Plate boundaries initially viewed as narrow

Now recognize that many plate boundaries - especially continental - aredeformation zones up to 1000 km wide, with motion spread beyond nominal

boundary

CONTINENTAL PLATE BOUNDARY ZONES

Gordon & Stein, 1992

Continental crust ismuch thicker, less

dense, and hasdifferent mechanical

properties thanoceanic crust. Thusplate boundaries in

continentallithosphere are

generally broader andmore complicated

than in oceaniclithosphere

Studies ofcontinentalplateboundaryzonesprovideimportantinsights intofundamentalgeologicalprocessescontrollingevolution ofcontinents

Studyphases ofthe Wilsoncycle indifferentplaces

East AfricanRift

Gulf ofAden, Gulfof California

Andes

SouthernEurope

HimalayaZagros

Continentalrifting

Youngocean

Closingocean

Continentalcollision

Ocean-continent convergence

Stein &Wysession,2003

CHALLENGE: NUBIA - SOMALIA: EAST AFRICAN RIFTOPENING

Boundary geometry & motions unclear

Extension began 15-35 Ma and may beaccelerating

Surprising given slowing of nearby plates

Geologic models infer opening fromdifferences in Nubia-Arabia (Red Sea) &Somalia-Arabia (Gulf of Aden) or Nubia-Antarctica & Somalia-Antarctica (SW IndianRidge) motion

Somalia GPS data from only 4 sites, one onvolcano, one in rift zone

Not yet clear if models agree or disagreeREVEL 2000

REVEL 2003

NUBIA

ARABIA

SOMALIA

ANTARCTICA

Chu &Gordon

ARABIA

SOMALIA

NUBIA

East African rift is spreadingcenter between the Nubian(West Africa) and Somalian(East Africa) plates.

Extension is so slow, < 10mm/yr, that it is hard toresolve in plate motionmodels, so two plates areoften treated as one.

Topography, active faulting,and seismicity show aboundary zone broader, morediffuse, and more complexthan at mid-ocean ridges.

For example, seismicity endsin southern Africa withno clear connection theSouthwest Indian ridge, wherethe boundary must go Stein & Wysession,

2003

Some of the complexity ofcontinental extensional zonesresults from the fact that, unlikemid-ocean ridges, lithospherestarts off with reasonablethickness and then is stretchedand thinned

Rifting can progress far enoughthat new oceanic spreadingcenter forms, as in Gulf ofAden and Red Sea, which arenewly formed (and hencenarrow) oceans separatingArabia from Somalia and Nubia

Whether EAR will evolve thisfar is unclear: geologic recordshows rifts that, though activefor some time, failed to developinto oceanic spreading centersand died. Fossil rifts can be locifor intraplate earthquakes. Stein & Wysession,

2003

CONTINENTAL STRIKE SLIP BOUNDARY ZONE

Stein, 1993

WESTERN NORTH AMERICA PLATE BOUNDARY ZONE -DEFORMATION INWARD OF NOMINAL BOUNDARY

Hebgen Lake, Montana 1959 Ms 7.5

Owens Valley, California 1872 Mw ~7.5Stein & Wysession, 2003

Find Euler vector using GPS,earthquake slip vector,magnetic, geologic data

Motion described by Eulervector predictions (smallcircles about pole, rate

increases as sin Δ)

Assess rigidity via fit of datato Euler vector predictions

Little (< 1 mm/yr rms) internaldeformation

Deformation where fit toEuler vector prediction fails

Add Euler vectors for otherplate motions

Stein & Sella, 2002

MICROPLATES OBEY RIGID PLATEKINEMATICS, LIKE MAJOR PLATES

RIGIDDEFORMING

PACIFIC-NORTH AMERICA PLATE BOUNDARYZONE: PLATE MOTION & ELASTIC STRAIN~ 50 mm/yr plate

motion spreadover ~ 1000km

~ 35 mm/yr elasticstrainaccumulationfrom locked SanAndreas inregion ~100 km wide

Locked strain willbe released inearthquakes

Since lastearthquake in1857 ~ 5 m slipaccumulated

Elasticstrain

BroadPBZ

Stein & Wysession,2003

GPS sitevelocities

relative to NorthAmerica

San AndreasFault system

Stable SierraNevada block

Great Basin

Intermountainseismic belt

EasternCaliforniashearzone

ColoradoPlateau

PACIFIC - NORTHAMERICA PLATE

BOUNDARYZONE

Central Nevadaseismic belt

Bennett et al., 1999

Wells and Simpson, 2001

NORTHWEST NORTHAMERICA

OREGON

SIERRA

BASIN & RANGE

Complex interaction ofsubduction of Juan de Fucaplate and Pacific-North Americastrike slip

Paleomagnetic, geologic, andearthquake data suggestedrigid Oregon and Sierra Nevadamicroplates

GPS data show this andresolve motion

Basin & Range may be rigid ordiffuse extension zone - GPSdata interpretations differ

REMOVING ELASTICSTRAIN ACCUMULATION

SHOWS OREGON MICROPLATEROTATION

Velocity field from campaign and continuousGPS sites.

Reference frame is North America andellipses are 1σ.

Data dominated by elastic strain on lockedsubduction zone

Use geodetic, earthquake, and geologic datato estimate simultaneously block angularvelocities, coupling on block-boundingfaults, and GPS reference frame

Removing estimated elastic strain showsmicroplate rotation

McCaffrey et al., 2003

CONTINENTAL CONVERGENCE ZONESOf the three boundary types, continental convergence zones may be

the most complicated compared to their oceanic counterparts.

One primary difference is that, because continental crust is muchless dense than the upper mantle, it is not subducted and a Wadati-

Benioff zone is not formed. As a result, continental convergencezones in general do not have intermediate and deep focus

earthquakes.

However, the plate boundary tectonics occur over a broader andmore complex region than in an oceanic case.

Ni and Barazangi, 1984

COLLISIONBETWEEN INDIAAND EURASIA

PLATES:EARTHQUAKES

Large destructivethrust earthquakesreflect convergenceon Himalayan frontalfaults such as MainCentral Thrust

Normal faultingearthquakes occurbehind convergentzone in the TibetanPlateau, due to alongstrike extension fromgravitational collapse

Strike slip earthquakesoccur further north

Ni and Barazangi, 1984

COLLISION BETWEEN INDIAN AND EURASIANPLATES: SPACE GEODETIC MOTIONS.Mountain building by

continental collisionproduced boundaryzone extending 1000’s ofkm northward from thenominal plate boundaryat the Himalayanfront.

Total plate convergencetaken up several ways.About half occursacross locked Himalayanfrontal faults such as theMain Central Thrust

These faults are part ofthe interface associatedwith the underthrustingIndian continental crust,which thickens crustunder high Himalayas.

Larson et al., 1999

COLLISION BETWEEN INDIAN AND EURASIANPLATES: SPACE GEODETIC MOTIONS.

GPS data also showalong-strike motionbehind the convergentzone, in the TibetanPlateau, presumablybecause the uplifted andthickened crust spreadsunder its own weight.

Extension is part ofa large-scale process ofcrustal "escape" or"extrusion" in which largefragments of continentalcrust are displacedeastward by the collisionalong major strike-slipfaults.

Larson et al., 1999

Collision process is thought to involve a complex interplay between forcesdue directly to the collision, gravitational forces due to the resulting uplift and

crustal thickening, and forces from the resulting mantle flow

Crustal "escape" or "extrusion" in which large fragments of continental crustdisplaced eastward by the collision along major strike-slip faults has beenmodeled assuming that India acts as a rigid block indenting a semi-infinite

plastic medium (Asia), giving rise to a complicated faulting and slip pattern.

Also modelled numerically as thin viscous sheet flow

Tapponnier et al., 1982

COMPARISON OF GEODETIC ANDSEISMOLOGICAL EVIDENCE FOR

CRUSTAL SHORTENING IN THE TIENSHAN

GPS data indicate that thisintracontinental mountain belt, 1000-

2000 km north of the Himalaya,accommodates about half the netconvergence between India and

Eurasia.

This shortening rate is approximatelytwice that inferred from seismic

moments.

Focal mechanisms reflect local strikeof structures, despite coherent

shortening direction shown by GPSdata.

Abdrakhmatov et al., 1996

EASTERN MEDITERRANEAN COLLISION ZONEARABIA - EURASIA CONVERGENCE

Complicatedsituation involvingAfrican, Arabian,and Eurasianplates.

Northern portionsof Arabia moveapproximatelyN40°W consistentwith global platemotion models.

Eastern Turkeydriven northwardinto Eurasia,causingcompression &thrust faultearthquakes inCaucasusmountains.

McClusky et al., 2000 GPS site velocities relative to Eurasia

NUBIA SINAI

ARABIA

EURASIA

EASTERN MEDITERRANEAN COLLISION ZONEARABIA - EURASIA CONVERGENCE

At

Anatolia (At)rotates as rigidmicroplate aboutpole near Sinai

Motion acrossNorth Anatolianfault, ~ 25mm/yr, givesright-lateralstrike-slipearthquakes like1999 M 7.4Izmit, about 100km east ofIstanbul thatcaused morethan 30,000deaths.

McClusky et al., 2000 GPS site velocities relative to Eurasia

NUBIA SINAI

ARABIA

EURASIA

EASTERN MEDITERRANEAN COLLISION ZONEARABIA - EURASIA CONVERGENCE

McClusky et al., 2000 GPS site velocities relative to Eurasia

NUBIA SINAI

ARABIA

EURASIA

W. Anatolia &Aegeaninterpreted asdiffuseextension,shown bysteadilyincreasingrates

Region may be"pulled" towardHellenic arc,perhaps by anextensionalprocess similarto oceanicback arcspreading

Extension

Strike Slip

Thrust

StrikeSlip

McClusky et al., 2000

McClusky et al., 2000

MICROPLATE VERSUS DIFFUSE DEFORMATION

Anatolia (At)rotates as arigidmicroplate,about polenear Sinai

AtAegeaninterpreted asdiffuseextension,shown bysteadilyincreasing rates

McClusky et al., 2000 GPS site velocities relative to Eurasia

NUBIA SINAI

ARABIA

EURASIA

NEWALTERNATEMODEL

Aegean alsointerpreted asmicroplate

Two othermicroplatesproposed

Nyst & Thatcher, 2004 Velocities relative to Eurasia

ANDES:

NAZCA -SOUTHAMERICAPBZ

ALTIPLA

NO

FTB

DEFORMATION IN NZ-SA PLATE BOUNDARY ZONE

Integrate GPS,earthquake, platemotion & geologicdata

Elastic strain fromtrench - primaryboundary segment -& permanentdeformationaway from it

Altiplano acts asrigid block betweenforearc & thrust belt

GPS site vectors relative to stable South AmericaNorabuena et al., 1998

USING GPS VELOCITYPROFILE TO ESTIMATE

LOCKING RATE ATTRENCH AND

SHORTENING RATE INFORELAND THRUST

BELT

Norabuena et al., 1998

DEFORMATION IN PLATE BOUNDARY ZONE

Why is GPS velocity across orogen much higher than long-termcrustal shortening?

Vinstaneous = Velastic + Vpermanent (GPS) (earthquakes) (topography/shortening)

Chacoregion,

Forelandthrust belt

TOPOGRAPHICEVOLUTION MODEL

Predicted elevation (colorbackground) and velocity fieldat the surface (arrows) for thetwo shortening models.

Lower crust assumed weak

In model A material flows fromnorth & south, where strain ratesare higher, to center

In model B material from centerflows south first due to earliershortening, reversed in past 10Ma

Yang et al., 2003

REGIONAL TECTONICS OF THE MEDITERRANEAN

Oldow etal., 2002

Nubia-Eurasia convergence causes complex geometry, many possibleblocks/microplates, boundaries & motion directions often unclear

Major challenge to sort out

Calais et al., 2003 Anderson & Jackson, 1987

CHALLENGE:ADRIATICBOUNDARIES &MOTIONS

Is Adria amicroplate?

What otherblocks exist?

How do theirmotions relate toNubia-Eurasiamotion?

GPS, earthquake& geological databeing integrated

Oldow et al., 2002

Geodetic and seismic data

Jenny, Hollenstein, et al., 2004

NUBIA

EURASIA

? ?

? ADRIA?

?

TRENCH?

ADRIA MICROPLATE: NOWMOVES NORTHEAST WRT

EURASIAFocal mechanisms and GPS find Adriabounded by convergent boundaries in theDinarides and the Venetian Alps,extensional boundary in the Apennines andmoving northeast away from western Italy(Eurasia).

Adria

GPS wrt Eurasia

Nubia

Eurasia

Adria subducted southwestwardbeneath Italy.

Apennines part of thrust beltextending south to Sicily.

Arc evolved in association withopening of Tyrrhenian sea since ~5 Ma, interpreted as back arcspreading associated with rollbackof Adria slab.

As subduction migrated eastward,western Italy microplate rotatedcounterclockwise with respect toEurasia

Nubia

Adria

Eurasia

MIO-PLIOCENE TIME: SOUTHWEST CONVERGENCE

After Rosenbaum & Lister, 2004

Western Italy

GPS & EARTHQUAKES vs. GEOLOGYSHOW PLEISTOCENE MOTION CHANGE

Subduction and back arcspreading ceased withinpast 2 Ma, making Italywest of the Apenninespart of Eurasia.

Slab may be detaching(Wortel & Spakman,2000)

Adria - Eurasia motionthen caused shift fromconvergence toextension in theApennines.

AdriaEurasia

Stein & Sella (2004); modified fromMalinverno and Ryan (1986)

W. ItalyEurasia Adria

SUMMARY

Continental boundary zone occurs where motion extends beyond elasticdeformation associated with earthquake cycle at nominal plate boundary

Thses zones may included discrete microplates and perhaps diffuse deformationzones

Integrating plate motion, GPS, earthquake & geologic data can resolve geometryand rates of motion

Differences between space geodesy & geologic plate motion models areincreasingly able to resolve changes in plate motions

Inferred changes often appear to be part of long-term trends

Can be associated with changes in plate boundary geometry: mountain building(Andes, Zagros (?)), rifting (East Africa), slab breakoff (Adria), etc.

Better distribution of space geodetic sites and longer time series will improveability to identify & confirm such changes

Testing for significance of additional rigid plates

χ 2

Do data show a rigidmicroplate?

How well do thepredictions of theEuler vector fit them?

Would a diffuse modelbe better?

Which data are poorlyfit?

TEST

F TESTIs the microplatenecessary?

Do the data require it?

Is the model toocomplicated?

Separate North & SouthAmerica, Nubia &Somalia, India & Australiapass test

In these cases, plategeometry inferred fromother data

Modify test (more freeparameters) if microplateinferred only from platemotion data

IS RIVERA DISTINCT FROM NORTHAMERICA & COCOS PLATES?Rates & directions from transform andearthquake slip vector azimuths alongpresumed Pacific-Rivera boundary misfit byPacific-North America and Pacific-Cocosmotion

Improved fit from a distinct Rivera plate passesF test, so plate can be resolved

DeMets& Stein,1990