Plate boundaries initially viewed as narrow Now recognize that many plate boundaries - especially continental - are deformation zones up to 1000 km wide, with motion spread beyond nominal boundary CONTINENTAL PLATE BOUNDARY ZONES Gordon & Stein, 1992 Continental crust is much thicker, less dense, and has different mechanical properties than oceanic crust. Thus plate boundaries in continental lithosphere are generally broader and more complicated than in oceanic lithosphere
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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
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
~ 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
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
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