Plate TectonicsEric CalaisPurdue UniversityDepartment of Earth and Atmospheric SciencesWest Lafayette, IN 47907-1397ecalais@purdue.eduhttp://www.eas.purdue.edu/~calais/

Plate tectonics• Developed in the 1960’s and 70’s from two basic ideas:

– Paleomagnetism ⇒ Apparent Polar Wander paths for continents⇒ Continental drift [plus paleontological and climatologicalevidence]

– Ocean floor magnetic anomalies (reversals) ⇒ Sea floorspreading

• Other pieces of the puzzle:– Earthquake distribution– Earthquake focal mechanisms– Structure of the Earth⇒ Definition of plates and of their motions

• Has become the unifying theory of geology and geophysicsfor explaining earthquakes, volcanoes, mountains, oceanbasins and other major earth phenomena

History…

• Fit between coasts of SouthAmerica and Africa: AbrahamOrtelius (1596) and FrancisBacon (1620)

• Alfred Wegener, 1912,observed mismatch of climatefeatures:

• Coal in Antarctica.• Glacial deposits in now arid

regions of Southern Continents.

History…• Additional observations:

– The same fossils found on severalcontinents: Glossopteris (Fern),Lystrosaurus and Cynognathus(Triassic reptiles), Triassic reptile(freshwater reptile)

– Paleozoic mountain belts• Wegener proposed that:

– Continents move about the Earth’ssurface = theory called ContinentalDrift.

– All continents were part of singlesupercontinent in Paleozoic = Pangea

• Pangaea starts to break apart 200million years ago:

– Two continents: Laurasia (Northern)and Gondwanaland (Southern)

– Oceans form in between.

History…

Wegener’s ideas were rejected because:– His theory proposed no physical mechanism to

move the continents– There was no direct measurement or

quantitative proof that continents had moved– It was unclear how continents could move

through ocean basins.

Mapping the ocean floor

• Prior to 1900s, sea floor was thought to be flat.• WWI and WWII: sonar used to map the ocean floor.• Linear underwater mountain ranges in every ocean = Mid-Ocean Ridges• 1947: the sediment layer on the ocean floor is very thin.• If oceans were the same age as the continents (billions of years), where was all

the sediment?• Oceanic crust not as old as continental crust.• Magnetic maps of the seafloor made in the 1950s showed evidence of polarity

reversals mirrored on both sides of a mid-ocean ridge.• Harry Hess, Princeton, 1962:

– The sea floor separates along the mid-ocean ridges = sea-floor spreading.– New crust is formed at the ridge from upwelling magma, pushed (or pulled?) laterally

away from the ridge.– Older crust destroyed at trenches.

Mapping the ocean floor

• Mid-ocean ridges• Fracture zones• Continental margins• Trenches• Seamounts:

– Isolated– In chains

Cont.margin

Abyssal plain Mid-oceanic ridge

Continental shelf

Continentalslope Seamount

Bathymetric profile across the Central Atlantic

Heezen and Tharp, 1977

Mapping the ocean floor

Earthquake distribution

• Vast aseismic areas• Earthquakes concentrated along narrow bands

Earthquake focal mechanisms

• Extension at mid-ocean ridges• Strike-slip perpendicular to MOR• Compression at trenches

Earth structure

• Cold and rigid crust andupper mantle =lithosphere, brittle

• Hot and weak layerunderneath =asthenosphere, plasticflow

• Lithospheric thicknessdepends on temperature:~1300oC at the base of thelithosphere (beginning ofpartial melting of mantleperidotites)

Putting it all together: 1. plates

Present-day plates and their boundaries

Putting it all together:2. Plate boundaries

Spreading centers• FAMOUS project, 1975 (French-

American Mid-Ocean Undersea Study)– Used Alvin to dive on a segment of the

Mid-Atlantic Ridge– Observed pillow basalts, normal faults

• 1979: discovery of hot springs (“blacksmokers”) at MOR (East Pacific Ridgenear Galapagos)

Spreadingcenters

• Anomalies mapped bymagnetometers towedbehind research vessels

• Vine and Matthew,1963:– Linear magnetic

anomalies– Parallel to ridges– Symmetrical pattern

w.r.t. the ridge

Spreading centers• Harry Hess proposed the idea of

sea floor spreading (“An essay ingeo-poetry”): seafloor is created atridges by volcanism and spreadsoutward from them

• Magnetic anomalies result fromnew oceanic crust formed at theridge axis

• Continuous process, as the Earth’smagnetic field changes polarityevery ~0.5 My or so

• Magnetic reversals give the age ofthe crust

• Distance from the ridge axis showshow fast spreading occurs.

Fast versus slow ridges

Spreadingcenters

• Magma chambers belowridges

• Magma rises (buoyant)forming dikes that feedpillow basalts

• Part of magma that slowlycools at depth forms gabbros

• Residual = dense minerals(= olivine) “sediment” in themagma chamber =>cumulates

• In addition:– Sedimentation– Hydrothermal system

Ophiolites

• “Green rocks” foundin mountain belts

• Show thesuperposition ofrocks found at mid-oceanic ridges:– Pillow basalts– Sheeted dikes– Gabbros– Peridotite cumulates

A famous ophiolitecomplex: Oman

Hydrothermal Systems

Depth and heat flow

• Oceanic lithosphere cools (and thickens) as it moves away from ridge axis =>contracts and becomes denser

• Heat flow decreases as the square-root of age• Isostasy => ocean basins deepen as the square root of age of the oceanic crust• Gravitational potential difference between ridges and basins => ridge push

Age of oceanbasins

• Ocean DrillingProgram

• Age of seafloorinferred frommagnetic anomaliesconfirmed

Transform faults

Transformfaults

Transform faults

• Arrows show velocities on plates A and B. Spreadingcenters are linked by transform faults.

• Transform faults accommodate strike-slip motionbetween plates A and B.

• There is no slip occurring outside of the transform fault(dashed line = fracture zone)

A famous transform fault:the San Andreas fault

Subduction zones

• 1930: earthquakedepth increases withdistance from thetrench

• Defines an inclinedplane, sometimeswith a complex shape

• Wadati-Benioff plane(1935) The Wadati-Benioff zone under northern

Honshu, Japan, showing two parallel planes ofearthquake loci. VF indicates the volcanicfront, at the center of the land area (fromHasegawa, 1989).

Earthquakes at subduction zones

• Usually compressive mechanisms dominate for the deepestearthquakes (depths > 300-350 km). This is due in part tothe increased resistance to slab penetration in response tohigher mantle viscosity; and in part to the presence of theolivine-spinel phase change.

Seismic tomography atsubduction zones

Seismic tomography shows the subduction ofcold oceanic lithosphere under continents

Subduction zones

Distribution of down-dip stresses in inclined seismic zones. Open circles show mechanisms with thecompressional axis parallel to the dip of the zone. Solid circles show mechanisms with the tensionalaxis parallel to the dip of the zone. Crosses indicate mechanisms with other orientations. Solid linesshow the approximate configuration of the seismic zone (from Isacks and Molnar, 1971).

Model of the thermal structure of a subduction zone(convergence at 6 cm/year)⇒ oceanic crust of subducted slab will only melt atdepths greater than 600 km⇒ BUT: volcanic arc above temperature of 200-300Cin the the slab (=depths of 120-140 km), which is toocool for the ocean crust too melt, but warm enough todrive off fluid into the overlying mantle wedge.⇒ Fluids decrease the melting point of mantle materialin mantle wedge => magmatism

Mantlewedge

Temperature structure

Back-arc (=marginal) basins

http://epsc.wustl.edu/seismology/old_website/Labatts/labatts.html

Accretionary prisms

(Nankai trench)

A “typical”subduction

• Trench• Accretionary prism• Magmatism:

– Volcanoes lie about 125 to175 km from the oceanictrench

– Produce andesitic lavas (moresilicic than basaltic)

– Some of the subductedmaterial (mostly sedimentsand recycled oceanic crust) isincorporated in these lavas.

• Back-arc basin

A “typical” subduction• The ultimate fate of subducted

slab is not certain:– The absence of earthquakes

below 700 km does not meanthat the slab has been totallyintegrated into the mantle

– There is evidence that beneathsome long-lived subductionzones, slabs have penetratedinto the lower mantle throughthe 670 km discontinuity

– There is evidence of large"anomalously cold blobs" inthe lower mantle that could bethe diffused remnants ofancient subducted slabs.