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01 Oct 1997
S K S Splitting beneath Continental Rift Zones
Stephen S. Gao
Missouri University of Science and Technology, sgao@mst.edu
Paul M. Davis
Kelly H. Liu
Missouri University of Science and Technology, liukh@mst.edu
Philip D. Slack
et. al. For a complete list of authors, see https://scholarsmine.mst.edu/geosci_geo_peteng_facwork/97
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Recommended Citation
S. S. Gao et al., "S K S Splitting beneath Continental Rift Zones," Journal of Geophysical Research, vol. 102,
no. B10, pp. 22781-22797, American Geophysical Union (AGU), Oct 1997.
The definitive version is available at https://doi.org/10.1029/97JB01858
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B10, PAGES 22,781-22,797,OCTOBER 10, 1997
$K$ splitting beneath continental rift zones
S. Gao,1 P.M. Davis,H. Liu,2 P. D. Slack,andA. W. Rigor
Department of Earth and SpaceSciences,University of California, Los Angeles
Y. A. Zorin, V. V. Mordvinova,V. M. Kozhevnikov,and N. A. Logatchev
Institute of the Earth's Crust, Siberian Branch, Russian Academy of Sciences,Irkutsk
Abstract. We presentmeasurements
of $KS splitting at 28 digital seismicstations
and 35 analog stations in the BaikM rift zone, Siberia, and adjacent areas, and at
17 stations in the East African Rift in Kenya and compare them with previous
measurements
from the Rio Grande
Rift of North
America.
Fast directions
in the
inner region of the Baikal rift zone are distributedin two orthogonaldirections,
NE •nd NW, •pproxim•tely p•r•llel •nd perpendicularto the NE strike of the rift.
In the adjacent Siberian platform and northern Mongolianfold belt, only the riftorthogonalfast directionis observed.In southcentralMongolia,the dominantfast
direction changesto rift-parMlel again, althougha small numberof measurements
are still rift-orthogonal. For the axial zonesof the East African and Rio Grande
Rifts, fast directionsare orientedon averageNNE, that is, rotated clockwisefrom
the N-S trending rift. All three rifts are underlain by low-velocityupper mantle
as determinedfrom teleseismictomography. Rift-related mantle flow providesa
plausibleinterpretation for the rift-orthogonMfast directions. The rift-parallel fast
directionsnear the rift axes can be interpreted by oriented magmatic cracksin
the mantle or small-scalemantle convectionwith rift-parallel flow. The agreement
betweenstressestimatesand correspondingcrackorientationslendssomeweight to
the suggestion
that the rift-parMlelfast directionsare causedby orientedmagmatic
cracks.
flow,and of volcanism,
magma-filled
orientedcracksin
the mantlecangiveriseto the observed
polarization.
A recent review of polarization anisotropymeasured
Polarizationanisotropydeterminedby SKS splitting
on the continents[Silver,1996]concludes
that in trans- isdeveloping
asa toolto examine
finitestraincaused
by
pressireregionsanisotropyis causedby vertically co- flowin the uppermost
mantle[Fuchs,1977;Andoet al.,
1.
Introduction
herent
deformation
of the mantle
with
fast directions
parallel to the compressional
features. While this behavior is observedfor compressireregionssuch as the
Alps, Tibet, and the Tien Shan, the sameprocessoperating in regionsof extensionis expectedto generatefast
directionsparallel to the extension.Contrary to this expectation,we presentresultsfrom three continentalrift
zones,the Rio GrandeRift (RGR), the Baikalrift zone
1983; Silver and Chan, 1988, 1991; Vinnik et al., 1989,
1992; Savageet al., 1990; Silver, 1996]. The earliest
indicationsthat seismicanisotropyis related to mantle
flow were observationsof fast mantle velocitiesperpen-
dicular to mid-oceanridges[Hess,1964; Raitt et al.,
1969; Forsyth,1975]. Azimuthal dependence
of velocities of long-periodRayleigh waveshas been observed
by Tanimotoand Anderson[1985]and Montagnerand
(BRZ) andthe EastAfricanRift (EAR) wherefast di- Tanimoto[1991],whonotedthat the globalmap of fast
rectionsare observedorthogonalto the extension. We phasevelocitiescorrelateswith absoluteplate motions.
proposethat a plausibleexplanationfor theseobserva- Subsequently,numerousobservationshave been made
tionsis that in regionsof mantleextension,of highheat of SKS splitting in diversetectonicsettings(see re-
viewsby Vinniket al. [1992]and Silver[1996]). Vinnik
•Now at Departmentof TerrestrialMagnetism,Carnegie et al. [1992]report singlestationmeasurements
from
Institution of Washington, D.C.
rift zones such as the Red Sea rift, the Baikal rift, the
2Also at Department of Terrestrial Magnetism, Carnegie continentalextensionof the Arctic ridge, and the RhineInstitution of Washington, D.C.
graben. They concludethat in regionsof rifting, the
inferred
Copyright 1997 by the American GeophysicalUnion.
Paper number 97JB01858.
0148-0227/ 97/ 97JB-01858509.00
directions
fast directions
of flow in the mantle from the
are close to the extension
directions
in
the crust. However, for the Rio Grande and the French
Massif Central, they find that the fast directionsare ori22,781
22,782
GAO ET AL.: $KS SPLITTING BENEATH CONTINENTAL RIFT ZONES
ented nearly 50o to the direction of extension. Recent with the a, b, and c axesof olivine becomingaligned
measurements
of near ridgesplittingin Iceland[Bjar- with the longest, shortest, and intermediate axes of
nasone! al., 1996]givefast directionsthat are closerto the finite strain ellipsoid.McKenzie's[1979]approach
waspursuedby Ribe[1992]who developed
orientation
parallel to the ridge than perpendicularto it. In this paper we examine about 80 continentalrift zone measurements in order to addressfurther the questionwhether
in regionsof continental extensionfast directionsare
parallel or perpendicularto the extensiondirection.
$K$ splitting can be used to characterizeseismic
anisotropybeneatha seismicstation[e.g.,Kind e! al.,
1985; Silver and Chan, 1988, 1991; Vinnik e! al., 1989;
Savage e! al., 1990; Makeyeva e! al., 1992; Silver and
Kaneshima, 1993; Gao e! al., 1994a; Liu e! al., 1995;
Gao, 1995;Silver, 1996]. The azimuthalanisotropyof
distribution
formulas for deformation-induced
LPO of
Ribe and Yu [1991]. Accordingto the theory,under
uniaxial compression,the a axis of olivine turns to be
perpendicularto the maximum compressional
strain direction; under pure shear, it becomesperpendicularto
the shorteningdirection; and under progressive
simple
shear, it aligns in the flow direction. This interpretation has been supported by numerousobservationsof
anisotropictextures in deformedmantle rockscomplemented by measurementsand theoretical estimatesof
the mantle is readily detectedby birefringenteffectson
$K$ phasesfrom earthquakeswith epicentral distance
greater than 85ø. The $K$ phasetravelsas an $ wave
in the crust and mantle and as a P wave through the
liquid outer core. At the core-mantleboundary on the
receiver side, the $ wave is radially polarized. If this
shear wave encountersanisotropicmaterial on its path
to the receiver, it may split into two different polarizations having differentvelocitiesdenotedfast and slow.
The splitting parametersare the polarizationof the fast
their seismicanisotropy[Hess,1964; Crossonand Lin,
P wave has the intermediate velocity with Ve - 8.65
cracks.
1971; Christensen,1971, 1984; Baker and Carter, 1972;
Nicolas et al., 1973; Peselnick et al., 1974; Christensen
and Salisbury,1979; Christensen,1984; Mercier, 1985;
Nicolas and Christensen,1987; Silver and Chan, 1991;
Chastelet al., 1993].
A continentalrift is defined as an elongatedtopographic depressionoverlying a place where the entire
lithospherehas been significantlymodifiedin extension
[OlsenandMorgan,1995].In continentalrift zoneswe
split shearwave• (measuredclockwise
fromthe north) wouldexpect that if the entire lithosphereis extended
and the travel time difference between the fast and slow
the a axes and thereforethe splitting fast directions
shear wavesSt. The method used to calculate the opti- would clusterin the directionof rift opening. Some
mal splitting parametersand their standarddeviations commonfeatures of a typical continentalrift zone inand the
wasdescribed
extensivelyby Silverand Chan[1991]. clude(1) the centralpart of rift valleysubsides
It is generallybelievedthat the main causefor $K$
edgesof the adjacentblocksare uplifted;(2) flanking
splitting in the mantle is the lattice preferredorienta- normalfaults;(3) negativeBouguergravityanomalies;
tion (LPO) of crystallographicaxes of elastically (4) higherthannormalheatflow;(5) shallow,
tensional
anisotropicmineralsin the upper mantle [e.g., Hess, andhigherthan normalseismicity;
and (6) thinningof
1964; Francis, 1969; Karato, 1989; Savageet al., 1990; the crustbeneaththe rift valley[TurcotteandSchubert,
Babuskaand Cara, 1991; Silver and Chan, 1991; Chas- 1982; Fowler,1990]. All thesefeaturesmark the initel et al., 1993; Silver, 1996]. Both olivine and or- tial stagesof continentalbreakupwherethe lithosphere
thopyroxeneare highly anisotropicupper mantle con- pulls apart and upwellinghot mantle risesbeneaththe
stituent minerals. Mantle peridotirestypically contain spreadingrift to eventually form oceaniclithosphere
morethan 65%olivineand 20%orthopyroxene
[Ander- and an ocean basin.
son,1989].Sinceorthopyroxene
is not asabundantand
The mechanisms
of continentalrifting can be sepanisotropicas olivine, it is generallyassumedthat the arated into two end-memberscalledpassiveand active
upper mantle beneath a station is composedof a certain rifting. For passiverifting the entirelithosphere
extends
percentageof oriented olivine. The rest is assumedto beneaththe rift zone. For this case,LPO is expected
be randomlyorientedolivine,orthopyroxene,and other to alignin the directionof extension.Activeriftinginmantle minerals.
It must be mentioned that in realvolveserosionof the lithosphereby mantle convection
ity the alignmentof orthopyroxenemay reduceoverall causingthe asthenosphereto upwarp beneath the rift
anisotropycausedby olivine alignment,sinceits fast axis. If a fabric developsin the asthenosphere,
its direcaxis doesn'talign parallelto olivinefast axisduringde- tion will depend on the details of the convection.Two
formation[Christensen
andLundquist,
1982].
mechanisms
for the developmentof anisotropyin upFor an individual olivine crystal, the fastest P wave wellingasthenosphere
havebeenconsidered
by Kendall
propagationdirectionis alongthe a axiswith Va - 9.87 [1994]for the mid-oceanridgecase.The first involves
km/s; alongthe b axis the P wavehas the slowestve- the preferentialalignmentof fast axesof olivinecrystals;
locity with ¬ - 7.73 km/s; and alongthe c axis the the secondinvolvespreferentiallyalignedmagma-filled
km/s [Verma,1960].The fastest$ wavesare thosepoWe compare$K$ and tomographicmeasurements
larizedalongthe a axis. McKenzie[1979]assumed
that from the three major Cenozoic continental rift areas
under sufficientlyhigh stressand finite strain, seismic (Figure1): theRioGrandeRift (RGR)of NorthAmeranisotropyin peridotiresis controlledby finite strain ica [Sandvol
et al., 1992],the EastAfricanRift (EAR)
GAO ET AL.: SKS SPLITTING
60' •
:' '"• ::•'::;'"
180'
5....
BENEATH CONTINENTAL
-90'
O'
RIFT ZONES
22,783
55'
45' •-•'••••d•t,'
180' 95' 100' 105' 110' 115' 120'
90'
.......
: 35'
.....
_..--•i•i...;.;-4!::
..
.......... .,--•!-:•:-• ;:::t?.-"
t•*' ,.. .
................
".'•*•;,.
"..••.::.•....
':.....
";-":",'""
*-:*;•'•-"';":'::,b.
-5' *
30'
..............
---.
................
-35'
40'
25'
45'
' 15'
-110'
-105'
-100'
-95'
Figure 1. Mercatorprojection
mapsshowing
locations
of the threeriftsandtheirtopography
andseismicity.
(a) Locations
of the 3 riftson a globalmap;(b)-(d) Stationsusedin the study
(solidsquares),
locationof My _>4.5 earthquakes
that occurred
1965-1992
(opencircles),and
topographyfor Baikal rift zone, East African Rift, and Rio Grande Rift, respectively. The
geographicareasin Figureslb, lc, and ld haveapproximatelythe samedimensions.
[Gao et al., 1994c],and alonga 1280km profileacross southernMongolianfold belt. The stationswithin an
the Baikalrift zone(BRZ) usingthe datafroma portable area are numberedfrom north to south, e.g., the northexperiment[Gao et al., 1994a;Gao, 1995]alongwith ernmost station in area A is A01 and the southernmost
new estimates from data provided by the Baikal Seis-
one is All.
mic Network.
The 63'stationsbelongto two seismicnetworks. Stations named 92xx in Table la were from a portable
Program for Array Seismic Studies of the Continen-
2.
Data
and
Results
2.1. Baikal Rift Zone (BRZ)
tal Lithosphere(PASSCAL) experimentconductedin
the summer of 1992 by a field team from the InstiIn this sectionwe extend our previousSKS splitting tute of Earth'sCrust (IEC), Irkutsk,Universityof Wis-
results[Gao et al., 1994a]to includea data set that consin(UW), and Universityof California, Los Anwe recently collectedfrom the permanent seismicarray geles(UCLA). SKS splittingparameterson most of
thesesiteswerepreviouslyreported[Gao et al., 1994a].
operated in the Baikal rift zone.
2.1.1. Data. Station locationsare shownin Figure 2 and listed in Table 1. We separatedthe region
into six subareas which approximately correspondto
the major tectonic provincesof the area. Lake Baikal
sits on the southeastmargin of the Siberian craton in a
regionthat underwentcollisionaltectonicsin the Mesozoic followedby regionalextensionin the Cenozoic.To
the southlies the Mongolianfold be]t, a regionof transpressionthought to be related to the collisionof India
with Asia. Areas A, B, and C comprisethe BRZ with
area A includingthe NE rift valleys,area B including
the central rift valleys,and area C includingthe southern rift valleys;area D is the Siberianplatform, area E
is the northern Mongolianfold be]t, and area F is the
The remaining stations with alphabetical names such
as "boda" belong to the Baikal SeismicNetwork op-
eratedby IEC. For this studythe analog,
seismograms
werescannedand digitized usingthe NXSCAN software
package[Humphreyand Helmberger,1993]. Locations
of the 31 eventsusedin the analysisare shownin Figure
3, and event information is listed in Table 2.
Figures4 and 5 showoriginal SKS signalsbefore
and after correctionfor splittingalongwith their particle motion patterns at station E01 for two events. One
of the evnets(event8) exhibitsexcellentsplitting,the
other(event17) doesnot. Whenthe difference
between
the radial direction of an event-stationpair and fast
SKS direction is near 0ø or 90ø, the anisotropyeffect
22,784
GAO ET AL' SKS SPLITTING BENEATH CONTINENTAL RIFT ZONES
99'
102'
105'
108'
111 ø
114'
117'
120'
I
I
I
I
I
I
I
I
r',,,.o
.:.,..:2•
o
I_
s o
•
.
•
.•,•:...•
....
o
•
.•
'
I
ß
'----'
,
60 ø
80'
lO0 ø 120'
•'/
•
o/-:.-•/•.•..•,,, I
F ø
•o.... •:"L
-?---'."•.•/•?:•i:;•i•!
•.......... n
0'
/,',_
o
""_
o
'•o /
,
i
...........
48'
/
,
45'
140'
leigure
2. A Mercator
projection
map
fortheBaikal-Mongolia
area
showing
locations
ofstations
used
inthestudy,
regional
stress
fields
associated
withBRZ,
and
local
events
thatoccurred
during
thefieldexperiment
(open
cirlcles).
Triangles
areBaikal
Seismic
Network
analog
stations,
and
squares
aredigitalstations
operated
in thesummer
of 1992.Solidsquares
represent
stations
withprevious
SKSsplitting
measurements
[Gaoetal.,1994a].
Arrows
show
regional
stress
fields
oftheBaikal
riftzone
obtained
from
surface
geological
structure
analysis
andearthquake
focal
mechanism
studies
[Sherman,
1992].
Thestudy
region
isdivided
intosixareas:
areas
A,B,
andCaretheBaikal
riftzone
which
isoutlined
bythethick
gray
lines.
Area
AistheNEpartof
theBRZ,areaB isthemainriftzone,andareaC istheSWpartoftheBRZ.AreaD islocated
ontheSiberian
Platform,
areaE isthenorthern
Mongolian
foldbelt,andareaF islocated
on
theMongolian
foldbelt.Thelowerleftinsetshows
coastlines
andnational
borders
ofAsiaand
the locationof the studyarea.
cannot
beobserved
andthemeasurement
isnull[Silver normalizedrosediagramsof the fast directionsfor the
andChan,1991].Threenondistinguishable
possibilities
six areas. The rosediagramsindicatethat thereare
may applyin sucha case:the uppermantlebeneath two dominantdirections.One of the directionsis about
thestationis (1) isotropic;
(2) anisotropic
withanun- 60o:t:20ø, whichis approximately
parallelto thestrike
known
splitting
timeanda fastdirection
parallel
tothe of the surfaceexpression
of the BRZ aswell asthe strike
radialdirection;
and(3) anisotropic
withan unknownof theuppermantlelow-velocity
structureinferredfrom
splitting
timeanda fastdirection
perpendicular
to the traveltimeinversion
[Gao,1995].The second
direction
transverse
direction.
Asdemonstrated
in Figures
4 and isabout140o:t:20ø,which
isapproximately
orthogonal
5, if onlyevent17wereavailable,
theresultfromstation
E01wouldbea nullmeasurement.
Theavailability
of
to the rift axis.
2.1.2.1. Area A:
The peakvalueson the rosedi-
event8, whichis froma differentazimuth,resolves
the agramfor thisareaare60o for the rift-parallel
group
measurement.
and 130o for the rift-orthogonal
group.Of the 11sta2.1.2.Results.Thefinalresults
ofSKSsplittingtionsin thisarea,two(A03,A10)havenullmeasure-
measurements
(Figure6 andTablela) wereobtainedments.
Threeofthefourstations
(A05,A06,A07,A08)
byweighted
averaging
according
to the95%confidence
in therift-parallel
grouparelocated
in themiddlepart
interval
ofeach
individual
measurement.
Thesplitting
of thisarea.Thetwostations
whichbelong
to therift-
ranges
from0.3to2.1swhich
could
have
been
caused
by orthogonalgroupare locatednear the northernboundlayers
of30to210kmthick,respectively,
characterized
ary(A01)andonthesouthern
boundary
(All).
by4%anisotropy.
Although
directions
inthevicinity
of
2.1.2.2.
Area B:
Most of the stations with null
theBRZappear
toberandomly
distributed,
examina-measurements are located within this area. Five stationofthescatter
reveals
some
trends.
Figure
7 showstions(B01,B02,B07,B12,andB15)neartherift axis
GAO ET AL.: $KS SPLITTING BENEATH CONTINENTAL RIFT ZONES
22,785
Table la. $K$ Splitting MeasurementResultsin the Baikal Rift Zone
Station
A01
A02
A03
A03
A04
A05
A06
A07
A08
A09
A10
A10
All
B01
B02
B03
B04
B04
B05
B05
B06
B06
B07
B08
B09
B09
B10
Bll
B12
B13
B14
B15
B15
C01
C01
C02
C03
C04
C05
C05
C06
C07
C07
C08
C09
C09
D01
D02
D03
D03
D04
D05
D06
D07
D07
D08
D08
D09
D10
Dll
Dll
D12
D12
Station
Name
Coordinates
Latitude,
Longitude,
deg
deg
Fast
Direction,
deg
boda
char
57.800
56.900
114.000
118.300
136.0J:13.0
107.9J: 6.0
nely
nely
56.500
56.500
115.700
115.700
29.0J: -119.0J: --
turi
56.400
113.100
86.0 J: 24.0
ozer
56.300
114.000
49.0J:
7.0
tonn
56.300
113.400
64.0J:
8.0
kovo
kala
kumo
uaki
uaki
56.100
55.900
55.900
55.500
55.500
113.100
117.400
111.200
113.600
113.600
57.0J: 9.0
61.0 J: 35.0
161.0 J:24.0
38.5J: -128.5J: --
Splitting
Time,
s
0.70J:0.50
0.82J:0.24
__
__
0.80-1-0.50
0.70-t-0.30
1.40-I-0.30
0.80-1-0.30
0.50-1-0.30
0.70•0.50
1.40J:0.90
0.84J:0.19
0.30J:0.20
0.60J:0.30
tsip
54.900
113.300
120.0J: 6.0
nizh
sbaJ
ulun
solo
solo
55.800
55.600
54.900
54.200
54.200
109.600
109.400
111.200
108.400
108.400
41.3J: 3.2
46.0J: 50.0
76.0 J: 20.0
26.0J: -116.0J: --
ongu
ongu
53.600
53.600
107.600
107.600
21.0:J:-111.0J: --
suvo
suvo
53.600
53.600
110.000
110.000
82.0 J:35.0
15.2J: 5.5
0.40J:0.30
1.80J:0.64
tyrg
52.800
106.300
44.6J: 3.3
9212
51.847
104.893
144.0J:19.0
0.85J:0.12
0.60J:0.40
9250
9250
9233
9224
51.799
51.799
51.541
51.526
106.015
106.015
104.942
105.121
22.0J: -112.0J: -127.0 J: 10.0
148.0 J: 17.0
9235
9221
51.320
51.292
105.761
105.339
45.7J: 8.7
131.5J: 6.3
9222
9223
51.021
50.791
105.682
105.970
137.9J: 6.2
37.6J: 3.8
9223
orli
orli
arsh
mond
t•.la
9270
9270
9272
9271
9271
9237
zaka
zaka
9200
9215
50.791
52.600
52.600
51.900
51.700
51.700
51.336
51.336
51.167
51.153
51.153
50.780
50.400
50.400
55.965
55.560
105.970
99.800
99.800
102.400
101.000
103.600
103.458
103.458
104.407
103.877
103.877
104.089
103.300
103.300
101.410
101.803
129.0J: 7.0
70.0J: 10.0
139.6J: 4.8
92. lJ: 11.9
125.5J: 2.9
128.7J: 4.3
24.0J: -114.0J: -141.0J: 7.1
21.0 :J:
111.0J: -58.8J: 5.5
66.5J: 8.3
127.0J: 12.0
133.0J: 7.0
170.0J: 13.0
9213
55.022
102.055
144.8J:
9213
9203
9204
9205
9206
9206
9207
9207
9209
55.022
54.516
54.193
53.929
53.649
53.649
53.243
53.243
52.778
102.055
102.070
102.649
102.934
103.255
103.255
103.767
103.767
104.105
3.5
144.8J: 3.5
127.5:k 3.7
143.8+ 2.6
149.2J: 8.9
20.0-4- -110.0:J: -20.0J:
110.0J: -137.7J: 4.8
9210
irku
52.622
52.200
104.234
104.300
138.2J: 7.4
25.0J: --
irku
52.200
104.300
115.0J: --
9211
9211
52.169
52.169
104.469
104.469
20.0J: -110.0J: --
__
__
__
__
0.70J:0.30
1.30J:0.30
0.39J:0.17
0.91J:0.25
1.09J:0.14
1.00-4-0.21
1.60J:0.41
0.80-1-0.20
1.00-1-0.17
1.18-1-0.50
1.48-I-0.20
0.82-1-0.20
1.10-1-0.21
__
0.83-1-0.17
0.86-I-0.24
1.70-1-0.50
1.00-1-0.50
0.90-1-0.40
0.82-1-0.07
0.82-t-0.07
1.18-1-0.12
0.72-1-0.09
0.58J:0.15
__
__
__
0.90J:0.16
0.60J:0.13
__
Number
Events
of
22,786
GAO ET AL.' $KS SPLITTING BENEATHCONTINENTAL RIFT ZONES
Table la. (continued)
Station
Station
Coordinates
Name
Fast
Sphtting
Number of
Latitude,
Longitude,
Direction,
Time,
Events
deg
deg
deg
s
E01
tupi
54.400
119.900
101.0-t- 8.0
0.90-t-0.30
1
E02
E02
E03
E04
E05
E06
chit
chit
9280
9291
9282
9283
52.000
52.000
50.193
49.747
49.738
49.288
113.600
113.600
106.254
106.188
106.202
106.412
30.0-t120.0-t- -137.0-t-16.0
145.3-t-16.0
147.24-23.5
134.14- 9.0
1.50-t-0.40
0.78-t-0.14
0.344-0.30
0.374-0.16
9
9
1
4
2
4
F01
hapc
49.700
112.400
71.24- 7.2
0.624-0.15
3
F02
F02
9284
9284
48.931
48.931
106.682
106.682
39.14-10.9
132.04- 9.9
1.044-0.39
0.844-0.57
2
2
F03
F04
F05
F06
F07
F07
F08
F09
9285
9286
9292
9287
9288
9288
9289
9290
48.383
47.921
47.866
47.209
46.635
46.635
46.115
45.262
106.783
106.954
107.051
107.422
107.758
107.758
107.619
108.260
105.24- 5.9
64.44- 4.5
69.24- 6.2
44.24- 6.7
56.24- 9.6
132.24- 3.1
54.74-21.8
84.7 4- 34.4
1.134-0.26
0.754-0.14
0.704-0.13
0.324-0.08
0.404-0.14
1.504-0.19
1.284-0.32
1.424-0.62
4
3
3
4
2
3
3
5
showrift-parallel fast directions. The peak value on the
rose diagram for this area is 50ø, which correspondsto
the rift-parallel group. Most of the other stationshave
fast directionsorthogonalto the rift axis, as evidenced
by the 130o fast direction peak on the rose diagram
trast liessignificantlydeeperthan this,the Fresnelzone
is broader and the transition in the fast directions would
spreadout over a longerdistancethan observed.
2.1.2.3. Area C: The rift-parallel fast directions
are not as commonas the rift orthogonalones,asshown
(Figure 7). The rapid changeof the fast directionsat in the rose diagram for this area. The most common
stationsB12 and B13, which are only 30 km apart, may valuefor the rift-parallel fast directionsis about 600,
indicate that the main part of the sourceof anisotropy and for the rift-orthogonalgroupit is 1300.
2.1.2.4.
Area D: Of the 12 stations in this area,
in the area is shallow, probably at a maximum depth
four
have
null
measurements. The rest of the measureof less than 50 km, as indicated by the Fresnel zone
forwardmodelingof Gao [1995].If the anisotropycon- ments belong to the rift-orthogonalgroup.
Table lb. SKS Splitting MeasurementResultsin the East Africa Rift Zone
Station
Coordinates
Latitude,
deg
Longitude,
deg
Fast
Direction,
deg
Splitting
Time,
s
K04
K07
-0.19
-0.24
35.44
35.75
20.4428.54-
7.3
8.5
0.864-0.21
1.434-0.31
K09
K10
-0.37
0.64
35.92
36.01
20.04- 3.0
32.24-10.3
2.004-0.20
K12
K15
K16
K24
K25
K31
K45
K47
-0.10
-0.62
0.40
-0.17
-0.47
-0.07
0.29
0.24
35.94
36.30
36.29
36.63
36.82
37.25
35.80
36.07
KS0
K51
K52
K53
-0.33
-0.11
0.06
0.12
36.32
36.46
36.67
37.02
26.04-12.0
19.04-30.0
176.04-13.0
35.04-19.0
26.44- 8.1
10.04- 7.0
26.04-12.0
55.04-28.0
22.04-27.0
33.04-20.0
43.04-13.0
10.04- 9.0
1.504-0.40
2.404-0.50
0.904-0.40
1.104-0.70
1.244-0.38
1.904-0.70
2.304-0.40
1.504-0.80
1.904-0.90
1.504-0.40
0.804-0.30
1.204-0.50
K54
-0.40
37.17
172.04-
0.804-0.30
7.0
1.554-0.38
Number
Events
of
GAO ET AL.: SKS SPLITTING
BENEATH CONTINENTAL
RIFT ZONES
22,787
Table lc. SKS Splitting MeasurementResultsin the Rio Grande Rift Zone
Station
Coordinates
Latitude,
Longitude,
deg
deg
Fast
Direction,
deg
Splitting
Time,
s
CZL
ANMO
36.28
34.95
-105.91
-106.46
26.0
41.0
0.9
1.5
SEVI
WTX
WSMR
ELPA
34.27
34.07
32.34
31.77
-106.73
-106.95
-106.49
-106.51
41.0
46.0
11.0
5.0
1.1
1.1
1.2
1.5
From Sandvolet al. [1992].
2.1.2.5.
Area
E:
The dominant
fast direction
is
rift-orthogonal. The peak value in the rosediagram for
this area is 1400 .
2.1.2.6.
Area
F:
The dominant
fast directions
in
this area range from 400 to 800. The fast direction for
the southernmost station is nearly E-W. At the northern part of this area, the transition from the dominant
rift-orthogonal fast directions in area E to the dominantly ENE fast directions in this area takes place over
a distanceof about 50-90 km. Once again, usingthe re-
across
the Tibetan Plateau[McNamaraet al., 1994]and
in the Tien Shaharea[Makeyevaet al., 1992].Both the
Tibetan and the MongolianPlateausand the Tien Shan
area have been deformed by Cenozoic deformation related to the collision between
the Indian
and Eurasian
plates. The observedfast directionsin these areasmay
have the same origin, i.e., the recent continental colli-
sion[Gao et al., 1994a].
sult of Fresnelzoneforwardmodeling[Gao, !995], the
2.2. East African Rift (EAR)
maximum depth of the source of the anisotropy near
Usingdata werecorded
duringthe 1989/1990Kenya
Rift InternationalSeismicProject(KRISP) [Slacket
this transition
is about 200 km.
In Figure 8 we comparethe BRZ resultswith other
clustered measurements
in Asia. The fast direction
for
the southern part of the BRZ profile is roughly ENE,
which is consistent
with
the dominant
direction
found
al., 199d],we found SKS arrivalsat 17 stationssuit-
ableforsplittinganalysis
(Figure9 andTablelb). The
backazimuth
for all of the eighteventsusedin measuring SKS splittinglieswithin a narrowrangeof 600to
0
(A)
31
ß
27
2O
21
.
,
.
24
0
20
50
180
180
Figure 3. Azimuthalequidistant
projection
mapsshowing
epicenters
ofeventsusedin thestudy
of $KS splittingfor (a) BRZ and (b) EAR. The centerof Figure3a is stationBll, and that
of Figure 3b is station K12. The three circlesrepresent45ø,90ø, and 1350epicentraldistances.
Numberson the peripheryindicateazimuthsof eventsrelativeto the centerof map.
22,788
GAO ET AL.: SKS SPLITTING BENEATH CONTINENTAL RIFT ZONES
Table 2a. Events Used in $K$ Splitting Studiesfor the Baikal Rift Zone
Event
Origin
Year
Day
Coordinates
Time, UT
Latitude,
Longitude,
øN
øE
1804
-23.466
-177.297
1453
0308
1237
0411
0003
1021
1811
1906
0128
18.211
-22.293
-18.083
11.866
-25.849
-15.860
-10.562
-20.190
-21.702
-98.240
171.742
-172.492
-13.529
-175.911
-173.643
41.424
178.860
-176.616
Depth,
Back
km
azimuth
Distance,
deg
deg
79
114.12
100.71
72
33
33
11
21
128
10
538
188
23.29
121.58
107.16
299.91
114.63
106.63
242.75
114.96
112.51
107.61
93.72
99.33
97.68
103.35
96.92
82.53
95.95
99.72
99.38
1
2
3
4
5
6
7
8
9
10
1980
1980
1981
1982
1983
1984
1984
1985
1986
1986
104
298
187
153
356
272
289
134
146
303
11
1987
279
0419
-17.940
-172.225
16
106.87
12
1988
282
0446
-18.771
-172.415
35
107.54
99.91
13
1988
284
1820
-28.644
-177.553
27
117.61
104.59
14
15
1989
1990
070
004
0505
0532
-17.766
-15.397
-174.761
-172.850
230
53
108.67
105.74
97.74
97.03
16
1990
050
0648
-15.465
166.385
12
121.96
85.26
17
1990
174
2138
-21.568
-176.483
180
112.33
99.70
18
1991
305
1623
-30.255
-177.981
21
118.96
105.60
19
1992
177
0630
-28.063
-176.735
18
116.65
104.60
20
1992
178
0318
-33.682
-179.076
33
121.97
107.65
21
1992
193
1044
-22.284
-178.507
381
114.27
99.09
22
23
24
25
1992
1992
1992
1992
217
217
238
241
0658
2108
0824
1715
-21.584
-12.023
-20.620
-22.154
-177.322
166.496
-175.151
-179.620
278
109
67
597
112.96
119.97
110.75
115.02
99.22
82.46
99.73
98.36
26
1992
241
1818
-0.996
-13.557
10
292.53
108.25
27
1992
243
2009
-17.738
-178.775
573
111.69
95.36
28
29
30
1992
1992
1992
254
255
259
1043
0357
2104
-22.518
-6.091
-14.122
-175.052
26.680
167.263
39
10
196
111.87
257.19
120.51
101.26
87.58
84.61
31
1992
285
1924
-19.276
168.913
157
122.06
89.75
900 (Table2b). The stationsusedweredistributedin a has the effectof orientingmantlecrystals[Gao et al.,
400by200km E-W elongated
areacentered
at (36.5øE, 1994c].
0øN). Most of the stationswerewithinor very closeto
the rift valley. Fourteenof the 17 stationsanalyzed 2.3. Rio Grande Rift (RGR)
havea fast directionof 30 •: 20ø, whichis subparallel In the RGR study by Sandvolet al., [1992],the six
to the N-S striking rift system.Thus fast directionsare stations used were within or very closeto the rift valcloserto orthogonalto the extensiondirection rather ley. The resultingfast $K$ polarizationdirectionsare
than parallel to it, whichwasexpectedif the extension NNE, i.e., parallelor subparallel
to the rift axis(Figure
Table 2b. Events Used in $K$ Splitting Studiesfor the East Africa Rift
Event
Year
Origin
Day
Time, UT
Coordinates
Latitude,
Longitude,
oN
1
2
3
4
5
6
7
8
1989
1989
1989
1989
1989
1989
1990
1990
328
342
343
350
350
354
048
073
0035
1023
2038
0033
0053
0835
0228
0344
0.989
10.094
0.141
8.431
8.396
8.192
29.533
4.575
Depth,
km
oE
126.007
126.495
123.340
126.942
126.848
126.852
130.732
122.620
Back
deg
25
43
151
36
29
39
65
639
Distance,
azimuth
deg
89.0
88.8
80.0
91.1
89.8
87.6
81.6
91.5
81.7
91.0
81.9
91.0
60.6
93.3
85.4
86.7
GAOET AL.' $KS SPLITTINGBENEATH
CONTINENTAL
RIFT ZONES
I
22,789
i
4
(A)
(B)
2
0
-2
-4
-5
I
o
5
lO
15
(c)
2o
-5
I
10
5
_
øt
_
i
I
i
(D)
-5-
-10
0
o
5
10
15
20
-10
I
I
-5
0
5
10
Time (S)
180 •_•..•1
,,..._f
(,E)
Station' tupi (E01)
120
EQ851341811 (No.08)
BAZ= 254.6 o, Dist= 91.9 o
60
$= 101.0 o, St= 0.9 s
(•_•)=8.0 o, (•_5t= 0.3 s
0
0
1 (•t(5;)2
3
Figure4. Diagrams
showing
example
dataandtheprocedure
forfinding
theoptimal
parameters
forevent
8. (a)theoriginal
seismograms
ofradial
(black)
andtransverse
(gray)
components,
(b)
theirparticle
motion
pattern,
(c) radialandtransverse
components
ofcorrected
seismograms,
(d)theparticle
motion
pattern
ofthecorrected
components,
and(e)thecontour
mapofanerror
function,
theminimum
ofwhich
corresponds
to theoptimal
parameter
pair.Theseismograms
in Figure4cwerecorrected
fromthose
shown
in Figure
4ausing
theoptimal
parameter
pair
indicated
bya dotonFigure
4e. Station
andeventnames,
backazimuth
(BAZ)anddistance
(DIST)oftheevent
relative
tothestation,
aswellastheoptimal
parameters
andtheirerrors,
areindicated
at thebottom
right.The95%confidence
region
isconfined
bya thickgraycontour
line in Figure 4e.
10andTablelc). The splittingtimewasfoundto range 3.
from 0.9 to 1.5 s; no measurementswere made on the
flanksof the rift zone. Westof the rift, in Californiaand
Discussion
We had expectedto find that rift zoneswould have
Nevada,the dominantfast directionis east-west
[e.g., fast directions in the extension direction if the manVinniket al., 1992;SavageandSilver,1993;Liu et al., tle beneaththe riffsextends,
aligning
olivinecrystals.
1995]. The rift-parallelfast directionwasinterpretedHowever,we havefoundthat for the Kenyaand Rio
by Sandvolet al. [1992]to resultfromrift-parallelflow GrandeRiffsandpartsof the Baikalriff zone,fastdiin a small-scale mantle convectioncell beneath the rift.
rectionsare orthogonalto the extensiondirection. For
22,790
GAO ET AL.' $KS SPLITTING
i
BENEATH CONTINENTAL
RIFT ZONES
i
(B)
4
2
_
0
-2
-4
i
o
5
lO
15
-5
2O
f
-5
0
5
1
lO
(D)
5
5
o
0
-5
-5
-lO
I
0
5
10
-
15
-10
2O
-5
0
5
10
Time (S)
!--)
180
Station' tupi (E01)
EQ901742138 (No. 17)
120
BAZ= 123.4 o, Dist= 93.1 o
(1)
= 28.0 o, St= 0.4 s
(•_(1)
= 104.0 o,(•_St=0.6 s
0
1(•t($)2
3
Figure 5. Sameas Figure4, exceptfor event17.
Baikal, fast directions are found in the rift zone that
are both parallel and perpendicular to extension. Interpretation of the S'K$ splitting measurementsis dif-
ficult in that the anisotropymay be a remnantof past
tectonic strain, or caused by mantle flow, or in regionswhere the mantle is melting, couldbe causedby
magma-filledcracks. Below we discussthese possibilities given what has been learned from recentrift zone
tomographicstudiesand work on paleostressand recent
stress directions.
3.1. Tomographic Results: Hot Mantle
beneath
Rifts
1994b;Gao, 1995]to probethe state of the mantlebeneath these rifts. Travel times in the crust determined
from refraction surveyswere removedfrom the data.
The remainingsignalsare causedby broad-scalelowvelocity anomaliesin the tippermostmantle beneath
the rifts whichextendto a muchgreaterdistanceei-
ther side than the surfaceexpression
of rifting. The
P wave velocity perturbations so determinedare-12%
for the EAR,-8% for the RGR, and-5% for the BRZ.
The greatestamountof volcanismis associatedwith the
largestvelocityanomalyin East Africa, aboutan order
of magnitudelessfor the RGR, and lesseramountsin
the vicinity of BRZ. Attenuation measurementsindicate
Tomographicexperimentshavebeenconductedacross that high-frequency
P wavesare preferentiallyabsorbed
the RGR [Davis et al., 1984, 1993;Slacket al., 1996] in the low-velocitymantle anomalies[Haldermanand
the EAR [Slacket al., 1994],and the BRZ [Gao et al., Davis,1991;Slacket al., 1994,1996;Gaoet aL,1994b].
GAO ET AL.' $KS SPLITTING BENEATH CONTINENTAL RIFT ZONES
53ø..•o•
22,791
'•O•o
D
....
ø-
C
99 ø 100 ø 101 ø 102 ø 103 ø 104 ø 105 ø 106 ø 107 ø 108 ø 109 ø 110 ø
A
57 ø
A09A
Ol
A06
"1]•,,
A03
•02
A00•A05:
B01
Bi
"..•.
A10
D03
D04
D05
54 ø
•D06
DO7
:"I•..D08
ß
,
.o
ß
•.o4
•o,•._.:"
........;"-- '....
F02
,'
,'
'•,F03
F04•F05
48 ø-
0.5 1.0 1.5 2.0 sec
!
F
{'.,........"
....'-.
,
F06
••_F07
•F08
ooo0
99 ø
;
i
102 ø
-•F09
i
105 ø
i
108 ø
..... '""
,.•
.--.
i
111 ø
i
,; .....
114 o
{"
i
117 ø
i
120 ø
Figure 6.
Maps showingSKS splitting resultsin the Baikal rift and adjacentareas. The
diagramon the top is a two times enlargementof the rectanglein the bottom diagram. The
line drawn through each.circle givesthe fast polarization direction. Those with two inconsistent
results are plotted'as double circles. Stations representedby squaresare null measurementson
which anisotropyeffect cannot be clearly observed.
Usinglaboratoryestimatesof the variationof velocity fer to bulk averagesand do not precludethe presence
of
and attenuationof mantlerockswith temperatureby pocketsof melt in isolatedareascorresponding
to the
Satoet al. [1989],ttaldermanandDavis[1991],Slack observedpatchesof Cenozoicvolcanismin the vicinity
et al. [1994,1996],andGaoet al. [1994b]
findthat the of the rift [Zonenshain
andSavostin,
1981].
tomographyresultscan be explainedif the mantle imRelation between Paleo and Recent
mediatelybeneaththe rifts containspartial melt with 3.2.
Stress Directions and Fast Directions
the fraction being 2-3% beneaththe EAR, possiblya
fractionof a percentbeneaththe RGR and essentially In continental
rift zonestheregionalpaleostress
operno melt beneath the BRZ. For the BRZ the results reating duringthe rifting processis inferredto be exten-
22,792
GAOET AL.:SKSSPLITTING
BENEATH
CONTINENTAL
RIFTZONES
sionin the directionof opening. For the EAR, Bosworth
et al. [1992]use boreholebreakoutsand alignedQuaternary vents to showthat the least principal horizontal
stress(LPHS) directionis alignedapproximatelyNWSE in Kenya(Figure9). In contrast,the maingeological
structures
54
ø .• I)
associated
with
the EAR
which have devel-
oped over the last 30 Myr have formed when the LPHS
wasalignedE-W resultingin the N-S striking rift direction. Thus the rotation of the stressfield is relatively
recentevent,thoughtto occurin the Quaternary(0.40.6 Ma), possiblyrelatedto the late uplift and doming
of the Kenyan and Ethiopian Rifts [Bosworthet al.,
1•2].
51 ø
48'
l•
In a similar fashion, the stressesin the RGR and
neighboring areas are thought to have undergone a
clockwiserotation. Paleostressdata throughoutmuch
-.........
..'....
,-.,.
of the western
ß
ß
.:
45 ø
,
-•
underwent
United
States indicate
a transition
from WSW-ENE
that
the LPHS
to WNW-ESE
,,.
at about 10 Myr ago, a time when Farallon subduction alongthe westernboundaryof North Americahad
•'igure 7. Rosediagramsof SKS fast polarization been replacedby significantdevelopmentof the transdirections for each of the six areas in the BRZ and ad- form boundary of the San Andreas fault. The modern
jacent areas. Null measurements
were excludedin the LPHS direction on the Rio Grande maintains an E-W
rosediagrams. The sectorwidth of the rosediagrams direction, but within the transition zone between the
is 200. The two most common fast direction ranges
are 400 -80 ø, which is approximatelyparallel to the rift and the Colorado Plateau, the LPHS is thought to
rift axis; and 1200- 1600, which is approximatelyrift- be WNW-rSr (Figure 9) [Zobackand Zoback,1980;
Aldrich et al., 1986]. The seismictomographyresults
orthogonal.
60'
30'
'•
20'
•
,-•:.•
ß
•"
10' '•
,,•,,.,
O'
,
60'
' ½'••••'•"
••
,•.
•
70'
::• " .... •
• •'•' ' ::' ""'
..-.... .
..--:::..,
'.:.
•-.,:..
t •.'•,....
'"'
'
."•
-'•.. ' '
'
'"'"'
ß
•..:•:....•.
%, ..'....,.,.-.,
.....,• ...: .
. .......
•"'.......
80'
' •,,
,•:'..•
•.'..:.-..• .•
90'
• •.
'...:.•..
....
100'
.
. '.,
110'
'..,".. J.. ,• •..
... .,,•=•
' .....• •, .•.'" . .-.,"
120'
130'
140'
•igure 8, Rosediagrams
of SKS splittingmeasurements
in the zoneof Tibet-TienShanMongolia-Baikal
fromportableseismic
arrays.Stationsarerepresented
by opensquares.•he
measurements
werefromt•e followingstudies:Ma•yeva • a•. [1992](Ticn S•an); Mcnamara
• a•. [1994](•i•t); 6ao • a•. [1994•]•nd t•i• •tudy(Si•ri•-•ongoli•).
GAO ET AL.' SKS SPLITTING BENEATH CONTINENTAL RIFT ZONES
22,793
o
o
o
Uganda
Kenya
O
o
.1 ø
_2 ø
Tanzania
ooO0
_3 ø
_4 o
33 ø
34 ø
35 ø
36 •,
37 ø
38 ø
39 ø
40 ø
Figure 9. Mapsshowing
SKS splittingresultsin the EastAfricanRift [Gaoet al., 1994c].
Arrowsrepresent
regional
stressdirections
[Bosworth
et al., 1992].
[Slacket al., 1996]reveala low-velocity
bodyin theup- localscale,Sherman[1992]findsthat the SW and NE
per 200 km of the mantlethat strikesNNE to NE, and regionsof the rift zone are dominated by shear at the
extends from beneath the rift into the transition zone.
centerwith a transition to tensionalstressesawayfrom
The resultswereinterpretedasarisingfrom lithospheric the center(Figure2). The centralpart of the BRZ has
extension
in the modern(<;5Ma) LPHS direction,i.e., the LPHS oriented NW-SE.
WNW-ESE causedby the modernstressfield [Slack
et al., 1996]. The magnitudeof the velocityanomaly 3.3. Splitting from Fossil Anisotropy,
and the associated attenuation in the mantle beneath
Rift-Related Mantle Flow or Magmatic Cracks?
the RGR were taken to imply that temperaturesin the
anomalouszone are closeto, or even just above, the
It is possiblefor all theserifts that SKS splitting
is causedby fossil anisotropy in which LPO of olivine
solidus.Thus Slacket al. [1996]concludethat it is crystalswascausedby pasttectoniceventsandhasbeen
probablythe mantle sourcezoneof the most recentvol- maintained in the lithosphereever since. Each rift zone
canicsthat lie alongthis trend (e.g., the Jemezlinea- wasthe siteof continentalconvergence
priorto rifting.
ment).
However,mobility of olivine crystalsat temperatures
The Baikal rift zoneis a regionof transtension.Fig- above900øCis high [Vinnik ½t al., 1992],and thereure 2 showsthe regionalstressfieldsof the BRZ [Sher- fore the survivalof fossilanisotropyis very unlikelyin
man, 1992]. The stressfieldswereobtainedfrom ge- the mantle beneaththe rifts, especiallyif the tomograologicalstructuralanalysisand from earthquakefocal phy interpretationis correctsuggesting
that the velocity
mechanisms.
Sherman
[1992]findsthattheresults
from anomaliesare causedby manfie at temperaturesdose
the two approaches
agreewith eachother. The regional to the solidus. The tomographyresultsindicate that
stresspattern is alsoin agreementwith the globalstress the 900øC isotherm may upwarp to a depth of about
map of Zoback[1992]and the subcrustalstresses
that 50 km beneath these rifts, as suggestedby Zorin and
of the layercoolerthan
havebeeninferredfromsatellitegravitydata [Liu, 1978, Osokina[1984].The thickness
1983].The globalstressmap indicatesthat the LPHS 900øC is probably too small to generatethe observed
lies in the direction of rift opening. However,on a more splitting if all the anisotropyis fossilanisotropy.
22,794
GAO ET AL.: $KS SPLITTING BENEATH CONTINENTAL RIFT ZONES
38 ø
36 ø
34 ø
32 ø
o o00
0.5
-110 ø
-108 ø
-106 ø
-104 ø
1.0
1.5 2.0sec
-102 ø
Figure 10. MapsshowingSKS splittingresultsin the Rio GrandeRift [Sandvolet al., 1992].
Arrowsrepresent
regionalstressdirections
[Zoback
andZoback,1980;Aldrichet al., 1986].
An alternative
model holds that the fast directions
of partial melt to generatethe observedanisotropy,proare caused by mantle flow. Three-dimensionalsmall- videdcrackaspectratiosare high [O'ConnellandBudiscaleconvectionin the mantle as suggestedby Sandvol ansky,1977;Anderson,1989]. This modelcan explain
et al. [1992]for the RGR, or by Makeyevaet al. [1992] both the P wave delays and why the fast directions
to explain anomalousdirectionsin the Tien Shan, could appear to be better correlated with the recent stressdi-
give rise to rift-parallel directions.At this stageof our rections on the RGR and EAR rather than the stress
understandingof mantle processesbeneath rifts, it is directionsrelated to the original rift events. We expect
difficult to rule out this possibility.
that the crack anisotropy would respondrapidly to a
A third model, which may be applicablein regionsof regionalstressrotation.
hot mantle beneath rift zones,holds that aligned magIn that the tomographyresultsimply that the BRZ
matic cracksin the mantle give rise to the fast direc- mantle is cooler than either the EAR or RGR, the mixtions. The effecton $KS splittingis similar to that of ture of rift-parallel and rift-orthogonaldirectionsin the
fluid-filledcracksin the rigiduppercruston splittingof immediate vicinity of that rift may arisefrom a combishearwavesfrom local earthquakes[e.g., Leafy et al., nation of LPO and orientedcracks,dependingon man1990;Liet al., 1994].Parallelverticalmagmaticcracks tle temperature. However,we add that resolvingbeform a transverseisotropywith rift-orthogonalaxis of tween fossilanisotropy,recent mantle flow, or aligned
symmetry. The fast direction of the anisotropyis par- cracksis sufficientlydifficult that favoringone over the
allel to the strike of the crac.
ks, i.e., parallel to the rift other cannot be made with a high degreeof certainty.
axis.
If the melt is distributedin crackssuchasalonggrain- 4.
grain contactsor in planar dike-likeregions,in a region
of extension
Conclusions
there will be a bias in the distribution of
Observationsof $KS splitting on the East African
crackplaneswith greater numbersorientedperpendicu- and Rio Grande Rifts give fast directionsthat are orlar to the extension.It takesjust a fractionof a percent thogonal to the least principal horizontal stress. For
GAO ET AL.: $KS SPLITTING
BENEATH CONTINENTAL
RIFT ZONES
22,795
calculated for ultramafic minerals and aggregates,in Flow
and Fracture o• Rocks,Geophys. Monogr. Set., Vol. 16,
with fast directionsboth orthogonaland parallelto
edited by H. C. Heard et al., pp. 157-166, AGU, Washthe leastprincipalhorizontal
stress.Tomography
reington, D.C., 1972.
sults and rift-related volcanismindicatethat the man- Bjarnason, I. T., C. J. Wolfe, S.C. Solomon, and G. Gudmundson, Initial results from the ICEMELT experimenttle underthe EAR is hottestand probablyabovethe
Body-wave delay times and shear-wave splitting across
solidus,
thatundertheRioGrande
it isnearthesolidus, Iceland,
Geophys. Res. Lett., œ3,459-462, 1996.
whereasthat beneath Baikal it is below the solidus.
Bosworth, W., M. R. Strecker, and P.M. Blisniuk, InteSuchhot mantlemaynot develop
LPO fabricdueto angration of east African paleostressand present-day stress
nealing
effects
such
asrotational
recrystallization
[Nico- data: Implications for continental stress field dynamics,
las andPoirier,1976]. Magmaticcracksin partially J. Geophys. Res., 97, 11,851-11,865, 1992.
moltenmantlecouldexplainthe splittingresultssince Chastel, Y. B., P. R. Dawson, H. R. Wenk, and K. Bennett,
Anisotropic convection with implications for the upper
a biasin crackpopulation
isexpected
withmorecracks mantle, J. Geophys.Res., 98, 17,757-17,771, 1993.
oriented
perpendicular
totheLPHS.ForRGRandEAR Christensen,N. I., Shear wave propagationin rocks, Nature,
Baikal,in thevicinityof therift, theresults
aremixed
thefastdirections
appearto havefollowed
therecentroœœ9,549-550, 1971.
tationof thestressfield. Thisisreadilyexplained
if they Christensen, N. I., The magnitude, symmetry and origin
arecaused
by a corresponding
redistribution
of oriented of upper mantle anisotropy based on fabric analyses of
cracks. For Baikal the inferred coolermantle may be ca-
pableof generating
a fabricawayfromtherift zone,as
observed,
but in patchesof hot mantlenearthe rift,
magmatic
crackscouldaccount
for the rift-parallel
directions.If mantlemagmaticcrackscauseanisotropy
in
regions
of extension,
thiscanexplainwhyobserved
fast
directions
areparallelto LPHSin compressive
zonesbut
perpendicular
to it in extension
zones.In the former,
temperatures
areloweranda fabriccandevelop.
In the
latter, temperatures
arehigher,possibly
annealing
out
the fabricbut causing
meltpocketsto formwhichgive
rise to volcanismand alignedcracks.
Acknowledgments. The fieldworkfor the Baikalrift
projectwasdonein conjunction
with the late BobMeyer,
Universityof Wisconsin,
whosepresence
is sorelymissed.
We dedicatethis paperto his memory.The analogseismo-
gramswererecorded
by the BaikalSeismic
Network
of the
RussianAcademyof Sciences
whichhasbeenmanagedby a
teamheadedby O. Masalskiof IEC. We thankR. Meyer's
groupmembers,
especially
P. Burkholder
andL. Delitsin
for
discussions
and field cooperation.The authorsare grateful
to R. Girdlerfor helpfuldiscussions.
D. Helmberger
andX.
Dingat Caltechkindlyprovided
assistance
for the digiti-
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RIFT ZONES
22,797
hliu@dtm.ciw.edu;
pslack@ess.ucla.edu;
arigor@ess.ucla.edu)
S.Gao, Department of Terrestrial Magnetism, Carnegie
Institution of Washington,5241 Broad BranchRoad, N.W.,
Washington,DC 20015. (e-mail:sgao@dtm.ciw.edu)
Y. A. Zorin, V. V. Mordvinova, V. M. Kozhevnikov,
and N. A. Logatchev, Institute of the Earth's Crust,
Siberian Branch• Russian Academy of Sciences, 128
Lermontov Street, Irkutsk 664 033, Russia. (e-mail:
zorin@crust.irkutsk.su)
P.M. Davis, H. Liu, P. D. Slack,and A. W. Rigor, De(ReceivedDecember3, 1996; revisedMay 30, 1997;
nia, Los Angeles,CA 90095. (e-mail: pdavis@ess.ucla.edu;acceptedJune 24, 1997.)
partment of Earth and Space Sciences,University of Califor-