Download Shape of the subducted Rivera and Cocos plates in southern Mexico

JOURNAL
OFGEOPHYSICAL
RESEARCH,
VOL.100,NO.B7,PAGES
12,357-12,373,
JULY10,1995
Shapeof the subductedRivera and Cocosplatesin southern
Mexico: Seismicand tectonicimplications
Mario Pardo and Germdo Sufirez
Insfitutode Geoffsica,UniversidadNacionalAut6nomade M6xico
Abstract.Thegeometry
of thesubducted
RiveraandCocos
plates
beneath
theNorthAmerican
plate
insouthern
Mexicowasdetermined
based
ontheaccurately
located
hypocenters
oflocaland
te!eseismic
earthquakes.
Thehypocenters
oftheteleseisms
wererelocated,
andthefocaldepths
of
21 eventswereconstrained
usinga bodywaveinversionscheme.Thesuduction
in southern
Mexico
maybeapproximated
asa subhorizontal
slabbounded
attheedges
bythesteep
subduction
geometry
of theCocos
platebeneath
theCaribbean
platetotheeastandoftheRiveraplatebeneath
NorthAmerica
tothewest.Thedipoftheinterplate
contact
geometry
isconstant
to a depthof 30
kin,andlateralchanges
in thedipofthesubducted
plateareonlyobserved
onceit isdecoupled
fromtheoverriding
plate.Onthebasisof theseismicity,
thefocalmechanisms,
andthegeometry
ofthedowngoing
slab,southern
Mexicomaybesegmented
intofourregions
ß(1) theJalisco
regionto thewest,wheretheRiveraplatesubducts
ata steepanglethatresembles
thegeometry
of
theCocosplatebeneath
theCaribbean
platein CentralAmerica;(2) theMichoacan
region,where
thedipangleof theCocosplatedecreases
gradually
towardthesoutheast,
(3) theGuerrero-Oaxac.a
region,bounded
approximately
by theonshore
projection
of theOrozcoandO'Gorman
fracture
zones,
wherethesubducted
slabis almostsubhorizontal
andunderplates
theuppercontinental
plate
forabout250 kin, and (4) thesouthern
OaxacaandChiapas
region,in southeastern
Mexico,
wherethedip of the subduction
graduallyincreases
to a steeper
subduction
in CentralAmerica.
Thesedrasticchangesin dip do notappearto takeplaceon tearfaults,suggesting
thatsmooth
contortions
accommodate
thesechanges
in geometry.The inferred80 and100 km depthcontours
of the subducted slab lie beneath the southernfront of the Trans-Mexican Volcanic Belt,
suggesting
it is directlyrelatedto thesubduction.
Thustheobserved
nonparallelism
with the
MiddleAmericanTrenchis apparently
dueto thechanging
geometry
of theRiveraandCocos
plates
beneath
theNorthAmerican
platein southern
Mexico,
andnottozones
ofweakness
inthe
crustof the North Americanplateassomeauthorshavesuggested.
Introduction
The seismicity and tectonics of southern Mexico are
characterized
by the subductionof the oceanicCocosand
Riveraplatesbeneaththe North Americanlithospherealong
the Middle American Trench.
Several studies have shown that
this subductionzone exhibits lateral variations in the dip of
takes place on a fold. They also suggestedthat to the
southeast,the changein dip may be relatedto differencesin
the age of oceanic lithosphere on either side of the
Tehuantepecridge.
Until recently, the acceptedview was that the Cocos plate
subductsbeneathcentral Mexico at a constantand relatively
shallowangleof ~12 to 15ø [Stoiberand Carr, 1973;Hanus
the subductedoceaniclithosphere[e.g., Molnar and Sykes, and Vanek, 1978; Havskov et al., 1982; Burbach et al., 19.84;
1969; Stoiber and Carr, 1973; Dean and Drake, 1978; Nixon,
1982; Beyis and lsacks, 1984; Burbach et aL, 1984; Ponce et
Bevis and lsacks, !984; Singh and Mortera, 1991]. Using
data from a permanentlocal network, Sutirez et al. [1990]
al., 1992]. A steep subductiongeometryin southeastern definedthe geometryof the subductedplate in more detail and
Mexico [Ponce et al., 1992] and in the Jaliscoregionof showed that in central Mexico the slab does not subduct at a
western
Mexico [Pardoand Sudrez,1993]bounda slabdipping
steadyshallowangle. Instead,after an initial dip of-15 ø, the
apparently
at a very shallowanglein centralMexico.
slab becomesalmostsubhorizontaland underplatesthe North
On the basis of teleseismic data, Burbach et aI. [19134]
American upper lithosphere.
This plate geometry is
concluded
that the subducted
Cocosplatecanbe dividedinto reminiscent of that found in central Peru and central Chile,
thesethreemajorregionsdippingat differentangles.Beyis wherethe Nazca plate also subductshorizontallybeneaththe
andlsacks[1984],usinga hypocentral
trendsurface
analysis, SouthAmericanupperlithosphere.
findno evidenceof tearfaultingseparating
thesesegments
of
Perhapsone of the more striking tectonic features in
thesubducted
lithosphere
•[nd"
suggested
thatthetransitionsouthern
Mexico is the location of the Trans-Mexican
Volcanic Belt. The Trans-Mexican Volcanic Belt crosses
southern Mexico from east to west at about I9øN.
The
distributionof the volcanicarc is oblique(-16 ø) relativeto the
1Now
atDepartamento
deGeoffsica,
Universidad
deChile,
Santiago.Middle American Trench (Figure 1), in an unusalgeometry
Copyright
1995by theAmerican
Geophysical
Union.
which is not parallel to the subductionzone. The Trans-
Paperhumber95JB00919.
MexicanVolcanicBelt is ~100 km wide andis cut by a
sequence
of grabensthat are obliquerelativeto the general
0148-0227/95/95JB.,00919505.00
trend of the arc [Nixon, 1982].
12,357
'
12,358
PARDO AND SUAREZ: SUBDUCTION
- 110 ø
-115"
- 105"
OF THE RIVERA AND COCOS PLATES
- 1O0 ø
-95 ø
-90 ø
-85 ø
-80 ø
25 ø
25
NORTH
AMERICA
CARIBE
20"
20 ø
PACIFIC
15"
15'
-'OCOS.'
10 ø
10'
-115'
-110 ø
-105 ø
-100 ø
-95 ø
-90 ø
-85 ø
-80 ø
Figure 1. General tectonic setting of the study area. Five lithosphericplates arc shown: Pacific, Rivera,
Coco.,,, North A•nerica, and Caribbean. Relative convergence rates (cm/yr) between the {•ceanic and
continental plates are indicated by open arrows, and absolute motions of the upper plates relative to a hot
spot rra•ne of reference ,ire shown by shadedarrows. The abrcviationsare EPR, East Pacific Rise; TFZ,
Tamayo Fracture Zone; RFZ, Rivera FractureZone; OFZ, Orozco Fracture Zone; OGFZ, O'Gorman Fracture
Zone; TR, TchuantcpecRidge; MAT, Middle AmericanTrench; JB, JaliscoBlock; CG, Colima Graben;EG,
E1 Gordo Graben. Solid triangles indicate Quaternaryvolcanism,and the crossedarea indicatesthe TransMexican Volcanic Belt (TMVB).
The volcanic arc erupted since the Miocene and mostly Volcanic Belt is presentedhere. The availability o1'datafrom
of
during the Plioccne and Quaternary. A diversity of structures permanentand temporary local networksand the occurrence
such as large strato-volcanoes, monogenetic cineritic cones, recentearthquakesrecordedat tcleseismicdistanceswhichhad
shield volcanoes, and several calderas are found on the
not been studied previously otter tin excellent opportunityto
volcanic belt. The chemical and petrologicai characteristics understand this region ol' complex subduction geometryin
.of the Trans-Mexican Volcanic Belt indicate that, in general, greater detail.
The purposeof this study is to determinethe geometryof
the volcanic sequences that form this belt are c,lic-alkaline,
although some localized zones of alealine volcanism exist the subducted Cocos and Rivera plate, based on all the
along the Trans-Mexican Volcanic Belt, especially in western available and reliable hypocenter locations of earthquakes
recorded with local and teleseismic networks, and to studythe
Mexico [e.g., Luhr et al., 1985; Allan et al., 1991]. The
radiometric dating of volcanic rocks and the broad nature of
regional distribution of stresseswith reported and newly
determined focal mechanisms. Together, these data are
the Trans-Mexican Volcanic Belt suggest the migration of
volcanism toward the trench sihce 5.3 Ma [Delgado et al.,
analyzedto determinean integralmodelof the subduction
in
1993].
southernMexico and its implicationson the seismicityand
A number of contrasting hypotheseshave been proposedto
account for the nonparallelism of the Trans-Mexican Volcanic
tectonics
Belt relative
Tectonic Setting
to the Middle
American
Trench and its relation to
of' the area.
the subductedoceanic plate. These hypothesescan be divided
This studyincludesthe subduction
of the Rivera andCocos
into two groups: (1) those which favor a direct association
plates in southernMexico, west of the intersectionof the
between subduction and volcanism [e.g., Molnar and Sykes,
1969; Demant and Robin, 1975; Nixon, 1982; Burbach et al.,
1984; Suc•rezand Singh, 1986], and (2) those which suggest
that the Trans-Mexican
Volcanic
Belt
bears no direct
tectonic
relation with the subduction along the Middle American
Trench and explain its origin as resulting from zones of
weakness within the crust of southern M6xico, inherited from
earlier episodes of deformation [e.g., Mooser, 1972; Gastil
and Jensky, 1973; Shurbet and Cebull, 1973; Johnson and
Harrison, 1989].
An integral study that takes into account the geometry of
the subducted Rivera and Cocos plates beneath the North
American lithosphere (NW of 94øW), the stressdistribution in
the subducted slab, and the relation between the subduction
process and the anomalous location of the Trans-Mexican
Tehuantepec
ridgewiththeMiddleAmerican
Trench(Figure1).
Accordingto Mammerickxand Klitgord[1982], the tectonic
historyof this regionfor the last25 m.y. hassufferedat least
three major plate reorganizations.The old Farallonplate
evolved first to the Guadalupe plate, which was later
segmented
into the actualRivera and Cocosplates. The
position
of thespreading
centers
movedeast,fromabout12.5
to 11 Ma, to the Mathematician seamounts. Later, between
6.3 to 3.5 Ma, it shifted to its actual positionof the East
Pacific Rise (EPR).
Rivera
Plate
Theexistence
of thesmallRiveraplatewasfirstsuggested
by Atwater [1970]. Sincethen, severalauthorshaveshown
PARDOANDSUAREZ:
SUBDUCTION
OFTIlERIVERAANDCOCOS
PLATES
thatthe Rivera plate is kinematicallydistinctfrom the North
AmericanandCocosplates[Bandyand Yah,1989;Eisslerand
12,359
Guerreroregion. Hypocentrallocationswere considered
reliableand includedin the final datasetwhentheymet the
McNaIly, 1984;DeMets and Stein, 1990]. The junction followingcriteria: (1) a minimumof sevenphasereadings
betweenthe East Pacific rise and Rivera fracturezoneis located withat leastoneS wavearrivaltime; (2) a rootmeansquare
165kmto thewestof theMiddleAmerican
Trench[Bourgois errorof lessthan0.25 s; (3) estimated
hypocentral
errorsof
andMichaud, 1991]. The preciselocationof the Rivera-Cocos lessthan 10 km; (4) a hypocentral
variationof lessthan 10
boundary
is still controversial,
and no clear bathymetric km whenthe initialtrial depthwasvariedfrom !0 to 150 km;
features
canbe associated
to a distinct
andclearplateboundary and(5) a maximumanglegapbetweenthe epicenter
andthe
[Eisslerand McNally,1984;Mamrnerickx,1984;Bourgois local stations of less than 230 ø. From a total of over 3000
and Michaud, 1991]. Bandy [1992] suggested
that the E1
events,a final subsetof only 1101 hypocenters
satisfiedthese
Gordograben
maybe partof theRivera-Cocos
plateboundary criteria (Figure 2).
(Figure1).
The Rivera lithosphere consumedat the trench is of late
Mioceneage (~9 Ma)[Klitgord and Mammerickx,1982].
Althoughthe seismicityrelatedto the subructionof theRivera
platebeneaththe Jaliscoblockis low (Figure2), at leastsix
Relocated Earthquakes
Earthquakeswith magnitudernb > 4.5 were relocatedin
southernMexico, usingthe JHD method[Dewey, 1971] and
the phase readings reported by the International
largeeathquakes
(Ms > 7.0) havebeendocumented
since! 837,
Seismological
Centre(ISC) between1964 andJanuary1990.
including
the great 1932 Jaliscoearthquake
(Ms=8.2)[Eissler
Several
calibration
eventswereusedalongthe region:(1) for
and McNally, 1984; Singh et al., 1985]. This contradictsthe
the zonebetween103ø and 105ø, we usedthe largestaftershock
suggestion
that the Riveraplatesubducts
aseismically
[Nixon,
of the 1973 Colima earthquake(Ms=7.5). This aftershock
1982]. On the basisof locallyrecordedmicroearthquakes
and
(Ms=6.2) was located with a temporary seismicnetwork
relocatedhypocentersof teleseismicevents,Pardo and Sudrez
deployedafter the mainshock[Reyeset al., 1979]; (2) for the
[1993]concluded
thatthe subduction
of the Riveraplatetakes
zone between100ø and 103øW, the 1979 Petatlanearthquake
placewith a steepangle(-50 ø) belowdepthsof 40 km.
(Ms=7.8)recorded
with portablestations
[Va!ddset aI., 1982]
Cocos
Plate
The Cocos plate subductsat a rate that increasesto the
southeast
from 4.8 cm/yr at 104.5øWto 7.5 cm/yr at 94øW
[DeMetset al., 1990]. The age of the Cocosplatealsovaries
alongthe Middle AmericanTrench, with jumps associated
to
fracturezones(Figure 1). To the east,the Tehuantepec
ridge
intersects
the Middle AmericanTrenchnear--.95øW.Thisridge
is the largestlinear bathymetricfeatureon the easternflank of
the East Pacific Rise. Apparently, it is the morphological
expressionof a fracture zone that presumablyseparatescrust
of significantlydifferentages. A youngerand shallowercrust
to the northwest differs in age to that southeastof the
Tehuantepec
ridgeby approximately10 to 25 m.y. [Couchand
Woodcock, 1981].
Data Usedin the Analysis
was used;and (3) the 1981 Ometepecearthquake(Ms=7.2)
[Nava, 1984] for the easternmost
region.
The JHD method [Dewey, 1971] determines relocated
hypocenters relative to the hypocenter of the calibration
event, applying time adjustmentsto the phasereadingsof P,
p P, and S.
These time adjustments of the reporting
seismologicalstationswere obtained from the varianceof the
different phasearrival times of the calibrationeventsand from
the earthquakes
reportedin this studyfor which the focal depth
was constrainedusing a long-period body wave inversion.
These calibration station-phaseswere then used in a singleevent location method [Dewey, 1971], in order to determine
the relocatedhypocenterswithin the zonesof each calibration
event. A sphericalEarth modelwasusedin the JHD methodto
relocate the hypocenters.
For the easternpart of the region, we used the relocated
hypocenters
obtainedby M. Cruz andG. Su•rez (manuscript
in
preparation)
who applieda similarprocedure.The precisionof
eachrelocatedhypocenterrelative to the calibrationevent is
estimatedby computingerror ellipsoidsfor the hypocentral
coordinatesat a 90% confidencelevel. A hypocentrallocation
was consideredreliablewhen the ellipsoidsemiaxeswere less
than 20 km; all eventswhich did not meet these criteria were
discarded. A final subset of 207 reliable hypocenterswas
The data used to infer the morphologyof the subducted
Rivera and Cocos plates beneathsouthernMexico are (1)
accurately determined hypocenters obtained from local
microearthquake
data recordedby permanentand temporary
seismological
stations,and (2) eventsrecordedat teleseismic
distances
and relocatedusingthe methodof JointHypocenter
Determination(JHD) [Dewey, 1971]. For someof the larger obtained from an initial set of over 300 events. The
andfocalparameters
of all theearthquakes
studied
events(m/•>_5.0), the focal depthwas constrained
from the hypocentral
are shown on Figure 2 and provided as an electronic
inversion
of long-period
bodywaves[Ndbelek,1984].
supplement
for thereader.
Local Microearthquake Data
All the available Iocal data from the permanent and
temporaryseismicnetworksinstalledin southernMexico were
Long-Period Body Wave Inversion
The formal inversion of P, S V, and S H
waveforms
used in order to accurately locate the hypocentersof
microearthquakes
within the zone [Nava, 1984; UNAM
SeisrnoIogy
Group,1986;Sansores,1990;PardoandSuc•rez,
1993;Ponceet al., 1992;Zu•iga et al., 1993].
constrainsthe focal mechanismand the centroidaIfocal depth
[NdbeIek, 1984]. Long-period seismogramsof teleseismic
The hypocenters
were locatedusingHYPO7! [Lee and
Valdes,1985]with thevelocitymodelreported
by Sudrezet al.
occurred between 1980 and 1987 and from the World-Wide
[1992],
anda velocity
ratio(Vp/V
s = 1.76)determined
forthe
earthquakes(25ø > A > 90ø) from the Global Digital
Seismograph
Network(GDSN) wereusedfor the eventsthat
StandarizedSeismograph
Network (WWSSN) for the events
that occurredprior to 1980. The analogseismograms
were
12,360
PARDO AND SUAREZ: SUBDUCTION OFTHE RIVERA AND COCOS PLATES
" .•
+-i--I-
ß
ß
ß
++
,
++.
.m
PARDO
ANDSUAREZ:
SUBDUCTION
OFTHERIVERA
ANDCOCOS
PLATES
12,361
hand-digitized
and filteredwith a high-pass
filter with cutoff the southeast,
themaximumdepthobserved
for earthquakes
in
frequency
of 0.017 Hz (60 s) to eliminatelong-period
noise. the Cocosplate,betweenthe E1 Gordograbenandthe point
All of theevents
modeled
arein themagnitude
range5.0>_rnt, where the Orozco FractureZone apparentlyintersectsthe
_>6.1, and therefore a point sourcewas assumedin all cases. trench, is 100 km. Farther east, in the region bounded
Keyexamples
of themodeled
eventsareshownonFigure3.
approximatelyby the onshoreprojectionof the Orozco and
Analysisof Hypocentral Data
the O'Gormanfracturezones,wherea goodcoverageof local
stationsexists, the maximum focal depths observedare
consistentlyless than 70 km. Betweenthe O'Gormanfracture
In plan view, the seismicityof southernMexico showsa
distributionin bands parallel to the trench [Sudrezet al.,
1990](Figure2). The first band alongthe coastis relatedto
zone and the TehuantepecRidge the maximum depth of
earthquakes
increases
continuoslydownto depthsof 180 km.
The depthextent of the various segmentsof the subduction
the interplateseismogeniccontact. North of it lies an almost zonedirectlycorrelateswith the observeddip of the slab.
aseismic zone, which becomes broader between 96øW and
Smooth contours of the Wadati-Benioff
zone were obtained
10!øW. Fartherinland,a secondbandof deeperintraplate using a two-dimensionalspline interpolation [Smith and
events(depths> 50 km) in the oceanicdowngoing
slablies at Wessel,1990] (Figure6). In centralMexico,however,to the
a distanceof 150-250km from the trench(Dt). Beyondthis northof 18øNandat distances
beyond300 km from the trench,
last band, which lies immediately to the south of the Trans-
MexicanVolcanicBelt, the seismicactivitydisappears.
In order to map the geometryof the subductedslab, 12 cross
sectionsof the seismicitywere drawn (Figures2 and4). The
originof all the crosssectionsis the trench,and they are all
orientedin the directionof convergence
predictedbetweenthe
Rivera-North America [DeMets and Stein, 1990] and CocosNorthAmerica [DeMetset al., 1990] platepairs(Figure2).
The shallow part of the subductionzone (h < 30 km)
indicatesa constantinterplate geometrythat initially dips at
~10ø and gradually increasesto --25ø at a depth of 30 km
(Figure 4). This geometry is observed throughout the
subruction zone in southern Mexico and appears to be
independentof the age and the relative convergencevelocity
of the subductedoceanic plate; no lateral changesof the
interplate geometry are observed where major bathymetric
thereis a remarkabledearthof seismicevents(Figures2 and
6). Thusthe 80- and 100-km-deep
contours
cannotbe mapped
directly in centralMexico.
These two contourswere drawn following the trend of the
slab at shallowerdepthsand extrapolatingtheir locationfrom
the regionswhere they are well defined by the seismicity:to
the westof 103øWand eastof 96øW (Figure6). The resulting
contoursshowthat the Cocosplate lies at depthsof 80 to 100
km beneath the southern front of the Trans-Mexican
Volcanic
Belt (Figure 6). This depth of the Wadati-Benioff zone of
approximately100 km beneaththe volcanicarc is observedin
most subructionzones of the world. This suggeststhat the
Trans-Mexican Volcanic Belt is directly related with the
subruction
of the oceanic
slab beneath the North
American
featuresare being subducted.
plate and that the observednonparallelismof the volcanicbelt
with the trenchis mainly due to the geometryof the subducted
plate.
Geometry of the Subducted Plate
Focal Mechanisms
The subductedRivera and Cocosplates show rapid lateral
variationsin dip at depthsgreaterthan-30 km (Figure4). In
the Isthmusof Tehuantepec(crosssectionAA' on Figures2
and 4a), the seismicitydefines a Wadati-Benioff zone dipping
at an angle of-30 ø. Immediatly towardthe west (Figure4b),
the dip of the downgoingslab is shallowerand decreases
to an
angle of-25 ø. In central Mexico, the geometry of the
downgoingslab becomesalmostsubhorizontal
at distances
of
between110 km to 275 km from the trench and at depthsof
about50 km (Figures4c to 4g). A similar geometrywas
In this study, 21 new focal mechanismsof earthquakes
within the subductingCocos plate are reported(see electronic
inferred
for theGuerrero
region(Figures
4f and4g) bySudrezet
al. [1990]andSinghand Pardo [ 1993]usinglocaldata.
In the westernmost
regionof the Cocosplate (Figures4h
and State of Stress
supplements).The lower hemispheric
projectionof the focal
mechanismsobtainedfrom a waveform inversionreportedin
this studyand in previousworks are shownon Figure7, and
severalkey examplesof the earthquakes
modeledin this study
are shownon Figure 3. These focal mechanismsare also
shownon lateralprojectionon the crosssections(Figure4).
Owingto the absence
of seismicstationsto the southandwest
of southern Mexico,
the focal
mechanisms of small
earthquakes
reportedfrom the first motionof P wavesare
poorlyconstrained.
Thereforeonly onemechanism
fromfirst
motion data alone was used; its small magnitudeprecludedthe
and4i), the dip of the subducted
slabchangesagainrapidly use of a formal inversion scheme (event 67 on the electronic
from the subhorizontalsubductionobservedin the region of
supp!ementl).
Guerrero
andOaxaca(Figures
4c to 4g)to a steeper
dipof-30 ø.
This dip angie is similarto that observedon Figure3a, near
the Isthmusof Tehuantepec.
Towardthe northwest,
the slab
In a manneranalogousto the seismicity,the stresspattern
observedalongthe subductionzone may be dividedin zones
continues
to steepenprogressively,
andbeneaththe Colima
volcano,the Wadati-Benioffis celarlyfolloweddippingat an
angleof ~50ø downto a maximumdepthof approximately
130
km (Figure4j).
A crosssection
parallelto thetrench,
from(19øN,106øW)
to (15øN,95øW), showsthe rapidand drasticvariationsin
maximumdepthextentof the hypocenters
within southern
Mexico(Figure5). In thecaseof theRiveraplatebeneath
the
Jaliscoregion,the maximumfocaldepthis 130 kin. Toward
1Anelectronic
supplement
of thismaterial
maybeobtained
ona
disketteor AnonymousFTP from KOSMOS.AGU.ORG. (LOGIN to
AGU's FTP account using ANONYMOUS as the usernameand
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APEND. TypeLS to seewhatfiles are available.TypeGET andthe
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(Paper95JB00919,
Shapeof thesubducted
RiveraandCocosPlatesin
southernMexico: Seismicand tectonic implications,by Mario Pardo
and Gerardo Su.4rez). Diskette may be ordered from American
Geophysical
Union,2000 FloridaAvenue,N.W., Washington,
DC
20009;$15.00.Payment
mustaccompany
order.
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Figure5. Relocated
hypocenters
projected
ontoa cross
section
parallel
tothetrench,
from(19øN,106øW)
to (15øN,95øW);symbols
ason Figure1. Southern
Mexicocanbe separated
intofourmajorsegments:
Jalisco,
Michoacan,
Guerrero,
andOaxaca.Thebottom
partshows
thevariations
in ageof thesubducted
slabat thetrench[KlitgordandMammerickx,
1982]andin therateof relativemotion
of theoceanic
plates
at thetrench[DeMetsand Stein,1990,DeMetset al., 1990]. Crosssections
A to L arethoseshownon
Figures2 and4.
parallel
to thetrench(Figures
4 and7). A shallow
depthzone subduction zone are the relative convergence rate in the
zone,the age of the subducted
slab,the absolute
closeto the trench showsinterplatethrustfaulting followed subduction
plate,andthe subduction
of aseismic
bydowndip,
normalfaultingevents.The thrustearthquakesmotionof theoverriding
features,
likeridgesor intraplate
seamounts
[e.g.,
areobserveddown to a maximumdepthof-25 km alongthe bathymetric
CrossandPilger,1982;Jarrard,1986].Except
fortheageof
Cocosplate (Figure 4).
correlatenegativelywiththe
Downdipfromtheinterplate
contact,
a zoneof lowseismic the slab,all of theseparameters
dip
of
the
downgoing
plate.
Thusyounglithosphere
is
activity
is observed
whichis followedfartherinlandby anarea
apparently
more
buoyant
than
old
plates
and
subduers
at
a
of intraplate
eventsshowingnormalfaultingat depthsgreater
than50 km. The normal faulting eventsin the downgoing lower dip angle.
slab(Figure7) show,in general,T axesoriented
in a direction Therefore, accordingto these empirical relations, the
zone of southern
parallelto thegradientof the subducted
oceanicplate(Figure shallowdip observedin the subduction
Mexico
could
be
explained
by
the
relatively
youngageof the
6). Only wherethe slabshowsa rapidchangein dip, near
96øW and 101øW, the stress distribution become more
complex(Figure 8).
subducting
slab,its moderate
convergence
rate,andtherapid
absolute
motionof theupperNorthAmericanplaterelativeto
ridge,
Beneaththe normalfaultingeventsnearthecoastline,
there thetrench(Figure1). To theeastof theTehuantepec
where
the
Cocos
plate
subducts
beneath
the
Caribbean
plate
is a sheetof compressional
events,withdepthsranging
from
dip angle,thesubducting
slabis 10 to 25 m.y.
40 km to 50 km (events47, 70, 158, 28, 152 and67 on the witha steeper
electronic
supplement;
Figures
4b,4d,4e,4f, 4h and4j). This olderthanto the westof TR [Couchand Woodcock,1981],and
doublezone in stressis probablyassociated
to flexural the relativemotionof the upperCaribbeanplate towardthe
stressesassociatedto the slab bending upward to become trench is lower than to the northwest[Gripp and Gordon,
1990] (Figure 1).
subhorizontal[SutSreaet al., 1990].
SouthwesternMexico is the region of the subductionzone
Practicallyno crustalseismicityis observedin the
overriding
plateoverthesubducted
Cocos
plate(Figure
4). In
contrast,
thereis a relativelyhighcrustalseismicity
within
theJalisco
regionin western
Mexico(Figures
2 and4j-41),
suggesting
thatthedeformation
of theJalisco
blockis greater
wheretheCocosplateis youngest.In thissamemanner,the
ageof thesubducting
Riveraplateis alsorelativelyyoung.
However,the dip of the downgoing
slabin this areais steep.
Thusthesimplecorrelation
between
theageof theslabandits
thanthatobserved
in the NorthAmericanplateovertheCocos dip doesnot holdin thiscase.The onlymajordifference
betweenthe Riveraandthe Cocossubduction
zonesis the slow
slab[Pardo and SutSrez,1993].
Discussion
convergence
rate betweenthe RiveraandNorthAmerican
plates(---2cm/yr)[DeMetsand Stein,1990],relativeto a
greaterCocos-North
Americaconvergence
rate(-6 cm/yr)
thislowrate
Although
the conditions
controlling
the geometry
of [DeMetset al., 1990](Figures1 and4). Perhaps
subduction
zonesare still controversial,severalauthorshave
suggested
thattheprincipal
factors
affecting
thegeometry
of a
of relativemotionbecomes
an important
parameter
affecting
thegeoinetry
of thesubducted
Riveraplate.
12,368
PARDO AND SUAREZ: SUBDUCTION OF THE RIVERA AND COCOS PLATES
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PARDO AND SUAREZ: SUBDUCYION OF THE RIVERA AND COCOS PLATES
12,369
12,370
PARDO AND SUAREZ: SUBDUCTION OF THE RIVERA AND COCOS PLATES
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PARDO AND SUAREZ: SUBDUCTION OFTHE RIVERA AND COCOS PLATES
12,371
earthquakesalong the coupled interplatecontactin southern
Mexico [Singh and Mortera, 1991] is probably related to
variationsin the strengthof couplingalongthe plate interface
and not to the geometryof the subductionor to the depth
extent of the coupledinterplatezone.
The depthextent of the seismogenicplate contactcan be
estimated from the maximum depth of the shallow thrust
earthquakes
[e.g., Sudrezand Comte,1993]. In the caseof the
50
kill
150
Cocosplate, this depth is consistentlyshallowerthan .--25
km. This resultsyield an unusuallynarrowseismogeniczone
which is only .-.60 km wide. This shallow depth of
seismogenic
couplingis abouthalf as deepas that observedin
most subductionzones of the world [e.g., Pacheco et al.,
0
50
km
-g
1993]
,
In the case of the Rivera plate, the maximum depth of
seismogenic
couplingappearsto be slightlydeeperand reach
ß-.40 km. This observation is suggested by a coastal
earthquakeshowinga nodal plane dippingto the northeastat
an angleof 34ø (event92 on Figure4j and on the electronic
supplement).If this earthquakestill reflectedthe interplate
contact,the width of the seismogeniczone of the Rivera plate
would be wider (.-.75 km) than that of the Cocos plate, and,
potentially, earthquakesof greater magnitude could be
generatedon its plate interface. Interestingly,the largest
earthquake
this centuryon the Middle AmericanTrench,the
1932, Jaliscoearthquake(Ms=8.2), occurredon the RiveraNorth America plate interface[Eissler and McNally, 1984;
2 (GUERRERO)
150
.'" ,'
.
,
.
,
.
,
.
',
.
, '.',
.
I
',,,,
. i
5O
h
km
I3(MICHOACAN)
i I l'"'I '
150 v .
,
.
,
.• . I
•
I
,
,
.
,
I
ß
1
.
Singh et al., 1985].
_
,•
The lateralvariationsin the dip angleof the subducted
plate
at depthsgreaterthan30 km (Figure4, 5, and6) suggest
that
southern Mexico can be divided into four regions bounded
5O
approximatelyby major bathymetric features identified
offshore(Figures6 and9). (1) The Jaliscoregion,wherethe
steep-dipping
Riveraplatesubducts
beneathNorthAmericais
kill
4 (JALISCO)
.............
..IxXj_,
150
0
I
i ,,,I .,
1
boundedto the eastby the subductionof the El Gordograben.
In fact, the significantdifferencesin the dip of the slab on
both sides of the El Gordo graben may indicate that this
tectonicfeaturedecouplesthe downgoingRivera and Cocos
plates. (2) The Michoacanregion, is a transitionzone
between the steep subductionof the Rivera plate and the
50
almost subhorizontal subduction to the east. As mentioned
before,it is boundedby the E1Gordograbento the westandby
the onshoreprojectionof the Orozcofracturezoneto the east.
(3) The Guerrero-Oaxaca region shows an almost
subhorizontal geometry of the subducted Cocos plate.
krn
150 ,
0
,
,
I00
,
,
200
300
km
400
Geographically,
thisregioncanbe definedas bounded
by the
Orozco and the O'Gorman fracture zones. (4) The southern
Oaxacaregion in southeastern
Mexico is a transitionzone
Figure 9. Comparison
of the crosssections
of the four betweenthe flat subductiongeometrybeneathGuerreroandthe
regions
into whichsouthernMexicowasdividedbasedon the
steepsubduction
geometryof the Cocosplate beneaththe
seismicity
andshapeof the subduction
zone. The seismicity Caribbeanplate towards the east.
andthe plate geometryare a superposition
of the cross
The remarkable absence of seismic events with depths
sections
on Figure4, andarelabeledwiththe sameletters. greaterthan70 km, northof---18øNandof theTrans-Mexican
Thebottomfigureshowsa comparison
of the geometry
of Volcanic Belt suggeststhat the downgoingplate lossesits
subduction
in eachof theseregionsin southern
Mexico.
brittle behavior and sinks aseismically into the mantle to the
north of the volcanic belt.
Seismic and Tectonic Implications
Perhaps low-magnitude
earthquakes
occurwhichare too smallto be detected
by the
Thedepthextentanddip of the interplate
megathrust
is insufficient seismicnetwork coverage that exists today. The
constant
and doesnot exhibitany clearcorrelation
with the inferred80- and 100-km-deepcontoursof the downgoingslab
to lie beneath
thesouthern
frontof theTrans-Mexican
ageof the subductedplate, with the relative convergence appear
velocity,
or withthesubduction
of bathymetric
features
along VolcanicBelt (Figure6). This observationstronglysuggests
thetrench
(Figures
4 and6). Thusthedifferent
rupture
modes the associationof this volcanic belt with the subductionof the
andcomplexity
of theseismic
sources
reported
forlargethrust oceanicplatebeneaththe North Americanplate. Thusthe
12,372
PARDOAND SUAREZ:SUBDUCTIONOFTHE RIVERA AND COCOSPLATES
observednonparallelismof the Trans-Mexican Volcanic Belt Bandy,
W.L.,andC. Y. Yan, Present-day
Rivera-Pacific
andRiverawith the trench is mainly due to the geometryof the subducted
Cocos
relative
plate
motions
(abstract)
Eos
Trans.
AGU,
70,1342,
1989.
Rivera and Cocosplatesin southernMexico.
Beyis,
M.,and
B.L.Isacks,
Hypocentral
trend
surface
analysis:
Probing'
thegeometry
of Benioffzones,
J. Geophys.
Res.,89,6153-6170,
1984.
Summary and Conclusions
Bourgois,J., and F. Michaud, Active fragmentation
of theNorth
The shapeof the subductedRivera and Cocosplatesbeneath
American
plateattheMexican
triple-junction
areaoffManzanillo,
Geo Mar. Lett., 11, 59-65, 1991.
the North American plate in southernMexico was determined
G.,C.Frolich,
W.Pennington,
andT.Matumoto,
Seismicity
usingaccuratelylocatedhypocentersof earthquakes.A total Burbach,
andtectonics
of thesubdueted
Cocosplate,J. Geophys.
Res.,89,
of 1101 microearthquakes hypocenters recorded by local
7719-7735, 1984.
networks and 207 relocated hypocenters of earthquakes Chael,E. P., andG.S.Stewart,Recentlargeearthquakes
along
the
recorded at teleseismic distances were used as the database. For
2! teleseismicevents (rob>5.0), the source parameterswere
determinedusing a formal long-period,body wave inversion
[Ndbelek, 1984]. In the relocationprocedure[Dewey, 1971],
the focal depth of these latter events was constrainedby the
inversion
and used within
the initial
subset of events
to
middle Americantrenchand their implicationsfor the subduction
process,J. Geophys.
Res.,87, 329-338, 1982.
Couch,R., and S. Woodcock, Gravity and structureof the continental
marginsof southwestern
Mexico and northwestern
Guatemala,
j.
Geophys.
Res.,86, 1829-1840,1981.
Cross,R.H., andT.A. Pilger, Controlsof subduction
geometry,
location
of magmaticarcs,andtectonicsof are and back-arcregions,
Geol.
Soc. Am. Bull., 93, 545-562, 1982.
generatethe variance of the residualsat each teleseismic
Dean, B.W., and C.L. Drake, Focal mechanism solutionsand tectonics
station, which were then applied to all the events in the
of the Middle America arc, J. Geol., 86, 111-128, 1978.
relocalization
scheme.
The subructionin southernMexico may be approximated
as
a subhorizontalslab subductingbeneaththe North American
plate,which is boundedby a steepsubructiongeometryof the
Cocosplate beneaththe Caribbeanplate to the east and the
Riveraplatebeneaththe North Americanplate to the west. No
lateral changesin the dip of the coupledinterplate zone are
observedat depthsof lessthan 30 km. The changesin dip of
the downgoing slab geometry are observed once it is
decoupled from the overriding plate. The width of the
seismogenic
zone of the Rivera plate is apparentlywider (--,75
km) than that of the Cocosplate (~60 kin).
The downgoingslabshowsdrasticchangesin the dip angle,
ranging from subhorizontal subduction beneath central
Mexico to a relativelysteepdip wheretheTehuantepec
ridge is
being subducted. These changesin the dip of the downgoing
slabdo not appearto take placeon tear faults,suggesting
that
smoothcontortionsaccomodatethese dip changes. The 80and 100-km-depthcontoursof the subductedplate lie beneath
the southern front of the Trans-Mexican
Volcanic Belt,
suggestingthere is a direct associationof this abnormally
aligned volcanic belt with the complex subruction geometry
of the subductedRivera and Cocosplatesin southernMexico.
Acknowledgments. We thank the ServicioSismo16gicoNacional,
the !nstitutode Geoffsica,and the Institutode Ingenieria,all of them at
UNAM, and the Universidadde Colima for facilitatingdata from their
local and temporarynetworks. We are gratefulto S.K. Singh and J.
Paehecofor helpful discussions
and a review of the manuscript.The
manuscript
was greatlyimpovedby comments
madeby D. E. Jamesand
an anonymous reviewer.
Thanks are due to W.H.F. Smith and P.
Wesselfor the use of the program GMT to draft the figures. This
research was partially supported by CONACYT and by project
UACPyP-UNAM 30309. One of us (M.P.) acknowledges
a fellowship
from the Secretaria de Relaciones Exteriores, M6xico.
References
Allan, J.F., S.A. Nelson, J.F. Luhr, I.S.E. Charmichael,M. Wopat, and
P.J. Wallace, Pliocene-Holocene
rifting and associatedvolcanismin
southwestMexico: An exotic terranein the making, in The Gulf and
Peninsular
Provincesof the Califomias,editedby J.P. Dauphinand
R.R.T. Simoneit, AAPG Mere., 47, 425-445, 1991.
Atwater, T., Implicationsof plate tectonicsfor the Cenozoictectonic
evolution of western North America, Geol. Soc. Am. Bull., 81, 35133536, 1970.
Bandy,W.L., Geologicaland geophysical
investigation
of the RiveraCocosplate boundary:Implicationsof plate fragmentation,Ph.D.
thesis,Tex. A&M Univ., CollegeStation,1992.
Delgado,H., P. Cervantes,and R. Molinero, Origende la faja
vo!e/inica trans-mexicanahate 8.3 May sus migracioneshaciael
W, SW y SSW desde5.3 Ma (abstract)GEOS Uni6n Geofs.Mex.,
13, 31-32, 1993.
Demant, A., and C. Robin, Las rasesdel volcanismoen M6xico;una
sfntesisen relaci6ncon !a evoluci6ngeodin•micadesdeel Cret•cico,
Rev. Inst. Geol., UNAM, 75, 70-82, !975.
DeMets, C., and S. Stein, Present-daykinematicsof the Riveraplate
andimplicationsfor tectonicsin southwestern
Mexico,J. Geophys.
Res., 95, 21,931-21,948,
1990.
DeMets, C., R.G. Gordon, D.F. Argus, and S. Stein, Currentplate
motions,Geophys.
Res.J. [nt.,101,425-478,1990.
Dewey,J.W., Seismicitystudieswith the methodof joint hypocenter
determination,Ph.D. thesis,Univ. of Calif., Berkeley, 1971.
Eissler,H., and K.C. MeNally, Seismicityandtectonics
of theRivera
plateand implicationsfor the 1932 Jalisco,Mexico,earthquake,
J.
Geophys.
Res.,89, 4520-4530,1984.
Gastil, R.G., and W. Jensky, Evidencefor strike-slipdisplacement
beneaththe Trans-Mexicanvolcanicbelt, StanfordUniv.Publ.Geol.
Sci., 13, 171-180, 1973.
Gonz•lez-Ruiz,
J., Earthquake
sourcemechanics
andtectonophysics
of
the middle America subructionzone in Mexico, Ph.D. thesis,Univ.
of Calif., Santa Cruz, 1986.
Gripp,A., andR. Gordon,Currentvelocities
relativeto thehotspots
incorporating
theNUVEL-1globalplatemotionmodel,Geophys.
Res.Lett., 17, I ! 09-1112, 1990.
Hanus,V., andJ. Vanek,Subduction
of theCocosplateanddeepactive
fracturezonesof Mexico, Geofis.Int., 17, 14-53, 1978.
Havskov,
J., S.K.Singh,andD. Novelo,Geometry
of theBenioff
zone
in theTehuantepec
areain southern
Mexico,Geofis.
!nt.,21,325330, 1982.
Jarrard,
R.D., Relations
amongsubduction
parameters,
Rev.Geophys.,
24, 2 ! 7-284, 1986.
Johnson,
C.A.,andC.G.A.Harrison,
Tectonics
andvolcanism
incentral
Mexico:A Landsatthematicmapperperspective,
RemoteSens.
Environ., 28, 273-286, 1989.
Klitgord,
K.,andJ.Mammerickx,
Northern
East
Pacific
Rise:
Magnetic
anomaly
andbathymetric
framework,
J. Geophys.
Res.,87,67256750, 1982.
Lee,W.H.K.,andC.Valdes,HYPO71PC:
A personal
computer
version
oftheHYPO71earthquake
location
program,
U.S.Geol.Surv.
Open
File Rep.85-749, 1985.
LeFevre,
L.V.,andK.C.McNally,Stress
distribution
andsubduction
on
aseismic
ridges
inthemiddle
America
subduction
zone,
J. Geophys.
Res., 90, 4495-4510, 1985.
Luhr,J.,S.Nelson,
J.Allan,andI.S.ECharmichael,
Active
rifting
in
southwestern
Mexico:Manifestations
of an incipient
east-ward
spreading-ridge
jump,Geology,
13, 54-57,1985.
Mammerickx,
J., Themorphology
of propagating
spreading
centers:
Newandold,J. Geophys.
Res.,89, 1817-1828,1984.
Mammerickx,
J.,andK.Klitgord,
Northern
East
Pacific
Rise:
Evolution
from25m.y.B.P.
tothepresent,
J.Geophys.
Res.,
87,6751-6759,
!982.
Molnar,
P.,Fault
plane
solutions
ofearthquakes
anddirection
ofmotion
PARDO
ANDSUARF.
Z:SUBDUCI•ON
OFTHERIVERA
ANDCOCOS
PLATES
12,373
intheGulfof California
andontheRivera
Fracture
zone,Geol.Soc. Singh,S.K.,L. Ponce,
andS.P.Nishenko,ThegreatJalisco,
Mexico,
Am. Bulk, 84, 1651-1658, 1973.
earthquakeof 1932: Subductionof the Rivera Plate, Bull. SeisrnoI.
Soc.Am.,75, 1301-1313,1985.
Molnar,P., andL. R. Sykes,Tectonics
of theCaribbean
andMiddle
American
regionfromfocalmechanisms
andseismicity,
Geol.Soc. Smith,W.H.F., andP. Wessel,Griddingwith continuous
curvature
Am. Bull., 80, 1639-1684, 1969.
splines
in tension,
Geophysics,
55, 293-305,1990.
Mooset,
F., TheMexican
volcanic
belt:Structure
andtectonics,
GeofiT StoiberR.E., and M.J. Carr, Quaternaryvolcanicand tectonic
Int., 12, 55-70, 1972.
segmentation
of CentralAmerica, Bull. Volcanol.,37, 304-325, 1973.
N/ibelek,
J.L., Determination
of earthquake
source
parameters
from Su/trez,G., andD. Comte,Commenton "Seismic
couplingalongthe
inversion
of bodywaves,Ph.D.thesis,346pp.,Mass.Inst.of
Chileansubductionzone" by B.W. Tichelaarand L.R Ruff, J.
Technol.,Cambridge,1984.
Nava,E., Estudio
delostemblores
deOmetepee
del7 deJuniode1982
y susreplicas,
thesis,
Univ.Nac.Aut6noma
deM6xico,
M6xieo
D.F.,
1984.
Nixon,G.T.,
Geophys.
Res.,98, 15,825-15,828,1993.
Su,Srez,
G., and S.K. Singh, Tectonicinterpretation
of the TransMexicanVolcanicBelt-Discussion,
Tectonophysics,
I27, 155-160,
1986.
The relationship
betweenQuaternary
volcanism
in
Sugrez,G., T. Mortfret,G. Wittlinger, and C. David, Geometryof
centralM6xicoandtheseismicity
andstructure
of subdueted
oceanic
subduction
and depthof the seismogenic
zonein the Guererogap,
lithosphere,
Geol.Soc.Am.Bull.,93, 514-523,1982.
Mexico,Nature, 345, 336-338, 1990.
Pathcoo,
J.F.,L.R.Sykes,
andC.H. Scholz,
Nature
ofseismic
coupling Su,Srez,
G., J.P.Ligorda,andL. Ponce,Preliminary
crustalstructure
of
along
simple
plateboundaries
of thesubduction
type,J. Geophys. thecoastof Guerrero,Mexico,usingtheminimumapparent
velocity
Res.,98, 14,133-14,159, 1993.
of refractedwaves,Geofis.Int., 31, 247-252, 1992.
Pardo,
M., andG. Su•ez, Steepsubduction
geometry
of theRivera UNAM SeismologyGroup, The September1985 Michoacan
platebeneath
theJalisco
blockin western
Mexico,Geophys.
Res.
earthquakes:
Aftershock
distribution
andhistoryof rupture,Geophys.
Lett., 20, 2391-2394, 1993.
Res.Lett., 13, 573-576, 1986.
Ponce,
L., Gaulon,G. Sufu'ez,
andE. Lomas,Geometry
andstateof
Vald6s,C., R. Meyer,R. Zufiiga,J. Havskov,andS.K. Singh, Analysis
stress
of thedowngoing
Cocos
platein theIsthmus
of Tehuantepec, of thePetarianaftershocks:
Numbers,energyreleaseandasperities,
Mexico, Geophys.Res.Lett., 19, 773-776, 1992.
J. Geophys.
Res.,87, 8519-8529, 1982.
Reyes,A., J. Brune, and C. Lomnitz, Sourcemechanismandaftershock Zufiiga,F.R., C. Gutierrez,E. Nava, J. Lermo,M. Rodriguezand R.
studyof the Colima,Mexicoearthquake
of January30,!973,Bull.
Coyoli, Aftershocks
of the sanMarcosearthquake
of April 25, 1989
SeismokSoc.Am., 69, 1819-1840, 1979.
(Ms=6.9) and some implicationsfor the Acapulco-SanMarcos
Sansores,
L., Sismicidad,
esfuerzos
y atenuaci6n
previos
y posteriores
al terremoro
del 29-Xi-1978,Oaxaca(Ms=7.8),thesis,Univ.Nac.
seismic
potential,
PureAppI.Geophys.,
1;10,
287-300,1993.
Aut6nomade M6xico, Mexico D.F., 1990.
M. Pardo,Departamento
de Geoffsica,Universidadde Chile, Casilla
Shurbet,D.H. and S.E. Cebull, Tectonicinterpretation
of the Trans- 2777, Santiago,Chile. (e-mail:mpardo@dgf.
uchile.cl)
Mexicanvolcanicbelt, Tectonophysics,
101, 159-165,1973.
G. Su,Srez,Instituto de Geoffsica, Universidad Nacional Aut6noma
Singh,S.K., andF. Mortera, Sourcetimefunctions
of largeMexican
subduction
earthquakes,
morphology
of theBenioffzone,ageof the
plate,andtheir tectonicimplications,
J. Geophys.
Res.,96, 21,487-
de M6xico, Apartado Postal 70-172, Mexico D.F., Mexico. (e-mail:
[email protected].
unam.mx)
21,502, 1991.
Singh,S.K., and M. Pardo, Geometryof theBenioffzoneandstateof
stress
in the overridingplatein centralMexico,Geophys.
Res.Lett.,
20, 1483-1486, 1993.
(ReceivedJuly 7, 1994;revisedMarch 1, 1995;
acceptedMarch 16, 1995.)