p. 94, Geological
Surveyof Canada,AtlanticGeescience
Centre,Dartmouth,
Nova
Scotia, 1989.
Talwani, M., and O. Eldholm, Evolution of the Norwegian-Greenland
Zelt, C. A., andR. B. Smith,Seismictraveltime
inversion
for 2-D crustal
Sea, Geol. Sec. Am. Bull, 88, 969-999, 1977.
velocitystructure,
Geephys.
J. Int., 108, 16-34,1992.
Tomoda, Y., K. Kobayashi,J. Segawa,M. Nomura, and K. KJmura,
Linear magnetic anomalies in the Shikoku Basin, northeastern
PhilippineSea,J. Geomagn.Geoelectr.,28,47-56, 1975.
K. E. Louden,Departmentof Oceanography,
DalhousieUniversity,
Toomey, D. R., S.C. Solomon,and G. M. Purdy,Microearthquakes
Halifax, Nova Scotia,CanadaB3H 4J1. (e-mail: keith•papa.ocean.
beneaththe median valley of the Mid-Atlantic Ridge near 23 øN:
dal.ca).
Tomographyandtectonics,J. Geephys.Res.,93, 9093-9112,1988.
J. C. Osier, Defence ResearchEstablishmentAtlantic, Dartmouth,
Van der Linden,W. J., Crustalattenuation
andseafloorspreading
in the
Nova
Scotia,CanadaB2Y 3Z7. (e-mail: osler•maggie.drea.dnd.ca)
Labrador Sea, Earth Planet. Sci. Lett., 27, 409-423, 1975.
Vogt, P. R., L. C. Kovacs,C. Bernere,and S. P. Srivastava,Asymmetric
geophysicalsignaturesin the Greenland-Norwegianand southern
LabradorSeas and the Eurasiabasin, Tectonophysics,89, 95-160,
1982.
Weissel, J. K., and D. E. Hays, Evolution of the Tasman Sea
reappraised,
Earth Planet.Sci.Lett.,36, 77-84, 1977.
(Received February 21, 1994; revised October 31, 1994;
accepted November 2, 1994.)